Stratospheric Circulation in the Southern Hemisphere: Links to Tropical Winds, Ozone, and the Hunga Eruption—Part 1: Evidence From ERA5 Reanalysis

Abstract

The interannual variability of the Southern Hemisphere polar vortex plays a key role in ozone depletion and recovery but this variability and its teleconnection patterns are less understood than in the Northern Hemisphere. Using ERA5 reanalysis data from 1980 to 2022 (43 years), we studied the variability of the Southern Hemisphere vortex by focusing on the location of the winter jet maximum in the upper stratosphere (1 hPa). We identify two dominant modes: In the first one, the core of the jet at 1 hPa remains at lower latitudes (40°S–50°S, LLJ) throughout austral winter (9 years, including 2022 following the Hunga eruption). In the second mode, the core of the jet shifts poleward (60°S–70°S, HLJ) in mid‐July and stays at high latitudes throughout August (15 years, including the 2002 and 2019 sudden warming events). In the remaining 19 years, the jet does not fall into either of these modes. LLJ years are linked to weaker upward propagating wave activity near the tropopause and a planetary wave index of refraction that inhibits propagation in the stratosphere; these years also have a stronger, colder polar vortex and greater ozone depletion during late winter and spring. HLJ years show the opposite behavior. Observations suggests a link with the Quasi‐Biennial Oscillation: LLJ years often occur when tropical westerlies are present at 10 hPa; HLJ years occur when tropical easterlies are present at that level. These findings suggest the latitude of the upper stratospheric jet provides a useful diagnostic for understanding Southern Hemisphere stratospheric dynamics.