Opposite decadal atmospheric trends in the Southern Hemisphere between winter and summer

【作者】 胡永云；

【机构】 北京大学物理学院大气科学系；

【Abstract】 The atmosphere demonstrates noticeable trends in the past few decades. In the present work, we study the decadal trends in the Southern Hemisphere between winter months (June-October) and summer months (December-April) over 1979-2005 using NCEP/NCAR, NCEP/DOE, and ECMWF reanalysis. It is found that the trends are nearly opposite between Southern winter and summer months since the late 1970s. In summer months, the trends are characterized by polar cooling in both the troposphere and the stratosphere, decrease in polar geopotential heights, and accelerated westerly zonal winds in the subpolar region, while the extratropics show warming and increasing in geopotential heights. This sea-saw spatial pattern is equivalent to the positive Antarctic oscillation trend found by previous studies. In contrast, in winter months the Antarctic polar region shows warming, increasing in geopotential heights, and decelerated westerly winds in the subpolar region, while the extratropical stratosphere show relatively weak cooling. It implies a negative trend in the Antarctic oscillation. The Southern-Hemisphere decadal trends are similar to the opposite trends in the Northern Hemisphere studied in our earlier work. Eliassen-Palm flux calculations show that in the Southern winter months the troposphere and stratosphere are dynamically coupled, and the polar warming is associated with increased planetary wave activity. It is likely that the negative trend in the Antarctic oscillation in winter months is related to sea surface temperature (SST) warming. In summer months, the polar cooling first occurs in the Antarctic stratosphere in November and slowly propagates downward into the troposphere. It has been suggested that the cooling is due to enhanced ozone depletion in the stratospheric Antarctic since the late 1970s. Questions remained are whether the opposite trends are caused respectively by SST warming and stratospheric Antarctic ozone depletion, and how the winter trends influence the summer trends. These are left to future work to answer.更多还原