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Observations of the Coastal Oceanic Response to Hurricane Ivan: Implications for Hurricane Intensity Change

Lynn K. Shay1, George R. Halliwell1, William J. Teague2

1

2 Division of Meteorology and Physical Oceanography, RSMAS, University of Miami, Miami, FL US Naval Research Laboratory, Oceanography Division, Stennis Space Center, MS

nshay@rsmas.miami.edu,ghalliwell@rsmas.miami.edu,teague@nrlssc.navy.mil

Hurricane Ivan (Sept 2004) was a classical hurricane of Cape Verde origin that reached Category 5 strength on three separate occasions in the Caribbean Sea. As Ivan moved over the NW Caribbean Sea, high ocean heat content (OHC) values exceeding 150 KJ cm-2 plus tropospheric outflow enhanced by upper atmospheric flow ahead of an approaching trough helped Ivan maintain category 5 strength. Upon entering the Gulf of Mexico (GOM) basin on 14 Sept as a category 5 hurricane, Ivan turned north- northwest and then northward as the high OHC of the Loop Current helped to maintain its intensity. Ivan subsequently weakened to a category 3 storm due to a combination of lower OHC associated with Gulf Common Water (GCW), vertical shear in the atmosphere associated with an upper-level trough, and dry air being drawn into its circulation. Within 24 hours of landfall, Ivan encountered a warm core ring shed by the Loop Current. High OHC within this ring supported intensification as surface pressure decreased by ~10 mb during this encounter (i.e. positive feedback). After passing the warm ring, cooler shelf water forced by hurricane Frances a few weeks earlier coupled with increasing atmospheric shear opposed intensification during an eye-wall replacement cycle (negative feedback) weakened Ivan at landfall.

In the northern GOM, Ivan fortuitously passed over 14 Acoustic Doppler Current Profiler (ADCP) moorings that were deployed as part of the Navy Research Laboratory Slope to Shelf Energetics and Exchange Dynamics (SEED) project from May through Nov 2004. In addition to three-dimensional currents, bottom temperature and pressure measurements were also acquired with the latter providing surface wave observations. Given the spatial mooring distribution from beyond the shelf break to over the s h e l f , t h e o c e a n i c r e s p o n s e w a s d o m i n a t e d b y v i g o r o u s n e a r - i n e r t i a l c u r r e n t s ( f - 1 w h e r e f i s t h e l o c Coriolis parameter) that contained both barotropic and baroclinic components. Coherent barotropic motions were fairly energetic with depth-independent currents of about 10 cm s-1 over the spatial mooring array. By removing this component, the baroclinic response contained strong currents and shears across the base of the ocean mixed layer. These currents rotated anticyclonically with depth and time indicative of rapid energy propagation out of the surface mixed layer into the thermocline. This ocean response persisted for several weeks following Ivan’s passage consistent with other moored hurricane measurements (Frederic 1979). Despite the growing importance of understanding the oceanic response to hurricane forcing, and the atmospheric response to the oceanic forcing, limited ocean current (and shear) and concurrent wave measurements have made it difficult to evaluate simulations from oceanic and coupled models. Such measurements are crucial in evaluating model parameterizations schemes that are being implemented into coupled operational models at the National Centers for Environmental Prediction. a l

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