Decadal change in the troposphere and atmospheric ...

work was supported in part by National Science Foundation grant number OPP 91-18961.

References

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Carroll, J.J. 1994. Observations and model studies of episodic events over the south polar plateau. Antarctic Journal of the U.S., 29(5). Dutton, E.G., R.S. Stone, D.W. Nelson, and B.G. Mendonca. 1991. Recent interannual variations in solar radiation, cloudiness, and surface temperature at the South Pole. Journal of Climate, 4(8), 848-858. Neff, W.D. 1980. An observational and numerical study of the atmospheric boundary layer overlying the east antarctic ice sheet. (Ph.D. dissertation, University of Colorado, Boulder, Colorado). Neff, W.D. 1986. On the use of sodars to study stably stratified flow influenced by terrain. Atmospheric Research, 20, 279-308. Neff, W.D. 1992. Synoptic influence on inversion winds at the South

Pole. Preprint Volume: Third Conference on Polar Meteorology and Oceanography, September 29 to October 2, 1992, Portland, Ore-

gon. American Meteorological Society, Boston, Massachusetts, J2, 24-28. Neff, W.D. 1994. Mesoscale air quality studies with meteorological 45Q3525 Temperature (C)

Figure 3. Time-height representation of temperatures from julian day 331 through 350 of 1993 through the lowest 3-km of the atmosphere during a prolonged warming event as observed from twice-a-day rawinsondes. (ASL denotes above sea level.) Technology Laboratory's winter-over project leader in 1993; and of Dan Gottas and Dan Wolfe, who supported the spring and summer operations, are gratefully acknowledged. This

remote sensing systems. International Journal

of Remote sensing,

15(2),393-426. Parish, T.R. 1982. Surface airflow over East Antarctica. Monthly Weather Review, 110(2),84-90. Parish, T.R., and D.H. Bromwich. 1991. Continental-scale simulation of the antarctic katabatic wind regime. Journal of Climate, 4(2), 135-146. Schwerdtfeger, W. 1984. Weather and climate of the Antarctic. New York: Elsevier. Stone, R.S., and J.D. Kahl. 1991. Variations in boundary layer properties associated with clouds and transient weather disturbances at the South Pole during winter. Journal of Geophysical Research, 96(D3),5137-5144.

Decadal change in the troposphere and atmospheric boundary layer over the South Pole W.D. NEFF,

National Oceanic and Atmospheric Administration/En vironmental Technology Laboratory, Boulder, Colorado 80303

uring the austral winter of 1993, the Environmental D Technology Laboratory in collaboration with the University of California at Davis (Carroll, Antarctic Journal, in this issue), carried out a detailed field study of the atmospheric boundary layer at Amundsen-Scott South Pole Station to determine the effect of transitory synoptic disturbances on the surface-energy budget. This study used newly developed 915-megahertz radar wind-profiling technology for the first time in the Antarctic in combination with conventional boundary layer instrumentation that included a short tower, sonic anemometer, microbarograph array, and doppler sodar (Neff, Antarctic Journal, in this issue). Recent discussions, however, of interdecadal variability in the circumpolar circulation around Antarctica (Hurrell and Van Loon 1994) and of decadal changes in summer cloudiness at the South Pole (Dutton et al. 1991), motivated our study of the long-term

variability in boundary layer characteristics, cloudiness, and tropospheric flow behavior to provide a climatological context for our single year's observations. During the austral spring, the atmosphere above 300 hectopascals (hPa) warms rapidly from temperatures less than -70°C to temperatures greater than -40°C because of the combination of dynamical effects and radiative heating (Kiehl, Boville, and Briegleb 1988). Because this increase is much greater than the warming of the troposphere below 300 hPa, a thermal tropopause forms, typically by mid-November, with a temperature minimum near 300 hPa. This change in stratification of the upper troposphere provides a dramatic contrast between winter and summer over Antarctica. In figure 1, we show the formation of this feature as a function of time starting from 1 September each year, from 1961 through 1993. For figure 1, our calculation of bulk tern-

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Figure 1. Time of formation of the thermal tropopause (time at which the bulk stability measured from the temperature minimum near 300 hPa to 150 hPa) increases to 100 per kilometer, measured in days from 1 September each austral spring from 1961 through 1993 (solid line). The dashed straight line is the less-squares-fit-line accounting for 77 percent of the variance after removal of interannual variability. The second dashed line represents time of increase of bulk stability between 200 and 150 hPa to at least 100 per kilometer. Because the stratosphere typically warms from aloft, this curve shows an earlier warming than at tropopause level (typically 300 hPa). perature lapse rates used the temperature difference from the height of the temperature minimum (usually about 300 hPa) to 150 hPa (the maximum reliable height normally obtained from the rawinsonde in the early spring), and then from 200 hPa to 150 hPa. A 20-day triangular filter was used to smooth data within a given season to eliminate synoptic time-scale variations. Furthermore, a 5-year triangular filter was used to remove the effect of variability at periods characteristic of the quasibiennial oscillation (QBO). The results in figure 1 show a delay of almost 30 days in the formation of the thermal tropopause over the last 3 decades with major excursions from the trend in the late 1970s and again in the late 1980s. The first of these excursions, from 1976 to 1980, coincides with a 15 percent reduction in the maximum antarctic sea-ice extent (Chapman and Walsh 1993); the later excursion does not. An interesting feature of the data in figure 1 is the systematic trend in the time of formation of the thermal tropopause with occasional interruptions at decadal timescales. After removal of interannual variability characteristic of the QBO, a linear trend accounts for 77 percent of the remaining variance. Of particular note in this result is the possibility that the unusual warming in the late 1970s may have obscured the linear trend in the time of formation of the thermal tropopause in earlier data sets. Decadal variations also appear in sky-cover observations from the South Pole as noted by Dutton et al. (1991) for January-March averaged sky cover. Schnell et al. (1991) argued that a coincident decrease in surface ozone arose from increased photochemical destruction (due to ozone depletion in its stratospheric source region) as well as increased transport of ozone-poor marine air from lower latitudes. Because their data extended only through 1989, we analyzed new data through the fall of 1994 (figure 2). Figure 2 shows daily averaged sky cover, smoothed as in figure 1, within seasons and

