Resolving Loop Current and Eddy Variability For Coupled Intensity Forecast Models

From 5 May to 9 July 2010, several research flights were conducted from NOAA WP-3D research aircraft in support of the Deepwater Horizon Oil Spill. These oceanographic flights acquired both oceanic and atmospheric structure measurements between 24 to 28N in the eastern Gulf of Mexico, capturing the variability of the Loop Current (LC) and a complex oceanic eddy field during a series of detachment and reattachment processes. This oceanic variability is oft-times difficult to capture with oceanic and coupled models that assimilate synthetic ocean profiles derived from satellite altimetry over a +/- 5-day envelope. The assimilation of the thermal profiles from expendable profilers improved the fidelity of the simulations from the forecast models at the operational centers. Between 28 May and 18 June, a large anticyclonically rotating warm core eddy (Franklin) was shed from the LC due to current instabilities that developed along the periphery of the LC and associated cold cyclones. Franklin's 20ºC isotherm depth (h20) reached maximum values of about 320 m, compared to values ranging from 280 to 300 m prior to shedding. In early July, Franklin experienced significant erosion from several cyclones along its periphery, where h20 values in Franklin's core ranged from 260 to 280 m, and levels of surface kinetic energy were depleted as suggested from the current profiles. These measurements have been used to evaluate satellite-based products such as upper ocean currents, isotherm depths and oceanic heat content variations.

Central to the question of the associated atmospheric variability is that even under relatively light wind conditions, these data suggest that equivalent potential temperature (Θ_e) variations over the warm and cold features differ by 4 to 7K. This has been observed in profiler data acquired over the LC and warm eddies in the Gulf of Mexico and Eastern Pacific Ocean. One working hypothesis is that these elevated levels of higher Θ_e air are due in part to more sustained heat and moisture fluxes to the lower part of the atmospheric boundary layer over these deeper warm ocean features. To address this question, we are comparing equivalent potential temperatures to estimated enthalpy fluxes relative to warm and cold oceanic regimes (e.g., oceanic mixed layer depths) from GPS sonde and upper ocean measurements (profilers and satellite-derived fields). In this context, such behavior can be investigated from coupled models where the ocean model is correctly initialized with both warm and cold oceanic features prior to hurricane passage.