A Positive Feedback Mechanism for Tropical Cyclone Intensification Over Warm Oceanic Mesoscale Features

Air-sea interactions during the intensification of tropical storm Isaac (2012) into a hurricane, over Gulf of Mexico's warm oceanic mesoscale features, are investigated using airborne oceanographic and atmospheric profilers deployed from NOAA WP-3D aircraft. Understanding these complex interactions is critical to correctly evaluating and predicting storm effects on marine and coastal facilities, wind-driven mixing and transport of suspended matter throughout the water column, and oceanic feedbacks on storm intensity.

Ocean profilers deployed during Isaac's intensification measured isotherms' downwelling up to 60 m over a 12-h interval (5 m h^-1), or twice the upwelling strength underneath the storm's center. This displacement occurred over a warm-core eddy (WCE) that extended underneath Isaac's left side, where the ensuing upper-ocean warming was ~8 kW m^-2. Rather than with just Ekman pumping, these observed upwelling-downwelling responses were consistent with a vertical velocity derived from the dominant vorticity balance that considers geostrophic flow strength and storm parameters. Sea surface warming (positive feedback mechanism) of ~0.5°C was measured over the WCE during Isaac's intensification, where enhanced bulk enthalpy fluxes were estimated due to an increase in moisture disequilibrium between the ocean and atmosphere. That is, the non-linear interaction of the wind stress with the surface oceanic geostrophic flow produced horizontal convergence of warmer waters underneath Isaac (sea surface warming), which enhanced the enthalpy fluxes into the storm. These results support the hypothesis that enhanced buoyant forcing from the ocean is an important intensification mechanism in tropical cyclones (TC) over warm oceanic mesoscale features. Thus, to improve TC intensity forecasting, coupled numerical models must be initialized with realistic ocean states to correctly resolve the three-dimensional upwelling-downwelling responses and ensuing enthalpy fluxes.