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INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 10: 1085–1104 (1998)
ATMOSPHERIC CIRCULATION PATTERNS AND SPATIAL CLIMATIC VARIATIONS IN BERINGIA CARY J. MOCKa,*, PATRICK J. BARTLEINa and PATRICIA M. ANDERSONb a Department of Geography, Uni6ersity of Oregon, Eugene, OR 97403, USA b Quaternary Research Center, Uni6ersity of Washington, Seattle, WA 98195, USA Recei6ed 28 May 1997 Re6ised 3 March 1998 Accepted 5 March 1998
ABSTRACT Analyses of more than 40 years of climatic data reveal intriguing spatial variations in climatic patterns for Beringia (North-eastern Siberia and Alaska), aiding the understanding of the hierarchy of climatic controls that operate at different spatial scales within the Arctic. A synoptic climatology, using a subjective classification methodology on January and July sea level pressure, and July 500 hPa height anomaly patterns, identified 13 major atmospheric circulation patterns (26 pairs consisting of 13 synoptic/temperature and 13 synoptic/precipitation comparisons) that occur over Beringia. Composite anomaly maps of circulation, temperature, and precipitation described the spatial variability of surface climatic responses to circulation. Results indicate that nine synoptic pairs yield homogeneous surface climatic anomaly patterns throughout most of Beringia. However, many of the surface climatic responses illustrate heterogeneous anomaly patterns as a result of variations in circulation controls, such as troughing over East Asia and the Pacific subtropical high superimposed over topography, with small shifts in atmospheric circulation dramatically altering spatial variations of anomaly patterns. Distinctive contrasts in climatic responses, as suggested from ten synoptic pairs, are clearly evident for Western Beringia versus Eastern Beringia. These results offer important implications for scholars interested in assessing late Quaternary climatic change in the region from interannual to millennial timescales. © 1998 Royal Meteorological Society. KEY WORDS: Beringia;
Northeast Siberia; Alaska; composite anomaly maps; 500 hPa heights; sea-level pressure; temperature; precipitation; synoptic climatology
1. INTRODUCTION Paleoclimatic records from the Arctic document surface climatic responses to past changes in large-scale controls of climate at a variety of timescales ranging from interannual to millennial (Bartlein et al., 1991; Briffa et al., 1996; Overpeck et al., 1997). Knowledge of the mechanisms that cause such changes is important for validating climate models as well as for understanding climatic impacts on ecosystems and society at northern high latitudes (Peterson and Johnson, 1995; Nichols et al., 1996). Large-scale controls, such as changes in solar insolation and ice sheet size, generally cause surface responses that are expressed up to the global scale (COHMAP Members, 1988). However, some regions in the Arctic also register heterogeneous climatic responses at the regional and smaller spatial scales, as the effects of synoptic-scale circulation features and physiography become more important (Barry and Hare, 1974). Beringia, extending from North-eastern Siberia across Alaska (Figure 1), represents a vast area of the Arcto-Boreal region. A growing body of paleoclimatic proxy data from Beringia offers opportunities to document paleoclimatic responses and to test paleoclimatic hypotheses (Barnosky et al., 1987; Bartlein et al., 1991; Anderson and Brubaker, 1994). Paleoclimatic investigations suggest that some climatic changes appar* Correspondence to: Department [email protected]
of
Geography,
University
of
Oregon,
Contract grant sponsor: NSF; Contract grant number: ATM-931769; ATM-9532074
CCC 0899–8418/98/101085 – 20$17.50 © 1998 Royal Meteorological Society
Eugene,
OR
97403,
USA;
e-mail:
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ently occur synchronously throughout the region, whereas at other times are characterized by considerable variation of spatial climate heterogeneity, particularly as it pertains to changes in Western Beringia (North-eastern Siberia) versus Eastern Beringia (Alaska) (Lozhkin et al., 1993; Anderson et al., 1997). A thorough knowledge of the climatic controls that operate at regional and smaller spatial scales today is an essential aid for evaluating possible mechanisms and feedbacks responsible for the inferred climatic variations of the past. A study of the modern synoptic climatology provides a framework for understanding the hierarchy of climatic controls that operate at various spatial scales. Although the length of modern climatic records in Beringia is shorter than in many other areas of the world, the availability of a fairly dense network spanning more than 40 years does provide an excellent opportunity for examining the controls of climate in the modern record and applying these findings to improving interpretations of the synoptic climatology of the past (Diaz and Andrews, 1982; Mock and Bartlein, 1995). The climatology of Beringia has been described previously, but most of this work focused on the description of general climatic characteristics (Lydolph, 1977), on particular subareas within Beringia (Streten, 1974), or climatic and circulation characteristics at larger spatial scales only (Reed and Kunkel, 1960). Thus, the synoptic and regional climatology of Beringia as a whole, dealing with both larger and smaller-scale spatial variations and their relationships to atmospheric circulation, remains poorly understood. For example, the comprehensive work of Trewartha (1981) on the regional climatology for selected areas around the world barely covers the edges of Beringia. This study presents a synoptic climatology of Beringia utilizing all available long-term temperature and precipitation records in order to examine the climatic controls that explain spatial climatic variations. We emphasize January and July because these 2 months generally represent the two extremes within the annual cycle when different climatic regimes occur. For example, precipitation bar graphs of all available
Figure 1. Precipitation bar graphs showing relative comparisons of averaged monthly precipitation for Beringia. For each bar graph, January is represented in the far left and December in the far right © 1998 Royal Meteorological Society
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stations in Beringia, with mean monthly precipitation covering at least a 20-year period, clearly exhibit a summer precipitation maximum over much of Eastern Beringia (Alaska) and interior Western Beringia (North-eastern Siberia). Specifically, many stations exhibit a July maximum, and some stations over the eastern half of Western Beringia exhibit a secondary winter maximum (Figure 1). We acknowledge that some locations, particularly along coastlines, exhibit minimum and maximum extremes within the annual cycle during February and August, respectively, due to a thermal lag effect, but the number of stations exhibiting this effect is lower than those with January and July extremes (World Meteorological Organization, 1981). Although this paper emphasizes the importance of understanding the modern climate to apply to understanding paleoclimate, it is also applicable for understanding spatial and temporal variations within the period of instrumental records. ENSO events, for example, are known to affect Alaska (Diaz and Kiladis, 1992), but have not been studied in detail for Western Beringia; results from this study can help address the possible synchroneity between climate responses of Western Beringia versus Eastern Beringia.
2. STUDY AREA AND DATA Beringia, as defined in this paper, is bounded by the 140°E meridian to the west, the Alaska/Canada border to the east, the 76°N parallel to the north, and the 50°N parallel to the south. Different physiographic contrasts are evident between Western Beringia and Eastern Beringia. Most of Western Beringia consists of tectonic depressions, with topography ranging from 1000 to 2000 m in the Upper Kolyma River region, and in the Sredinny and Koryak Ranges of the Kamchatka Peninsula. Localized rainshadow effects are evident in south-western Beringia, and the topography also restricts maritime climatic effects from penetrating inland. In contrast, the generally east–west oriented Brooks Range in Northern Alaska and the Alaska Range in southern Alaska have elevations that exceed 3500 m, and these relatively higher mountain ranges play important roles on creating heterogeneous climatic responses. During January, averaged monthly temperatures range from − 47 to − 5°C in Western Beringia and − 30 to 0°C in Eastern Beringia, with the lowest values centered in the continental interiors because of persistent temperature inversions (World Meteorological Organization, 1981). July averaged monthly temperatures exhibits generally an increase with decreasing latitude, with values ranging from 0 to 16°C. Precipitation also generally increases with decreasing latitude (Figure 1), but the superimposition of topography and coastal land/sea breezes creates some spatially heterogeneous patterns. January averaged precipitation ranges from 1 to 120 mm in Western Beringia and 6 to 348 mm in Eastern Beringia, but most stations have values less than 30 mm. July averaged precipitation for both western and Eastern Beringia generally falls in the same range of variability, with values varying from 20 to 225 mm. Monthly circulation data consist of gridded sea-level pressure (in hPa) and 500 hPa heights (in geopotential meters). The data encompass a region from central Asia to central North America so that large-scale atmospheric circulation can be described adequately. These data cover the period from January 1946 to January 1989 (Mass, 1993). January sea-level pressure was examined in order to clearly represent variations of the Siberian high as well as other persistent large-scale temperature inversions that are prevalent during the winter months (Mock and Anderson, 1997). Averaged sea-level pressure for January indicates two main pressure centers that are important for influencing winter Beringian climate: the Siberian high to the west and the Aleutian low just south of the Bering Sea (Figure 2). Higher levels, such as at the 500 hPa level, show that the polar jetstream is normally centered south of Beringia in January, enabling the formation of the Siberian high. However, winter 500 hPa variations were found to not represent surface climatic variations as closely as sea-level pressure since they do not respond as directly to surface diabatic processes, and thus were not discussed in this paper. The 500 hPa level was used for July because this level clearly represents upper level troughs that affect surface climate over Beringia which sea-level pressure data does not depict as clearly. Sea-level pressure data was also examined since they may add further insight in some cases. Averaged July 500 hPa heights indicate that the ‘East Asian trough’ is normally centered through the Beaufort and Bering Seas (Moritz, © 1998 Royal Meteorological Society
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Figure 2. Averaged January and July sea-level pressure in hPa (right), and averaged January and July 500 hPa heights in geopotential meters (left) for the period 1946 – 1989
1979), associated with generally north-westerly flow over Siberia and south-westerly flow over Alaska (Figure 2). July sea-level pressure shows that the Pacific subtropical high is normally prevalent over the North-eastern Pacific Ocean, bringing southerly flow in Western Beringia and westerly flow to Eastern Beringia. Fifty-nine temperature and eighty-seven precipitation stations were ultimately included in the analyses. These data were also expressed as monthly averages and available from the period 1946–1990 (Groisman et al., 1991; Vose et al., 1992). Only stations with less than 10% of missing data for the period were used; in most cases the amount of missing data is much less. The precipitation data from Western Beringia were provided by Pavel Groisman (Groisman et al., 1991). Careful screening and corrections were made on the data concerning changes in instrument exposure, elevation, and location with time. Some of the precipitation data for winter were found to be unreliable as a result of wind-blown snow and problems in converting snow amounts to liquid precipitation; they were simply deleted from the dataset (Groisman et al., 1991). The temperature data for all of Beringia and the precipitation data for Eastern Beringia were taken from the Global Historical Climatology Network (GHCN) (Vose et al., 1992). The data were subjected to a large amount of quality control, with all impossibly extreme values interpreted as ‘missing’ data, and obvious discontinuities were detected as well. Three stations in North-central Alaska from the national climatic data center’s cooperative station network, with data mostly continuous since 1946, were added to the station network. © 1998 Royal Meteorological Society
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3. METHODS The synoptic climatology of Beringia was examined by first classifying synoptic patterns followed by constructing composite anomaly maps of temperature and precipitation. This approach is similar in design for both January and July, differing only because of the different synoptic patterns that prevail between these months. The classification of synoptic types was done by examining anomaly centers of circulation maps for each month from 1946 to 1989 (Muller, 1977; Maddox et al., 1995). a subjective classification was used as opposed to automated methods because of the relatively lower number of months available (43 months) for classification. This qualitative approach is justified in that other similar synoptic classification studies with similar goals have proved robust (Yarnal, 1993; Maddox et al., 1995). The framework for generating the synoptic types described herein was formulated from the existing literature (Streten, 1974; Lydolph, 1977) that describes the main modes of circulation variability and thus the most active anomaly centers (e.g., Siberian high). Since our subjective classification is based on anomaly centers, a single month can be classified under several different synoptic types (e.g., a month may have both an intensified Aleutian low and an intensified Pacific subtropical high). Conversely, some months contain no synoptic type due to weak anomalies within and surrounding Beringia. Composite anomaly maps depict spatial climatic patterns that represent differences between averaged and anomalous values of circulation (Yarnal, 1993). Because the objective of this research is the identification of primary synoptic patterns, composite anomaly maps were constructed for the synoptic types that occurred at least seven times out of the 43 months available for classification. The selection of seven was chosen because this is a conservative number that is high enough to produce meaningful monthly composite anomaly maps when dealing with a 43 year record (Ely et al., 1994; Hirschboeck et al., 1996). The interpretation of composite circulation anomaly maps is similar to standard climate anomaly maps, with increased clockwise (anticyclonic) flow around positive centers and increased counter-clockwise (cyclonic) flow around negative centers. Temperature and precipitation composite anomaly maps illustrate the spatial patterns and responses to each of the synoptic types. These maps were constructed in similar fashion as the circulation composite anomaly maps. Stations with missing data for more than one month were not included on the temperature and precipitation composite maps. Temperature anomalies are expressed as standardized z-scores, with values of less than − 0.65 indicative of very abnormally cold conditions, − 0.65 to 0.00 as abnormally cold, 0.00 to 0.65 as abnormally warm, and greater than 0.65 as very abnormally warm. Since some of the synoptic climate classifications occurred more than ten times out of 43 cases possible, the probabilities of z-scores at individual stations being greater than 9 1.00 diminish dramatically, thus this criterion was not used. The 9 0.65 cutoff was used instead because it corresponds closely with the top and bottom 25% probability of a normal distribution (Gregory, 1978), and mapping of patterns provides a much clearer representation of regional-scale anomaly patterns (Hirschboeck et al., 1996). Precipitation anomalies are expressed as percentage departures from normal, with values of less than − 30% indicative of very abnormally dry conditions, − 30 to − 10% as abnormally dry conditions, −10 to 10% as no significant anomaly, 10 to 30% as abnormally wet, and greater than 30% as very abnormally wet conditions. This criterion of expressing anomalies clearly illustrates spatial patterns of precipitation responses to atmospheric circulation patterns.
