A modulation of the atmospheric annual cycle in the Southern Hemisphere

Abstract

The annual cycle in pressure and winds at the middle and high latitudes of the Southern Hemisphere changed appreciably in the troposphere after the late 1970s. Before that time, its major component over most of the southern oceans and the Antarctic was a semiannual oscillation (SAO) which had maxima in the equinoctial seasons over the middle latitude oceans and in the solstitial seasons over the Antarctic south of 60°S. The SAO weakened after the late 1970s because of significant decadal changes in the monthly means primarily during the second half of the year. A result was that the polar vortex in the troposphere, which normally weakens after a peak in late September and early October, remained strong into November, and the breakdown of the stratospheric polar vortex was similarly delayed. The pressure in the circumpolar trough in the 1980s was generally lower than in the 1970s, which is similar to a documented change in the North Pacific during northern winter. We suggest that the changes in the SAO, the circumpolar trough, and the polar vortex are related to the concurrent rise of sea surface temperatures at low latitudes. The delayed breakdown of the polar vortex in the troposphere and lower stratosphere which happened after the late 1970s was coincident with the beginning of the ozone deficit in the Antarctic spring. This points to a strong dynamical influence on ozone amounts.