During September and October 1995, the existence of a warm core ring in the Gulf of Mexico contributed to the intensification of Hurricane Opal, when its path crossed directly over this warm ring. Opal's wind field increased from 35 m/s to 65 m/s in only 9 hours, and fluxes doubled to more than 2000 w/(m*m), a response subsequently simulated in numerical model studies. This case was the motivation to investigate the characteristics of warm mesoscale features in regions where tropical storms and hurricane occur.

Regarding the influence of the ocean on tropical cyclone intensity changes, upper ocean temperatures higher than 26C are necessary but not sufficient condition for strengthening. Mesoscale features, such as warm rings, the Loop Current, the Caribbean Current and the Gulf Stream are characterized by higher upper ocean temperatures than their surrounding. These features represent a source of enhanced air-sea fluxes to the atmospheric boundary layer that may cause strengthening of tropical storms and hurricanes. The integrated temperature between the sea surface and the depth of the 26C isotherm, proportional to the energy (heat) content in the upper ocean, is defined as the hurricane heat potential. These warm layers exceeding 26C, which extend up to 250 meters beneath the surface in the region of study, represent areas of high hurricane heat potential.

Satellite altimetry data from TOPEX/POSEIDON has been proven to be a useful tool to study mesoscale dynamics, and can provide information on the vertical structure of the upper ocean when complemented with historical hydrographic data. A two-layer model approximation is used to monitor in near-real time the upper layer thickness and the hurricane heat potential in the western North Atlantic Ocean, Caribbean Sea and Gulf of Mexico. This two-layer model assumes that changes in the sea surface height are compensated by variations in the upper layer thickness, which goes from the sea surface to the depth of the main thermocline. The variations in upper layer thickness are then added to the climatological values of this parameter to obtain the absolute field. The hurricane heat potential is then computed by integrating the temperature between the sea surface and the altimeter-derived depth of the 26C isotherm. The near-real time altimeter data is provided by NOAA/NESDIS and the sea surface temperature data used are the OI analyzed Reynold's fields. Daily maps of upper layer thickness and hurricane heat content are produced at NOAA/AOML and made available through the www (http://www.aoml.noaa.gov/phod/cyclone/data/).

As part of a jointly supported NSF/NOAA study, a warm core ring experiment will be conducted during the summer of 1999 using Airborne eXpendable Current Profilers (AXCPs) and Airborne eXpendable Conductivity Temperature and Depth Profilers (AXCTDs). These profilers will provide three dimensional velocity, temperature and salinity data, which will be compared with the fields derived from T/P altimetry.

This research is supported by the National Science Foundation and NOAA in support of the United States Weather Research Program on Hurricanes at Landfall.