The Competing Forces of Hurricane-Induced Ocean Cooling

It is known that oceanic conditions can play a crucial role in the intensification of tropical cyclones (TCs) when atmospheric conditions are conducive. However, the relative roles of ocean temperature and salinity stratification on ocean mixing and TC-induced sea surface temperature (SST) cooling are unclear. Temperature stratification has competing effects on cooling from ocean mixing: stronger stratification leads to cooler water near the surface which can enhance SST cooling (thermodynamic effect), yet it also leads to resistance to vertical mixing due to a stronger density gradient and increased static stability (mixing effect). In contrast, salinity stratification almost always acts to reduce mixing and cooling. To investigate the mechanisms that control the amount of ocean cooling under a storm, we use a one-dimensional mixed layer model, initialized with different oceanic profiles and forced with cyclones of various intensities, translation speeds, and sizes. We then compare output from the mixed layer model with observations. Results consistently show that the thermodynamic effect (changes in vertical temperature gradient with density gradient held constant) is 2-3 times that of the mixing effect (changes in density stratification with temperature stratification held constant). Furthermore, we find that translation speed and storm size are the two most important factors for SST cooling, followed by temperature stratification, maximum wind speed, and mixed layer depth, respectively. These results emphasize the importance of temperature stratification over most of the tropical cyclone basins and the often overlooked role of storm size.