Straightforward dynamics-based mixed layer models underestimate the deepening seen in long-term observational records. A mechanism that augments mixing is apparently missing. One possible mechanism involves penetration of the thermocline by eddy motions from the mixed layer. Another is enhanced shear and/or overturns due to internal waves (which may themselves be generated either locally or elsewhere). In either case, it appears important to include the compliance of the thermocline in models of wind-driven mixing; i.e., to study the interaction of the actively forced Langmuir circulations with high-frequency internal waves.
In order to answer these questions, this project will analyze high resolution data collected as part of the Hawaiian Ocean Mixing Experiment (HOME). The instrumentation includes a multi-beam surface-sidescan Doppler sonar system that is designed to define the length and time scales of the mixed layer motion (e.g. Langmuir circulation), directional surface waves, and surface strain fields associated with high-frequency internal waves, a rapid-profiling CTDs that provides density profiles with resolution in both time and depth sufficient to estimate mixing via overturns (Thorpe scales). an 8-beam coded pulse Doppler sonar system that samples flows and shears over the water column (upward and downward from 400m). and wind speed and direction instruments. The central question addressed by this project is "Do internal waves affect the mixed layer motions, and hence alter entrainment and mixing in the uppermost pycnocline?" To test the hypothesis that they do, the investigators will look at effects of high-frequency internal waves on the form and dynamics of the surface velocity structures like those seen in the data. An integral component of this proposal is employing a post-doc drawn from the Langmuir circulation modeling community, incorporating internal wave physics of incrementally increasing complexity, and re-sampling the output to mimic the data. The focus is on in-depth data/model comparisons, rather than on model development per se: the strategy involves re-sampling and statistical comparisons. The anticipated results include improved understanding and models of oceanic surface layer mixing.
Intellectual merit. The project focuses on a link in the air/ocean system that is vitally important yet incompletely understood: net mixing through the surface layer of the sea. The mixed layer and uppermost pycnocline forms a crucial barrier between the atmosphere and deep ocean, 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.
Broader impacts. The results will help improve air-sea exchange estimates, and hence climate effects, thereby helping improve the information by which policy decisions are made. The data sets, interpretations, and results will be disseminated broadly to enhance scientific understanding and facilitate further re-examination and interpretation. Summaries accessible to the public and policy makers will be made available. The proposed activity will provide training and teaching opportunities for a post-doc. Partnerships and collaborations across disciplines and institutions will be actively developed.
