The cause of the strengthening of the Antarctic polar vortex during October November periods

The stratospheric polar vortex strength in spring determines to a great extent the duration and intensity of ozone depletion in the polar regions. The size of the Antarctic ozone hole usually reaches its maximum in September, and then drops off during October and November. However, in 1987, 1998, 1999, 2001, 2006, 2011 and especially in 2015, a significant increase in the ozone hole area relative to climatological mean values was observed in October and November under strong polar vortex conditions. Furthermore, in these years it occurred simultaneously with a temperature increase in the subtropical lower stratosphere in the Southern Hemisphere. Based on the ERA-Interim reanalysis temperature and zonal wind data, we reveal a high correlation between interannual variations of the subtropical temperature and the zonal wind at 60 degrees S in October and November (the Pearson correlation coefficients r equal 0.71 and 0.82, respectively). Thus, a temperature increase in the subtropical lower stratosphere from October to November can strengthen the Antarctic polar vortex in this period.