Interannual variability in the troposphere and stratosphere of the southern hemisphere winter

Abstract

Interannual variability in the tropospheric and stratospheric circulation of the southern-hemisphere winter is studied over the 16-year period from 1979 to 1994. In particular, the interannual mode of variability in the seasonal march from autumn to spring is investigated by means of a multiple empirical orthogonal function analysis. The extracted dominant mode of variability is characterized by the poleward and downward propagation of anomalous zonal-mean zonal winds, accompanied by changes in wave activity. The overall features of the variability are similar to those found in the northern hemisphere except that the entire sequence of evolution is closely locked to the annual cycle and requires a longer period of 6 months. The time coefficients of this mode exhibit a very clear biennial oscillation and increasing trend. A similar spatial pattern of variability is also found in the simulated data of an atmospheric general circulation model with climalogical sea surface temperatures as the lower boundary conditions. The similarities in the time evolution of the spatial pattern suggest that this mode is essentially an internal mode of variability in the atmosphere, which is modulated by external conditions. Copyright 1998 by the American Geophysical Union.