Effects of Ambient Stratification and Shelfbreak Topography on Cross-shelf Transport of Dense Water: Implications for Arctic Shelf-Basin Exchange
Glen Gawarkiewicz
Woods Hole Oceanographic Institution
MS#21 Woods Hole, MA 02543, U.S.A
Tel: 1-508-289-2913
Fax: 1-508-457-2181
glen@paddle.whoi.edu
The stratification within the halocline of the central Arctic basins has long been thought to result from the lateral injection of water of intermediate density from the adjoining continental shelves. In order to examine possible exchange mechanisms, a three-dimensional primitive equation numerical model is used with an idealized coastal polynya, shelfbreak topography, and ambient stratification.
The presence of the ambient stratification does not inhibit the formation of dense water eddies in the manner described previously by Gawarkiewicz and Chapman (JGR, 1995). The maximum density attained within the negative buoyancy forcing region (the idealized polynya) is consistent with previous estimates from Chapman and Gawarkiewicz (JPO, 1997), limited by a balance between surface negative buoyancy fluxes and offshore eddy fluxes of dense water.
The dense water is carried offshore within eddies. However, the presence of linear bottom friction leads to the spin-down of near-bottom eddies, so that the remaining eddies are primarily sufface-rapped when these eddies reach the shelfbreak, the fate of the dense water is strongly dependent on the ambient stratification. If the dense water within the eddies is much denser the ambient density at the level of the shelfbreak, the dense fluid descends the continental slope in a thin layer along the bottom. However, if the densities are comparable, a portion of the dense water is carried along the shelfbreak in a gravity current, while another portion is carried across the shelfbreak along with the surface-trapped eddies. The resulting density structure is quite similar to the few shelfbreak sections collected from the Arctic, with isopycnals passing from mid-depths over the continental slope onto the continental shelf. These results suggest that the dense water transport from the continental shelves that maintains the Arctic halocline may occur in the form of small-scale eddy fluxes.
