On The Hyperbolicity Of The Bulk Air-Sea Heat Flux Functions: Insights Into The Efficiency Of Air-Sea Moisture Disequilibrium For Tropical Cyclone Intensification

Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulas, these fluxes are a function of surface wind speed U-10 and air-sea temperature and moisture disequilibrium (Delta T and Delta q, respectively). Although many studies have explained TC intensification through the mutual dependence between increasing U-10 and increasing sea-to-air heat fluxes, recent studies have found that TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under conditions of relatively low wind. Herein, a new perspective on the bulk aerodynamic formulas is introduced to evaluate the relative contribution of wind-driven (U-10) and thermodynamically driven (Delta T and Delta q) ocean heat uptake. Previously unnoticed salient properties of these formulas, reported here, are as follows: 1) these functions are hyperbolic and 2) increasing Delta q is an efficient mechanism for enhancing the fluxes. This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady-state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat uptake was found during periods of SS, SI, and RI. Compensation by larger values of Delta q > 5 g kg(-1) at moderate values of U-10 led to intense inner-core moisture fluxes of greater than 600 W m(-2) during RI. Peak values in Delta q preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Delta q is a very effective way to increase surface heat fluxes-this can easily be achieved as a TC moves over deeper warm oceanic regimes.