Competing Effects of Elevated Vertical Mixing and Freshwater Input on the Stratification and Sea Ice Cover in a Changing Arctic Ocean

The heat contained within the Atlantic Water layer of the Arctic Ocean is sufficient to melt all sea ice in the Arctic within a few years. Whilst the sea ice cover is currently isolated from this heat by the cold halocline, the Arctic is undergoing a period of rapid transition, with the freshwater input to the ocean projected to increase, and the decline in Arctic sea ice cover likely to drive periodic increases in vertical mixing. Using a 1D model of the Arctic Ocean we examine how these competing processes will affect the stratification, the stability of the cold halocline, and the vertical heat flux from the Atlantic Water layer, and we explore whether elevated vertical mixing will represent a significant positive feedback for the ongoing melting of Arctic sea ice. We find that elevated vertical mixing initially warms the mixed layer due to an enhanced diffusive heat flux, but the change in mixed layer temperature is too small to significantly affect the sea ice cover. Most importantly, the elevated vertical mixing also strengthens the stratification over the Atlantic Water layer. After about a decade this effect dominates, and the mixed layer begins to cool. Overall, the sea-ice cover can only be significantly affected if the elevated vertical mixing is sufficient to completely erode the cold halocline, allowing heat to be directly entrained into the mixed layer from the Atlantic Water layer.