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Polar amplification is a widely discussed phenomenon, and a range of mechanisms have been proposed to contribute to it, many of which involve atmospheric and surface processes. However, substantial questions remain regarding the role of ocean heat transport. Previous studies have found that ocean heat transport into the Arctic increases under global warming, but the reasons behind this remain unresolved. Here, we investigate changes in oceanic heat fluxes and associated impacts on polar amplification using an idealized ocean-sea ice-climate model of the Northern Hemisphere. We show that beneath the sea ice, vertical temperature gradients across the halocline increase as the ocean warms, since the surface mixed layer temperatures in ice-covered regions are fixed near the freezing point. These enhanced vertical temperature gradients drive enhanced horizontal heat transport into the polar region and can contribute substantially to polar amplification.

Plain Language Summary The Arctic region is warming at a faster rate than the rest of the globe. A number of mechanisms that may contribute to this have been identified, the most well-known being the surface albedo feedback that occurs due to the higher reflectivity of ice compared to open water. However, substantial gaps remain in our understanding of what drives the polar amplification of global warming, and projections of how much the polar regions will warm in the future vary widely. Here, we look at the contribution to Arctic warming from the vertical transfer of heat in the upper ocean. In the Arctic Ocean, a large amount of heat is stored in relatively warm waters at depth, with a cold layer of water and sea ice cover above. The results indicate that the amount of heat from this warm water that reaches the sea ice cover will increase under global warming, enhancing the rate of warming in the Arctic region.