4. JANUARY CLIMATE
4.1. Circulation controls The three primary circulation controls that affect Beringian climate during January are the Siberian high, the Pacific subtropical high, and the Aleutian low. Spatial variations of these circulation controls in winter are clearly apparent in anomaly maps over three primary locations: over Northern Siberia, the central North Pacific Ocean, and the Eastern Pacific Ocean. The climate over most of the area of © 1998 Royal Meteorological Society
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Northern Siberia has been explained by the anomalous behavior of the Siberian high (Keegan, 1958; Dmitriev, 1970). Serreze et al. (1993) also noted that anticyclonic frequencies tend to be relatively high in this region during winter. Lydolph (1977) implied that storms originating from the nearby Kara Sea tend to move south-eastward, south of the Siberian high. Pressure anomalies directly over Western Beringia during any particular month are generally low as a result of persistent temperature inversions. The central North Pacific Ocean is a region that also affects surface climate over both Western and Eastern Beringia (Lydolph, 1977; Overland and Pease, 1982), since variations in the Aleutian low here determine predominant wind directions and, thus, temperature and precipitation anomalies. An easterly shift of pressure anomalies into the north-eastern Pacific Ocean also often occurs, representing variations in the Pacific subtropical high or Aleutian low, and affecting surface climatic variations over Eastern Beringia (Fahl, 1975; Mock, 1996). Therefore, six synoptic types were classified for all January months, with three types corresponding to positive anomalies of the three main pressure centers, and the three other types corresponding to negative anomalies of the same pressure systems (Table I). Positive and negative anomalies were treated separately when compared with surface climate, because opposite variations in pressure do not necessarily correspond identically with opposite variations in temperature or precipitation (Cayan, 1996). The synoptic types are termed as the following in relation to location and circulation anomalies: Siberia positive, Siberia negative, North-central Pacific positive, North-central Pacific negative, Northeast Pacific positive, and Northeast Pacific negative.
4.2. Siberia types Results for the Siberia positive and Siberia negative types generally show opposite synoptic patterns, but the temperature and precipitation responses are not perfectly opposite between these two different patterns (Figure 3). The sea-level pressure composite anomaly map for Siberia positive, based on ten events (23%), shows positive anomalies encompassing most of the northeast Asian coast into the Arctic Ocean. Associated with the northern expansion of this Siberian high are negative anomalies over the central Pacific Ocean which represent a stronger Aleutian low. Temperature anomalies are negative in central Beringia due to the increased frequencies of cold north-easterly winds around the Siberian high. However, throughout the remainder of Beringia, temperature anomalies are mostly positive. The flow around the western edge of the Siberian high becomes more southerly and warmer by the time it reaches the western Beringian interior. Over south-eastern Beringia, positive temperature anomalies are explained by the stronger Aleutian low, as increased warmer southerly flow results from the north-eastern Pacific. This southerly flow also explains the positive precipitation anomalies in south-eastern Beringia, with the Alaska Range both restricting moist air masses from penetrating inland and causing orographic enhancements. Most of western Beringia exhibits negative precipitation anomalies as a result of the dominance of an enhanced Siberian high, with the largest negative values in the coastal areas of Western Beringia. The Siberia negative synoptic composites are based on eight events (19%). The negative sea-level pressure anomalies centered off northern Asia in the sea-level pressure composite anomaly map suggest increased north-westerly flow as cold air masses predominate throughout most of Western Beringia Table I. List of synoptic types for January from 1946 to 1989 Synoptic type
Years
Siberia positive Siberia negative North-central Pacific positive North-central Pacific negative Northeast Pacific positive Northeast Pacific negative
1953, 1948, 1947, 1946, 1949, 1952,
© 1998 Royal Meteorological Society
1960, 1949, 1954, 1955, 1950, 1958,
1961, 1957, 1956, 1960, 1957, 1961,
1970, 1962, 1959, 1966, 1963, 1964,
1974, 1964, 1962, 1977, 1969, 1976,
1975, 1967, 1968, 1978, 1972, 1978,
1977, 1973, 1971, 1979, 1975 1983,
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Figure 3. January composite anomaly maps for the Siberia positive (left) and Siberia negative (right) types. Sea-level pressure composite anomaly maps are in the top row (hPa). Temperature composite anomaly maps are in the middle row (large pluses and minuses represents z-scores equal or greater than 0.65 and −0.65 respectively, and smaller pluses and minuses represent from 0.00 to 0.65 and − 0.65 to 0.00, respectively). Precipitation composite maps are in the bottom row, expressed as percentage departures from normal (large pluses and minuses represent equal or greater than −30%, small pluses and minuses represent equal or greater than 10 to 30% and − 10 to − 30%, respectively, and dots represent from −10 to 10%)
(Figure 3), as indicated by widespread negative temperature anomalies. The negative temperature anomalies are likely due to radiation losses caused by clear skies and light winds of a persistent (not necessarily stronger) Siberian anticyclone. Positive temperature anomalies in Eastern Beringia are as a © 1998 Royal Meteorological Society
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result of increased maritime influence from stronger south-westerly flow around the northern end of a weakened Aleutian low. The increased advection of winds from the southwest also cause periodic weakening of the temperature inversion, allowing warmer air to mix down to the surface (Kahl, 1990). Precipitation anomalies are negative in far Western Beringia as a result of drier air masses and anomalously colder temperatures, but generally increase eastward as warmer and moist southerly airflow becomes more predominant.
4.3. North-central Pacific types The North-central Pacific positive synoptic type occurred during eight of the months in the modern record (19%). The associated composite anomaly map of sea-level pressure shows positive anomalies centered in the Bering Sea, expanding into the North-central Pacific and most of the land areas of Beringia (Figure 4). These anomalies indicate a weaker Aleutian low as compared to normal. Temperature anomalies show opposite signs in Western Beringia as opposed to Eastern Beringia. The positive temperature anomalies throughout Western Beringia and the negative anomalies throughout Alaska are associated with the weaker Aleutian low. This synoptic situation causes weaker flow of warm air masses from the south to Alaska and also weaker flow of cold air masses from Alaska and the Beaufort Sea to Western Beringia. Positive precipitation anomalies are evident over most of south-western Beringia, resulting from the warmer and wetter flow from the Sea of Okhotsk feeding into cyclones traversing through the low-elevation corridor located just to the west. The magnitude of positive precipitation anomalies decreases dramatically inland since topographic effects restrict the relatively warmer and moister air along the Sea of Okhotsk. Over most of Eastern Beringia, precipitation anomalies exhibit little variability. The higher precipitation anomalies along the North Alaska coast and to the southeast may relate to occasional storms traveling from the southwest around the high pressure center, but winter precipitation is generally very low for this region. The North-central Pacific negative type was classified for eleven months (26%). Sea-level pressure anomalies represent a strengthened Aleutian low (Figure 4). Widespread positive temperature anomalies are evident over all of Eastern Beringia and somewhat weaker positive temperature anomalies in Western Beringia. These anomalies are due to warmer south-westerly flow around a strengthened Aleutian low traversing through Eastern Beringia and eventually moving from a north-easterly direction into Western Beringia, as turbulent down-mixing of warmer boundary-layer air breaks down the temperature inversion. Anomalies change to negative along the Kamchatka Peninsula, reflecting flow from the north over land which is normally colder than the more typical southerly flow over the Sea of Okhotsk and the Western Pacific Ocean. Because sea-level pressure anomalies indicates an eastward shift in circulation features as compared to normal, allowing intensification of the Aleutian low in the Bering Sea, negative precipitation anomalies persist throughout most of Beringia due to increased northerly flow, especially in south-western Beringia. Onshore flow from the strengthened Aleutian low, coupled with orographic enhancements in areas south of the Alaska Range in Eastern Beringia, result in positive precipitation anomalies.
