Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere

Abstract

Extratropical cyclones are responsible for much of the extreme weather in the midlatitudes; thus, how these cyclones may change under increasing greenhouse gas forcing is of much general interest. Previous studies have suggested a poleward shift in the location of these cyclones, but how the intensity may change remains uncertain, especially in terms of maximum wind speed. In this study, projected changes in extreme cyclones in the Southern Hemisphere, based on 26 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), are presented. Multiple definitions of extreme cyclones have been examined, including intensity exceeding constant thresholds of sea level pressure perturbations, 850-hPa vorticity, and 850-hPa winds, as well as variable thresholds corresponding to a top-5 or top-1 cyclone per winter month in these three parameters and the near-surface winds. Results presented show that CMIP5 models project a significant increase in the frequency of extreme cyclones in all four seasons regardless of the definition, with over 88% of the models projecting an increase. Spatial patterns of increase are also consistent, with the largest increase projected between 45° and 60°S, extending from the South Atlantic across the south Indian Ocean into the Pacific. The projected increases in cyclone statistics are consistent with those in Eulerian statistics, such as sea level pressure (SLP) variance. However, while the projected increase in SLP variance can be linked to increase in the mean available potential energy (MAPE), the increases in cyclone statistics are not well correlated with those in MAPE.