Volume exchange across the shelf break: the role of the internal tide and an integrated understanding
The shallow shelf seas support 16% of the global oceanic primary production despite only being 9% of the global ocean surface. There is an open question about how the nutrients to support this enhanced productivity are supplied to the shelf seas. Previous budgets have shown that a large portion of these nutrients are supplied from the open ocean. Here the role of lateral exchange of volume between the nutrient deplete shelf seas and the comparatively nutrient rich open ocean is explored for the European Shelf, both considering a specific physical process and an integrated view point.
Previous theoretical studies of internal waves have shown a Stokes’ Drift with a clear vertical structure, which can be split into two distinct components: the bolus transport, and the vertical displacement of an isopycnal through shear. Here the role of this internal wave driven Stokes' drift at the continental shelf break is assessed. Initially, we use an idealised model simulation to compare two formulations of the Stokes' drift: using particle tracking, and taking a layered approach. A three layered approach is then applied to a series of moorings located near the shelf edge. For many of these moorings we see a vertical structure consistent with both the theoretical and modelling calculations of the Stokes' drift however for some moorings the internal wave field, and thus the Stokes’ Drift, is more complex.
The Walin Framework is used to evaluate the transformation of volume across density surfaces from the combination of surface forcing and mixing. The surface forcing on the European Shelf over an annual cycle results in a strengthening of contrast, i.e. a gain of high and low density water and a loss of middle densities. Observations of mixing are insufficient to counter this surface forcing. As a result a transport across the shelf edge is invoked to balance the air-sea forcing and maintain steady-state over inter-annual time scales. This reveals three distinct regimes: an off-shelf transport in the lightest classes of 0.5 Sv, an on-shelf transport in the middle density classes of 1.5 Sv, and an off shelf transport in the densest classes of 1 Sv. Mapping these density classes along the shelf edge reveals that for much of the shelf there is a down-welling circulation of 1 Sv and an across shelf transport of 0.5 Sv, on shelf across much of the boundary and off shelf at the Norwegian Trench.