Oceanic And Atmospheric Measurements In Hurricanes Isidore And Lili (2002)

Lynn K. "Nick" Shay

RSMAS, University of Miami


Sea surface temperatures (SSTs) have long been recognized as important for coupled modeling not only because they set the oceanic surface boundary conditions, but SST cooling is readily observable in the hurricane's wake. In oceanic regimes with thin oceanic mixed layers (OML), the SST cooling causes negative feedback on storms that reduce air-sea fluxes. This strong SST cooling is due primarily to wind-forced, current shear-induced mixing across the OML. This feedback has been shown to be important in simulating the intensity change to the cooler SSTs in models. By contrast, positive feedback regimes occur because SST cooling is suppressed by deep warm layers, that require substantially more current shear to further deepen and cool the OML as observed in the Loop Current (LC) and warm core rings. In this context, the SST response and oceanic heat content (OHC) variability strongly depend on oceanic current (and shear) response to storm forcing. Thus, models must have realistic 3-D conditions capable of resolving fronts, eddies and boundary currents. Radar altimetry (i.e. surface height anomaly) SST, and OHC from space-based measurements provide the locations of these features; however, in situ oceanic current, salinity and temperature measurements are needed not only to improve initial model fields, but to carefully evaluate the veracity of model simulations.

To examine these scientific issues, a joint NSF and NOAA sponsored hurricane air-sea interaction experiment was conducted during the passage of hurricanes Isidore (18-25 Sept) and Lili (28 Sept - 4 Oct) from the NOAA WP-3Ds where concurrent oceanic and atmospheric measurements were acquired. The oceanic measurement strategy profiled ocean current, temperature and salinity conditions in the Caribbean Sea and in the Gulf of Mexico using airborne expendable oceanic profilers. In addition to the large-scale Gulfstream-IV flights setting the environmental context from atmospheric dropsondes, this strategy resulted in one of the most comprehensive data sets ever acquired during intensifying hurricanes.

Isidore intensified to a Category 3 hurricane moving across the Yucatan Straits, and as the tropical storm moved northward a few days later across the Gulf of Mexico, a broad area of SST cooling of ~28oC was created. Subsequently, Lili formed in the northwest Caribbean Sea and followed Isidore's track. In the Caribbean Sea, SSTs decreased by less than 0.5oC due to deep, warm layers, which provided more positive feedback to the storm. As Lili moved over the Cuba and into the Gulf of Mexico where Pre and Post-Isidore ocean data grids were acquired in the LC, she rapidly intensified to category 4 storm resulting in surface winds exceeding 60 m s-1 on 2 Oct. In the LC, Lili did not significantly induce current shear as SST decreases were < 1oC and OHC losses were 10 KJ cm-2. Preliminary surface flux estimates in the experimental domain approached 2 KW m-2 when atmospheric shears between 200 to 850 mb were ~10 m s-1. Lili then began a steady weakening cycle due to less favorable atmospheric conditions (dry air, atmospheric shear), and the cooler wake of Isidore prior to landfall (3 Oct). Lili experienced both positive and negative feedback from the upper ocean during her intensity changes through the Gulf of Mexico.