Topographic Rectification on the Continental Margin: Toward Detectability from Observations

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Over the continental shelf and slope, mean (over months) flow is very often found to be in the direction of free subinertial coastal-trapped wave (CTW) propagation (e.g. northward off the west coast of the United States). There are many potential mechanisms that could generate flow in this sense, and some of these are sufficiently well-studied that we have practical diagnostics to use with observations, and thus we can test whether a given mechanism (e.g. tidal rectification) is responsible. One potentially important mechanism that is not readily diagnosed from observations is form stress. This mechanism is essentially a reflection of the underlying asymmetry of flows along a coastline. Flows in the opposite sense to CTW (these can only propagate phase in one direction) generation can encounter a bump and set up a standing wave (one that has zero phase velocity and appears steady). This standing wave exerts a drag on the flow. On the other hand, flow in the sense of CTWs does not allow a standing wave to form (since the flow only accelerates wave propagation), and no form stress can result. This underlying asymmetry means that zero-mean forcing over a bumpy bottom can result in a mean flow generation. This mechanism has previously been addressed in a number of idealized contexts, and its use has been shown to improve performance of coarse-grid simulation models. The present study calls for advancing our knowledge by extending form stress model development into the realistic parameter regime with a finite shelf and slope, and with density stratification. The Regional Ocean Modeling System (ROMS) will be used systematically to study this realistic parameter range, and to generate a quantitative empirical expression for rectified flow, given observable inputs. This set of numerical experiments is necessary in order to reach the main goal of this project: to define tests that can be used with existing or realizable observations to determine unambiguously whether topographic rectification is important in a particular setting.The intellectual merit of a successful execution of this project is twofold: 1) a systematic extension of topographic rectification models into a realistic parameter range not previously explored, and 2) the definition of a set of diagnostic tests that can used unambiguously to detect (or eliminate) topographic rectification as a means for driving mean alongshore flow and upwelling. Mean flow is particularly important since it dominates advective processes.The broader impact of this project is likewise twofold: 1) making improvements in ocean models that should be useful in climate and ecological forecasting, and 2) serving as a means to introduce an undergraduate student to scientific research and methodology. Should this proposal be funded, Woods Hole Oceanographic Institution will contribute $5,000 to this projectto help defray the costs of the summer student.