4.4. Northeast Pacific types The Northeast Pacific positive type occurred during 7 months in the modern record (16%). The sea-level pressure composite anomaly map represents a weaker Aleutian low and more frequent anticyclonic activity, centered south of Alaska (Figure 5). Positive temperature anomalies are distributed over most of Beringia, with the highest anomalies prevalent in the central portion of the study area. Warmer, southerly flow around the anomalous high pressure center explain these temperature anomalies in a similar manner as for the North-central Pacific positive type, except that the eastward location of sea-level pressure anomalies allow the positive temperature anomalies to spread eastward. Precipitation anomalies are mostly positive throughout Beringia as a result of the higher temperatures, generally enabling the air masses to be relatively warmer and wetter as compared to normal. Isolated negative precipitation anomalies along the Western Beringian coast are probably related to localized land-sea breeze effects. © 1998 Royal Meteorological Society
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The Northeast Pacific negative type occurred in eight months of the modern record (19%), and the sea-level pressure composite anomaly map shows a stronger and eastward shift of the Aleutian low. Temperature anomalies are mostly positive in Eastern Beringia and negative in most of the region to the west. The positive temperature anomalies along the coast correspond with the distribution of positive precipitation anomalies; this positive correlation is restricted to south of the Alaska Range and related to anomalously stronger south-westerly flow around the eastern end of the Aleutian low. North of the
Figure 4. January composite anomaly maps for the North-central Pacific positive (left) and North-central Pacific negative (right) types. Symbol representation is the same as listed in Figure 3 © 1998 Royal Meteorological Society
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Figure 5. January composite anomaly maps for the Northeast Pacific positive (left) and Northeast Pacific negative (right) types. Symbol representation is the same as listed in Figure 3
Alaska Range and in parts of Western Beringia, slightly colder and drier conditions result as compared to normal, perhaps affected by dry airstreams associated with easterly and north-easterly flow along the northern side of the stronger Aleutian low. © 1998 Royal Meteorological Society
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5. JULY CLIMATE
5.1. Circulation controls During July, the normal pattern of 500 hPa heights illustrates a dramatically weaker flow as compared to January, with the number of longwaves increases from 3 to 4 or 5 over the Northern Hemisphere (Harman, 1991). The summer climate of Beringia is strongly affected by the configuration of the ‘East Asian Trough’ (Moritz, 1979) and adjacent ridges as well as the Pacific subtropical high, as the jetstream and the associated ‘polar front’ account for latitudinal summer temperature gradients over Beringia (Krebs and Barry, 1970). Cyclogenesis normally occurs in Eastern Siberia (Whittaker and Horn, 1982; Serreze et al., 1993) and around Mongolia (Chen et al., 1991), with the storms generally move eastward to affect Beringia. Northeast of Beringia, anticyclonic frequencies are generally highest around the Beaufort Sea (Reed and Kunkel, 1960; Serreze et al. 1993). However, variations in the longwave pattern occasionally occur. For example, trough-ridge contrasts may occur over Beringia and the adjacent Pacific and Arctic Oceans, causing different climatic contrasts between conditions in Western Beringia versus Eastern Beringia (Streten, 1974; Lydolph, 1977). Ridges can also provide some precipitation along their adjacent edges by advecting moisture from the oceans over land, as exhibited for example by moist flow along the western edge of the Pacific subtropical high into south-eastern Russia (Lydolph, 1977). Anomalous variability of July 500 hPa heights as well as sea-level pressure data tend to occur in four areas within the study area: Western Beringia, the North-western Pacific, the North-eastern Pacific, and off the North Alaska coast (Mock and Anderson, 1997). Initially, eight synoptic types were classified for all of the July months, with four types each corresponding to positive and negative anomalies associated with the four main pressure centers (Table II). However, only 5 months were found that depict positive anomalies centered over Western Beringia. Since this number is too low to conduct a meaningful composite anomaly analysis, this synoptic type is not discussed any further. Therefore, the remaining seven synoptic types are termed as the following: Western Beringia negative, North-western Pacific positive, North-western Pacific negative, North-eastern Pacific positive (‘North-eastern’ differentiates from ‘Northeast’ which was used in the January classification), North-eastern Pacific negative, Alaska positive, and Alaska negative. 500 hPa maps are emphasized more in the following discussion of summer synoptic types since they exhibit stronger anomaly patterns as compared to sea-level pressure. However, all the composite anomaly maps of sea-level pressure maps are provided for comparisons (Figures 6 and 7).
5.2. Western Beringia negati6e type This synoptic type was classified for ten Julys in the modern record (23%), with its main characteristic being an area of negative anomalies of 500 hPa heights centered over Western Beringia (Figure 6). Negative anomalies of sea-level pressure also occur over Western Beringia, but the magnitudes of anomalies are relatively lower as compared with the 500 hPa pattern (Figure 6). Associated with these negative anomalies is a weaker center of positive anomalies over the Beaufort Sea, north of Alaska. These two different anomaly centers explain contrasts of surface climatic responses between Western Beringia Table II. List of synoptic types for July from 1946 to 1988 Synoptic type
Years
Western Beringia negative North-western Pacific positive North-western Pacific negative North-eastern Pacific positive North-eastern Pacific negative Alaska positive Alaska negative
1946, 1952, 1947, 1949, 1950, 1947, 1953,
© 1998 Royal Meteorological Society
1950, 1953, 1964, 1951, 1958, 1949, 1956,
1955, 1956, 1965, 1954, 1960, 1950, 1961,
1963, 1957, 1970, 1955, 1962, 1955, 1963,
1964, 1959, 1971, 1963, 1968, 1958, 1964,
1966, 1961, 1974, 1966, 1973, 1968, 1967,
1968, 1962, 1982, 1967, 1975, 1972, 1969,
1976, 1969, 1985, 1979, 1976, 1977, 1973,
1978, 1984 1987 1980, 1978, 1978, 1981,
1979 1981, 1986 1983, 1988 1983, 1985, 1987 1984
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Figure 6. July composite anomaly maps for the Western Beringia type. The 500 hPa composite anomaly map is at the upper left (geopotential meters), sea-level pressure anomaly map at the upper right, temperature composite anomaly map at the lower left (large pluses and minuses represents z-scores equal or greater than 0.65 and −0.65, respectively, and smaller pluses and minuses represent from 0.00 to 0.65 and −0.65 to 0.00, respectively), and precipitation composite map at the lower right and expressed as percentage departures from normal (large pluses and minuses represent equal or greater than − 30% respectively, small pluses and minuses represent equal or greater than 10 to 30% and −10 to −30%, respectively, and dots represent from −10 to 10%)
and Eastern Beringia. The temperature anomaly map shows widespread negative anomalies over Western Beringia as a result of increased colder north- westerly flow from the increased troughing. This East Asian trough also steers more storms from the west into the region, as shown by mostly positive precipitation anomalies over Western Beringia. Somewhat higher precipitation anomalies tend to occur in the south-eastern portion of Western Beringia, perhaps due to storms intensifying as they enter the Sea of Okhotsk. Over Eastern Beringia, temperature anomalies are mostly positive in the interior as a result of increased north-easterly flow around the anticyclone centered in the Beaufort Sea. This synoptic pattern also explains the negative precipitation anomalies, but anomalies are generally weak.
5.3. North-western Pacific types The North-western Pacific positive type was identified for nine months (21%). Although the classification is based on positive 500 hPa height anomalies centered in the Western Pacific off the Kamchatka Peninsula, the 500 hPa anomaly map illustrates clearly that negative anomalies to the north of Eastern Beringia are associated with these positive anomalies (Figure 8). Sea-level pressure anomalies exhibit a similar pattern (Figure 7). The positive anomalies reflect an expansion and strengthening of the western Pacific subtropical high, and the negative anomalies represent a northward shift of the East Asian trough. © 1998 Royal Meteorological Society
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Temperature anomalies show opposite contrasts between Western and Eastern Beringia, with positive anomalies predominant in the west and negative anomalies throughout the east. The highest positive temperature anomalies are along the eastern coast of Western Beringia due to increased warm southerly flow along the western end of the Pacific subtropical high. Negative temperature anomalies over Eastern Beringia, especially in the interior, result because of stronger north-westerly flow from the Arctic Ocean. Precipitation anomalies over most of Beringia are weak, with the highest anomalies restricted along the coast of the Sea of Okhotsk from increased moisture advected from the south. The North-western Pacific negative type occurred on nine of the months in the modern record (21%). The 500 hPa anomaly map
Figure 7. Sea-level pressure composite anomaly maps for six July synoptic types © 1998 Royal Meteorological Society
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generally shows the opposite pattern as for the North-western Pacific positive type, but the zone of positive anomalies to the north is centered farther west over the Northern Siberia coast. Sea-level pressure anomalies are generally weak (Figure 7). The 500 hPa positive anomalies represent increased ridging in the region, while the negative anomalies off Kamchatka Peninsula indicate a weaker western Pacific subtropical high. Along the eastern portion of Western Beringia, temperature anomalies are negative because of weaker and perhaps less frequent southerly flow around the subtropical high. To the west, the occurrence of a ridge perhaps explains the positive temperature anomalies. With less moisture advected from the weaker subtropical high, precipitation anomalies in Western Beringia are mostly negative. Over Eastern Beringia, the positive temperature anomalies along the North-western Alaskan coast may be due to increased south-westerly flow from the weaker Pacific subtropical high. Positive anomalies are weak in the interior because positive 500 hPa anomalies to the north are centered farther west, thus only slightly stronger easterly flow occurs. The negative temperature anomalies along the south-eastern Beringian coast south of the Alaska Range are perhaps a result of prevalent westerly flow. Precipitation anomalies in Alaska, however, are mostly small, as the important circulation anomalies for precipitation in this area are centered too far westward.
5.4. North-eastern Pacific types The North-eastern Pacific positive type, based on eleven events in the modern record (26%), represents a strengthened and expanded Eastern Pacific subtropical high south of the Aleutian Islands. The 500 hPa anomaly map, and the sea-level pressure anomaly map to a lesser degree, illustrate this pattern, associated with a weak center of negative anomalies over Western Beringia (Figures 7 and 9). Temperature anomalies are negative over most of Beringia with the exception of south-eastern Beringia. Perhaps the stronger ridging in the Eastern Pacific enhances a trough centered over Western Beringia, causing colder air to predominate over most of Beringia. The positive anomalies in south-eastern Beringia are the result of warm south-westerly flow along the northern portion of the ridge. Precipitation anomalies are generally positive over Beringia, caused by the trough over Western Beringia and the stronger, wetter south-westerly flow over most of Eastern Beringia. The North-eastern Pacific negative type, based on eleven events (26%), shows negative 500 hPa and sea-level pressure anomalies that suggests a weaker Pacific subtropical high in the north-eastern Pacific Ocean. However, interestingly, an associated area of 500 hPa positive height anomalies is evident to the north, perhaps indicative of a weakened general circulation that includes the circumpolar vortex. Less frequent westerly flow from a weakened general circulation may explain small positive temperature anomalies in Western Beringia, and the high temperature anomalies over Eastern Beringia are the result of less frequent westerly flow from the weaker subtropical high and increased easterly flow around the anticyclone north of Eastern Beringia. Precipitation anomalies are generally weak, especially in Western Beringia as circulation anomalies near the region are weak as well. Over Eastern Beringia, negative precipitation anomalies prevail, related to airflow associated with the weaker Pacific subtropical high and the anticyclone north of Eastern Beringia as discussed for temperature.
