Loop Current Interactions to Hurricanes Isidore and Lili

As part of an NSF and NOAA-sponsored research project, ocean and atmosphere measurements were acquired during the passage of hurricanes Isidore (18-25 Sept 02) and Lili (28 Sept-4 Oct 02) from the NOAA WP-3Ds. Aircraft-based strategy from the NOAAWP-3Ds sampled pre-storm ocean current, temperature and salinity conditions in the northwest Caribbean Sea and in the Gulf of Mexico just north of the Yucatan straits using airborne expendable ocean current, temperature and salinity profilers. As Isidore moved slowly across the Yucatan Straits, there was little evidence of SST cooling as horizontal advection of the thermal gradients by the Loop Current dominated the ocean's mixed layer (OML) balance. By contrast, upwelling and shear-induced mixing caused significant cooling over the Yucatan Shelf. Tropical Storm Isidore moved northward across the Gulf of Mexico and induced a broad area of 28oC SSTs where oceanic heat content (OHC) decreased by less than 10 KJ cm-2 prior to landfall in Louisiana based on satellite-based estimates.

Hurricane Lili (28 Sept) formed in the northwest Caribbean Sea and followed Isidore's track. Due to deep, warm layers in this oceanic regime, no significant SST cooling (< 0.5oC) was observed from the ocean profilers. Lili moved over the western tip of Cuba across the Loop Current and directly over the Isidore ocean grids. In the Loop Current, OHC loss was less than 7 KJ cm-2 with a decrease in SST of 1oC. On 2 Oct, Lili rapidly intensified to a Category 4 storm across a strong OHC gradient where surface wind speeds exceeded 50 m s-1. In the Loop Current regime, the oceanic response was considerably less than that observed to northwest of the Loop Current due to the horizontal advection of deep warm layers by currents of 1.5 m s-1. In situ and satellite-derived fields indicated an additional OHC loss of >30 KJ cm-2 north of the Loop Current where the upper ocean cooled by more than 2oC due to strong vertical current shears across the base of the oceanic mixed layer. Subsequently, Lili decreased to a cat-1 storm upon encountering the cooler wake of Isidore in the northern Gulf of Mexico, increased wind shear, and drier air prior to landfall on 3 Oct. Measurements acquired from these research flights are providing insights into the role of the three-dimensional atmospheric structures encountering a warm and sustained heat source such as the Loop Current and warm subtropical water emanating from the northwest Caribbean Sea.