Polar Amplification Due to Enhanced Heat Flux Across the Halocline

Polar amplification describes how the surface temperature in the polar regions warms more than the global mean. A range of mechanisms have been proposed to contribute to polar amplification, including the surface albedo feedback and other surface and atmosphere processes. However, there is still substantial uncertainty about the relative magnitudes of these contributions, and no clear consensus has emerged regarding what are the main drivers of polar amplification. Here, we investigate changes in oceanic heat fluxes across the halocline using an idealized ocean--sea ice--climate model of the Northern Hemisphere. Based on the model results, we identify an oceanic feedback process that can contribute to polar amplification that has been largely absent from previous discussions. In regions with sea ice, the surface mixed layer temperature is constrained to remain near the melting point, rather than adjust according to the radiative forcing at the surface. Under global warming, the ocean below the halocline steadily warms while the waters above are fixed near the melting point, thereby increasing vertical heat fluxes and driving enhanced warming in regions of sea ice cover. We evaluate the contribution of this process to polar amplification using a feedback locking approach. The results indicate that this enhanced heat flux across the halocline contributes 20% to polar amplification in the model simulations.