Sack Lunch Seminar (SLS)

Radiational cooling over the Greenland Ice Sheet and warming over the ocean result in large temperature gradients within the atmospheric boundary layer, especially in winter. Flow of air perpendicular to the coast of Greenland, therefore, can give rise to a large heat flux. In southeast Greenland, this occurs through substantial cold air drainage off of the ice sheet, onto the ocean, by downslope wind events that can reach hurricane intensity.
Here, data from two meteorological stations, the reanalysis product ERA-Interim (ERA-I), and remote sensing data are used to identify and characterize these wind events, and to describe their influence on the regional sea ice cover and air-sea fluxes. Based on a composite of the events it is concluded that they last approximately a day, and seven to eight events occur each winter on average. They are triggered by a synoptic-scale cyclone whose geostrophic flow is approximately in the downslope direction. The advection of cold air from the ice sheet results in heat fluxes of up to 1000 W/m2 over a broad swath of the Irminger Sea extending south of Iceland and near the Irminger Gyre — an important deep water formation site — and it is estimated that they drive one fifth of the net wintertime buoyancy loss over the Irminger Sea. In addition, the strong winds advect sea ice away from the coast and out of a local fjord (Sermilik Fjord) where one of Greenland’s major outlet glaciers discharges.
Next, the momentum balance of the downslope flow is analyzed using ERA-I and the atmospheric weather research and forecasting (WRF) model with different model and topography resolutions. It is found that the winds are accelerated by the gravitational and synoptic-scale pressure gradient acceleration as well as channeling by the topography and the pressure drag associated with a large amplitude mountain wave. Simulations of individual events suggest that lower resolution products underestimate the wind speed because they misrepresent the steepness of the topography and do not account for the underlying wave dynamics. If a 5 km instead of a 60 km resolution is used over the slope, the flow can be faster by up to 20 m/s. The effects extend far downstream over the Irminger Sea with differences in the heat fluxes amounting to 20%.