Dynamic Potential Intensity: An Improved Representation of the Ocean's Impact on Tropical Cyclones

To incorporate the effects of tropical cyclone (TC) induced upper-ocean mixing and sea surface temperature (SST) cooling on TC intensification, a vertical average of temperature down to a fixed depth was proposed as a replacement for SST within the framework of air-sea coupled Potential Intensity. However, the depth to which TC-induced mixing penetrates may vary substantially with ocean temperature and salinity stratification and storm state. To account for these effects, here we develop a `dynamic potential intensity' (DPI) based on considerations of stratified fluid turbulence. For the Argo period 2004-2013 and the three major TC basins of the Northern Hemisphere, we show that the DPI explains 11-32% of the variance in TC intensification, compared to 0-16% using previous methods. One of the main advantages of using DPI for climate-scale analyses is that it explicitly accounts for changes in TC-induced SST cooling and Potential Intensity when upper-ocean salinity stratification is present. Using the DPI framework, we show that surface freshening in the western North Pacific in recent decades has acted to increase Potential Intensity significantly and is likely to continue to do so in the future in response to global warming-induced intensification of the hydrological cycle.