Near-N Internal Waves and Transition-Layer Mixing

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

The oceanic mixed layer is crucial to air-sea exchange. The enhanced stratification at the base of the mixed layer (or 'transition layer') is also crucially important, as it affects the rate of mixed-layer deepening and mediates vertical exchanges between the mixed-layer and deeper ocean. Understanding the processes that control turbulent mixing of momentum and tracers in this transition layer is an outstanding problem in upper ocean physics. Based on new modeling results, we hypothesize that turbulent structures in an active mixed layer combined with inertial shear across the upper pycnocline can generate energetic Near-N Internal Waves (NNIW). These NNIW are confined to the region of elevated buoyancy frequency in the uppermost pycnocline, where they must eventually break and contribute their energy to turbulent dissipation and mixing. Two likely candidates for this kind of NNIW generation are Langmuir Cells and buoyancy-driven convection, both of which may generate waves by impinging on the stratified mixed-layer base. At the same time, the turbulence makes the eddy-viscosity in the surface layer large, which would damp these short internal waves. The project will look at the three-way interactions between the surface mixed layer, inertial shear, and high-frequency internal waves, and to examine the resulting effects on upper pycnocline mixing. Our approach combines numerical simulations (LES) that resolve both turbulent structures in the Mixed Layer (ML) and the NNIW generated in the upper pycnocline, along with detailed comparisons with suitable data collected over the past 2 decades. These data sets generally include estimates of length and time scales of the mixed layer motion, directional surface wave spectra, strain fields associated with NNIW, density profiles and overturns, velocity profiles over the upper few hundred meters, heat-flux estimates, and vector wind stress. The central questions are: How do high-frequency internal waves interact with mixed layer motions? and Do they affect entrainment and mixing in the upper pycnocline? An integral component is Large-Eddy Simulation (LES) modeling of the upper ocean, including wind and wave forced mixed layer motions, inertial shear, and the resulting NNIW in the stratified layer just below. The ultimate objective is to make in-depth data/model comparisons, more than on model development per se. However, some further model development is needed before comparisons with data can be realistic. The model-data comparison strategy involves data analogous re-sampling of model output and statistical comparisons. The anticipated results include improved understanding of upper ocean and surface layer mixing and dynamics, and hence improved models on all scales. Intellectual merit. The project focuses on a link in the air/ocean system that is vitally important yet incompletely understood: net mixing through the uppermost layers of the sea. The mixed layer and uppermost pycnocline together form a crucial barrier between the atmosphere and deepocean, controlling the net fluxes of heat, momentum, nutrients, and greenhouse gases. This uppermost layer is also vital to marine life, setting the scene where light and nutrients meet, and where pollutants and contaminants are dispersed. Appropriate scaling of these fluxes has been shown to be important to global air-sea coupled simulations, which can be sensitive to both the degree of surface mixing and to the ML depth. Broader impacts. The results will help improve air-sea exchange estimates, and hence both environmental and climate effects, thereby helping improve the information by which long-term policy decisions are made. The results and interpretations will be disseminated broadly to enhance scientific understanding and facilitate both decision making and further scientific work. The activity will provide training and scientific opportunities for a graduate student and a post-doc. Partnerships and collaborations across institutions will be actively developed. As an economic stimulus, salaries and supplies are spent locally and steadily; scientific equipment is by necessity selected for the highest quality with sustainable support and availability.