DEEPENING AND POLEWARD SHIFT OF THE ANTARCTIC CIRCUMPOLAR PRESSURE TROUGH FROM ERA5 REANALYSIS DATA

Abstract

The Antarctic circumpolar pressure trough (ACT) is a prominent feature of the Southern Hemisphere polar atmosphere. This band of low atmospheric pressure is formed as a net effect of individual storms emerging in, developing, and traversing the high-latitude region of the Southern Ocean. The location and depth of the trough directly influence regional weather patterns, atmospheric circulation, ocean currents, and sea ice dynamics. The north-south movement of the ACT presents a key aspect of the Southern Annular Mode (SAM). In this study we used monthly mean sea level pressure data from ERA5 atmospheric reanalysis to characterize the current properties of the ACT, its seasonal variability and trends in the last 45 years. Two major metrics of the ACT were considered, the sea level pressure and the meridional position of the ACT axis. Our analysis has shown the dominance of decreasing trends in the ACT pressure and gradual shifting of the ACT axis towards the continent. The largest changes in the ACT position and pressure occur in Western Antarctica, particularly in Weddell, Amundsen and Ross Seas. The estimated overall drop in the annually and zonally averaged pressure at the ACT axis in the last 45 years amounted to around 3 hPa in Western Antarctica and to around 1.5 hPa in Eastern Antarctica while the ACT axis in these sectors shifted poleward correspondingly by 0.45 and 0.27 degrees latitude. Deepening of the ACT and its poleward contraction were mostly due to their strong changes in the austral summer and fall seasons whereas in winter and in spring the trends were mixed and close to neutral. Trends in both the meridional position of the ACT axis and the sea level pressure were found to accelerate in the last 10–15 years.