Enthalpy and Momentum Fluxes during Hurricane Earl Relative to Underlying Ocean Features

Abstract

Abstract Using dropsondes from 27 aircraft flights, in situ observations, and satellite data acquired during Tropical Cyclone Earl (category 4 hurricane), bulk air–sea fluxes of enthalpy and momentum are investigated in relation to intensity change and underlying upper-ocean thermal structure. During Earl’s rapid intensification (RI) period, ocean heat content (OHC) variability relative to the 26°C isotherm exceeded 90 kJ cm−2, and sea surface cooling was less than 0.5°C. Enthalpy fluxes of ~1.1 kW m−2 were estimated for Earl’s peak intensity. Daily sea surface heat losses of , , and kJ cm−2 were estimated for RI, mature, and weakening stages, respectively. A ratio of the exchange coefficients of enthalpy (CK) and momentum (CD) between 0.54 and 0.7 produced reliable estimates for the fluxes relative to OHC changes, even during RI; a ratio overestimated the fluxes. The most important result is that bulk enthalpy fluxes were controlled by the thermodynamic disequilibrium between the sea surface and the near-surface air, independently of wind speed. This disequilibrium was strongly influenced by underlying warm oceanic features; localized maxima in enthalpy fluxes developed over tight horizontal gradients of moisture disequilibrium over these eddy features. These regions of local buoyant forcing preferentially developed during RI. The overall magnitude of the moisture disequilibrium (Δq = qs − qa) was determined by the saturation specific humidity at sea surface temperature (qs) rather than by the specific humidity of the atmospheric environment (qa). These results support the hypothesis that intense local buoyant forcing by the ocean could be an important intensification mechanism in tropical cyclones over warm oceanic features.