Modeling the Impact of Glacial Runoff on Fjord Circulation and Submarine Melt Rate using a New Parameterization for Glacial Plumes

Abstract:

The injection at depth of ice sheet runoff into fjords may be an important control on the frontal melt rate of tidewater glaciers. Runoff influences submarine melt rate directly, by generating buoyant plumes adjacent to the ice front, and indirectly, by influencing the circulation of heterogeneous water masses within the fjord. Understanding these processes has been held back, however, by the computational expense of modeling both the fine scale turbulent processes occurring within the buoyant plumes and the coarser scale circulation throughout the remainder of the fjord.

Here, we develop a new parameterization for ice marginal plumes within the Massachusetts Institute of Technology General Circulation Model (MITgcm), allowing three-dimensional simulation of large (500 km²) glacial fjords on annual (or longer) timescales. We find that for an idealised fjord (without shelf-driven circulation), subglacial runoff produces a thin, strong and warm down-fjord current in the upper part of the water column, balanced by a thick and slow up-fjord current at greater depth. Submarine melt rates increase with runoff due to higher melt rates where the plume is in contact with the ice front. We find however that annual submarine melt rate across the ice front is relatively insensitive to variability in annual runoff. Better knowledge of the spatial distribution of runoff, controls on melt rate in those areas not directly in contact with plumes and feedback mechanisms linking submarine melting and iceberg calving are necessary to more fully understand the sensitivity of glacier mass balance to runoff-driven fjord circulation.