5.5. Alaska types The Alaska positive type occurred on 12 months in the modern record (28%), characterized primarily by the center of positive 500 hPa height and sea-level pressure anomalies off the Northern Alaska coast (Figures 7 and 10). Associated with these positive anomalies are negative anomalies over south-western Beringia, representative of an intensified East Asian trough. These 500 hPa anomaly patterns are similar to the map from the Western Beringia negative synoptic type, but differs obviously with the highest anomalies instead being associated with the positive anomalies over Eastern Beringia. Temperature anomalies show mostly opposite signs of the responses in Western Beringia versus Eastern Beringia. The negative temperature anomalies over Western Beringia result from the increased troughing, with colder air coming from the northwest. The positive temperature and negative precipitation anomalies over Eastern Beringia, especially the continental interior, are the result of increased warmer and drier easterly flow © 1998 Royal Meteorological Society
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Figure 8. July composite anomaly maps for the north-western Pacific types. 500 hPa composite anomaly maps are in the top row (geopotential meters). Temperature composite anomaly maps are in the middle row (large pluses and minuses represents z-scores equal or greater than 0.65 and − 0.65, respectively, and smaller pluses and minuses represent from 0.00 to 0.65 and −0.65 to 0.00, respectively). Precipitation composite maps are in the bottom row, expressed as percentage departures from normal (large pluses and minuses represent equal or greater than − 30%, respectively, small pluses and minuses represent equal or greater than 10 to 30% and − 10 to − 30%, respectively, and dots represent from −10 to 10%)
around the anticyclone centered to the north. In Western Beringia, precipitation anomalies are small but generally positive, particularly along the Eastern Siberian coast because of the increased troughing and more frequent storms coming from the west. © 1998 Royal Meteorological Society
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The Alaska negative type, which occurred during ten months in the modern record (23%), is characterized with negative 500 hPa height and sea-level pressure anomalies to the north of Eastern Beringia. However, unlike the Alaska positive type, the opposite 500 hPa anomalies in sign are centered over the Bering Sea as opposed to in Western Beringia; this explains the lack of a west–east contrast of opposite anomaly signs. The positive anomalies indicate a northern movement and expansion of the subtropical high, associated with a shift of the trough to the north of Eastern Beringia. Temperature
Figure 9. July composite anomaly maps for the North-eastern Pacific positive (left) and North-eastern Pacific negative (right) types. Symbol representation is the same as listed in Figure 8 © 1998 Royal Meteorological Society
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Figure 10. July composite anomaly maps for the Alaska positive (left) and Alaska negative (right) types. Symbol representation is the same as listed in Figure 8
responses are negative throughout most of Beringia, as the influence of stronger westerlies spreads from Western Beringia through most of Eastern Beringia. The positive temperature anomalies in Western Beringia are due to the increased warm and southerly flow around the center of positive height anomalies. The south-eastern Beringian coast also exhibits positive temperature anomalies, perhaps because the colder air northward cannot penetrate southward through the Alaska Range too frequently. Precipitation anomalies are weak but generally positive throughout most of Beringia, because of the increased troughing, especially in interior Eastern Beringia. © 1998 Royal Meteorological Society
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6. DISCUSSION AND IMPLICATIONS FOR UNDERSTANDING SPATIAL CLIMATIC VARIATIONS This paper provides the first comprehensive description of the relationships between synoptic-scale atmospheric circulation patterns and associated spatial variations of surface responses at both large and small spatial scales for Beringia. The results are summarized as follows: (i) Out of the 26 different pairs of synoptic/surface responses in the 13 synoptic classifications (13 synoptic/temperature and 13 synoptic/precipitation comparisons) examined in this paper, nine clearly exhibit homogeneous responses of the same anomaly sign over most of Beringia (e.g. July Northeastern Pacific synoptic types). Generally, temperature responses to circulation exhibit clearer patterns of homogeneity as compared with precipitation because heterogeneous precipitation responses are complicated due to topographic features causing orographic, rainshadow, and channeling effects. (ii) Surface climatic responses between Western and Eastern Beringia often exhibit opposite anomaly signs from the different synoptic climatic controls that are responsible. For example in July, anomalous high precipitation in Western Beringia may result from a stronger trough centered in the region, whereas high precipitation in south-central Alaska may require a strengthened Eastern Pacific subtropical high. Seven synoptic/temperature and three synoptic/precipitation of the 26 total different pairs of synoptic/surface responses examined in this paper clearly depict an inverse relationship of anomalies, and the magnitude of the anomalies tend to be higher when compared with spatial patterns of climatic responses in most other synoptic types; (iii) Considerable spatial variability of surface climatic responses is also clearly evident at smaller scales because of topographic features and coastal-inland contrasts. Several synoptic types yield contrasting surface responses on opposite sides of the Alaska Range, and other synoptic types show that surface responses tended to weaken or change sign as distance increases northward from the Sea of Okhotsk; (iv) Small shifts of circulation patterns can dramatically alter the spatial variations of surface climatic responses. For example, a strengthened Aleutian low centered over the Bering Sea (January North-central Pacific negative type), is associated with generally warmer temperatures throughout most of Beringia. An easterly shift of the strengthened Aleutian low to south of the Aleutian Islands (January North-east Pacific negative type), changes the spatial pattern of temperature responses, with negative anomalies over Western Beringia and positive anomalies over Eastern Beringia. The results of this paper provide several important implications for studying past climates and climatic change of Beringia. Most importantly, Beringia cannot be treated as a single homogeneous climatic unit. The synoptic climatic controls that govern temperature and precipitation anomalies differ spatially for different locations, particularly as they relate to Western Beringia versus Eastern Beringia. Furthermore, the combination of characteristics of different synoptic controls (e.g. Aleutian low, Siberian high) can differ in the past, leading to the possibility of numerous different patterns of spatial climatic responses. Physiographic controls also cause considerable spatial variability of surface climatic responses, warning against the dangers of generalizing paleoclimate patterns or trends from only a few sites. Useful next steps in investigating the synoptic climatology of Beringia would be to examine other climatic variables such as radiosonde, solar radiation, surface wind, and cloud observations, and examine the data at the daily timescale. At various times during the late Quaternary, external climatic controls such as ice sheet size and insolation were dramatically different that today. For example, increased sea ice adjacent to the land areas and a large ice sheet east of Beringia were present around 18000 years ago, and summer insolation was approximately 8% greater than today around 9,000 years ago (COHMAP Members, 1988). These external controls undoubtedly affected the magnitude and spatial variability of circulation controls. Consequently, temperature and precipitation anomalies may have exhibited homogeneous responses at times in anomaly sign throughout Beringia. Atmospheric general circulation models and regional climate models can be used to simulate synoptic-scale circulation patterns and explain some surface climatic responses for past © 1998 Royal Meteorological Society
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and present Beringian climates (Bartlein et al., 1991; Lynch et al., 1995), but such simulations may not explain some paleoclimatic reconstructions for particular sites that respond to both large-scale and localized climatic controls. Since different external controls occurred in the past, no exact modern analogues occur in the modern instrumental record that resembles past climates. However, many synoptic controls that occur today also most likely similarly occurred in the past. Thus an understanding of the modern synoptic climatology may be used as a conceptual guide to complement climate models to explain spatial variations of anomalies, and to test paleoclimatic hypotheses of synoptic patterns. For example, fossil paleoclimatic data suggest that the paleoclimatic histories of Western and Eastern Beringia for much of the late Quaternary were most likely much different from one another (Lozhkin et al., 1993; Anderson and Brubaker, 1994; Anderson et al. 1997), consistent with the information from the modern synoptic climatology that shows the climatic differences of the two regions is the rule rather then the exception. Paleoclimatic evidence also shows distinct heterogeneous spatial variations within Eastern Beringia, also relating with some of the climatic responses to some of the synoptic types discussed in this paper. The authors plan to apply the information from modern synoptic climatology to understand past variations in the late Quaternary by examining modern climatic extremes that may serve as ‘process climate analogues’ of the past (Alt, 1983; Mock and Bartlein, 1995). Providing that suitable modern synoptic analogues can be found that correspond with hypotheses implied from climate models, we would expect surface climatic responses in the modern analogues to correspond closely with reconstructed paleoclimatic conditions as suggested from paleoclimatic fossil data.
ACKNOWLEDGEMENTS
We gratefully acknowledge Pavel Groisman for providing some of the data, Lynn Songer and Bev Lipsitz for technical assistance, and Feng Sheng Hu and Linda Brubaker for comments. This research was supported by NSF Grants ATM- 9317569 and ATM9532074. This is PALE Contribution c111. REFERENCES Alt, B.T. 1983. ‘Synoptic analogs: a technique for studying climatic change in the Canadian High Arctic’, Syllogeous, 49, 70 – 107. Anderson, P.M. and Brubaker, L.B. 1994. ‘Vegetation history of North Central Alaska: A mapped summary of late Quaternary pollen data’, Quat. Sci. Re6., 13, 71–92. Anderson, P.M., Lozhkin, A.V., Belaya, B.V., Glushkova, O.Y. and Brubaker, L.B. 1997. ‘A lacustrine pollen record from near altitudinal forest limit, Upper Kolyma Region, northeastern Siberia’, The Holocene, 7, 331 – 335. Barnosky, C.W., Anderson, P.M. and Bartlein, P.J. 1987. ‘The northwestern U.S. during deglaciation; vegetational history and paleoclimatic implications’, in Ruddiman, W.F. and Wright, H.E. (eds), North America and the Adjacent Oceans During the Last Deglaciation, Geological Society of America, Denver, 289 – 321. Barry, R.G. and Hare, F.K. 1974. ‘Arctic climate’, in Ives, J.D. and Barry, R.G. (eds), Arctic and Alpine En6ironments, Metheun, London, 17 – 54. Bartlein, P.J., Anderson, P.M., Edwards, M.E. and McDowell, P.F. 1991. ‘A framework for interpreting paleoclimatic variations in eastern Beringia’, Quat. Int., 10–12, 73–83. Briffa, K.R. Jones, P.D., Schweingruber, F.H., Shiyatov, S.G. and Vaganov, E.A. 1996. ‘Development of a North Eurasian chronology network: rationale and preliminary results of comparative ring-width and densitometeric analyses in Northern Russia’, in Dean, J.S., Meko, D.M. and Swetnam, T.W. (eds), Tree Rings, En6ironment, and Humanity, Radiocarbon, Department of Geosciences, University of Arizona, pp. 25–41. Cayan, D.R. 1996. ‘Interannual climate variability and snowpack in the western United States’, J. Climate, 9, 928 – 948. COHMAP Members, 1988. ‘Climatic changes of the last 18,000 years: Observations and model simulations’, Science, 241, 1043 – 1052. Chen, S.J., Kuo, Y.H., Zhang, P.Z. and Bai, Q.F. 1991. ‘Synoptic climatology of cyclogenesis over East Asia, 1958 – 1987’, Mon. Wea Re6., 119, 1407–1418. Diaz, H.F. and Andrews, J.T. 1982. ‘Analysis of the spatial pattern of July temperature departures (1943 – 1972) over Canada and estimates of the 700 mb mid-summer circulation during the middle and late Holocene’, J. Climatol., 2, 251 – 265. Diaz, H.F. and Kiladis, G.N. 1992. ‘Atmospheric teleconnections associated with the extreme phase of the Southern Oscillation’, in Diaz, H.F. and Markgraf V. (eds), El Nino, Historical and Paleoclimatc Aspects of the Southern Oscillation, Cambridge University Press, Cambridge, pp. 7–28. Dmitriev, A.A. 1970. ‘Situation characterizing various states of the Siberian anticyclone and its northeastern extension in the cold period of the year’, in Treshnikov A.F. (ed.), Problems of the Arctic and Antarctic 29 – 32, Israel Progrom for Scientific Translations, Jerusalem, pp. 448–455. © 1998 Royal Meteorological Society
