Dynamics of the summer hydrographic regime at Elephant Island
AMLR program: Dynamics of the summer hydrographic regime at Elephant Island A. F. AMOS AND M. K. LAVENDER University of Texas at Austin Marine Sciences Institute Port Aransas, Texas 78373
The 1991-1992 field season of the U.S. Antarctic Marine Living Resource program (AMLR) (Rosenberg et al. this issue) was the third where physical oceanographic measurements were made by University of Texas Marine Science Institute researchers aboard the National Oceanic and Atmospheric Administration (NOAA) ship Surveyor in conjunction withbiological studies of other AMLR groups. The physical oceanography component of the AMLR program provided the means to identify contributing water masses and environmental influences within the study area, including meteorological conditions (Amos this issue). To accomplish this aim, 190 conducting-temperature-depth (CTD)/rosette casts were made, providing over 2,000 samples in the water column to other AMLR programs (see collected AMLR papers in this issue). As our knowledge of the Elephant Island region oceanography has increased, a grid sampling plan and station transect locations have been developed (Rosenberg et al. figures 2 and 3, respectively) to study the spatial and temporal variability. The large scale survey of 72 stations on a quarter-degree latitude by half-degree longitude grid-spacing (total area approximately 36 x 10 square kilometers) was occupied twice. CTD stations were made to within 10 meters of the bottom (or to 750 meters in deeper water). The Sea-Bird SBE-9 CTD instrumentation also included oxygen, beam-transmission, fluorometry, and downwelling (Photosynthetic Active Radiation) light sensors. However, we concentrate here on the standard oceanographic parameters to describe the dynamics of the circulation during the 1991-1992 austral summer. We will also briefly examine the temporal changes observed in the month separating the two cruise legs in the position of a front north of Elephant Island. In previous AMLR cruises aboard Surveyor we have described five water-mass types grouped by similarity of the temperature/ salinity characteristics of the upper water column (Amos and Lavender this issue). Identified by roman numerals as "Type II water," for example, AMLR researchers have used these designations to relate the distribution of the phyto- and zooplankton, krill, and fish larvae to the summer hydrography (AMLR papers in this issue). To review briefly: Drake Passage water (Type I) is warm, low in salinity water, and has a sub-surface temperature minimum ("Winter water," approximately -1 'C; salinity 34.0 ppt), Circumpolar deep water (CDW, T-max near 500 meters). Transition water (Type II) has a temperature minimum near 0 'C, isopycnal mixing below T-min, and CDW is evident at some locations. Weddell-Scotia Confluence (Type III) shows little evidence of a temperature minimum, mixes with Type II, has no CDW, and its temperature is at a depth of generally greater than 0 'C. Eastern Bransfield Strait water (Type IV) has a deep temperature near -i 'C, salinity of 34.5 ppt, and cooler surface temperatures. Wed-
228
dell Sea water (Type V) has little vertical structure and cold surface temperatures (near 0 'C). Figure 1 maps the implied geostrophysically-balanced flow as computed from the network of CTD stations from survey A (January 1992) and survey D (March 1992). The streamlines are labeled with dynamic height anomaly in dynamic centimeter referenced to two levels, the 200 decibar (figures 1A and 1B) and 500 decibar (figures 1C and 1D). The flow's direction is predominantly from southwest to northeast and streamline patterns are quite similar whether referenced to 200 or 500 decibar. There is a difference from survey A to survey D. By March the trend of the streamlines has rotated clockwise to a more easterly direction. There are two streams evident, one in the Drake Passage, and one in the Bransfield. The latter is an extension of the Bransfield Current, identified by Niiler et al. 1991. A persistent meander in the northwest corner of the large scale survey grid seems to have some relationship with the various biomass measurements made by AMLR scientists. The