Hyperbolicity of Bulk Air-Sea Enthalpy Fluxes in Major Hurricanes

Air-sea enthalpy fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulae, these fluxes are a function of wind speed and air-sea thermodynamic disequilibrium (i.e. moisture and temperature differences between the sea surface and near-surface atmosphere). Assessing the relative contribution of surface wind speed and thermodynamic disequilibrium to sea-to-air enthalpy fluxes and ensuing TC intensity change remains an open issue. For instance, the theories for TC intensification often invoke a wind-driven positive feedback mechanism, in which intensifying surface wind speeds progressively extract more heat from the ocean, while the increased heat transfer leads to increasing storm winds. However, a new scientific finding indicates that bulk air-sea enthalpy fluxes occur in hyperbolic spaces determined by surface wind speeds and thermodynamic disequilibrium. The curvature of these spaces indicates that thermodynamic disequilibrium provides the most efficient pathway to extract large amounts of heat from the ocean, even at relatively moderate surface wind speeds.

We investigate bulk air-sea enthalpy fluxes in the two-parameter hyperbolic space (surface wind-dependence and thermodynamic-dependence) in which the fluxes take place, for two category 1 hurricanes (Isaac of 2010 and Nate of 2017), and ten major hurricanes (Isidore of 2002; Lili of 2002; Ivan of 2004; Emily of 2005; Dean of 2007; Felix of 2007; Gustav of 2008; Ike of 2008; Earl of 2010; and Edouard of 2014). The goal is identifying the relative contribution of surface wind speeds and thermodynamic disequilibrium to heat extraction from the ocean during steady intensification, rapid intensification, steady-state, and secondary eyewall formation in major hurricanes. Preliminary results indicate that intense latent heat fluxes of more than 1200 W m^-2 and 800 W m^-2 were supported by moisture disequilibrium of ~9 g Kg^-1 during rapid intensification and secondary eyewall formation, respectively, in major Hurricane Earl of 2010, under relatively moderate surface wind speeds. By contrast, moderate latent heat fluxes of ~500 W m^-2 were supported by moisture disequilibrium of ~6 g Kg^-1 during the moderate intensification of tropical storm Isaac into a hurricane. Both during Earl and Isaac, peak values of moisture disequilibrium and enthalpy fluxes occurred over warmer oceanic features. These results indicate that acquiring accurate measurements of 10-m wind speeds, as well as air-sea moisture and temperature differences, is critical to improve our scientific understanding of intensity change in TCs, as well as to provide the optimal forcing at the sea surface in forecasting models of TC intensity.