Dynamical Downscaling Projections of Late 21st Century Atlantic Hurricane Activity: CMIP3 and CMIP5 Model-based Scenarios

The robustness of late 21st century dynamical model projections of intense Atlantic hurricane activity is examined. Multi-model ensemble climate change scenarios from CMIP3/A1B and CMIP5/RCP4.5 are compared. Ten individual CMIP3 models are downscaled. Dynamical downscalings are compared from a 18 km grid regional model and a 50 km grid global model. To simulate intense hurricanes, each storm from the regional model is downscaled using the GFDL hurricane model, which has grid spacing as fine as 9 km and ocean coupling. A significant (p=0.05) reduction in the frequency of tropical storms and hurricanes is projected for both CMIP3 (-23%) and CMIP5 (-27%) ensembles, and by five of ten individual CMIP3 models that were downscaled. However, for strong category 4+ hurricanes (winds of at least 65 m s-1), we find a significantly increased frequency for the CMIP3 ensemble (+250%); for the CMIP5 ensemble we find a smaller (nonsignificant) increase (+84%). For the frequency of Category 4-5 hurricanes (winds >= 59 m s-1) the Bender et al. (2008) CMIP3-based study had projected a significant increase (+87%); the CMIP5 ensemble projects a smaller (non-significant) increase (+39%). Three of ten individual CMIP3 models show a significant increase in frequency of both Category 4-5 hurricanes and hurricanes stronger than 65 m s-1. Tropical cyclone-related rainfall rates, averaged within 100 km of the storm, increase significantly by 23% (CMIP3) and 14% (CMIP5). The fractional increase in precipitation approximates that expected from water vapor content scaling considerations for relatively large radii (200-400 km), but is substantially higher at relatively smaller radii (50-150 km).