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Ely, L.L., Enzel, Y. and Cayan, D.R. 1994. ‘Anomalous North Pacific atmospheric circulation and large winter floods in the Southwestern United States’, Climate, 7, 977–987. Fahl, C.B. 1975. ‘Mean sea level pressure patterns relating to glacier activity in Alaska’, in Weller G. and Bowling, S.A. (eds), Climate of the Arctic, American Meteorological Society, Boston, pp. 339 – 346. Gregory, S. 1978. Statistical Methods and the Geographer. Longman Group Limited, London. Groisman, P.Y., Koknaeva, V.V., Belokrylova, T.A. and Karl, T.R. 1991. ‘Overcoming biases of precipitation measurement: A history of the USSR experience’, Bull. Am. Meteorol. Soc., 72, 1725 – 1733. Harman, J.R. 1991. Synoptic Climatology of the Westerlies: Process and Patterns. Association of American Geographers, Washington, D.C. Hirschboeck, K.K., Ni, F., Wood, M.L. and Woodhouse, C.A. 1996. ‘Synoptic dendroclimatology: Overview and outlook’, in Dean, J.S., Meko, D.M. and Swetnam, T.W. (eds), Tree Rings, En6ironment, and Humanity, Radiocarbon, Department of Geosciences, University of Arizona, pp. 205–223. Kahl, J.D. 1990. ‘Characteristics of the low-level temperature inversion along the Alaskan arctic coast’, Int. J. Climatol., 10, 537 – 548. Keegan, T.J. 1958. ‘Arctic synoptic activity in winter’, J. Meteorol., 15, 513 – 521. Krebs, J.S. and Barry, R.G. 1970. ‘The arctic front and the tundra-taiga boundary in Eurasia’, Geogr. Re6., 60, 548 – 554. Lozhkin, A., Anderson P.M., Eisner, W.R., Ravako, L.G., Hopkins, D.M., Brubaker, L.B., Colinvaux, P.A. and Miller, M.C. 1993. ‘Late Quaternary lacustrine pollen records from South Western Beringia’, Quat. Res., 39, 314 – 324. Lydolph, P.E. 1977. ‘Climates of the Soviet Union’, World Sur6ey of Climatology Volume 7, Elsevier Scientific Publishing, New York. Lynch, A., Chapman, W.L., Walsh, J.E. and Weller, G. 1995. ‘Development of a regional climate model of the western Arctic’, J. Climate., 8, 1555 – 1570. Maddox, R.A., McCollum, D.M. and Howard, K.W. 1995. ‘Large-scale patterns associated with severe summertime thunderstorms over central Arizona’, Wea. Forecast., 10, 763–778. Mass, C.F. 1993. ‘The application of compact discs (CD-ROM) in the atmospheric sciences and relate fields: and update’, Bull. Am. Meteorol. Soc., 74, 1901–1908. Mock, C.J. 1996. ‘Avalanche climate of Alyeska, Alaska, U.S.A.’, Arctic Alpine Res., 28, 502 – 508. Mock, C.J. and Anderson P.M. 1997. ‘Some perspectives on the late Quaternary paleoclimate of Beringia’, in Isaacs, C.M. and Tharp, V. (eds), Proceedings of the Thirteenth Annual Pacific Climate (PACLIM) Workshop, Technical Report 53 of the Interagency Ecological Program, California Department of Water Resources, pp. 193 – 200. Mock, C.J. and Bartlein, P.J. 1995. ‘Spatial variability of late-Quaternary paleoclimates in the western United States’, Quat. Res., 44, 425 – 433. Moritz, R.E. 1979. Synoptic Climatology of the Beaufort Sea Coast of Alaska, Occasional Paper No. 30, Institute of Arctic and Alpine Research, University of Colorado. Muller, R.A. 1977. ‘A synoptic climatology for environmental baseline analysis: New Orleans’, J. Appl. Meteorol., 16, 20 – 33. Nichols, N., Gruza, G.V., Jouzel, J., Karl, T.R., Ogallo, L.A., and Parker, D.E. 1996. ‘Observed climate variability and change’, in Houghton, J.T., Meira Filho, L.G., Callander, B.A., Harris, N., Kattenberg, A. and Maskell, K. (eds), Climate Change 1995, The Science of Climate Change, Contribution of Working Group I to the Second Assessment Report of the Intergo6ernmental Panel on Climate Change, Cambridge University Press, Cambridge, pp. 133 – 192. Overland, J.E. and Pease, C.H. 1982. ‘Cyclone climatology of the Bering Sea and its relation to sea ice extent’, Mon. Wea. Re6., 110, 5 – 13. Overpeck, J., Hughen, K., Hardy, D., Bradley, R., Case, R., Douglas, M., Finney, B., Gajewski, K., Jacoby, G., Jennings, A., Lamoureux, S., Lasca, A., MacDonald, G., Moore, J., Retelle, M., Smith, S., Wolfe, A. and Zielinski, G. 1997. ‘Arctic environmental change of the last four centuries’, Science, 278, 1251 – 1256. Peterson, D.L. and Johnson, D.R. 1995. Human Ecology and Climate Change. Taylor and Francis, Washington, D.C. Reed, R.J. and Kunkel, B.A. 1960. ‘The arctic circulation in summer’, J. Meteorol., 17, 489 – 506. Serreze, M.C., Box, J.E., Barry, R.G. and Walsh, J.E. 1993. ‘Characteristics of arctic synoptic activity, 1952 – 1989’, Meteorol. Atmos. Phys., 51, 147–164. Streten, N.A. 1974. ‘Some features of the summer climate of interior Alaska’, Arctic, 27, 273 – 286. Trewartha, G.T. 1981. The Earth’s Problem Climates. University of Wisconsin Press, Madison. Vose, R.S., Schmoyer, R.L., Steurer, P.M., Peterson, T.C., Heim, R., Karl, T.R. and Eischeid, J. 1992. 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ATMOSPHERIC CIRCULATION PATTERNS AND SPATIAL CLIMATIC VARIATIONS IN BERINGIA CARY J. MOCKa,*, PATRICK J. BARTLEINa and PATRICIA M. ANDERSONb a Department of Geography, Uni6ersity of Oregon, Eugene, OR 97403, USA b Quaternary Research Center, Uni6ersity of Washington, Seattle, WA 98195, USA Recei6ed 28 May 1997 Re6ised 3 March 1998 Accepted 5 March 1998
ABSTRACT Analyses of more than 40 years of climatic data reveal intriguing spatial variations in climatic patterns for Beringia (North-eastern Siberia and Alaska), aiding the understanding of the hierarchy of climatic controls that operate at different spatial scales within the Arctic. A synoptic climatology, using a subjective classification methodology on January and July sea level pressure, and July 500 hPa height anomaly patterns, identified 13 major atmospheric circulation patterns (26 pairs consisting of 13 synoptic/temperature and 13 synoptic/precipitation comparisons) that occur over Beringia. Composite anomaly maps of circulation, temperature, and precipitation described the spatial variability of surface climatic responses to circulation. Results indicate that nine synoptic pairs yield homogeneous surface climatic anomaly patterns throughout most of Beringia. However, many of the surface climatic responses illustrate heterogeneous anomaly patterns as a result of variations in circulation controls, such as troughing over East Asia and the Pacific subtropical high superimposed over topography, with small shifts in atmospheric circulation dramatically altering spatial variations of anomaly patterns. Distinctive contrasts in climatic responses, as suggested from ten synoptic pairs, are clearly evident for Western Beringia versus Eastern Beringia. These results offer important implications for scholars interested in assessing late Quaternary climatic change in the region from interannual to millennial timescales. © 1998 Royal Meteorological Society. KEY WORDS: Beringia;
Northeast Siberia; Alaska; composite anomaly maps; 500 hPa heights; sea-level pressure; temperature; precipitation; synoptic climatology
1. INTRODUCTION Paleoclimatic records from the Arctic document surface climatic responses to past changes in large-scale controls of climate at a variety of timescales ranging from interannual to millennial (Bartlein et al., 1991; Briffa et al., 1996; Overpeck et al., 1997). Knowledge of the mechanisms that cause such changes is important for validating climate models as well as for understanding climatic impacts on ecosystems and society at northern high latitudes (Peterson and Johnson, 1995; Nichols et al., 1996). Large-scale controls, such as changes in solar insolation and ice sheet size, generally cause surface responses that are expressed up to the global scale (COHMAP Members, 1988). However, some regions in the Arctic also register heterogeneous climatic responses at the regional and smaller spatial scales, as the effects of synoptic-scale circulation features and physiography become more important (Barry and Hare, 1974). Beringia, extending from North-eastern Siberia across Alaska (Figure 1), represents a vast area of the Arcto-Boreal region. A growing body of paleoclimatic proxy data from Beringia offers opportunities to document paleoclimatic responses and to test paleoclimatic hypotheses (Barnosky et al., 1987; Bartlein et al., 1991; Anderson and Brubaker, 1994). Paleoclimatic investigations suggest that some climatic changes appar* Correspondence to: Department [email protected]
of
Geography,
University
of
Oregon,
Contract grant sponsor: NSF; Contract grant number: ATM-931769; ATM-9532074
CCC 0899–8418/98/101085 – 20$17.50 © 1998 Royal Meteorological Society
Eugene,
OR
97403,
USA;
e-mail:
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ently occur synchronously throughout the region, whereas at other times are characterized by considerable variation of spatial climate heterogeneity, particularly as it pertains to changes in Western Beringia (North-eastern Siberia) versus Eastern Beringia (Alaska) (Lozhkin et al., 1993; Anderson et al., 1997). A thorough knowledge of the climatic controls that operate at regional and smaller spatial scales today is an essential aid for evaluating possible mechanisms and feedbacks responsible for the inferred climatic variations of the past. A study of the modern synoptic climatology provides a framework for understanding the hierarchy of climatic controls that operate at various spatial scales. Although the length of modern climatic records in Beringia is shorter than in many other areas of the world, the availability of a fairly dense network spanning more than 40 years does provide an excellent opportunity for examining the controls of climate in the modern record and applying these findings to improving interpretations of the synoptic climatology of the past (Diaz and Andrews, 1982; Mock and Bartlein, 1995). The climatology of Beringia has been described previously, but most of this work focused on the description of general climatic characteristics (Lydolph, 1977), on particular subareas within Beringia (Streten, 1974), or climatic and circulation characteristics at larger spatial scales only (Reed and Kunkel, 1960). Thus, the synoptic and regional climatology of Beringia as a whole, dealing with both larger and smaller-scale spatial variations and their relationships to atmospheric circulation, remains poorly understood. For example, the comprehensive work of Trewartha (1981) on the regional climatology for selected areas around the world barely covers the edges of Beringia. This study presents a synoptic climatology of Beringia utilizing all available long-term temperature and precipitation records in order to examine the climatic controls that explain spatial climatic variations. We emphasize January and July because these 2 months generally represent the two extremes within the annual cycle when different climatic regimes occur. For example, precipitation bar graphs of all available
Figure 1. Precipitation bar graphs showing relative comparisons of averaged monthly precipitation for Beringia. For each bar graph, January is represented in the far left and December in the far right © 1998 Royal Meteorological Society
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stations in Beringia, with mean monthly precipitation covering at least a 20-year period, clearly exhibit a summer precipitation maximum over much of Eastern Beringia (Alaska) and interior Western Beringia (North-eastern Siberia). Specifically, many stations exhibit a July maximum, and some stations over the eastern half of Western Beringia exhibit a secondary winter maximum (Figure 1). We acknowledge that some locations, particularly along coastlines, exhibit minimum and maximum extremes within the annual cycle during February and August, respectively, due to a thermal lag effect, but the number of stations exhibiting this effect is lower than those with January and July extremes (World Meteorological Organization, 1981). Although this paper emphasizes the importance of understanding the modern climate to apply to understanding paleoclimate, it is also applicable for understanding spatial and temporal variations within the period of instrumental records. ENSO events, for example, are known to affect Alaska (Diaz and Kiladis, 1992), but have not been studied in detail for Western Beringia; results from this study can help address the possible synchroneity between climate responses of Western Beringia versus Eastern Beringia.
