Evolution of a tropical cyclone is strongly affected by the supply of heat from a warm ocean surface. When strong tropical cyclone wind blows and enhances upper ocean currents and turbulence, it may cool the sea surface temperature, decrease the heat supply to the storm, and weaken the storm. In this study we have investigated how such upper ocean responses to a tropical cyclone are modified by ocean surface waves, by combining field observations and numerical model simulations under five past tropical cyclones. Our results clearly show that ocean surface waves enhance upper ocean turbulence and reduce upper ocean currents. Our results also show that when ocean surface waves propagate in a different direction from the wind direction, the wind forcing and the resulting upper ocean responses are weakened. These results strongly suggest that accurate predictions of tropical cyclones and ocean responses below need to account for the state of and changes in ocean surface gravity wave conditions. Such improvements made to tropical cyclone forecast models will help to avoid future losses due to these deadly storms and assist with municipal preparedness activities. Results from this proposed study also inform not only the tropical cyclone modeling community but also the entire weather and climate modeling community on the leading order impacts from surface waves on upper ocean processes in high wind conditions.

 On the ocean mixing side, data from Lagrangian floats in typhoons was reprocessed to a higher level of quality control and used to compare with mixing parameterizations in the ocean components of wave-coupled models. Work initiated in two prior NSF projects, on coupling wave and ocean models and on the outgassing of bubbles under a hurricane, were both brought to completion with publications. Further progress was made on coupling the wave models to subgrid ocean mixing, but in evaluating this progress, as well as divergent developments in studies of near-surface Large Eddy Simulation results, we were led to turn our attention to establishing a less ambiguous understanding of the surface and mixed layer scaling behavior of turbulent vertical velocity in ocean surface and boundary layers from archived observations. Further reanalysis of archived Lagrangian float data has yielded clear empirical scaling expressions for vertical turbulent kinetic energy and for vertical temperature gradients in the surface layer that depart markedly from traditional Monin-Obukhov similarity theory, with additional dependence on surface wave forcing. The new analysis results form part of a PhD thesis defending July 2023, with manuscripts for submission to publication.

At University of Rhode Island, the project trained one PhD student, and at UW the project contributed to training an second PhD student during his final years. Through course work, frequent meetings with the principal investigators, and hands-on research, the graduate student learned basic concepts of the project related science and methodology (fluid dynamics and physical oceanography, upper ocean mixing, wave dynamics, numerical modeling, high performance computing). Based on this developed understanding, the student became capable of asking and addressing critical science questions independently. In addition, the student participated in outreach training activities, engaging public audiences and U.S. 4-6th grade classrooms, facilitated by our broader impacts professionals. The experience that the graduate student received as they worked side by side with education and outreach professionals to translate complex scientific concepts into non-expert language will assist her in future education and outreach efforts.

Last Modified: 06/29/2023
Modified by: Ramsey R Harcourt