Abstract

Large-scale environmental variables known to be linked to the formation of tropical cyclones, have previously been used to develop empirical indices as proxies for assessing cyclone frequency from large-scale analyses or model simulations. Here we examine the ability of two recent indices, the Ge... Show moreLarge-scale environmental variables known to be linked to the formation of tropical cyclones, have previously been used to develop empirical indices as proxies for assessing cyclone frequency from large-scale analyses or model simulations. Here we examine the ability of two recent indices, the Genesis Potential and the Genesis Potential Index to reproduce observed North Atlantic cyclone annual frequency variations and trends. These skillfully estimate the mean seasonal variation of observed cyclones, but they struggle with reproducing interannual frequency variability and change. Examination of the independent contributions by the four terms that make up the indices finds that potential intensity and shear have significant skill, while moisture and vorticity either do not contribute or degrade the indices capacity to reproduce observed interannual variability. We also find that for assessing basin wide cyclone frequency, averaging indices over the whole basin is less skillful than its application to the general area off the coast of Africa broadly covering the Main Development Region (the MDR). These results point to a revised index, the Cyclone Genesis Index (CGI) comprised of only potential intensity and vertical shear. Application of the CGI averaged over the MDR demonstrates high and significant skill at reproducing interannual variations and trends in all-basin cyclones across both reanalyses. The CGI also provides a more accurate reproduction of seasonal variations than the original GP. Future work applying the CGI to other tropical cyclone basins and to the downscaling of relatively course climate simulations is briefly addressed. Show less