Seasonal impact of submesoscale eddies on the ocean heat budget near the sea ice edge

Abstract. Oceanic submesoscale mixed layer eddies (SMLEs), with horizontal scales of 0.1–10 km, are not captured in climate models. SMLEs energized in the marginal ice zone (MIZ) have been shown to be of importance to sea ice melt rates in summer and to sea ice transport through notably a dynamical coupling with sea ice. Here our focus is on the thermodynamical coupling, which has received comparatively little attention. We aim to quantify, for the first time, the impact of eddies on both sea ice and the heat budget in the MIZ, contrasting different seasons and different background stratifications. To this end, we set up SMLE-resolving simulations of the ocean mixed layer (ML) near the ice edge using the MITgcm, representing a lead or the MIZ. We isolate the effect of eddies by comparing 3D simulations with eddies to 2D (latitude-depth) simulations without eddies. In summer (i.e., melting conditions) and regardless of the background stratification, SMLEs act as a heat pump from the atmosphere over the open ocean to the sea ice. On average over a season, SMLEs triple the meridional heat transport to the ice covered region, increase melting over their meridional extent, and trigger a positive radiative feedback by increasing shortwave absorption over the thinner ice. These changes are in the range 20–60 % for reasonable choices of shortwave forcing and initial ice thickness. In winter (i.e., freezing conditions), SMLEs have a relatively small impact on sea ice growth due to compensation between vertical and horizontal eddy heat transports. However, they reduce ML deepening by 80/50 % in the open/ice-covered ocean. Overall, our results reveal up to order one impacts of SMLEs on the heat and sea ice budgets in the MIZ, which will require the development of a SMLE parameterization tailored for polar regions.