2. STUDY AREA AND DATA Beringia, as defined in this paper, is bounded by the 140°E meridian to the west, the Alaska/Canada border to the east, the 76°N parallel to the north, and the 50°N parallel to the south. Different physiographic contrasts are evident between Western Beringia and Eastern Beringia. Most of Western Beringia consists of tectonic depressions, with topography ranging from 1000 to 2000 m in the Upper Kolyma River region, and in the Sredinny and Koryak Ranges of the Kamchatka Peninsula. Localized rainshadow effects are evident in south-western Beringia, and the topography also restricts maritime climatic effects from penetrating inland. In contrast, the generally east–west oriented Brooks Range in Northern Alaska and the Alaska Range in southern Alaska have elevations that exceed 3500 m, and these relatively higher mountain ranges play important roles on creating heterogeneous climatic responses. During January, averaged monthly temperatures range from − 47 to − 5°C in Western Beringia and − 30 to 0°C in Eastern Beringia, with the lowest values centered in the continental interiors because of persistent temperature inversions (World Meteorological Organization, 1981). July averaged monthly temperatures exhibits generally an increase with decreasing latitude, with values ranging from 0 to 16°C. Precipitation also generally increases with decreasing latitude (Figure 1), but the superimposition of topography and coastal land/sea breezes creates some spatially heterogeneous patterns. January averaged precipitation ranges from 1 to 120 mm in Western Beringia and 6 to 348 mm in Eastern Beringia, but most stations have values less than 30 mm. July averaged precipitation for both western and Eastern Beringia generally falls in the same range of variability, with values varying from 20 to 225 mm. Monthly circulation data consist of gridded sea-level pressure (in hPa) and 500 hPa heights (in geopotential meters). The data encompass a region from central Asia to central North America so that large-scale atmospheric circulation can be described adequately. These data cover the period from January 1946 to January 1989 (Mass, 1993). January sea-level pressure was examined in order to clearly represent variations of the Siberian high as well as other persistent large-scale temperature inversions that are prevalent during the winter months (Mock and Anderson, 1997). Averaged sea-level pressure for January indicates two main pressure centers that are important for influencing winter Beringian climate: the Siberian high to the west and the Aleutian low just south of the Bering Sea (Figure 2). Higher levels, such as at the 500 hPa level, show that the polar jetstream is normally centered south of Beringia in January, enabling the formation of the Siberian high. However, winter 500 hPa variations were found to not represent surface climatic variations as closely as sea-level pressure since they do not respond as directly to surface diabatic processes, and thus were not discussed in this paper. The 500 hPa level was used for July because this level clearly represents upper level troughs that affect surface climate over Beringia which sea-level pressure data does not depict as clearly. Sea-level pressure data was also examined since they may add further insight in some cases. Averaged July 500 hPa heights indicate that the ‘East Asian trough’ is normally centered through the Beaufort and Bering Seas (Moritz, © 1998 Royal Meteorological Society
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Figure 2. Averaged January and July sea-level pressure in hPa (right), and averaged January and July 500 hPa heights in geopotential meters (left) for the period 1946 – 1989
1979), associated with generally north-westerly flow over Siberia and south-westerly flow over Alaska (Figure 2). July sea-level pressure shows that the Pacific subtropical high is normally prevalent over the North-eastern Pacific Ocean, bringing southerly flow in Western Beringia and westerly flow to Eastern Beringia. Fifty-nine temperature and eighty-seven precipitation stations were ultimately included in the analyses. These data were also expressed as monthly averages and available from the period 1946–1990 (Groisman et al., 1991; Vose et al., 1992). Only stations with less than 10% of missing data for the period were used; in most cases the amount of missing data is much less. The precipitation data from Western Beringia were provided by Pavel Groisman (Groisman et al., 1991). Careful screening and corrections were made on the data concerning changes in instrument exposure, elevation, and location with time. Some of the precipitation data for winter were found to be unreliable as a result of wind-blown snow and problems in converting snow amounts to liquid precipitation; they were simply deleted from the dataset (Groisman et al., 1991). The temperature data for all of Beringia and the precipitation data for Eastern Beringia were taken from the Global Historical Climatology Network (GHCN) (Vose et al., 1992). The data were subjected to a large amount of quality control, with all impossibly extreme values interpreted as ‘missing’ data, and obvious discontinuities were detected as well. Three stations in North-central Alaska from the national climatic data center’s cooperative station network, with data mostly continuous since 1946, were added to the station network. © 1998 Royal Meteorological Society
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3. METHODS The synoptic climatology of Beringia was examined by first classifying synoptic patterns followed by constructing composite anomaly maps of temperature and precipitation. This approach is similar in design for both January and July, differing only because of the different synoptic patterns that prevail between these months. The classification of synoptic types was done by examining anomaly centers of circulation maps for each month from 1946 to 1989 (Muller, 1977; Maddox et al., 1995). a subjective classification was used as opposed to automated methods because of the relatively lower number of months available (43 months) for classification. This qualitative approach is justified in that other similar synoptic classification studies with similar goals have proved robust (Yarnal, 1993; Maddox et al., 1995). The framework for generating the synoptic types described herein was formulated from the existing literature (Streten, 1974; Lydolph, 1977) that describes the main modes of circulation variability and thus the most active anomaly centers (e.g., Siberian high). Since our subjective classification is based on anomaly centers, a single month can be classified under several different synoptic types (e.g., a month may have both an intensified Aleutian low and an intensified Pacific subtropical high). Conversely, some months contain no synoptic type due to weak anomalies within and surrounding Beringia. Composite anomaly maps depict spatial climatic patterns that represent differences between averaged and anomalous values of circulation (Yarnal, 1993). Because the objective of this research is the identification of primary synoptic patterns, composite anomaly maps were constructed for the synoptic types that occurred at least seven times out of the 43 months available for classification. The selection of seven was chosen because this is a conservative number that is high enough to produce meaningful monthly composite anomaly maps when dealing with a 43 year record (Ely et al., 1994; Hirschboeck et al., 1996). The interpretation of composite circulation anomaly maps is similar to standard climate anomaly maps, with increased clockwise (anticyclonic) flow around positive centers and increased counter-clockwise (cyclonic) flow around negative centers. Temperature and precipitation composite anomaly maps illustrate the spatial patterns and responses to each of the synoptic types. These maps were constructed in similar fashion as the circulation composite anomaly maps. Stations with missing data for more than one month were not included on the temperature and precipitation composite maps. Temperature anomalies are expressed as standardized z-scores, with values of less than − 0.65 indicative of very abnormally cold conditions, − 0.65 to 0.00 as abnormally cold, 0.00 to 0.65 as abnormally warm, and greater than 0.65 as very abnormally warm. Since some of the synoptic climate classifications occurred more than ten times out of 43 cases possible, the probabilities of z-scores at individual stations being greater than 9 1.00 diminish dramatically, thus this criterion was not used. The 9 0.65 cutoff was used instead because it corresponds closely with the top and bottom 25% probability of a normal distribution (Gregory, 1978), and mapping of patterns provides a much clearer representation of regional-scale anomaly patterns (Hirschboeck et al., 1996). Precipitation anomalies are expressed as percentage departures from normal, with values of less than − 30% indicative of very abnormally dry conditions, − 30 to − 10% as abnormally dry conditions, −10 to 10% as no significant anomaly, 10 to 30% as abnormally wet, and greater than 30% as very abnormally wet conditions. This criterion of expressing anomalies clearly illustrates spatial patterns of precipitation responses to atmospheric circulation patterns.
4. JANUARY CLIMATE
4.1. Circulation controls The three primary circulation controls that affect Beringian climate during January are the Siberian high, the Pacific subtropical high, and the Aleutian low. Spatial variations of these circulation controls in winter are clearly apparent in anomaly maps over three primary locations: over Northern Siberia, the central North Pacific Ocean, and the Eastern Pacific Ocean. The climate over most of the area of © 1998 Royal Meteorological Society
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Northern Siberia has been explained by the anomalous behavior of the Siberian high (Keegan, 1958; Dmitriev, 1970). Serreze et al. (1993) also noted that anticyclonic frequencies tend to be relatively high in this region during winter. Lydolph (1977) implied that storms originating from the nearby Kara Sea tend to move south-eastward, south of the Siberian high. Pressure anomalies directly over Western Beringia during any particular month are generally low as a result of persistent temperature inversions. The central North Pacific Ocean is a region that also affects surface climate over both Western and Eastern Beringia (Lydolph, 1977; Overland and Pease, 1982), since variations in the Aleutian low here determine predominant wind directions and, thus, temperature and precipitation anomalies. An easterly shift of pressure anomalies into the north-eastern Pacific Ocean also often occurs, representing variations in the Pacific subtropical high or Aleutian low, and affecting surface climatic variations over Eastern Beringia (Fahl, 1975; Mock, 1996). Therefore, six synoptic types were classified for all January months, with three types corresponding to positive anomalies of the three main pressure centers, and the three other types corresponding to negative anomalies of the same pressure systems (Table I). Positive and negative anomalies were treated separately when compared with surface climate, because opposite variations in pressure do not necessarily correspond identically with opposite variations in temperature or precipitation (Cayan, 1996). The synoptic types are termed as the following in relation to location and circulation anomalies: Siberia positive, Siberia negative, North-central Pacific positive, North-central Pacific negative, Northeast Pacific positive, and Northeast Pacific negative.
4.2. Siberia types Results for the Siberia positive and Siberia negative types generally show opposite synoptic patterns, but the temperature and precipitation responses are not perfectly opposite between these two different patterns (Figure 3). The sea-level pressure composite anomaly map for Siberia positive, based on ten events (23%), shows positive anomalies encompassing most of the northeast Asian coast into the Arctic Ocean. Associated with the northern expansion of this Siberian high are negative anomalies over the central Pacific Ocean which represent a stronger Aleutian low. Temperature anomalies are negative in central Beringia due to the increased frequencies of cold north-easterly winds around the Siberian high. However, throughout the remainder of Beringia, temperature anomalies are mostly positive. The flow around the western edge of the Siberian high becomes more southerly and warmer by the time it reaches the western Beringian interior. Over south-eastern Beringia, positive temperature anomalies are explained by the stronger Aleutian low, as increased warmer southerly flow results from the north-eastern Pacific. This southerly flow also explains the positive precipitation anomalies in south-eastern Beringia, with the Alaska Range both restricting moist air masses from penetrating inland and causing orographic enhancements. Most of western Beringia exhibits negative precipitation anomalies as a result of the dominance of an enhanced Siberian high, with the largest negative values in the coastal areas of Western Beringia. The Siberia negative synoptic composites are based on eight events (19%). The negative sea-level pressure anomalies centered off northern Asia in the sea-level pressure composite anomaly map suggest increased north-westerly flow as cold air masses predominate throughout most of Western Beringia Table I. List of synoptic types for January from 1946 to 1989 Synoptic type
Years
Siberia positive Siberia negative North-central Pacific positive North-central Pacific negative Northeast Pacific positive Northeast Pacific negative
1953, 1948, 1947, 1946, 1949, 1952,
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1960, 1949, 1954, 1955, 1950, 1958,
1961, 1957, 1956, 1960, 1957, 1961,
1970, 1962, 1959, 1966, 1963, 1964,
1974, 1964, 1962, 1977, 1969, 1976,
1975, 1967, 1968, 1978, 1972, 1978,
1977, 1973, 1971, 1979, 1975 1983,
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Figure 3. January composite anomaly maps for the Siberia positive (left) and Siberia negative (right) types. Sea-level pressure composite anomaly maps are in the top row (hPa). Temperature composite anomaly maps are in the middle row (large pluses and minuses represents z-scores equal or greater than 0.65 and −0.65 respectively, and smaller pluses and minuses represent from 0.00 to 0.65 and − 0.65 to 0.00, respectively). Precipitation composite maps are in the bottom row, expressed as percentage departures from normal (large pluses and minuses represent equal or greater than −30%, small pluses and minuses represent equal or greater than 10 to 30% and − 10 to − 30%, respectively, and dots represent from −10 to 10%)
(Figure 3), as indicated by widespread negative temperature anomalies. The negative temperature anomalies are likely due to radiation losses caused by clear skies and light winds of a persistent (not necessarily stronger) Siberian anticyclone. Positive temperature anomalies in Eastern Beringia are as a © 1998 Royal Meteorological Society
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result of increased maritime influence from stronger south-westerly flow around the northern end of a weakened Aleutian low. The increased advection of winds from the southwest also cause periodic weakening of the temperature inversion, allowing warmer air to mix down to the surface (Kahl, 1990). Precipitation anomalies are negative in far Western Beringia as a result of drier air masses and anomalously colder temperatures, but generally increase eastward as warmer and moist southerly airflow becomes more predominant.
