The Importance of Accurately Representing the Surface Momentum Flux Driving Upper Ocean Currents for Coupled Hurricane Prediction

The ocean response to tropical cyclone (TC) forcing is sensitive to the surface momentum flux that forces ocean currents because of the dominant importance of shear-driven entrainment cooling of the ocean mixed layer. This sensitivity is explored using two approaches. First, idealized simulations of the ocean response to Hurricane Frances (2004) are compared to repeat ocean profiles of temperature, salinity, and current observed by EM-APEX profiling floats (Sanford et al, 2011, JPO, 1041-1056). The mixed layer deepening and cooling response is most accurate if the bulk momentum drag coefficient used to calculate momentum flux driving ocean currents is reduced by ~30% from values used by the HWRF atmospheric model to calculate surface drag on the atmosphere. This reduction leads to ocean momentum flux estimates that agree with in-situ estimates obtained by Sanford et al. (2011) beneath Hurricane Frances using the EM-APEX profiles. This reduction can be justified in part because a fraction of the momentum flux at the ocean surface forces the time-evolving surface gravity wave field beneath storms. Adjustment of momentum flux also serves to correct for physical processes not adequately represented by the ocean model such as Langmuir mixing. The second approach examines the sensitivity of realistic coupled forecasts of 2014 storms Edouard and Gonzalo to changes in the bulk drag coefficient. These cases are evaluated against ocean observations collected before, during, and after these storms by field programs supported by the 2013 Disaster Relief Appropriations Act. These results demonstrate the critical need to obtain high-quality subsurface ocean measurements of temperature, salinity, current, and turbulence along with surface flux measurements to rigorously evaluate and improve the performance of ocean models in coupled TC prediction systems.