most dramatic hydrographic difference in the region is between the oceanic Drake Passage water and the waters of the Elephant Island shelf, namely between types I and III water masses. The narrow transition or mixing zone (type II) between them appears to have some significance for phytoplankton distribution (Helbling et al. in press; Holm-Hansen et al. this issue) and krill stock (Loeb and Seigel this issue). Along this boundary a density gradient (density increasing towards Elephant Island) reflects the change from warm, low-salinity to cold, more saline surface waters. At the same location the much deeper CDW core ends. The intensification of the flow in the Drake Passage (figure 1) coincides with this boundary and is perhaps related to the topography of the continental shelf-break. We have investigated this front by making transects of closely-spaced stations normal to the topographic trends. Figure 2 shows that the frontal boundary moves progressively seaward from early February to mid-March. Finally, the dynamic nature of the water column is illustrated in the change in water-structure at the same location from January to March (figure 3). The water overlying station 45, due north of Elephant Island (Rosenberg et al. this issue, figure 2 for locations), has a classic Type I structure on 26 January 1992 but is considerably modified to Type H water by 3 March 1992. Our analyses of these data will concentrate on the inter-annual and temporal changes in the hydrography and its relationship with the biomass distribution (Amos in press). The present work was performed under NOAA award NA27FRO01-10 to the UTMSI. I wish to thank the officers and crew of the NOAAS Surveyor, especially Lt. Cmdr. Fred Rossma and Lt. John Miller; ET's Mark May and Frank Gomes; Surve Techs Katherine Simmons, Ridge Liepens, and Tammy Tyner; AB's Mark Davidoff, Tim Patrico, and Harry Guist. CTD personnel were Margaret Lavender (legs I and II), Jeff Heimann (leg I) and Tony Amos (leg II). References Amos, A. F. and M. K. Lavender. 1992. AMLR program: Water masses in the vicinity of Elephant Island. Antarctic Journal of the U.S., 26(5):210213. Amos, A. F. 1992. AMLR program: A comparison between meteorologi. cal conditions at Seal Island and over the adjacent waters of the Drake Passage. Antarctic Journal of the U.S., this issue. Amos, A. F. Submitted. Interannual variability in the summer hydrogra-
ANTARCTIC JOURNAL
5e 0 w 57°W 56°W 55°W 54°W 530 SB°W 57°W 1 I I I 59°30
56°W 55°W 54°W 53°W
600 0S
50° 30
51° 0S
61°
30
.I I I 620
0S R. . 57°W 56°W 55°W 54°W 53°W 53° 58°W 56 0 W 57°W 56°W 55°W 54°W
AMLR 92. SURVEY 0 DYNAMIC HEIGHT (200 db) 28 FEB to 10 MAR 1992
AMLR 92, SURVEY A DYNAMIC HEIGHT (200 db) 19 JAN to 02 FEB 1992
FIGURE i—B
FIGURE i —A
58°W 57°W 56°W 55°W 54°W 53° 59°W 57°W 56°W 55°W 54°W .I I fg 59° 30
53° W
60° 0S
60° 30
61°
0S
61°
30
.'52°0S 1 " . 54°W 53°W 56°W 55°W 56W 530 56W 57°W 55°W 54°W 56°W 57°W
AMLR 92. SURVEY A DYNAMIC HEIGHT (500 db) 19 JAN to 02 FEB 1992 FIGURE 1 —C
AMLR 92. SURVEY 0 DYNAMIC HEIGHT (500 db) 28 FEB to 10 MAR 1992 FIGURE 1-0
Figure 1. Dynamic heights and geostrophic flow during the two large-scale surveys of AMLR 1991-1992. (A) Surface relative to 200 dcclbar, survey A; (B) surface relative to 200 decibar, survey 0; (C) surface relative to 500 decibar, survey A; (D) surface relative to 500 decibar, survey B.
1992 REvIEw
229
/
57 0 W
60 30 S
660K
560W
540
+
+ + +
++++/ +4:.._-
34
1111,1
IA
34.5
li,i
1
/
I1
400' -
+ 61005
I 66°W
i35 -3
/26.6
7..4//2/so
w 55°W
2
p.4/27.8
27
57°W
II
+ +
C + 61 0 OS
33.5 3- 1,1
60030 S +
AMLR 92 SURVEY STA 45A (DOTTED). STA 45D, (SOLID)
54°W
SURVEYOR CRUISE AMLR 92. SURVEYS .X LOCATION OF ELEPHANT I. FRONT 03 FEB to 04 FEB 1992 (A—A') 22 FEB to 25 FEB 1992 (B—B) 10 MAR to 12 MAR 1992 CC—C')
Figure 2. Location of frontal boundaries north of Elephant Island from cross-shell CTD transects.
-
-o
a.I
7 j5
I I-
I I I I
/ I I I I I I I —
2 i 33.5
s I'
I
I'l
34
I
I
I
34.5
/ I I
I'—I---2
35
SALINITY (ppt) Figure 3. T/S relationship of water column at station 45 on 26 January and 3 March 1992. Depth of major 1/S inversions are annotated on the curves.
phy of the surface water at Elephant Island, Antarctica. Continental
Shelf Research.
