Projected Changes of Tropical Cyclone Formation Regions in the Southern Hemisphere

A future reduction in global tropical cyclone (TC) numbers is a robust result emerging from the majority of projection studies published in the last decade. However, few studies have considered potential future changes in TC formation geographic distributions. A method for analysing geographic boundaries of TC formation is applied to CMIP5 model data, to identify potential changes in southern hemisphere TC formation regions in the future. The method was developed using 34 years of ERA-Interim data and TC detections in those data (using the OWZP TC detector). Thresholds of 700 hPa relative humidity (RH₇₀₀=40%) and Emanuel’s maximum potential intensity (V_max=40ms^-1) defined the poleward boundaries, and a composite parameter made up of the 850 hPa absolute vorticity and its meridional derivative defined the equatorward boundaries.

Applied to CMIP5 data, significant TC formation distribution errors were apparent for a number of models, including frequent formation in the South Atlantic and eastern South Pacific (where TCs rarely or never form), and a lack of TC formation in the central North Atlantic. Those southern regions are in reality too dry (RH₇₀₀<40%) for TC formation, but are moist in the models, allowing frequent formation. The opposite is true in the central North Atlantic where TC formation is frequent in reality, but too dry in the models.

Results from seven models with good southern hemisphere TC formation distributions, are being analysed for changes in TC formation boundaries between the historical (1970—2000) and future (2070—2100, RCP8.5 scenario) climate. Preliminary results suggest little change in the boundaries will be identified, except in the eastern South Indian Ocean basin where there is general consensus between the models of an equatorward migration of the RH₇₀₀ boundary that will thin the already narrow latitudinal band in which TCs form. Reduced TC detection numbers in this region support the result.