Recent changes in energy and freshwater budgets for the Godthåbsfjord catchment simulated in a 5 km regional climate model
Abstract:Freshwater input to the Godthåbsfjord (Southwest Greenland) is analyzed with special attention on the melt and runoff from the ice sheet. We use the high resolution (5.5 km) HIRHAM5 regional climate model covering all of Greenland, forced by the ERA-Interim reanalysis at the lateral boundaries over the period 1989-2012.
The horizontal model resolution (5.5 km) allows a high degree of detail in the representation of topography and surface types. This makes the model particularly suited for local catchment-scale analysis. The model output is compared to a range of different hydro-meteorological observations both on and off the ice sheet and is found to represent weather variability well.
During this period, the ice sheet up to elevations of 2000 m experienced increasing energy input from the surface turbulent heat flux and the ice sheet above 1000 m experienced increasing energy input due to shortwave radiation. Large-scale trends show an increase in atmospheric pressure over North Greenland, southerly wind anomalies and declining cloudiness. These factors contributed to increased summer melt, which outweighed changes in annual accumulation, resulting in a decline in surface mass balance (SMB) and upward migration of the equilibrium line of more than 100 m. The associated increase in runoff to Godthåbsfjord dominates over changes in precipitation directly over the fjord and surface runoff from non-glacier land.
Regressing simulated SMB on observed temperature and precipitation, we reconstruct SMB back to 1955. In this 58-year perspective, the recent SMB changes are found to be unprecedented. While low SMB has occurred before, this has previously been driven by low precipitation. The current low SMB, on the other hand, is driven by increased melting.