Multi-sensor satellite and HYCOM analysis of the upper ocean response to Hurricane Katrina in the Gulf of Mexico

This study aims to investigate the biological and physical responses of the upper ocean to Hurricane Katrina (2005) in the Gulf of Mexico (GoM) through the use of satellite observations and 1/25° horizontal resolution nested GoM HYbrid Coordinate Ocean Model (HYCOM) simulations. It is important that both simulations and observations be used to provide a complete spatial and temporal evolution of the biophysical responses exhibited. Satellite and/or model simulations depict sea surface cooling of 3°-7°C, salinity freshening of 0.2-0.3 psu, and chlorophyll-a enhancement (i.e., increase in phytoplankton biomass accumulation at the ocean's surface) of approximately 3 mg m^-3 within a region from 23.5°-25.5°N and 85°-83°W. Model and satellite sea surface height illustrates that the region transpires in a cold-core eddy, which contributes to the enhanced observed and simulated responses. Analysis of model surface and subsurface dynamics in this region reveal rapid upwelling/downwelling of 2-2.5 x 10^-4 m s^-1, hurricane force wind-driven currents dominating the surface circulation, and near-inertial oscillations following Hurricane Katrina.

A nitrate-temperature relationship is derived to illustrate the potential contribution that nitrate influx has upon the satellite-observed phytoplankton bloom. Analytical-derived nitrate indicates nutricline ventilation as a result of Hurricane Katrina, leading to nitrate concentrations of 3-4 µM at the surface. Using an approximate nitrate to chlorophyll-a ratio, analytical-derived nitrate is converted into chlorophyll, and yields a chlorophyll-a increase of 3-4 mg m^-3. Comparison of approximated and observed chlorophyll-a agrees reasonably well, and thereby suggests that the bloom is greatly influenced by nitrate entrainment. Additionally, comparison of thermocline displacement estimates and depth approximations of the nitracline and deep chl-a maximum suggests that the increases in surface phytoplankton biomass associated with Hurricane Katrina is the result of new production from nutrient influx, as well as entrainment of phytoplankton from the deep chlorophyll maximum. However, the contribution from chlorophyll-a entrainment to the bloom is considered to be small, since it mainly accounts for initial increase in satellite-detected chlorophyll-a concentrations.