Impact of shallow sills on heat transport and stratification regimes in proglacial fjords

Abstract. The increased melting and rapid retreat of glaciers is a main contributor to sea level rise. In shallow-silled fjords common in Patagonia, Alaska, and other systems, these bathymetric features may act as the first-order control on the dynamics, constraining fjord-shelf exchange and thereby modulating glacial melting. However, we still lack a clear understanding of how circulation and associated heat transport in shallow-silled glacial fjords are modulated by fjord-glacier geometry and fjord-shelf properties. To address this, idealized numerical simulations are conducted using a coupled plume-ocean fjord model. The steady-state fjord exhibits strong mixing and vertical transport over the sill. Relatively colder water from the upper-layer outflow is refluxed into the deeper layer, cooling the incoming warm oceanic water and modifying water properties near the glacier front. Driven by a shallow sill, up to ~70 % of the outflow is refluxed downward and leads to ~10 % cooling of the inflow and the deep fjord. A range of sensitivity experiments indicate that sill depth, subglacial discharge, ambient fjord temperature and stratification are key parameters that modulate the heat transport to the glacier terminus. In particular, the relative depth of the fjord, the sill, and the terminal height of meltwater plume are used to characterize four circulation and heat transport regimes. The sill-driven reflux is found to result in a decrease of both deep fjord temperature and stratification, which have opposite effects on the glacial melt rate. These results underscore the importance of sill bathymetry and associated fjord processes in the variability of oceanic heat supply to melting glaciers.