Basin scale forcing of flows on western-boundary shelves

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The flow along western boundary shelves drive regionally significant heat fluxes with significant regional climate impacts, and is an important biogeochemical transport pathway linking high and mid-latitude regions. The exchange of water between continental shelves and the deep sea is an important transport pathway between land and sea for carbon and other important chemical species. Recent work has shown that the magnitude and inter-annual variability of these flows structures the spatial distribution of benthic species and their genetic variability, and determines which life-history strategies are evolutionarily stable. It is difficult to understand, predict, or model these flows, and their effects, without a quantitative understanding of the forces that drive them. From a modeling perspective, embedding a regional model in a larger scale model is problematic if one does not understand what the larger-scale model must simulate accurately to provide sufficient boundary conditions. It is also hard to define observational strategies to understand variability in shelf flows without understanding the timescale of their variability, and how the phase and amplitude of this variability may vary across the shelf. A satisfactory understanding of shelf flows cannot be claimed if the source of the mean alongshore flow, and its interannual variation, is not known. To address these questions, a series of process-oriented investigations will be conducted to clarify the mechanisms leading to the mean and long timescale alongshore flows on these shelves and quantify the time-scale and origin of their variability. The main goal is to re-establish a link between the flows along western boundary continental shelves and the dynamics of the deep ocean. With a clear understanding of the source of observed alongshore pressure gradients, it will be possible to understand the links between large-scale variation in the ocean/atmosphere system such as the North Atlantic Oscillation and shelf dynamics. This will place the coastal oceans back into the broader context of the global ocean, and allow for a better quantitative understanding of how they are coupled. The project will support the training of a graduate student and 2 to 3 undergraduates to prepare them for careers in the sciences or industry. The graduate student will participate in the "Preparing Future Faculty" program that develops teaching, mentoring and writing skills in our students. The undergraduates will be progressively trained, and will be mentored by both the PI and graduate student. Each undergraduate will work on a simple starting project for the school year, and then a more independent project over the summer. This will culminate in a capstone project required for graduation. Studies of the alongshore flow along western-boundary continental shelves have failed to explain the dynamics of the mean and long-time scale flows along these shelves. The best studied example is the eastern shelf of North America. It has long been noted there that to close local momentum balances an along-isobath pressure gradient of unclear origin must be invoked. Mean winds would drive a circulation opposite to that observed; along and cross-shore buoyancy gradients are of insufficient magnitude to explain the circulation. Interannual variations in the winds explain less than half of the inter-annual variability in the alongshore transport. The main hypothesis of this project is that this "mean" flow (and perhaps its interannual variation) is largely driven by basin scale circulations, and in particular the swift currents along the Western Boundaries. While this idea is not new, recent advances in the understanding of Western Boundary Current dynamics provide a plausible and quantitatively tractable mechanism for how these basin scale flows can drive shelf flows. Support for this hypothesis is provided by recent modeling efforts for the Argentinean and Chinese shelves. A combination of analytical work and idealized numerical modeling will explain how changes in basin scale forcing alter the mean and interannually varying flow along western-boundary shelves. This will reveal the links between basin scale forcing and exchange between the shelf and ocean, how Western Boundary Currents modify shelf hydrography, and the role of along-shore topographic features in enhancing the interactions between the shelf and slope.