Impact of mesoscale eddy parameterization on Arctic Atlantic Water circulation and heat transport in the eddy-permitting grey zone

Abstract. The Arctic Ocean is undergoing rapid change, yet many CMIP-type climate models struggle to accurately represent its circulation and water masses. A key feature of the system is the topographically controlled boundary currents that transport warm, saline Atlantic Water northward at intermediate depths into the Atlantic Water layer. An important process affecting these boundary currents is the lateral flux of heat and salt driven by mesoscale eddies. Because the deformation radius is relatively small in the Arctic Ocean, numerical simulations require kilometer-scale resolution to fully capture eddy dynamics. Most of the current climate models, however, operate in a non-eddying regime, relying on mesoscale eddy parameterizations – typically combining isopycnal diffusion (Redi) with eddy-induced advection (Gent & McWilliams, GM). As horizontal resolution increases, future models will shift from an eddy-parameterized to eddy-permitting regime, entering a grey zone where eddies are only partially resolved and the role of GM parameterization becomes less straightforward. This study investigates the use of GM parameterization in eddy-permitting models, focusing on its effect on the northward transport of Atlantic Water in the Nordic Seas and Arctic Ocean. We conduct realistic simulations where we vary GM diffusivity strength and test two different GM scalings. These experiments are compared with a high-resolution model-truth simulation and observational data. Consistent with earlier idealistic studies, we find that omitting the GM parameterization excludes important eddy–mean flow interactions, which in our case results in stronger meridional overturning, barotropic circulation, and northward transport. Conversely, applying GM results in weaker circulation and dampens resolved eddy fluxes, while the parameterized fluxes introduce biases, particularly in the temperature distribution in the Greenland Sea.