The influence of ozone forcing on blocking in the Southern Hemisphere

We investigate the influence of ozone depletion and recovery on tropospheric blocking in the Southern Hemisphere. Blocking events are identified using a persistent positive anomaly method applied to 500 hPa geopotential height. Using the National Institute for Water and Atmospheric Research‐United Kingdom Chemistry and Aerosols chemistry‐climate model, we compare reference runs that include forcing due to greenhouse gases (GHGs) and ozone‐depleting substances to sensitivity simulations in which ozone‐depleting substances are fixed at their 1960 abundances and other sensitivity simulations with GHGs fixed at their 1960 abundances. Blocking events in the South Atlantic are shown to follow stratospheric positive anomalies in the Southern Annular Mode (SAM) index; this is not the case for South Pacific blocking events. This relationship means that summer ozone depletion, and corresponding positive SAM anomalies, leads to an increased frequency of blocking in the South Atlantic while having little effect in the South Pacific. Similarly, ozone recovery, having the opposite effect on the SAM, leads to a decline in blocking frequency in the South Atlantic, although this may be somewhat counteracted by the effect of increasing GHGs.