Thermal and Momentum Response of the Loop Current Complex to Hurricanes

The 3-dimensional oceanic velocity response of the Loop Current and its complex warm and cold eddy field to hurricanes is critical to accurately evaluate coupled forecast models of hurricane intensity including the impacts of vertical mixing on the sea surface temperature response. Direct velocity measurements of ocean current (temperature and salinity) fields during hurricanes is critical to better understand these physical processes, and acquire reference data sets to initialize, evaluate, and validate numerical ocean-atmosphere models. As part of NOAA Intensity Forecasting Experiments, airborne expendable bathythermographs (AXBT), Conductivity-Temperature-Depth (AXCTD), and Current Profilers (AXCP) probes have been deployed in several major hurricanes from the NOAA research aircraft over the Gulf. Over the last decade, profilers were deployed in Isidore and Lili, Katrina and Rita, Gustav and Ike and Isaac-all of which interacted with the LC and WCE field.

Central to these interactions under hurricane forcing is the level of sea surface cooling (typically about 1^oC) induced by the wind-forced current response in the LC complex. It has been found that vertical current shear and instability (e.g., Richardson number) at the base of the oceanic mixed layer is often arrested by the strong upper ocean currents associated with the LC of 1 to 1.5 m s^-1. By contrast, the SST cooling response often exceeds 3.5 to 4^oC away from the LC complex in the Gulf Common Water. A second aspect of the interaction between the surface wind field and the LC is that the vorticity of the background flows (based on altimetry) enhances upwelling and downwelling processes by projecting onto the wind stress. The upwelling process modulates the vertical mixing process at depth by keeping the Richardson numbers above criticality (larger buoyancy frequencies). Thus, the ocean cooling is less in the LC allowing for a higher and more sustained enthalpy flux as determined from global positioning system sondes deployed in these storms. This level of cooling (or lack thereof) impacts the intensity of hurricanes that can reach severe status which can have major impacts on offshore structures, oil dispersion and coastal communities at landfall.