Drivers of glacial fjord-shelf temperature differences in the reduced-physics model FjordRPM

Greenland fjords connect and modulate exchanges between its outlet glaciers and the open ocean. Subglacial meltwater and icebergs discharged from the ice sheet cause convection and mixing inside the fjord, upwelling warmer waters from depth and cooling down the upper layers. As a result of these processes, the water temperatures that effectively melt outlet-glacier termini are different than what is observed close to the fjord mouths and on the continental shelf. Data coverage to study such processes, however, is limited in both space and time, and a Greenland-wide assessment of fjord behaviour using general circulation models is computationally prohibitive. To tackle these issues, we use the Fjord Reduced-Physics Model (FjordRPM) to simulate 37 fjords around Greenland forced by available observations between 2016 and 2020. Its low computational cost enables large-ensemble analyses (i.e., 27000 simulations in total for this study) to explore the effects of icebergs, exchanges between the fjord and the continental shelf, and meltwater plumes. We show that, while fjords cool down at the surface under increased icebergs, the effect of fjord-shelf exchanges and the meltwater plume are less straightforward. The depth where the plume reaches neutral buoyancy will determine which water masses are mixed, which might either warm or cool the fjord. The intensity of exchanges between the fjord and the shelf will affect the fjord stratification, therefore affecting the plume’s role in mixing water masses and homogenising the water column. Finally, we match our simulations to available observations to find the best-fitting model parameters, and highlight that, although there is a high variability in the best-fit model parameters between fjords and between years for a given fjord, values for the model parameters tested follow known statistical distributions, which can be used to refine prescribed model-parameter ranges. Although fjord systems are highly complex and the effects of different processes are not linear, a reduced-physics model can elucidate how fjords modulate ice-sheet-ocean exchanges at a Greenland-wide scale, provided observations are available to force the model.