Global Analogues of Deep Warm Upper Ocean Layers: Hurricane Heat Potential Estimates

Subtropical gyres in the Atlantic, Pacific and Indian Ocean basins represent part of the oceanic climate cycle. Strong upper ocean currents advect and transport sizeable amout of heat from the equator northward. The 26° C water, which extends to about 100-120 m beneath the surface, represent a major source of energy for tropical cyclones. Global forecasting of these events have enormous consequences for the public who rely not only on just track forecasting, but certainly intensity changes within 24-36 h of striking the coastline. These deep warm features can be detected by satellite altimetry from TOPEX and the new follow-on missions such as Jason. This approach is needed during the summer months when many of the deep warm ocean features are masked by incoming solar radiation and cannot be detected with just an SST.

Of particular relevance to storm intensity issue then is not just SST, but rather a integrated measure of the upper ocean thermal structure. In the Gulf of Mexico, deep warm isothermal structures were observed this past summer using a combination of airborne expendable profilers including the new Airborne eXpendable Conductivity Temperature Depth (AXCTD) profilers. Within the deep warm region of the Loop Current/Warm Core Ring Complex, isothermal layers exceeding 28° C extended to about 90 m. Within this isothermal layer was a halocline with two different salinities. That is while this layer was not well mixed in density (T,S), the reduced gravity or density differential was weak, and with a sudden burst of wind-driven current or near-inertial shear detected by Airborne eXpendable Current Profilers (AXCP) this halocline can become well mixed within a slightly cooler isothermal layer. For example, in hurricane Opal, net cooling was only about 0.5-1° C. A similar scenario has been hypothesized for hurricane Camille in 1969, which may have moved up the throat of this warm oceanic complex and intensified to a category 5 storm with wind speeds in excess of 190 mph.

To place this effect in global context, a well known example of a halocline effect in a deep warm isothermal layer is the western Pacific Ocean warm pool dubbed the barrier layer by Lukas and Lindstorm. Thus, these deep warm ocean features are global, and may not necessarily be obvious in just an SST image. Thus, perhaps the community should begin to at least use altimetry and sea surface temperatures to locate these deep warm ocean features for use in forecasting intensity in all basins.