The Response of Tropical Cyclone Statistics to an Increase in CO₂ with Fixed Sea Surface Temperatures

The effects on tropical cyclone statistics of doubling CO₂, with fixed sea surface temperatures (SSTs), are compared to the effects of a 2-K increase in SST, with fixed CO₂, using a 50-km resolution global atmospheric model. Confirming earlier results of Yoshimura and Sugi, a significant fraction of the reduction in globally averaged tropical storm frequency seen in simulations in which both SST and CO₂ are increased can be thought of as the effect of the CO₂ increase with fixed SSTs. Globally, the model produces a decrease in tropical cyclone frequency of about 10% due to doubling of CO₂ and an additional 10% for a 2-K increase in SST, resulting in roughly a 20% reduction when both effects are present. The relative contribution of the CO₂ effect to the total reduction is larger in the Northern than in the Southern Hemisphere. The average intensity of storms increases in the model with increasing SST, but intensity remains roughly unchanged, or decreases slightly, with the increase in CO₂ alone. As a result, when considering the frequency of more intense cyclones, the intensity increase tends to compensate for the reduced total cyclone numbers for the SST increase in isolation, but not for the CO₂ increase in isolation. Changes in genesis in these experiments roughly follow changes in mean vertical motion, reflecting changes in convective mass fluxes. Discussion of one possible perspective on how changes in the convective mass flux might alter genesis rates is provided.