26th Conference on Hurricanes and Tropical Meteorology

16A.4

Observed Modulation of Upper Ocean Heat Content Variability by Upper Ocean Current Shears During Hurricanes Isidore and Lili

Lynn K. Shay, University of Miami - Rosenstiel School of Marine and Atmospheric Science, Miami, FL; and T. Cook, R. Evans, E. W. Uhlhorn, S. Guhin, J. Cione, S. White, and M. L. Black

As part of an NSF and NOAA sponsored research project, coupled 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 oceanís mixed layer balance. By contrast, upwelling and shear-induced mixing caused significant cooling over the Yucatan Shelf. A weakened Isidore moved northward across the Gulf of Mexico and caused a broad area of 28oC SSTs where oceanic heat content decreased by ~20 KJ cm-2 prior to landfall in Louisiana.

Hurricane Lili (28 Sept) formed in the northwest Caribbean Sea and followed Isidoreís 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 and into the Gulf of Mexico across the Loop Current and directly over the pre- and post-Isidore ocean grids. During this period on 2 Oct, Lili rapidly intensified to a Category 4 storm across a strong oceanic heat content gradient where surface wind speeds exceeded 50 m s-1. Lili decreased to a Category 1 storm upon encountering the cooler wake of Isidore in the northern Gulf of Mexico, increased wind shear, and cooler drier air prior to landfall on 3 Oct. In situ and satellite-derived upper ocean heat content indicated an additional loss of heat 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. Observations acquired from these research flights are providing insights into the role of the three-dimensional atmospheric and oceanic processes on intensity and structure change.

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Session 16A, sea-air interaction: Hurricanes Isidore and Lili
Friday, 7 May 2004, 8:00 AM-9:45 AM, Le Jardin Room

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