Heibling, E. W., A. F. Amos, N. Silva, V. Villafane, and 0. Holm-Hansen. In press. Phytoplankton distribution and abiotic measurements over a frontal system north of Elephant Island, Antarctica. Antarctic
Science.
Holm-Hansen, 0., V. E. Villafane, and E. W. Helbling. 1992. AMLR program: Phytoplankton abundance and rates of primary production
AMLR program: A comparison between the summer meteorological conditions at Seal Island and those over the adjacent waters of the Drake Passage A. F. AMOS Marine Science Institute University of Texas at Austin Port Aransas, Texas 78373
Seal Island is the largest of a small group of islands located north of Elephant Island, the northernmost of the South Shetland Islands (see Rosenberg et al. for location map). It is the site 230
around Elephant Island area. Antarctic Journal of the U.S., this issue. Loeb, V. and V. Siegal. 1992. AMLR program: Krill stock structure in the Elephant Island area. Antarctic Journal of the U.S., this issue. Niiler, P. P., A. F. Amos, and J. -H. Hu. 1991. Water masses and 200m relative geostrophic circulation in the western Bransfield Strait region. Deep Sea Research, 38(8/9):943-959. Rosenberg, J., R. P. Hewitt, and R. S. Holt. 1992. The U.S. AMLR program 1991-1992 field season activities. Antarctic Journal of the U.S., this issue.
of an annual study of seals and penguins by the Antarctic Marine Living Resource (AMLR) program (Croll et al. this issue). The waters of the adjacent continental shelf and the Drake Passage are the primary study area of the AMLR program's fieldwork aboard the National Oceanic and Atmospheric Administration (NOAA) ship Surveyor. The University of Texas's program in physical oceanography is part of the AMLR multidisciplinary study of the distribution of krill (Euphausia superba). Our work has two major tasks: to study the hydrography of the upper water column as it relates to the observed distribution of the biological organisms (Helbling et al. in press; Amos and Lavender this issue) and to monitor the surface meteorological conditions that may affect the water structure (Amos this issue). To accomplish the second task, continuous measurements of winds, air temperature, humidity, pressure, solar radiation parameters, sea temperature, salinity, beam transmission, and chlorophyll fluorescence are made while the ship is under way. This year a Coastal Climate automatic weather station was installed on Seal Island and operated from mid-December 1991 to midMarch 1992. Weather data were of interest for the seal and
ANTARCTIC J
The 1991-1992 field season of the U.S. Antarctic Marine Living Resource program (AMLR) (Rosenberg et al. this issue) was the third where physical oceanographic measurements were made by University of Texas Marine Science Institute researchers aboard the National Oceanic and Atmospheric Administration (NOAA) ship Surveyor in conjunction withbiological studies of other AMLR groups. The physical oceanography component of the AMLR program provided the means to identify contributing water masses and environmental influences within the study area, including meteorological conditions (Amos this issue). To accomplish this aim, 190 conducting-temperature-depth (CTD)/rosette casts were made, providing over 2,000 samples in the water column to other AMLR programs (see collected AMLR papers in this issue). As our knowledge of the Elephant Island region oceanography has increased, a grid sampling plan and station transect locations have been developed (Rosenberg et al. figures 2 and 3, respectively) to study the spatial and temporal variability. The large scale survey of 72 stations on a quarter-degree latitude by half-degree longitude grid-spacing (total area approximately 36 x 10 square kilometers) was occupied twice. CTD stations were made to within 10 meters of the bottom (or to 750 meters in deeper water). The Sea-Bird SBE-9 CTD instrumentation also included oxygen, beam-transmission, fluorometry, and downwelling (Photosynthetic Active Radiation) light sensors. However, we concentrate here on the standard oceanographic parameters to describe the dynamics of the circulation during the 1991-1992 austral summer. We will also briefly examine the temporal changes observed in the month separating the two cruise legs in the position of a front north of Elephant Island. In previous AMLR cruises aboard Surveyor we have described five water-mass types grouped by similarity of the temperature/ salinity characteristics of the upper water column (Amos and Lavender this issue). Identified by roman numerals as "Type II water," for example, AMLR researchers have used these designations to relate the distribution of the phyto- and zooplankton, krill, and fish larvae to the summer hydrography (AMLR papers in this issue). To review briefly: Drake Passage water (Type I) is warm, low in salinity water, and has a sub-surface temperature minimum ("Winter water," approximately -1 'C; salinity 34.0 ppt), Circumpolar deep water (CDW, T-max near 500 meters). Transition water (Type II) has a temperature minimum near 0 'C, isopycnal mixing below T-min, and CDW is evident at some locations. Weddell-Scotia Confluence (Type III) shows little evidence of a temperature minimum, mixes with Type II, has no CDW, and its temperature is at a depth of generally greater than 0 'C. Eastern Bransfield Strait water (Type IV) has a deep temperature near -i 'C, salinity of 34.5 ppt, and cooler surface temperatures. Wed-
