Modeling the impact of sill depth on glacier submarine melting in narrow fjords

The geometry of pro-glacial fjords plays an important role in modulating fjord currents and heat transport. Many such fjords (e.g., LeConte in Alaska, Jorge Montt in Patagonia) have shallow sills modulating exchange with the surrounding ocean. Here, idealized model runs are conducted to investigate the constraining effects of the sill on the fjord circulation and submarine melt rate. A coupled plume-ocean fjord model based on MITgcm is set up with horizontal and vertical resolutions of <200 m and 0.5~8 m respectively. The fjord width is 2 km. We vary the sill depth, subglacial discharge, and ambient properties. As expected, the results show that for a given subglacial runoff the exchange volume flux at the mouth is strongly reduced by the sill. However, this process only partially explains the resulting cooling of the deep fjord temperature and the corresponding reduction in submarine melting. A strong frontal region is formed near the constriction point as the lower layer accelerates down the sill, with enhanced mixing and vertical advection. Relatively colder water from the upper layer is entrained into the deeper layer, cooling the latter as a result. With shallower sills, the fronts become steeper and more water is transported downward, feeding a recirculation within the fjord. In most cases, this recirculating flow largely compensates for the reduction of inflow at the sill, but it lowers the deep layer temperature (and thus the heat flux), resulting in diminished melting. The submarine melting flux at the glacier front with a shallow, 10 m sill, is ~25% smaller than the case with no sill. The parameters dependency relating the forcing, the fjord geometry, and the melting are discussed. These results highlight the impacts of sill bathymetry and associated fjord processes on the variability of oceanic heat supply to melting glaciers.