Drivers of Marine Heatwaves in the Arctic Ocean.

Among the documented consequences of anthropogenic global warming are the increased frequency and duration of marine heatwaves in the global ocean. The literature dedicated to Arctic marine heatwaves corroborates those results, but fails to identify the heat sources and sinks. Because of the numerous feedbacks impacting polar regions, understanding the processes triggering and dissipating those extreme events is particularly important to predict their occurrence in a fast changing ocean. A three‐dimensional regional ice‐ocean numerical model is used to calculate a surface mixed layer heat budget and to investigate mechanisms generating and dissipating marine heatwaves. The majority of the marine heatwaves are onset by surface heat fluxes and decayed by bottom and surface heat fluxes. The dominant processes are spatially and seasonally heterogeneous: lateral heat flux can become the primary process when advecting heat anomalies at the main Arctic gateways or by triggering temperature extremes in winter. Using a Reynolds decomposition, it can be determined that the shoaling of the surface mixed layer induced by ice melt can significantly lengthen and intensify Arctic marine heatwaves. In winter, the analysis of marine heatwaves poses unique challenges, with the long term freshening of the Arctic inducing a positive trend of 0.1°C per decade for the freezing point. Arctic marine heatwaves are expected to keep increasing in duration and intensity due to the increased trend of the primary process, the surface heat flux, and their dissipation by bottom heat flux provides a pathway for heat from the atmosphere to the Arctic subsurface water masses. Plain Language Summary: Extreme warm temperature anomalies in the ocean, called marine heatwaves are becoming more frequent, longer and more intense around the globe. In the Arctic Ocean, the complex links between sea ice, the atmosphere and the ocean could both amplify or hinder the impact of those events. Yet, understanding where the heat comes from and where it goes is critical to anticipate further changes in an already fast‐changing environment. To this aim, we calculate the sources and sinks of heat in a numerical model simulating the ocean and sea ice. Most marine heatwaves in the Arctic are generated by taking atmospheric heat up and disappear when this heat is transferred to the deeper ocean. In doing so, marine heatwaves provide a pathway for heat from the atmosphere to the subsurface ocean. Oceanic circulation can also propagate marine heatwaves along the continental shelf and toward the marginal ice zone. When ice melts, it shoals the upper layer of the ocean, concentrating the atmospheric heat and lengthening and intensifying marine heatwaves. The excess heat stored in the subsurface can be expected to resurface later in the season and will then delay the freezing of new sea ice. Key Points: A heat budget analysis is applied to a regional ice‐ocean model to determine the dominating drivers of marine heatwaves in the ArcticMarine heatwaves in the Arctic provide a pathway for heat from the atmosphere to the subsurface oceanSea ice melt shoals the surface mixed layer, and this prolongs and intensifies marine heatwaves [ABSTRACT FROM AUTHOR]