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Abstract Winter Arctic sea ice loss has been simulated with varying degrees of abruptness across global climate models (GCMs) run in phase 5 of the Coupled Model Intercomparison Project (CMIP5) under the high-emissions extended RCP8.5 scenario. Previous studies have proposed various mechanisms to explain modeled abrupt winter sea ice loss, such as the existence of a wintertime convective cloud feedback or the role of the freezing point as a natural threshold, but none have sought to explain the variability of the abruptness of winter sea ice loss across GCMs. Here we propose a year-to-year local positive feedback cycle in which warm, open oceans at the start of winter allow for the moistening and warming of the lower atmosphere, which in turn increases the downward clear-sky longwave radiation at the surface and suppresses ocean freezing. This situation leads to delayed and diminished winter sea ice growth and allows for increased shortwave absorption from lowered surface albedo during springtime. Last, the ocean stores this additional heat throughout the summer and autumn seasons, setting up even warmer ocean conditions that lead to further sea ice reduction. We show that the strength of this feedback, as measured by the partial temperature contributions of the different surface heat fluxes, correlates strongly with the abruptness of winter sea ice loss across models. Thus, we suggest that this feedback mechanism may explain intermodel spread in the abruptness of winter sea ice loss. In models in which the feedback mechanism is strong, this may indicate the possibility of hysteresis and thus irreversibility of sea ice loss.
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