Loop Current Response to Hurricanes Isidore and Lili

Abstract

Recent hurricane activity over the Gulf of Mexico basin has underscored the importance of the Loop Current (LC) and its deep, warm thermal structure on hurricane intensity. During Hurricanes Isidore and Lili in 2002, research flights were conducted from both National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft to observe pre-, in- and poststorm ocean conditions using airborne expendable ocean profilers to measure temperature, salinity, and current structure. Atmospheric thermodynamic and wind profiles and remotely sensed surface winds were concurrently acquired as each storm moved over the LC. Observed upper-ocean cooling was about 1°C as Isidore moved across the Yucatan Straits at a speed of 4 m s?1. Given prestorm ocean heat content (OHC) levels exceeding 100 kJ cm?2 in the LC (current velocities >1 m s?1), significant cooling and deepening of the ocean mixed layer (OML) did not occur in the straits. Estimated surface enthalpy flux at Isidore?s eyewall was 1.8 kW m?2, where the maximum observed wind was 49 m s?1. Spatially integrating these surface enthalpy fluxes suggested a maximum surface heat loss of 9.5 kJ cm?2 at the eyewall. Over the Yucatan Shelf, observed ocean cooling of 4.5°C was caused by upwelling processes induced by wind stress and an offshore wind-driven transport. During Hurricane Lili, ocean cooling in the LC was ?1°C but more than 2°C in the Gulf Common Water, where the maximum estimated surface enthalpy flux was 1.4 kW m?2, associated with peak surface winds of 51 m s?1. Because of Lili?s asymmetric structure and rapid translational speed of 7 m s?1, the maximum surface heat loss resulting from the surface enthalpy flux was less than 5 kJ cm?2. In both hurricanes, the weak ocean thermal response in the LC was primarily due to the lack of energetic near-inertial current shears that develop across the thin OML observed in quiescent regimes. Bulk Richardson numbers remained well above criticality because of the strength of the upper-ocean horizontal pressure gradient that forces northward current and thermal advection of warm water distributed over deep layers. As these oceanic regimes are resistive to shear-induced mixing, hurricanes experience a more sustained surface enthalpy flux compared to storms moving over shallow quiescent mixed layers. Because ocean cooling levels induced by hurricane force winds depend on the underlying oceanic regimes, features must be accurately initialized in coupled forecast models.