Effects of a Warm Oceanic Feature on Hurricane Opal

TLDR

Hurricane Opal intensified from 965 to 916 hPa over 14 h when it passed a warm core ring shed by the Loop Current, illustrating that warm ocean features can markedly strengthen tropical cyclones, while prior studies show that 10–15 % of upper‑ocean cooling is driven by surface heat fluxes. Using historical hydrographic data, a two‑layer model, and TOPEX radar‑altimeter surface‑height anomalies, the authors identified two warm core rings in September–October 1995 that likely influenced Opal’s intensification. When Opal traversed a warm core ring, its 1‑min surface winds rose above 60 m s⁻¹, the radius of maximum wind shrank to 25 km, and the storm extracted ~24 kcal cm⁻² of heat—about six times the threshold needed to sustain a hurricane—while surface heat fluxes of 2 000–3 000 W m⁻² and only modest SST cooling confirmed a positive feedback from the warm core ring rather than the negative cooling seen elsewhere.

Abstract

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 (WCR) in the ocean shed by the Loop Current during an upper-level atmospheric trough interaction. Based on historical hydrographic measurements placed within the context of a two-layer model and surface height anomalies (SHA) from the radar altimeter on the TOPEX mission, upper-layer thickness fields indicated 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 winds increased from 35 to more than 60 m s−1, and the radius of maximum wind decreased from 40 to 25 km. Pre-Opal SHAs in the WCR exceeded 30 cm where the estimated depth of the 20°C isotherm was located between 175 and 200 m. Subsequent to Opal’s passage, this depth decreased approximately 50 m, which suggests upwelling underneath the storm track due to Ekman divergence. The maximum heat loss of approximately 24 Kcal cm−2 relative to depth of the 26°C isotherm was a factor of 6 times the threshold value required to sustain a hurricane. Since most of this loss occurred over a period of 14 h, the heat content loss of 24 Kcal cm−2 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 2000 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°–3°C cooling associated with vertical mixing in the along-track direction of Opal except over the WCR where AVHRR-derived and buoy-derived SSTs decreased only by about 0.5°–1°C. Thus, the WCR’s effect was to provide a regime of positive feedback to the hurricane rather than negative feedback induced by cooler waters due to upwelling and vertical mixing as observed over the Bay of Campeche and north of the WCR.