Tropical Cyclone Air-Sea-Wave Coupling in COAMPS-TC: Interaction Between Atmospheric Wind Stress and the Upper Ocean Stokes' Drift Current

Tropical cyclone (TC) ocean-wave model interaction is examined using the state-of-the-art ESMF-based (Earth System Modeling Framework) tropical cyclone version of the Coupled Oceanic and Atmospheric Mesoscale Prediction System (COAMPS-TC) that was jointly developed by the Marine Meteorology and Oceanography Divisions of the Naval Research Laboratory. The ocean and wave model components of COAMPS-TC, the Navy Coastal Ocean Model (NCOM) and Simulating Waves Nearshore (SWAN), are coupled through several variables, such as the Stokes' drift current (SDC), wave radiation stress gradients, and ocean currents. This study focuses on the SDC interaction between NCOM and SWAN in intense Hurricane Ivan (September 2004). The SDC profile is generated from the SWAN wave frequency spectrum and is used in the total kinetic energy (TKE) parameterization outline by Kantha and Clayson (2004) in NCOM. Early studies of the SDC assumed that the SDC is mostly aligned with the surface wind stress; however, extreme situations such as TCs, tend to produce a misalignment of these vectors. Large swell and waves generated by TCs are susceptible to this misalignment with quick changes in wind direction, which may greatly alter the Langmuir turbulence in the upper ocean and subsequent mixing generated by SDC. A non-TC idealized study by Van Roekel et al. (2012) shows that a greater misalignment angle between the SDC and wind stress works to inhibit mixing. The simulation of Hurricane Ivan shows that the misalignment angle between the SDC and wind stress is greatest (> 90o) to the left of the forward translating TC. Calculations of the Stokes' drift shear production term in the TKE equation indicate that within the region of the TC associated with the highest misalignment angle, the Stokes' shear production becomes very small or even switches sign, which may act to decrease mixing in this region.