Tidally-Driven Exchange at the Shelf Break

Enhanced productivity in the shelf seas is dependent on a supply of nutrients from the open ocean, across the steep topography of the shelf break. The exchange at the shelf break is limited as the geostrophic flow is constrained to following isobaths, thus we need to explore processes that break the assumption of geostrophy. Using the thickness-weighted volume transport, we show that the covariance of thickness and velocity can drive a volume transport across the shelf break, usually referred to as the eddy bolus transport. We propose that the internal tide drives a covariance resulting in a bolus transport of both volume and tracers in the direction of propagation, analogous to the Stokes Drift. The theory is also extended to consider the interaction between the internal and barotropic tide, generating a large, but spatially variable, bolus transport. Whilst these eddy bolus transports are smaller than the transport by the time-mean velocity, they can make a substantial contribution to the cross-shelf component, as they are free from the limitations of geostrophy. The bolus transport is diagnosed for a series of near shelf break moorings in the Celtic Sea, Malin Shelf and the North-East New Zealand shelf, where there are transports equivalent to eddy bolus velocities O($1 – 0.01 cm s^{-1)$), primarily controlled by the strength of the internal tide linked to the M2 period in velocity and thickness. The diagnosed eddy bolus transports are comparable to estimates of the transport from our theory. For the Celtic Sea, integrating the bottom layer transports from the moorings to the whole of shelf break gives a total volume transport of approximately $0.1 Sv$ and, when combined with nitrate measurements, then provides an onshore nitrate transport of $1 Kmol s^{-1}$. This onshore nitrate transport acts to sustain shelf-sea productivity and is comparable in magnitude to the vertical diapycnal nitrate supply over a 50km wide adjacent shelf.