Atmospheric Sensitivity to Marginal-Ice-Zone Drag: Local and Global Responses

The impact of a physically-based parameterization of atmospheric drag over the marginal-ice-zone (MIZ) is evaluated through a series of regional and global atmospheric model simulations. The sea-ice drag parameterization has recently been validated and tuned based on a large set of observations of surface momentum flux from the Barents Sea and Fram Strait. The regional simulations are from March 2013 and make use of a collection of cold-air outbreak observations in the vicinity of the MIZ for validation. The global model analysis uses multiple 48-hour forecasts taken from a standard test suite of simulations. Our focus is on the response of the modelled atmosphere to changes in the drag coefficient over the MIZ. We find that the parameterization of drag has a significant impact on the simulated atmospheric boundary layer: for example, changing the surface momentum flux by typically 0.1-0.2 N m^-2 (comparable to the mean) and low-level temperatures by 2-3 K in the vicinity of the MIZ. Comparisons against aircraft observations over and downwind of the MIZ show that simulations with the new sea ice drag scheme generally have the lowest bias and lowest root-mean-square errors. The wind speed and temperature biases are reduced by up to 0.5 m s^-1 and 2 K respectively, compared to simulations with two settings of the previous drag scheme. In the global simulations the atmospheric response is widespread – impacting most of the Arctic and Antarctic sea-ice areas – with the largest changes in the vicinity of the MIZ and affecting the entire atmospheric boundary layer.