Physical and Biological Responses to Hurricane Katrina (2005) in a 1/25 Degrees Nested Gulf of Mexico HYCOM

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Marine Science


Recent studies indicated sea surface temperature (SST) cooling of 6-7 degrees C and a phytoplankton bloom of 3 mg chlorophyll-a m(-3) during the passage of Hurricane Katrina (23-30 August 2005) in a region from 23.5 degrees to 25.5 degrees N and 85 degrees to 83 degrees W in the Gulf of Mexico (GoM). Employing a 20-layer. 1/25 degrees horizontal resolution nested CoM HYbrid Coordinate Ocean Model (HYCOM), the evolving three-dimensional ocean response to Hurricane Katrina in the GoM was examined. During the passage of Hurricane Katrina, analysis of model surface and subsurface dynamics in this region revealed strong upwelling/downwelling of 1.5-2 x 10(-4) m s(-1), wind-driven currents dominating the surface circulation, and near-inertial oscillations following Hurricane Katrina. Associated with the storm, the 26 degrees C isotherm was raised by 28 m, generating SST cooling of 3-4 degrees C and salinity freshening of 0.1-0.2 in less than 24 h. Comparison of model-simulated SSTs with in situ buoy data and satellite observations revealed that model SSTs were cooler by 1-2 degrees C and had a greater spatial extent of cooling. Analysis of heat budget terms in the mixed layer (20 m) indicated that surface heat flux accounted for pre-storm temperature changes, and wind-driven mixing (-3375 W m(-2)) dominated net upper-ocean cooling (-2464 W m(-2)) during Hurricane Katrina. At 50 m depth, temperature changes were largely due to vertical advection associated with upwelling and downwelling processes. A temperature-nitrate relationship was derived to illustrate the potential contribution that nitrate influx had upon the satellite-observed phytoplankton bloom associated with Hurricane Katrina. Comparison of calculated nitrate agreed reasonably well with in situ nitrate profiles in the interest region. Nitrate concentrations of 3.7 mu M were entrained from 30 m depth during hurricane passage. An approximate nitrate to chlorophyll-a ratio provided a chlorophyll-a value of 3 mg m(-3), which was consistent with that derived from satellite. Thus, the elevated chlorophyll-a concentration following the passage of Katrina was greatly influenced by nitrate entrainment into the surface layer through vertical mixing and Ekman divergence. (C) 2009 Elsevier B.V. All rights reserved.

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Journal of Marine Systems





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