228
dell Sea water (Type V) has little vertical structure and cold surface temperatures (near 0 'C). Figure 1 maps the implied geostrophysically-balanced flow as computed from the network of CTD stations from survey A (January 1992) and survey D (March 1992). The streamlines are labeled with dynamic height anomaly in dynamic centimeter referenced to two levels, the 200 decibar (figures 1A and 1B) and 500 decibar (figures 1C and 1D). The flow's direction is predominantly from southwest to northeast and streamline patterns are quite similar whether referenced to 200 or 500 decibar. There is a difference from survey A to survey D. By March the trend of the streamlines has rotated clockwise to a more easterly direction. There are two streams evident, one in the Drake Passage, and one in the Bransfield. The latter is an extension of the Bransfield Current, identified by Niiler et al. 1991. A persistent meander in the northwest corner of the large scale survey grid seems to have some relationship with the various biomass measurements made by AMLR scientists. The most dramatic hydrographic difference in the region is between the oceanic Drake Passage water and the waters of the Elephant Island shelf, namely between types I and III water masses. The narrow transition or mixing zone (type II) between them appears to have some significance for phytoplankton distribution (Helbling et al. in press; Holm-Hansen et al. this issue) and krill stock (Loeb and Seigel this issue). Along this boundary a density gradient (density increasing towards Elephant Island) reflects the change from warm, low-salinity to cold, more saline surface waters. At the same location the much deeper CDW core ends. The intensification of the flow in the Drake Passage (figure 1) coincides with this boundary and is perhaps related to the topography of the continental shelf-break. We have investigated this front by making transects of closely-spaced stations normal to the topographic trends. Figure 2 shows that the frontal boundary moves progressively seaward from early February to mid-March. Finally, the dynamic nature of the water column is illustrated in the change in water-structure at the same location from January to March (figure 3). The water overlying station 45, due north of Elephant Island (Rosenberg et al. this issue, figure 2 for locations), has a classic Type I structure on 26 January 1992 but is considerably modified to Type H water by 3 March 1992. Our analyses of these data will concentrate on the inter-annual and temporal changes in the hydrography and its relationship with the biomass distribution (Amos in press). The present work was performed under NOAA award NA27FRO01-10 to the UTMSI. I wish to thank the officers and crew of the NOAAS Surveyor, especially Lt. Cmdr. Fred Rossma and Lt. John Miller; ET's Mark May and Frank Gomes; Surve Techs Katherine Simmons, Ridge Liepens, and Tammy Tyner; AB's Mark Davidoff, Tim Patrico, and Harry Guist. CTD personnel were Margaret Lavender (legs I and II), Jeff Heimann (leg I) and Tony Amos (leg II). References Amos, A. F. and M. K. Lavender. 1992. AMLR program: Water masses in the vicinity of Elephant Island. Antarctic Journal of the U.S., 26(5):210213. Amos, A. F. 1992. AMLR program: A comparison between meteorologi. cal conditions at Seal Island and over the adjacent waters of the Drake Passage. Antarctic Journal of the U.S., this issue. Amos, A. F. Submitted. Interannual variability in the summer hydrogra-
ANTARCTIC JOURNAL
5e 0 w 57°W 56°W 55°W 54°W 530 SB°W 57°W 1 I I I 59°30
56°W 55°W 54°W 53°W
600 0S
50° 30
51° 0S
61°
30
.I I I 620
0S R. . 57°W 56°W 55°W 54°W 53°W 53° 58°W 56 0 W 57°W 56°W 55°W 54°W
AMLR 92. SURVEY 0 DYNAMIC HEIGHT (200 db) 28 FEB to 10 MAR 1992
AMLR 92, SURVEY A DYNAMIC HEIGHT (200 db) 19 JAN to 02 FEB 1992
FIGURE i—B
FIGURE i —A
58°W 57°W 56°W 55°W 54°W 53° 59°W 57°W 56°W 55°W 54°W .I I fg 59° 30
53° W
60° 0S
60° 30
61°
0S
61°
30
.'52°0S 1 " . 54°W 53°W 56°W 55°W 56W 530 56W 57°W 55°W 54°W 56°W 57°W
AMLR 92. SURVEY A DYNAMIC HEIGHT (500 db) 19 JAN to 02 FEB 1992 FIGURE 1 —C
AMLR 92. SURVEY 0 DYNAMIC HEIGHT (500 db) 28 FEB to 10 MAR 1992 FIGURE 1-0
Figure 1. Dynamic heights and geostrophic flow during the two large-scale surveys of AMLR 1991-1992. (A) Surface relative to 200 dcclbar, survey A; (B) surface relative to 200 decibar, survey 0; (C) surface relative to 500 decibar, survey A; (D) surface relative to 500 decibar, survey B.
