Tropical Cyclogenesis Factors in a Warming Climate

Understanding the underlying causes of tropical cyclone formation is crucial to predicting tropical cyclone behavior in a warming environment, given the Earth's current warming trend. This study examines two sets of simulations from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 3.1 (CAM3): one with aerosol forcings and one without. We looked at how four factors known to be important to tropical cyclone formation vary as carbon dioxde and the ensuing temperature changes increase to very high levels. These factors include Maximum Potential Intensity (MPI), mid-tropospheric moisture content, 200-850 mb vertical wind shear, and 850 mb absolute vorticity. We considered different representations of mid-tropospheric moisture by examining both relative humidity and chi, a non-dimensional measure of the saturation entropy deficit at 600 mb. We also looked at different combinations of these factors, including several variations of a Genesis Potential Index (GPI) and an incubation parameter, gamma, that is related to the length of time required to saturate the middle troposphere and aid tropical cyclogenesis. Higher MPI, lower saturation deficits and higher relative humidity, lower wind shear, and higher absolute vorticity all act to enhance the GPI and lower the incubation time, meaning larger environmental support for tropical cyclone development and intensification.

In areas where tropical cyclone development is prevalent today, we found that shear generally decreased, but MPI decreased, absolute vorticity decreased, and the saturation deficit increases. Thus, in today's prevalent tropical cyclone regions, conditions become less favorable for development and intensification as the climate warms. On the other hand, genesis regions tend to push northward into the subtropics, as conditions become much more favorable for development up to ~40 degrees North due to both decreased wind shear and much higher MPI values.