On the Mechanism of Tropical Cyclone Frequency Changes Due to Global Warming

In order to explore the hypothesized mechanisms for the reduction of global tropical cyclone (TC) frequency due to greenhouse warming, an experiment has been conducted using a most recent version of MRI-AGCM with a new convection scheme. In addition to a present climate run (HPA run) and a future climate run (HFA run), two more runs are conducted. In CO2F run, future values of CO2 and other greenhouse gas (GHG) concentrations are used with present value of sea surface temperature (SST), while in the SSTF run, future value of SST is used with present values of CO2 and other GHG concentrations. The reductions of global TC frequency in HFA run, CO2F run and SSTF run from HPA run are 25%, 9% and 18%, respectively. These results are basically consistent with previous studies. Based on the results of the experiment, we examined three key relations in our hypothesized mechanism for the reduction of TC frequency. First, the relation between changes in atmospheric radiative cooling and precipitation is confirmed to be valid in the experiment, in which not only CO2 but other GHG is increased. It is also confirmed that the effect of increasing CO2 is decreasing precipitation, while the effect of increasing other GHG is increasing precipitation. Second, the relation between changes in precipitation and upward mass flux is clarified by using a simple approximate thermodynamic equation. Third, regarding the relation between changes in upward mass flux and TC genesis frequency, we examined the changes in four parameters (precipitation, upward mass flux, vertical wind shear and mid-troposphere saturation deficit) which are closely related to deep convective activities in the tropics and may affect TC genesis frequency. The results of our experiment support the idea suggested by the previous studies that the reduction of TC frequency is closely related to a reduction of upward mass flux, although the chain of causality linking the two remains unclear. In addition, our experiment suggests a possibility that the changes in mid-troposphere saturation deficit may also contribute to the changes in TC genesis frequency.