Exchange across the shelf break at high southern latitudes

J M Klinck,M S Dinniman

doi:10.5194/os-6-513-2010

Abstract

Abstract. Exchange of water across the Antarctic shelf break has considerable scientific and societal importance due to its effects on circulation and biology of the region, conversion of water masses as part of the global overturning circulation and basal melt of glacial ice and the consequent effect on sea level rise. The focus in this paper is the onshore transport of warm, oceanic Circumpolar Deep Water (CDW); export of dense water from these shelves is equally important, but has been the focus of other recent papers and will not be considered here. A variety of physical mechanisms are described which could play a role in this onshore flux. The relative importance of some processes are evaluated by simple calculations. A numerical model for the Ross Sea continental shelf is used as an example of a more comprehensive evaluation of the details of cross-shelf break exchange. In order for an ocean circulation model to simulate these processes at high southern latitudes, it needs to have high spatial resolution, realistic geometry and bathymetry. Grid spacing smaller than the first baroclinic radius of deformation (a few km) is required to adequately represent the circulation. Because of flow-topography interactions, bathymetry needs to be represented at these same small scales. Atmospheric conditions used to force these circulation models also need to be known at a similar small spatial resolution (a few km) in order to represent orographically controlled winds (coastal jets) and katabatic winds. Significantly, time variability of surface winds strongly influences the structure of the mixed layer. Daily, if not more frequent, surface fluxes must be imposed for a realistic surface mixed layer. Sea ice and ice shelves are important components of the coastal circulation. Ice isolates the ocean from exchange with the atmosphere, especially in the winter. Melting and freezing of both sea ice and glacial ice influence salinity and thereby the character of shelf water. These water mass conversions are known to have an important effect on export of dense water from many Antarctic coastal areas. An artificial dye, as well as temperature, is used to diagnose the flux of CDW onto the shelf. Model results for the Ross Sea show a vigorous onshore flux of oceanic water across the shelf break both at depth and at the surface as well as creation of dense water (High Salinity Shelf Water) created by coastal polynyas in the western Ross Sea.

Highlights

The exchange between the deep ocean and continental shelves is typically thought to be important for the processes over the shallower shelf
Cross-shelf exchange brings in nutrients from the ocean, which fuels biological production, as well as salt which balances the effect of river discharge and excess precipitation
In the Ross Sea, we are interested in the onshore movement of Circumpolar Deep Water (CDW), which provides a source of heat and nutrients

Summary

Introduction

The exchange between the deep ocean and continental shelves is typically thought to be important for the processes over the shallower shelf. Cross-shelf exchange brings in nutrients from the ocean, which fuels biological production, as well as salt which balances the effect of river discharge and excess precipitation. At high southern latitudes around the continent of Antarctica, continental shelves exchange water with the open ocean. The geometry, processes, and even the direction of water exchange can be very different from those on non-polar continental shelves. Antarctic shelves are active participants in the Global Overturning Circulation; some Antarctic shelves (e.g., the Weddell and Ross Seas) are places where dense water is formed. Deep water surfaces just off the shelf break in many areas (e.g., west side of the Antarctic Peninsula) and is converted into either surface or dense water (Rintoul et al, 2001)

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