The Response of the Nordic Seas to Wintertime Sea-ice Retreat

The ocean response to wintertime sea ice retreat is investigated in the coupled climate model HiGEM. We focus on the marginal ice zone and adjacent waters of the Nordic seas, where the air-sea temperature difference can be large during periods of off-ice winds promoting high heat flux events. Both control and transient climate model ensembles are examined, which allows us to isolate the ocean response due to sea ice retreat from the response due to climate change. As the wintertime sea ice edge retreats toward the Greenland coastline, it exposes waters that were previously covered by ice, which enhances turbulent heat loss and mechanical mixing, leading to a greater loss of buoyancy and deeper vertical mixing in this location. However, under global warming, the buoyancy loss is inhibited as the atmosphere warms more rapidly than the ocean, which reduces the air-sea temperature difference. This occurs most prominently farther away from the retreating ice edge, over the Greenland Sea Gyre. Over the gyre the upper ocean also warms significantly, resulting in a more stratified water column and, as a consequence, a reduction in the depth of convective mixing. In contrast, closer to the coast the effect of global warming is overshadowed by the effect of the sea ice retreat, leading to significant changes in ocean temperature and salinity in the vicinity of the marginal ice zone.