Impact of CO2-induced warming on hurricane intensities as simulated in a hurricane model with ocean coupling

Recent high resolution simulations of the impact of CO2-induced warming on hurricane intensity indicate a modest increase (5-12%) in maximum surface wind speeds, comparing strong simulated storms in a high CO2 climate with those in the control (present day) climate (Knutson and Tuleya, Clim. Dyn., in press). These simulation results are in reasonable accord with theoretical predictions using the Maximum Potential Intensity methodologies of Emanuel or Holland. One assumption of the simulations to date and of the MPI theories is the neglect of the effect of hurricane/ocean coupling (i.e., the local SST cooling induced by the hurricane) on the intensity change. This study explores how a CO2 warming-induced enhancement of hurricane intensity could be altered by hurricane/ocean coupling.

A series of idealized hurricane experiments were carried out using the GFDL Hurricane Prediction System coupled to a 1/5 degree resolution regional version of the Princeton Ocean Model (POM). For each of a series of 72-hr experiments, a specified initial storm disturbance was placed in an idealized basic state consisting of a uniform easterly flow of 5 m/sec. The initial SST, atmospheric lapse rate, and atmospheric moisture conditions for the experiments were derived from long simulations of a global climate model. Separate experiments were run for atmosphere and ocean environmental conditions from each of six tropical storm basins for both control and high CO2 forcing conditions. The ocean thermal stratifications for the control cases were based on observed (GDEM) climatologies. For the high CO2 cases, a slight increase in upper ocean stratification was included, based on the global climate model simulations.

Preliminary results confirm that a CO2 warming-induced intensification still occurs even when the hurricane/ocean coupling effects are included. However, a larger ensemble of experiments appears to be necessary to quantify how the hurricane/ocean coupling effects the degree of CO2 warming-induced intensification. These experiments are now in progress, and results will be reported at the meeting.