Recent cases have demonstrated that sudden unexpected intensification in tropical cyclones often occurs within 24 to 48 hours of striking the coast over oceanic regimes such as the Gulf Stream, Florida Current, Loop Current or warm core rings (WCRs) in the western North Atlantic Ocean and Gulf of Mexico. Sea surface temperatures (SSTs) remain a necessary, yet insufficient condition on the ocean's influence on the tropical cyclone pressure and wind variations.
Temperatures distributed over the Oceanic Planetary Boundary Layer
(OPBL), defined as the well-mixed upper ocean layer, represent a more effective means of assessing oceanic regimes where intensification is likely to occur. In the presence of warm baroclinic features such as the Florida Current, Gulf Stream and WCRs, the OPBL and depth of the warm isotherms (~ 26 degrees C) are deeper due to warm water transport from the tropics to the higher latitudes as part of the subtropical gyre circulation. These warm currents transport heat in the upper ocean from the equator towards the poles just as Hadley Cells and tropical cyclones redistribute heat and moisture in the atmosphere.
A common perception of air-sea coupling in tropical cyclones is that SST represents the only important oceanic parameter for the maintenance of tropical cyclones (Palmen 1948). Even simple mixed layer models indicate that for nominal wind speeds of 4 to 10 ms-1, thin SST layers are eroded away in minutes to hours. From an oceanographic perspective, AVHRR-derived SSTs represent only the temperature of a thin layer of less than a few centimeters (not even 0.5 m) thick that mixes with the underlying OPBL as the winds increase beyond 5 ms-1. As winds increase to gale force, the tropical cyclone structure removes heat from the OPBL, not from just the thin SST layer that has already been mixed. Thus, the underlying oceanic current and temperature structure has far more importance to the heat and moisture fluxes feeding the storm than just an SST.
Momentum and thermal structure measurements acquired during hurricane Gilbert inside and outside a warm core ring demonstrate the marked spatial variations in the upper oceanic layers. Typically, the mixed layer depth is 30 to 40 m, and across the mixed layer base strong current shears develop as the currents reverse direction in the thermocline and force mixing events by lowering the Richardson number.
By contrast in a WCR, the depth of the isothermal layer is about 60 m with fairly weak stratification below. In both shallow and deeper mixed layers, current shears are strong enough to deepen the layer through shear-induced instabilities. Beneath this upper ocean isothermal layer is a deeper isothermal layer to depths of 150 m where warm temperatures exceed the threshold temperatures exceed 25 degrees C. That is, warmer water with temperatures exceeding this temperature extend to greater depths in the ocean feature than outside of the WCR. These higher temperatures at depth have a significant influence on the heat content contrast since outside the WCR the heat content is less than half that inside the WCR. This is the type of isothermal structure found in WCR, and as such creates a condition where a significant amount of heat is available for the atmosphere through the air-sea fluxes. An indirect confirmation of this effect was the Opal case using the TOPEX data.