Wave Impacts in Upper Ocean Mixing

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Intellectual Merit. Surface waves are believed to play a key role in the upper ocean boundary layer, yet do not appear explicitly in any of the major boundary layer parameterizations used in ocean circulation or climate models. This project will assess whether this neglect is important by testing the following hypothesis:(1) Mixed layer turbulence intensity and entrainment efficiency, scaled by wind stress, will increase with surface wave age, in the presence of swell, and with decreasing boundary layer depth as predicted by recent and proposed Large Eddy Simulations (LES).(2) A boundary layer model that includes these sea state dependencies, in addition to the usual dependencies on surface stress, buoyancy flux, and subsurface shear, will be significantly more accurate than one which does not.Multi-year data from the NOAA/PMEL long-term mooring at OWS Papa, will be used to test the second hypothesis. In collaboration with the OWS-P team at NOAA/PMEL, a dedicated surface wave measuring buoy will be deployed at the OWS-P site. Combined with existing measurements of air-sea fluxes, ocean stratification and shear, these data will provide complete, high quality data for the forcing and testing of 1-D ocean boundary layer models with waves. Existing boundary layer parameterizations, including KPP and a second moment turbulence closure, will be modified for the effect of variations in sea state on mixed layer entrainment, via critical bulk Richardson numbers criteria or through Turbulence Kinetic Energy (TKE) production rates or equilibrium levels. This ensemble of models will be compared against 3 years of enhanced OWS-P data to test the second hypothesis.Data from water-following Lagrangian floats operated in the upper ocean boundary layer will be used to test the first hypothesis. These instruments, deployed over the last 17 years, have produced measurements from which profiles of vertical kinetic energy and flux profiles of heat, salt and buoyancy can be estimated. These data span almost the entire range of oceanic wind speeds [0-57 m/s] and a wide range of mixed layer depths [0-250m], but do not span a wide range of wave conditions at each wind speed. The quality of wind speed, surface wave and ocean shear measurements also vary greatly among these data. The data set will be enhanced with an additional float deployment in mature, big seas near OWS-P, and on very young, small seas in the lee of a floating bridge over Lake Washington. Remote sensing and operational wind/wave products will be used to supplement both the new and existing float data. This enhanced data, coupled with both idealized and observationally-based LES case studies for model-data comparison, will be analyzed uniformly to test the first hypothesis.Broader Impacts. Turbulent mixing in the oceanic boundary layer is a key component in the climate system, playing a crucial role in setting the surface values and thus fluxes of temperature and CO2 as well as the transition layer nutrient fluxes supporting upper ocean productivity. Operational models now routinely make global predictions of surface wave fields. Incorporation of this information into models could have significant impacts on physical and biogeochemical models on a wide range of scales.The OWS-P data used in this work will be available publicly in near-real time thus providing a long-term, high quality data set for future studies. The project will support continuing outreach and education efforts to K-12 students and their teachers. The project will support training and research experience for one undergraduate engineering major, and a unique forecasting experience for an undergraduate meteorology student.