Influence of the Antarctic Ozone Hole on Seasonal Changes in Climate in the Southern Hemisphere

Climate change in the Southern Hemisphere (SH) in the latter half of the 20th century has been marked by the strengthening of the circumpolar westerlies in both the lower stratosphere over Antarctica and in the troposphere. The deep vertical coupling between the stratosphere and the troposphere is linked to changes in the SH atmospheric circulation where the dominant mode of variability, the summertime Southern Annular Mode (SAM), has shifted towards indexes of positive polarity in recent decades and is largely due to anthropogenic ozone depletion. The strong seasonal character of the ozone hole motivates our study of changes in the seasonal amplitude of changes in Southern Hemisphere temperatures and its links to atmospheric circulation. Using the amplitude of the annual cycle greatly reduces the sensitivity to instrumental or siting changes, and illuminates broad impacts on areas such as agriculture, the economy, and ecology. In this study, changes in the seasonal amplitude of temperature in the SH, defined as the difference between the mean summer (December-February) and winter (June-August) months, over the period from 1979 to 2012 were investigated through regression and correlation analysis to determine the linkage to related changes in ozone and the SAM using reanalysis datasets and surface station observations. Aloft in the troposphere at 500 hPa, a zonally symmetric pattern, with an increase in seasonal amplitude located in the mid-latitudes and a decrease over the pole was found that is highly congruent with the change in the SAM index over the same period. In contrast, at the surface, the seasonal amplitude has significantly decreased over areas of eastern Antarctica, southeast and south-central Australia, and inland parts of Southern Africa. This is juxtaposed with areas of increased seasonal amplitude on the western coastal regions of the same landmasses. Analysis of the near surface winds shows that many of the changes in mid-latitudes are linked to anomalous easterly flow and accompanying precipitation, while in the Antarctic the changes are explained by the enhanced westerly flow and resulting adiabatic changes in temperature. Furthermore, strong correlations between the surface seasonal amplitude and the total column ozone of the preceding spring were found in many areas over land, with the strongest signal emerging in south-central and eastern Australia. This study suggests that springtime Antarctic ozone can be a useful predictor of the subsequent summer temperature over important regions of the Southern Hemisphere, and reveals that the pending recovery of the ozone hole should be expected to lead to an increasing frequency of extremes such as the Australian summer of 2012/3.