Volume exchange across the shelf edge : the role of the internal tide and other physical processes

• Main Author: Spingys, C. P.
• Published: University of Liverpool 2017
• Subjects: 551.46
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.722081

The world ocean can broadly be split into two regimes, the shallow shelf seas up to a few hundred metres deep and the deep open ocean which is typically 4-5 km deep. These two regimes are separated by a narrow region of steep topography. This steep topography acts to limit the exchange to those processes that break the assumptions of geostrophy. This exchange, however, is important on both a local and global scale. The shelf seas are an important source of dense water masses, ventilating the deep ocean, control of the flux of mid-depth open ocean heat to glacial fronts, supply of nutrients from the open ocean to sustain enhanced productivity on the shelf, and export of the carbon drawn-down by this productivity into the deep ocean. This problem will be explored through two themes, one exploring the role of the bolus transport and one exploring an integral framework. The bolus transport is de ned as the transport as a result of co-variances in layer thickness and layer velocity. This has previously been extensively applied for the atmosphere and the action of eddies in the Southern Ocean. Here the bolus transport driven by the action of the internal tide near the shelf edge is considered. First we consider a sinusoidal internal wave on the interface of a two well mixed layer ocean. This leads to a theoretical scaling for the bolus transport of (ca2)=(2hi) in the same direction as the propagation of the wave. The bolus transport was then calculated for eight moorings across four locations. For the surface layer these transports were in the range 1.09 to 0.1 m2s 1 and for the bottom layer were in the range 1.06 to 0.04 m2s 1. These observational estimates compare favourably with the estimates from the theory, with the theory within 40% of the observed values. This bolus transport is balanced locally by a time-mean velocity of equal magnitude but directed off shelf in both layers. The Walin Framework is an integral framework describing the rate at which water crosses density surfaces. This is used to diagnose the formation, or destruction, rate of water within density classes across the European Shelf, bounded by the 200m isobath, the southern limit of the Celtic Sea and the Norwegian Coast. This formation rate makes the largest contribution to the exchange across the shelf edge. 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 along the shelf edge indicates that much of the shelf does not reach the lightest classes so only the intermediate and dense regime is applicable. The lightest regime primarily exists along the southern section of the Norwegian Trench. This implies two circulations: a down-welling circulation of 1 Sv across much of the European Shelf and an across shelf transport of 0.5 Sv, on-shelf across much of the shelf edge balanced by an off shelf transport in the Norwegian Trench. This down-welling circulation is similar to previous estimates of the total transport across the European Shelf Break.