4.3. North-central Pacific types The North-central Pacific positive synoptic type occurred during eight of the months in the modern record (19%). The associated composite anomaly map of sea-level pressure shows positive anomalies centered in the Bering Sea, expanding into the North-central Pacific and most of the land areas of Beringia (Figure 4). These anomalies indicate a weaker Aleutian low as compared to normal. Temperature anomalies show opposite signs in Western Beringia as opposed to Eastern Beringia. The positive temperature anomalies throughout Western Beringia and the negative anomalies throughout Alaska are associated with the weaker Aleutian low. This synoptic situation causes weaker flow of warm air masses from the south to Alaska and also weaker flow of cold air masses from Alaska and the Beaufort Sea to Western Beringia. Positive precipitation anomalies are evident over most of south-western Beringia, resulting from the warmer and wetter flow from the Sea of Okhotsk feeding into cyclones traversing through the low-elevation corridor located just to the west. The magnitude of positive precipitation anomalies decreases dramatically inland since topographic effects restrict the relatively warmer and moister air along the Sea of Okhotsk. Over most of Eastern Beringia, precipitation anomalies exhibit little variability. The higher precipitation anomalies along the North Alaska coast and to the southeast may relate to occasional storms traveling from the southwest around the high pressure center, but winter precipitation is generally very low for this region. The North-central Pacific negative type was classified for eleven months (26%). Sea-level pressure anomalies represent a strengthened Aleutian low (Figure 4). Widespread positive temperature anomalies are evident over all of Eastern Beringia and somewhat weaker positive temperature anomalies in Western Beringia. These anomalies are due to warmer south-westerly flow around a strengthened Aleutian low traversing through Eastern Beringia and eventually moving from a north-easterly direction into Western Beringia, as turbulent down-mixing of warmer boundary-layer air breaks down the temperature inversion. Anomalies change to negative along the Kamchatka Peninsula, reflecting flow from the north over land which is normally colder than the more typical southerly flow over the Sea of Okhotsk and the Western Pacific Ocean. Because sea-level pressure anomalies indicates an eastward shift in circulation features as compared to normal, allowing intensification of the Aleutian low in the Bering Sea, negative precipitation anomalies persist throughout most of Beringia due to increased northerly flow, especially in south-western Beringia. Onshore flow from the strengthened Aleutian low, coupled with orographic enhancements in areas south of the Alaska Range in Eastern Beringia, result in positive precipitation anomalies.
4.4. Northeast Pacific types The Northeast Pacific positive type occurred during 7 months in the modern record (16%). The sea-level pressure composite anomaly map represents a weaker Aleutian low and more frequent anticyclonic activity, centered south of Alaska (Figure 5). Positive temperature anomalies are distributed over most of Beringia, with the highest anomalies prevalent in the central portion of the study area. Warmer, southerly flow around the anomalous high pressure center explain these temperature anomalies in a similar manner as for the North-central Pacific positive type, except that the eastward location of sea-level pressure anomalies allow the positive temperature anomalies to spread eastward. Precipitation anomalies are mostly positive throughout Beringia as a result of the higher temperatures, generally enabling the air masses to be relatively warmer and wetter as compared to normal. Isolated negative precipitation anomalies along the Western Beringian coast are probably related to localized land-sea breeze effects. © 1998 Royal Meteorological Society
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The Northeast Pacific negative type occurred in eight months of the modern record (19%), and the sea-level pressure composite anomaly map shows a stronger and eastward shift of the Aleutian low. Temperature anomalies are mostly positive in Eastern Beringia and negative in most of the region to the west. The positive temperature anomalies along the coast correspond with the distribution of positive precipitation anomalies; this positive correlation is restricted to south of the Alaska Range and related to anomalously stronger south-westerly flow around the eastern end of the Aleutian low. North of the
Figure 4. January composite anomaly maps for the North-central Pacific positive (left) and North-central Pacific negative (right) types. Symbol representation is the same as listed in Figure 3 © 1998 Royal Meteorological Society
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Figure 5. January composite anomaly maps for the Northeast Pacific positive (left) and Northeast Pacific negative (right) types. Symbol representation is the same as listed in Figure 3
Alaska Range and in parts of Western Beringia, slightly colder and drier conditions result as compared to normal, perhaps affected by dry airstreams associated with easterly and north-easterly flow along the northern side of the stronger Aleutian low. © 1998 Royal Meteorological Society
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5. JULY CLIMATE
5.1. Circulation controls During July, the normal pattern of 500 hPa heights illustrates a dramatically weaker flow as compared to January, with the number of longwaves increases from 3 to 4 or 5 over the Northern Hemisphere (Harman, 1991). The summer climate of Beringia is strongly affected by the configuration of the ‘East Asian Trough’ (Moritz, 1979) and adjacent ridges as well as the Pacific subtropical high, as the jetstream and the associated ‘polar front’ account for latitudinal summer temperature gradients over Beringia (Krebs and Barry, 1970). Cyclogenesis normally occurs in Eastern Siberia (Whittaker and Horn, 1982; Serreze et al., 1993) and around Mongolia (Chen et al., 1991), with the storms generally move eastward to affect Beringia. Northeast of Beringia, anticyclonic frequencies are generally highest around the Beaufort Sea (Reed and Kunkel, 1960; Serreze et al. 1993). However, variations in the longwave pattern occasionally occur. For example, trough-ridge contrasts may occur over Beringia and the adjacent Pacific and Arctic Oceans, causing different climatic contrasts between conditions in Western Beringia versus Eastern Beringia (Streten, 1974; Lydolph, 1977). Ridges can also provide some precipitation along their adjacent edges by advecting moisture from the oceans over land, as exhibited for example by moist flow along the western edge of the Pacific subtropical high into south-eastern Russia (Lydolph, 1977). Anomalous variability of July 500 hPa heights as well as sea-level pressure data tend to occur in four areas within the study area: Western Beringia, the North-western Pacific, the North-eastern Pacific, and off the North Alaska coast (Mock and Anderson, 1997). Initially, eight synoptic types were classified for all of the July months, with four types each corresponding to positive and negative anomalies associated with the four main pressure centers (Table II). However, only 5 months were found that depict positive anomalies centered over Western Beringia. Since this number is too low to conduct a meaningful composite anomaly analysis, this synoptic type is not discussed any further. Therefore, the remaining seven synoptic types are termed as the following: Western Beringia negative, North-western Pacific positive, North-western Pacific negative, North-eastern Pacific positive (‘North-eastern’ differentiates from ‘Northeast’ which was used in the January classification), North-eastern Pacific negative, Alaska positive, and Alaska negative. 500 hPa maps are emphasized more in the following discussion of summer synoptic types since they exhibit stronger anomaly patterns as compared to sea-level pressure. However, all the composite anomaly maps of sea-level pressure maps are provided for comparisons (Figures 6 and 7).
5.2. Western Beringia negati6e type This synoptic type was classified for ten Julys in the modern record (23%), with its main characteristic being an area of negative anomalies of 500 hPa heights centered over Western Beringia (Figure 6). Negative anomalies of sea-level pressure also occur over Western Beringia, but the magnitudes of anomalies are relatively lower as compared with the 500 hPa pattern (Figure 6). Associated with these negative anomalies is a weaker center of positive anomalies over the Beaufort Sea, north of Alaska. These two different anomaly centers explain contrasts of surface climatic responses between Western Beringia Table II. List of synoptic types for July from 1946 to 1988 Synoptic type
Years
Western Beringia negative North-western Pacific positive North-western Pacific negative North-eastern Pacific positive North-eastern Pacific negative Alaska positive Alaska negative
1946, 1952, 1947, 1949, 1950, 1947, 1953,
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1950, 1953, 1964, 1951, 1958, 1949, 1956,
1955, 1956, 1965, 1954, 1960, 1950, 1961,
1963, 1957, 1970, 1955, 1962, 1955, 1963,
1964, 1959, 1971, 1963, 1968, 1958, 1964,
1966, 1961, 1974, 1966, 1973, 1968, 1967,
1968, 1962, 1982, 1967, 1975, 1972, 1969,
1976, 1969, 1985, 1979, 1976, 1977, 1973,
1978, 1984 1987 1980, 1978, 1978, 1981,
1979 1981, 1986 1983, 1988 1983, 1985, 1987 1984
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Figure 6. July composite anomaly maps for the Western Beringia type. The 500 hPa composite anomaly map is at the upper left (geopotential meters), sea-level pressure anomaly map at the upper right, temperature composite anomaly map at the lower left (large pluses and minuses represents z-scores equal or greater than 0.65 and −0.65, respectively, and smaller pluses and minuses represent from 0.00 to 0.65 and −0.65 to 0.00, respectively), and precipitation composite map at the lower right and expressed as percentage departures from normal (large pluses and minuses represent equal or greater than − 30% respectively, small pluses and minuses represent equal or greater than 10 to 30% and −10 to −30%, respectively, and dots represent from −10 to 10%)
and Eastern Beringia. The temperature anomaly map shows widespread negative anomalies over Western Beringia as a result of increased colder north- westerly flow from the increased troughing. This East Asian trough also steers more storms from the west into the region, as shown by mostly positive precipitation anomalies over Western Beringia. Somewhat higher precipitation anomalies tend to occur in the south-eastern portion of Western Beringia, perhaps due to storms intensifying as they enter the Sea of Okhotsk. Over Eastern Beringia, temperature anomalies are mostly positive in the interior as a result of increased north-easterly flow around the anticyclone centered in the Beaufort Sea. This synoptic pattern also explains the negative precipitation anomalies, but anomalies are generally weak.
