Projected changes in the length of the tropical cyclone season

In response to increasing greenhouse gases, global climate models consistently project, alongside an annual mean warming, a delay and amplification of the annual cycle of SST in the tropics, indicating extrema reached later in the year and a larger annual range. Using the GFDL global High Resolution Atmospheric Model (HiRAM) and downscaled tropical cyclone tracks from a few CMIP5 models, we investigate whether these seasonal SST changes and other projected climate changes impact the length of the tropical cyclone season globally and in the major basins. We find that the fraction of the year for which the tropical cyclone potential intensity is greater than a chosen threshold is projected to increase. However, if we define the length of the active season as when the average number of tropical cyclones is above a threshold, HiRAM and the downscaled tropical cyclones yield opposite projections on how the length of the tropical cyclone season might change in most of the basins. HiRAM projects a shorter tropical cyclone season, while the downscaling method projects a longer season. We find that these changes are well explained by changes in the annual mean number of tropical cyclones in the models. We also find a relationship between the timing delay of the annual cycle of SST and the timing delay of the peak potential intensity and peak number of storms.