Effects of a warm oceanic feature on Hurricane Opal

On 4 October 1995, Hurricane Opal deepened from 965 to 916 hPa in the Gulf of Mexico over a 14-h period upon encountering a warm core ring (\NCR) in t he ocean shed by the Loop Current during an upper-level atmospheric trough interaction. Based on historical hydrographic measurements placed within th e context of a two-layer model and surface height anomalies (SHA) from the radar altimeter on the TOPEX mission, upper-layer thickness fields indicate d the presence of two warm core rings during September and October 1995. As Hurricane Opal passed directly over one of these WCRs, the 1-min surface w inds increased from 35 to more than 60 m s(-1), and the radius of maximum w ind decreased from 40 to 25 km. Pre-Opal SHAs in the WCR exceeded 30 cm whe re the estimated depth of the 20 degrees C isotherm was located between 175 and 200 m. Subsequent to Opal's passage, this depth decreased approximatel y 50 m, which suggests upwelling underneath the storm track due to Ekman di vergence. The maximum heat loss of approximately 24 Kcal cm(-2) relative to depth of the 26 degrees C isotherm was a factor of 6 times the threshold value requi red to sustain a hurricane. Since most of this loss occurred over a period of 14 h, the heat content loss of 24 Kcal cm equates to approximately 20 kW m(-2). Previous observational findings suggest that about 10%-15% of upper -ocean cooling is due to surface heat fluxes. Estimated surface heat fluxes based upon heat content changes range from 1000 to 3000 W m(-2) in accord with numerically simulated surface heat fluxes during Opal's encounter with the WCR. Composited AVHRR-derived SSTs indicated a 2 degrees-3 degrees C c ooling associated with vertical mixing in the along-track direction of Opal except over the WCR where AVHRR-derived and buoy-derived SSTs decreased on ly by about 0.5 degrees-1 degrees C. Thus, the WCR's effect was to provide a regime of positive feedback to the hurricane rather than negative feedbac k induced by cooler waters due to upwelling and vertical mixing as observed over the Bay of Campeche and north of the WCR.