5.3. North-western Pacific types The North-western Pacific positive type was identified for nine months (21%). Although the classification is based on positive 500 hPa height anomalies centered in the Western Pacific off the Kamchatka Peninsula, the 500 hPa anomaly map illustrates clearly that negative anomalies to the north of Eastern Beringia are associated with these positive anomalies (Figure 8). Sea-level pressure anomalies exhibit a similar pattern (Figure 7). The positive anomalies reflect an expansion and strengthening of the western Pacific subtropical high, and the negative anomalies represent a northward shift of the East Asian trough. © 1998 Royal Meteorological Society
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Temperature anomalies show opposite contrasts between Western and Eastern Beringia, with positive anomalies predominant in the west and negative anomalies throughout the east. The highest positive temperature anomalies are along the eastern coast of Western Beringia due to increased warm southerly flow along the western end of the Pacific subtropical high. Negative temperature anomalies over Eastern Beringia, especially in the interior, result because of stronger north-westerly flow from the Arctic Ocean. Precipitation anomalies over most of Beringia are weak, with the highest anomalies restricted along the coast of the Sea of Okhotsk from increased moisture advected from the south. The North-western Pacific negative type occurred on nine of the months in the modern record (21%). The 500 hPa anomaly map
Figure 7. Sea-level pressure composite anomaly maps for six July synoptic types © 1998 Royal Meteorological Society
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generally shows the opposite pattern as for the North-western Pacific positive type, but the zone of positive anomalies to the north is centered farther west over the Northern Siberia coast. Sea-level pressure anomalies are generally weak (Figure 7). The 500 hPa positive anomalies represent increased ridging in the region, while the negative anomalies off Kamchatka Peninsula indicate a weaker western Pacific subtropical high. Along the eastern portion of Western Beringia, temperature anomalies are negative because of weaker and perhaps less frequent southerly flow around the subtropical high. To the west, the occurrence of a ridge perhaps explains the positive temperature anomalies. With less moisture advected from the weaker subtropical high, precipitation anomalies in Western Beringia are mostly negative. Over Eastern Beringia, the positive temperature anomalies along the North-western Alaskan coast may be due to increased south-westerly flow from the weaker Pacific subtropical high. Positive anomalies are weak in the interior because positive 500 hPa anomalies to the north are centered farther west, thus only slightly stronger easterly flow occurs. The negative temperature anomalies along the south-eastern Beringian coast south of the Alaska Range are perhaps a result of prevalent westerly flow. Precipitation anomalies in Alaska, however, are mostly small, as the important circulation anomalies for precipitation in this area are centered too far westward.
5.4. North-eastern Pacific types The North-eastern Pacific positive type, based on eleven events in the modern record (26%), represents a strengthened and expanded Eastern Pacific subtropical high south of the Aleutian Islands. The 500 hPa anomaly map, and the sea-level pressure anomaly map to a lesser degree, illustrate this pattern, associated with a weak center of negative anomalies over Western Beringia (Figures 7 and 9). Temperature anomalies are negative over most of Beringia with the exception of south-eastern Beringia. Perhaps the stronger ridging in the Eastern Pacific enhances a trough centered over Western Beringia, causing colder air to predominate over most of Beringia. The positive anomalies in south-eastern Beringia are the result of warm south-westerly flow along the northern portion of the ridge. Precipitation anomalies are generally positive over Beringia, caused by the trough over Western Beringia and the stronger, wetter south-westerly flow over most of Eastern Beringia. The North-eastern Pacific negative type, based on eleven events (26%), shows negative 500 hPa and sea-level pressure anomalies that suggests a weaker Pacific subtropical high in the north-eastern Pacific Ocean. However, interestingly, an associated area of 500 hPa positive height anomalies is evident to the north, perhaps indicative of a weakened general circulation that includes the circumpolar vortex. Less frequent westerly flow from a weakened general circulation may explain small positive temperature anomalies in Western Beringia, and the high temperature anomalies over Eastern Beringia are the result of less frequent westerly flow from the weaker subtropical high and increased easterly flow around the anticyclone north of Eastern Beringia. Precipitation anomalies are generally weak, especially in Western Beringia as circulation anomalies near the region are weak as well. Over Eastern Beringia, negative precipitation anomalies prevail, related to airflow associated with the weaker Pacific subtropical high and the anticyclone north of Eastern Beringia as discussed for temperature.
5.5. Alaska types The Alaska positive type occurred on 12 months in the modern record (28%), characterized primarily by the center of positive 500 hPa height and sea-level pressure anomalies off the Northern Alaska coast (Figures 7 and 10). Associated with these positive anomalies are negative anomalies over south-western Beringia, representative of an intensified East Asian trough. These 500 hPa anomaly patterns are similar to the map from the Western Beringia negative synoptic type, but differs obviously with the highest anomalies instead being associated with the positive anomalies over Eastern Beringia. Temperature anomalies show mostly opposite signs of the responses in Western Beringia versus Eastern Beringia. The negative temperature anomalies over Western Beringia result from the increased troughing, with colder air coming from the northwest. The positive temperature and negative precipitation anomalies over Eastern Beringia, especially the continental interior, are the result of increased warmer and drier easterly flow © 1998 Royal Meteorological Society
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Figure 8. July composite anomaly maps for the north-western Pacific types. 500 hPa composite anomaly maps are in the top row (geopotential meters). Temperature composite anomaly maps are in the middle row (large pluses and minuses represents z-scores equal or greater than 0.65 and − 0.65, respectively, and smaller pluses and minuses represent from 0.00 to 0.65 and −0.65 to 0.00, respectively). Precipitation composite maps are in the bottom row, expressed as percentage departures from normal (large pluses and minuses represent equal or greater than − 30%, respectively, small pluses and minuses represent equal or greater than 10 to 30% and − 10 to − 30%, respectively, and dots represent from −10 to 10%)
around the anticyclone centered to the north. In Western Beringia, precipitation anomalies are small but generally positive, particularly along the Eastern Siberian coast because of the increased troughing and more frequent storms coming from the west. © 1998 Royal Meteorological Society
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The Alaska negative type, which occurred during ten months in the modern record (23%), is characterized with negative 500 hPa height and sea-level pressure anomalies to the north of Eastern Beringia. However, unlike the Alaska positive type, the opposite 500 hPa anomalies in sign are centered over the Bering Sea as opposed to in Western Beringia; this explains the lack of a west–east contrast of opposite anomaly signs. The positive anomalies indicate a northern movement and expansion of the subtropical high, associated with a shift of the trough to the north of Eastern Beringia. Temperature
Figure 9. July composite anomaly maps for the North-eastern Pacific positive (left) and North-eastern Pacific negative (right) types. Symbol representation is the same as listed in Figure 8 © 1998 Royal Meteorological Society
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Figure 10. July composite anomaly maps for the Alaska positive (left) and Alaska negative (right) types. Symbol representation is the same as listed in Figure 8
responses are negative throughout most of Beringia, as the influence of stronger westerlies spreads from Western Beringia through most of Eastern Beringia. The positive temperature anomalies in Western Beringia are due to the increased warm and southerly flow around the center of positive height anomalies. The south-eastern Beringian coast also exhibits positive temperature anomalies, perhaps because the colder air northward cannot penetrate southward through the Alaska Range too frequently. Precipitation anomalies are weak but generally positive throughout most of Beringia, because of the increased troughing, especially in interior Eastern Beringia. © 1998 Royal Meteorological Society
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6. DISCUSSION AND IMPLICATIONS FOR UNDERSTANDING SPATIAL CLIMATIC VARIATIONS This paper provides the first comprehensive description of the relationships between synoptic-scale atmospheric circulation patterns and associated spatial variations of surface responses at both large and small spatial scales for Beringia. The results are summarized as follows: (i) Out of the 26 different pairs of synoptic/surface responses in the 13 synoptic classifications (13 synoptic/temperature and 13 synoptic/precipitation comparisons) examined in this paper, nine clearly exhibit homogeneous responses of the same anomaly sign over most of Beringia (e.g. July Northeastern Pacific synoptic types). Generally, temperature responses to circulation exhibit clearer patterns of homogeneity as compared with precipitation because heterogeneous precipitation responses are complicated due to topographic features causing orographic, rainshadow, and channeling effects. (ii) Surface climatic responses between Western and Eastern Beringia often exhibit opposite anomaly signs from the different synoptic climatic controls that are responsible. For example in July, anomalous high precipitation in Western Beringia may result from a stronger trough centered in the region, whereas high precipitation in south-central Alaska may require a strengthened Eastern Pacific subtropical high. Seven synoptic/temperature and three synoptic/precipitation of the 26 total different pairs of synoptic/surface responses examined in this paper clearly depict an inverse relationship of anomalies, and the magnitude of the anomalies tend to be higher when compared with spatial patterns of climatic responses in most other synoptic types; (iii) Considerable spatial variability of surface climatic responses is also clearly evident at smaller scales because of topographic features and coastal-inland contrasts. Several synoptic types yield contrasting surface responses on opposite sides of the Alaska Range, and other synoptic types show that surface responses tended to weaken or change sign as distance increases northward from the Sea of Okhotsk; (iv) Small shifts of circulation patterns can dramatically alter the spatial variations of surface climatic responses. For example, a strengthened Aleutian low centered over the Bering Sea (January North-central Pacific negative type), is associated with generally warmer temperatures throughout most of Beringia. An easterly shift of the strengthened Aleutian low to south of the Aleutian Islands (January North-east Pacific negative type), changes the spatial pattern of temperature responses, with negative anomalies over Western Beringia and positive anomalies over Eastern Beringia. The results of this paper provide several important implications for studying past climates and climatic change of Beringia. Most importantly, Beringia cannot be treated as a single homogeneous climatic unit. The synoptic climatic controls that govern temperature and precipitation anomalies differ spatially for different locations, particularly as they relate to Western Beringia versus Eastern Beringia. Furthermore, the combination of characteristics of different synoptic controls (e.g. Aleutian low, Siberian high) can differ in the past, leading to the possibility of numerous different patterns of spatial climatic responses. Physiographic controls also cause considerable spatial variability of surface climatic responses, warning against the dangers of generalizing paleoclimate patterns or trends from only a few sites. Useful next steps in investigating the synoptic climatology of Beringia would be to examine other climatic variables such as radiosonde, solar radiation, surface wind, and cloud observations, and examine the data at the daily timescale. At various times during the late Quaternary, external climatic controls such as ice sheet size and insolation were dramatically different that today. For example, increased sea ice adjacent to the land areas and a large ice sheet east of Beringia were present around 18000 years ago, and summer insolation was approximately 8% greater than today around 9,000 years ago (COHMAP Members, 1988). These external controls undoubtedly affected the magnitude and spatial variability of circulation controls. Consequently, temperature and precipitation anomalies may have exhibited homogeneous responses at times in anomaly sign throughout Beringia. Atmospheric general circulation models and regional climate models can be used to simulate synoptic-scale circulation patterns and explain some surface climatic responses for past © 1998 Royal Meteorological Society
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and present Beringian climates (Bartlein et al., 1991; Lynch et al., 1995), but such simulations may not explain some paleoclimatic reconstructions for particular sites that respond to both large-scale and localized climatic controls. Since different external controls occurred in the past, no exact modern analogues occur in the modern instrumental record that resembles past climates. However, many synoptic controls that occur today also most likely similarly occurred in the past. Thus an understanding of the modern synoptic climatology may be used as a conceptual guide to complement climate models to explain spatial variations of anomalies, and to test paleoclimatic hypotheses of synoptic patterns. For example, fossil paleoclimatic data suggest that the paleoclimatic histories of Western and Eastern Beringia for much of the late Quaternary were most likely much different from one another (Lozhkin et al., 1993; Anderson and Brubaker, 1994; Anderson et al. 1997), consistent with the information from the modern synoptic climatology that shows the climatic differences of the two regions is the rule rather then the exception. Paleoclimatic evidence also shows distinct heterogeneous spatial variations within Eastern Beringia, also relating with some of the climatic responses to some of the synoptic types discussed in this paper. The authors plan to apply the information from modern synoptic climatology to understand past variations in the late Quaternary by examining modern climatic extremes that may serve as ‘process climate analogues’ of the past (Alt, 1983; Mock and Bartlein, 1995). Providing that suitable modern synoptic analogues can be found that correspond with hypotheses implied from climate models, we would expect surface climatic responses in the modern analogues to correspond closely with reconstructed paleoclimatic conditions as suggested from paleoclimatic fossil data.
ACKNOWLEDGEMENTS
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