Positive feedback regimes during tropical cyclone passage

Coupled oceanic and atmospheric models to accurately predict hurricane intensity and structure change will eventually be used to issue forecasts to the public who increasingly rely on the most advanced weather forecasting systems to prepare for landfall. Early ocean-atmosphere studies have emphasized the negative feedback between tropical cyclones and the ocean due to the cold wake. The extent of this cooling is a function of wind-forced current shears that reduce Richardson numbers to below criticality, which in turn cool and deepen the mixed layer through entrainment mixing. Unfortunately, most of these early studies did not consider the relative importance of deep warm mixed layers associated with Caribbean Current, Loop Current, Florida Current, Gulf Stream and the warm core eddy field. These oceanic features are characterized as deep, warm thermal regimes with high Oceanic Heat Content (OHC) associated with energetic ocean currents that transport heat from the tropics to the northern latitudes are part of the annual cycle.

As part of a NSF/NOAA sponsored Hurricane Air-Sea Interaction Experiment, dual aircraft experiments concurrently mapped ocean-atmosphere fields using expendable profilers deployed from NOAA aircraft as Isidore and Lili moved into the Gulf of Mexico in Sep 02. As the storms encountered the Loop Current, Isidore intensified to a category 3 and Lili rapidly intensified to a category 4 storm. Even at these levels of intensity, the upper ocean response indicated minimal SST decrease of less than 1oC and OHC loss of 6 KJ cm-2. In the Loop Current, advection of thermal gradients dominated the upper ocean heat balance compared to shear-induced mixing events resulting in positive feedback to the atmosphere. As Lili moved northwest of the Loop Current, the upper ocean cooled by more than 2oC with a net OHC loss of 30 KJ cm-2 due to shear-induced mixing across the base of a thin ocean mixed layer. Subsequently, Lili weakened prior to a category 1 storm due to dry air entrainment and negative feedback from an ocean previously cooled by tropical storms Hanna and Isidore earlier.

Recently, hurricanes Ivan (04), Dennis and Emily (05) exemplified this positive feedback between the ocean and tropical cyclones where these storms reached category 4/5 status over deep warm layers. During the passage of Ivan, the net change in post-storm SST and OHC was less than 1oC and 10 KJ cm-2 in the NW Caribbean Sea as Ivan reached a category 5 status on three separate occasions. Similarly, Dennis decreased in intensity from category 4 to category 1 moving over Cuba, and just prior to landfall when he decreased from category 4 to category 3 moving over shelf water previously cooled by Arlene and Cindy. Along the track of Dennis, atmospheric conditions were favorable. Emily initially experienced unfavorable atmospheric conditions which limited her intensity. As atmospheric conditions improved, she intensified to category 5 in the NW Caribbean, decreased to category 1 over the Yucatan Peninsula land mass, then re-intensified to category 3 storm over a warm core eddy in the western Gulf of Mexico. These early season intensity modulations are linked to warm SSTs and OHC variability associated with positive feedback regimes compared to thin ocean mixed layers where negative feedback occurs.