from year-to-year. A period of increased cloudiness occurs in the early 1980s during both spring and fall seasons. Of note is the minimum in sky cover between days 70 and 100 (measured from 1 September: we assumed that twilight was sufficient by this time to make consistent sky cover estimates). Comparison with figure 1 suggests that this minimum occurs following the formation of the thermal tropopause; additional analysis (not shown) shows that the minimum corresponds roughly to the period of maximum static stability above the tropopause. A possible physical explanation for this phenomena is that the horizontal scale of weather systems is directly related to their vertical scale: when a strong tropopause forms for only a month or so during the early austral summer, the vertical scale and consequently the horizontal scale of atmospheric motions is reduced. Thus, the transport of marine air from the distant coastal areas becomes less likely. If this hypothesis is correct, then the weakening of the tropopause in the summer as a result of ozone depletion should influence transport processes over Antarctica, including the meridional transport of heat and moisture. Lastly, as a final highlight of our analyses of decadal changes, figure 3 shows time-series of • sky cover during spring (mid-September to mid-November) and summer (January to mid-March), • the annual average of 300-150-hPa thickness providing a measure of the variability of the mean temperature just above tropopause level, and

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Figure 2. As described in the text, the seasonal and long-term variability in sky cover at the South Pole from 1957 through early 1994 obtained from averaging and smoothing daily weather observations. The timescale is in days from 1 September.

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The time-series of sky cover shows an increase in cloudiness in both spring and summer during the late 1970s and early 1980s following the decrease in antarctic sea-ice extent (after 1975) noted earlier. Similarly, the lower stratosphere shows a warming (increased thickness) during the same general period. Of greatest interest to our experimental work, however, is the near-surface lapse rate time-series that shows systematic decreases in lapse rate from 1961 to 1994 with distinct recoveries at roughly decadal timescales. In general, changes in the lapse rate follow changes in the 300-150-hPa thickness; however, examination of individual seasons does not show any distinct inverse relationship between cloudiness and lapse rate as one might expect. In summary, the atmosphere over the South Pole shows strong variability at decadal timescales, distinct trends in surface lapse rate, and delays of almost a month in the time of formation of the thermal tropopause over the South Pole during the period from 1961 to 1994. Explanations for the changes in the surface lapse rate will likely depend on the more detailed analysis of data obtained during the austral winter of 1993 as described elsewhere in this issue. Further analysis of the effect of stratospheric ozone depletion on tropospheric thermal structure over the South Pole and the consequences for tropospheric weather also appears warranted. This research was supported in part by National Science Foundation grant OPP 91-18961.

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References

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Carroll, J.J. 1994. Observations and model studies of episodic events over the south polar plateau. Antarctic Journal of the U.S., 29(5). Chapman, W.L., and J.E. Walsh. 1993. Recent variations of sea ice and air temperatures in high latitudes. Bulletin of the American Meteorological Society, 74(1), 33-47. Dutton, E.G., R.S. Stone, D.W. Nelson, and B.G. Mendonca. 1991. Recent interannual variations in solar radiation, cloudiness, and surface temperature at the South Pole. Journal of Climate, 4(8), 848-858. Hurrell, J.W., and H. Van Loon. 1994. A modulation of the atmospheric annual cycle in the Southern Hemisphere. Tellus, 46A(3), 325-338. Kiehl, J.T., B.A. Boville, and B.P. Briegleb. 1988. Response of a general circulation model to a prescribed antarctic ozone hole. Nature, 332(6164), 501-504. Neff, W.D. 1994. Studies of variability in the troposphere and atmospheric boundary layer over the South Pole: 1993 experimental design and preliminary results. Antarctic Journal of the U.S., 29(5). Schnell, R.C., S.C. Liu, S.J. Oltmans, R.S. Stone, D.J. Hofmann, E.G. Dutton, T. Deshler, W.T. Sturges, J.W. Harder, S.D. Sewell, M. Trainer, and J.M. Harris. 1991. Decrease of summer tropospheric ozone concentrations in Antarctica. Nature, 351(6329), 726-729.

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Figure 3. Top panel: Time-series of sky cover in spring (solid black line) and summer (gray line). Middle panel: Time-series of annual averaged 300-150-hPa thickness. (m denotes meters.) Bottom panel: Time-series of annual average surface lapse rate obtained from daily rawinsondes at the South Pole. (C/i OOm denotes degrees Celsius per 100 meters.) • the annual averaged surface lapse rate computed using the lowest two significant levels from the South Pole rawinsonde profiles obtained from 1961 through mid-1994.

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