1992 REvIEw
229
/
57 0 W
60 30 S
660K
560W
540
+
+ + +
++++/ +4:.._-
34
1111,1
IA
34.5
li,i
1
/
I1
400' -
+ 61005
I 66°W
i35 -3
/26.6
7..4//2/so
w 55°W
2
p.4/27.8
27
57°W
II
+ +
C + 61 0 OS
33.5 3- 1,1
60030 S +
AMLR 92 SURVEY STA 45A (DOTTED). STA 45D, (SOLID)
54°W
SURVEYOR CRUISE AMLR 92. SURVEYS .X LOCATION OF ELEPHANT I. FRONT 03 FEB to 04 FEB 1992 (A—A') 22 FEB to 25 FEB 1992 (B—B) 10 MAR to 12 MAR 1992 CC—C')
Figure 2. Location of frontal boundaries north of Elephant Island from cross-shell CTD transects.
-
-o
a.I
7 j5
I I-
I I I I
/ I I I I I I I —
2 i 33.5
s I'
I
I'l
34
I
I
I
34.5
/ I I
I'—I---2
35
SALINITY (ppt) Figure 3. T/S relationship of water column at station 45 on 26 January and 3 March 1992. Depth of major 1/S inversions are annotated on the curves.
phy of the surface water at Elephant Island, Antarctica. Continental
Shelf Research.
Heibling, E. W., A. F. Amos, N. Silva, V. Villafane, and 0. Holm-Hansen. In press. Phytoplankton distribution and abiotic measurements over a frontal system north of Elephant Island, Antarctica. Antarctic
Science.
Holm-Hansen, 0., V. E. Villafane, and E. W. Helbling. 1992. AMLR program: Phytoplankton abundance and rates of primary production
AMLR program: A comparison between the summer meteorological conditions at Seal Island and those over the adjacent waters of the Drake Passage A. F. AMOS Marine Science Institute University of Texas at Austin Port Aransas, Texas 78373
Seal Island is the largest of a small group of islands located north of Elephant Island, the northernmost of the South Shetland Islands (see Rosenberg et al. for location map). It is the site 230
around Elephant Island area. Antarctic Journal of the U.S., this issue. Loeb, V. and V. Siegal. 1992. AMLR program: Krill stock structure in the Elephant Island area. Antarctic Journal of the U.S., this issue. Niiler, P. P., A. F. Amos, and J. -H. Hu. 1991. Water masses and 200m relative geostrophic circulation in the western Bransfield Strait region. Deep Sea Research, 38(8/9):943-959. Rosenberg, J., R. P. Hewitt, and R. S. Holt. 1992. The U.S. AMLR program 1991-1992 field season activities. Antarctic Journal of the U.S., this issue.
of an annual study of seals and penguins by the Antarctic Marine Living Resource (AMLR) program (Croll et al. this issue). The waters of the adjacent continental shelf and the Drake Passage are the primary study area of the AMLR program's fieldwork aboard the National Oceanic and Atmospheric Administration (NOAA) ship Surveyor. The University of Texas's program in physical oceanography is part of the AMLR multidisciplinary study of the distribution of krill (Euphausia superba). Our work has two major tasks: to study the hydrography of the upper water column as it relates to the observed distribution of the biological organisms (Helbling et al. in press; Amos and Lavender this issue) and to monitor the surface meteorological conditions that may affect the water structure (Amos this issue). To accomplish the second task, continuous measurements of winds, air temperature, humidity, pressure, solar radiation parameters, sea temperature, salinity, beam transmission, and chlorophyll fluorescence are made while the ship is under way. This year a Coastal Climate automatic weather station was installed on Seal Island and operated from mid-December 1991 to midMarch 1992. Weather data were of interest for the seal and
ANTARCTIC J
