Evaluation and Sensitivity Analysis of an Ocean Model Response to Hurricane Ivan

Abstract

An ocean model response to Hurricane Ivan (2004) over the northwest Caribbean Sea and Gulf of Mexico is evaluated to guide strategies for improving performance during strong forcing events in a region with energetic ocean features with the ultimate goal of improving coupled tropical cyclone forecasts. Based on prior experience, a control experiment is performed using quasi-optimal choices of initial ocean fields, atmospheric forcing fields, air?sea flux parameterizations, vertical mixing parameterizations, and both horizontal and vertical resolutions. Alternate experiments are conducted by altering one single model attribute and comparing the results to SST analyses and moored ADCP current measurements to quantify the sensitivity to that attribute and identify where to concentrate model improvement efforts. Atmospheric forcing that does not resolve the eye and eyewall of the storm (scales >10 km) substantially degrades the ocean response. Ordering other model attributes from greatest to least sensitivity, ocean model initialization with regard to the accuracy of upper-ocean temperature?salinity profiles along with accurate location of ocean currents and eddies is the most important factor for ensuring good ocean model performance. Ocean dynamics ranks second in this energetic ocean region because a one-dimensional ocean model fails to capture important physical processes that affect SST cooling. Wind stress drag coefficient parameterizations that yield values exceeding 2.5 ? 10?3 at high wind speeds or that remain <2.0 ? 10?3 over all wind speeds reduce the realism of wind-driven current profiles and have a large impact on both SST cooling and the heat flux from ocean to atmosphere. Turbulent heat flux drag coefficient parameterizations substantially impact the surface heat flux while having little impact on SST cooling, which is primarily controlled by entrainment at the mixed layer base. Vertical mixing parameterizations have a moderate impact on SST cooling but a comparatively larger impact on surface heat flux. The impacts of altering the horizontal and vertical resolutions are small, with horizontal resolution of ≈10 km and vertical resolution of ≈10 m in the mixed layer being adequate. Optimal choices of all attributes for simulating the ocean response to Ivan are identified.