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

Extratropical cyclones form an important part of the global circulation. They are responsible for much of the high impact weather in the mid-latitudes, including heavy precipitation, strong winds, and coastal storm surges. They are also the surface manifestation of baroclinic waves that are responsible for much of the transport of momentum, heat, and moisture across the mid-latitudes. Thus how these storms will change in the future is of much general interest. In particular, how the frequency of the extreme cyclones change are of most concern, since they are the ones that cause most damages.

While the projection of a poleward shift of the Southern Hemisphere (SH) storm track and cyclone activity is widely accepted, together with a small decrease in the total number of extratropical cyclones, as discussed in the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5), projected change in cyclone intensity is still rather uncertain. Several studies have suggested that cyclone intensity, in terms of absolute value of sea level pressure (SLP) minima or SLP perturbations, is projected to increase under global warming. However, other studies found no increase in wind speed around extratropical cyclones.

It is difficult to directly compare the results of the different studies discussed in the previous paragraph, since different ensembles of simulations using different models were examined, and different studies defined cyclone intensity in different ways. In this study, projected changes in extreme cyclones in the SH, based on 27 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), are presented. Multiple definitions of extreme cyclones have been examined, including intensity exceeding constant thresholds of SLP, 850 hPa vorticity, and 850 hPa winds, as well as variable thresholds corresponding to a top-5 or top-1 cyclone per 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. The largest increase is projected between 45 and 60 °S, extending from the South Atlantic across the South Indian Ocean into the Pacific. The projected increases in extreme cyclone statistics are consistent with the projected increases in Eulerian statistics such as SLP variance, frequency of extreme winds, and total kinetic energy. However, while the projected increases in these Eulerian statistics can be linked to increase in the mean available potential energy (MAPE), the increases in cyclone statistics are not well correlated with those in the MAPE. Thus there is still a challenge in finding mean flow metrics that can quantitatively explain the changes in extreme cyclone frequency.