Tropical cyclone frequency in simulations of the Last Glacial Maximum

While global tropical cyclone frequency typically decreases in projections of anthropogenic warming over the next century, the physical mechanism underlying the change in frequency is not clear. The extent to which to the critical factors are thermodynamic (e.g., normalized saturation deficit or potential intensity) or dynamic (e.g., changes in the magnitude of vertical velocity, position of convergence zones, or vertical wind shear) is under active investigation. One approach to address this question is to examine alternative climate change scenarios, which may have different sensitivities in the environmental factors controlling tropical cyclone genesis than those of anthropogenic warming.

Here, I present the results of simulations of the Last Glacial Maximum (LGM) with the tropical cyclone permitting, 50-km horizontal resolution Geophysical Fluid Dynamics Laboratory global atmospheric general circulation model HiRAM. Comprehensive simulations, in which the ensemble-mean sea surface temperature (SST) anomaly from the LGM simulations of the third phase of the Paleoclimate Intermodel Comparison Project, greenhouse gas concentration, and ice sheet orography are simultaneously perturbed, are compared to simulations in which these boundary conditions and forcing are individually perturbed. The global tropical cyclone frequency increases in the LGM simulations, with offsetting changes between ocean basins, but at a weaker rate than the 10% change per degree of tropical temperature change that is typical of future projections.