This project addresses the dissipation of mesoscale, balanced flow in the ocean through the use of high resolution nesting experiments in two different numerical models (ROMS and the MIT general circulation model). The work specifically addresses the generation of sumesoscale, ageostrophic flows by mesoscale interactions with topography by explicitly resolving the relevant scales of 0.1-10 km in the horizontal. Numerical simulations will be designed to calculate the cascades of energy, enstrophy, and tracer variance and compare with theories, which will also be developed and improved within the scope of the project. The high resolution simulations will be performed in the Kuroshio region where strong topographic features and the the western boundary current dynamics will allow robust testing of the hypotheses. It is hoped that the work will lead to new understandings of the submesoscale regime, the dynamics that connect balance and unbalanced flows, and perhaps even lead to new parameterizations which can be used to represent these processes in ocean and climate models.
The ocean is full of eddies and sworls (essentially the equivalent in the ocean of 'weather') and an important question in oceanography concerns their fate. We know these features eventually erode, but we don't know how they erode. Figuring this out would help us to produce more accurate climate projections, because the ocean weather plays an important role is setting the ocean state. And the ocean state feeds into the state of the climate. It is quite likely the erosion of ocean weather occurs at the boundaries of the ocean, either the surface or the bottom. The objective of this proposal is to examine the dynamics of ocean currents (like the Gulf Stream) and eddies with the ocean bottom in a combined theoretical and modeling study. There are unique types of waves that exist on strongly sloping bottoms. We have found that for currents like the Gulf Stream, the interactions involving these waves have dramatic consequences, including sizable weakenings of the currents and redirection from the topography. We have also found that these interactions generate a churning of the ocean interior, resulting in ocean mixing. This is an exciting discovery directly linking ocean weather to the maintenance of the thermocline. The first three plots indicate the sequence of events associated with these dynamics. They show plots of a surface inside the ocean and how it is affected when a strong current meets topography. The topography appears in the distance as a grayish slope, and the surface as the sheet connecting to it. Note that as time progresses, the surface develops in a manner very similar to a breaking wave on the beach. As a result of the breaking and its associated turbulence, energy is lost from the strong flow and the interior ocean water is mixed. There are also classes of currents that differ from the Gulf Stream in that they occur on the west coasts of the continents, rather than the east coast. We have also examined these interactions and found they can churn the water, but in a way distinct from the Gulf Stream. Specifically, boundary friction destabilizes the flow, so that when it moves into the interior, the flow becomes turbulent and mixes. A planview of ocean weather looking down from space offshore of Monterey, California shows an example state. Note the strong offshore eddy, which is the result of the mixing process we describe, and the strong flow emanating from Point Sur into the Monterey Bay. It is the latter flow that mixes and eventually organizes into features like the offshore eddy. We speculate this behavior is commonplace on the US and Southern America western coastline, and assists in maintaining the near surface ecosystem by bringing nutrients from the deep ocean to the surface.
