Climate and tropical cyclone activity: A new model downscaling approach

While there is a pressing need to understand and predict the response of tropical cyclones to climate change, global climate models are at present too coarse to resolve tropical cyclones to the extent necessary to simulate their intensity, and their ability to simulate genesis is questionable. For these reasons, a downscaling approach to modeling the effect of climate change on tropical cyclones is desirable. Here a new approach to downscaling is introduced that consists of generating a large set of synthetic storm tracks whose statistics are consistent with the large-scale general circulation of the climate model, and then running a deterministic, coupled tropical cyclone model along each track, with atmospheric and upper-ocean thermodynamic conditions taken from the global climate model. As a first step in this direction, this paper explores the sensitivity of the intensity of a large sample of tropical cyclones to changes in potential intensity, shear, and ocean mixed layer depth, fixing other variables, including the space-time probability distribution of storm genesis. It is shown that a 10% increase in potential intensity leads to a 65% increase in the power dissipation index, a measure of the total amount of mechanical energy generated by tropical cyclones over their life spans. This is consistent with the observed increase of power dissipation over the past 50 yr. Storms are somewhat less influenced by equivalent fractional changes in environmental wind shear or ocean mixed layer depth.