Three-Way Coupling of Surface Currents, Waves, and Wind Stress Over the Gulf Stream

This study provides the first detailed analysis of the oceanic and atmospheric response to the wave-current-stress interaction at the Gulf Stream using a high-resolution three-way coupled regional modeling system (COAWST). This study is motivated by recent studies showing that two-way coupling of currents and wind stress introduces substantial negative bias to the wind stress and surface eddy kinetic energy. We suspected that the negative wind stress bias due to current-stress interaction might be canceled out by adding wave-stress interaction to the coupled system. To evaluate the impact of three-way coupling, we perform a 1-month long simulation with COAWST over the Gulf Stream and east coast of America by using four different model configurations, which differ in whether include surface currents and sea-state parameters in the wind stress calculation. We find that 30-day averaged wind stress increases by 15% at the Gulf Stream, with only small changes elsewhere. Part of the wind stress increase is used to grow waves, which results in increased wave height in the three-way coupled experiment. By comparing each term in the equation of log wind profile, we find that current-induced surface roughness and near-surface wind changes play important roles in the sea-state response to the three-way coupling. We also find that 30-day averaged SST increases up to 0.4 ^oC (95^th percentile) at the Gulf Stream, which is slightly smaller than SST change in two-way coupling of currents/waves experiments. When waves and currents are considered in the stress parameterization, the vertical entrainment instead of horizontal advection is the major contributor to the SST change at the Gulf Stream. This is consistent with the strong wind stress curl generated at the Gulf Stream and seen in satellite images. The wind stress curl and SST gradient are well coupled in the three-way coupled experiment, with largest coupling coefficient (0.89 × 10^-2 N m^{-2 o}C^-1) among four experiments, which is plausible in comparison to satellite observations. In a three-way coupled model, the stress parameterization is sufficiently sensitive to waves and particularly currents, that satellite observations of the mesoscale features associated with the currents could be used to test the stress parameterization.