Climate Changes in Tropical Cyclone Activity Simulated by GFDL CM2.5

Global tropical cyclone (TC) activity is simulated by Geophysical Fluid Dynamics Laboratory CM2.5 that is a fully coupled global climate model with horizontal resolution of about 50km for atmosphere and 25 km for ocean. Firstly, the climatological characteristics of the simulated TC activity are analyzed. The present climate simulation results show the realistic global TC frequency, seasonal cycle, and global distributions in agreement with the observation, but CM2.5 have some bias in regional TC activity, including notable negative bias in the TC count over the North Atlantic. The regional bias in TC activity is highly associated with the simulated bias in large-scale environments such as sea surface temperature, vertical wind shear, and 500 hPa vertical velocity. Despite of the biases, the CM2.5 has outstanding skill to simulate the realistic variation of TC activity induced by El Nino/Southern Oscillation over the globe. Possible changes in TC activity in the future are investigated by the comparison between the present climate and CO2 doubling experiments. Globally, the TC formations are reduced (-18.7%) while TC intensity increases (+2.7%) in response to CO2 doubling, which is consistent with the previous studies. Accordingly, the TC lifetime decreases by -4.6% and the spatial TC size and TC-induced rainfalls significantly increase by about 3% and 12%, respectively. These changes show consistent signal over the basins but the amounts vary from basin to basin. The analysis suggests that the regional changes of several large-scale environments complexly affect the different changes of TC activities over the basins. For Atlantic basin, the TC frequency reduced in associated with the response of the relative SST and vertical wind shear over the main development region to the global warming. However, increasing interannual variation of Atlanitc SST forced the Atlantic TC activity up in the warming climate, resulting in severe TC damages during the warmer Atlantic SST seasons.