NSF/GEO-NERC: Stirring at the Walls - A Dynamical Boundary Model for the Ocean

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Much of ocean circulation variability occurs over time scales of days to months and length scales of 10s to 100s of km, a range known as the 'mesoscale'. These features can feedback on the larger scales of circulation in the ocean (up to 'basin scale' - 6000 km), and are important in determining ocean circulation on interannual to decadal timescales. Yet mesoscale features require much finer grids to represent them in models than basin scale features, so their direct numerical resolution over the entire globe for the long times required in climate simulations is far beyond current computer resources. This project will use a 'Dynamical Boundary Model' (DBM) instead of high resolution grids at model boundaries. This project will first develop a DBM that can be incorporated into existing realistic global climate models. Then performance will be evaluated by comparison to models with high resolution near boundaries. Finally, the project will investigate mixing at boundaries using realistic models with the DBM implementation. The overall collaboration will support a post-doctoral fellow, a graduate student, and an undergraduate from an under-represented demographic. The work has broad social relevance, as success would represent a significant technical step forward in numerical modeling and prediction of global climate. The basic methodology is to split the ocean into separate interacting regions: an outer region that encompasses the general ocean circulation and a narrow boundary layer with different dynamics, referred to as a DBM. Mathematically, this is accomplished by the introduction of a small parameter that measures the ratio of the thin boundary layer thickness to the large horizontal scale of the ocean basin. At smaller scales, the boundary layer equation becomes a nonlinear wave equation allowing coastal waves to break, energy to be dissipated and fluid to mix at the boundary, and feeds back into the interior dynamics. To extend this work to non-idealized geometry, the equations for the boundary layer will be generalized to include coastal curvature and cross-shore depth profiles, and Kelvin waves on vertical walls will be replaced by coastal trapped waves. Once a reliable and rapid method has been derived for the boundary/interior interaction, the DBM will be inserted directly into global climate primitive equation (PE) models. Side-by-side numerical experiments using PE models with dense nested grids to explicitly resolve the boundary layer will be compared with those run at coarser resolution using the DBM. This project is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with their own country.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.