A Glacial Fjord Box Model: Derivation and Representation of Glacially Modified Water

Fjords are a crucial connection between the Greenland Ice Sheet and the ocean, but remain unrepresented at the scale of many Earth System Models. To parameterize mixing within them, we modify a two-layer estuary box model to include buoyant plumes, which are common features of glacial fjords. This fjord box model allows us to estimate primarily the salinity of the water mass exported to the open ocean. We compare high-resolution MITgcm simulations and observations to both the fjord box model and results from buoyant plume theory. The volume transport out of the fjord is increased in the fjord box model compared to a buoyant plume alone. However, the volume of shelf water entrained into the buoyant plume is a key control on the properties of the water mass exported from the fjord into the open ocean. This model is also used to estimate the temperature of glacially modified water by using the temperature of the water mass entering the fjord as a tuning parameter. We find that the fjord box model could improve the salt flux into the ocean from fjords, but that temperature is highly sensitive to the effective temperature of the freshwater flux, which is based on a ratio of meltwater and subglacial discharge and is challenging to estimate.