Air-Sea Momentum Flux in a Coupled Ocean-Wave Model and its impact on the Ocean Response to a Hurricane

Brandon Reichl,
Colin Hughes,
Biju Thomas,
Isaac Ginis,
Tetsu Hara

This project is funded by NOAA HFIP program.

Increased understanding of hurricanes (also known as tropical cyclones) is critical to improving our ability to predict both the storm track and the intensity. Both the sea surface temperature (SST) and the upper mixed layer thickness are important to hurricane formation and maintenance. Following the passage of a tropical cyclone, there is a marked cooling in the SST known as the cold wake. This is due to turbulent mixing and upwelling. Thus, to effectively model tropical cyclones, it is important to also accurately model the ocean currents and temperature.

Current operational hurricane models do not account for the effect of waves on the momentum flux into the ocean. My research seeks to improve current hurricane models by using a Coupled Ocean-Wave model to investigate the impact of waves on the momentum flux into the ocean and in turn the SST. We want to know how much each component of the waves (Wave Momentum from a growing or decaying wave field and Coriolis Stokes forcing) contributes. The specific issues to resolve are: the extent that each component of the waves contributes, the role of the speed with which the hurricane moves (translation speed), the impact of currents on the wave related fluxes, and any effects on the SST magnitude or spatial distribution in the cold wake.

Here is an example of two different experiments for an idealized hurricane translating westward along a constant latitude of 22.4 degrees N at a constant speed of 4.8 m/s. The top plot compares the momentum flux into the ocean, the middle plot compares the SST, and the bottom one the currents in the two experiments. One experiment, (which I refer to as Experiment 10), includes both effects of waves on the momentum flux into the ocean as well as full coupling between the ocean and wave models. The other experiment (which I refer to as Experiment 1) sets the momentum flux into the ocean equal to the wind stress and does not involve any coupling.