Mesoscale features dominating the Denmark Strait overflow and our plan to improve the representation of overflows in global models

Ocean currents affecting the global climate are sustained by cold and dense water that sinks in the North Atlantic Ocean. A large portion of this water overflows through Denmark Strait, the channel located between Greenland and Iceland. Mesoscale anomalies play a major role in controlling the amount of overflow water entering the global circulation. The observed mesoscale anomalies are well represented by regional general circulation models of O(1) km resolution. MITgcm numerical solutions show that the yearly mean southward volume flux of dense water is about 30% greater in the presence of these mesoscale features [1], and reveal a causal relationship between the overflow surges at the sill and cyclones controlling the overflow variability further downstream [2]. Global simulations at kilometric scale are computationally expensive. Therefore, the difficulty to resolve the mesoscale dynamics dominating overflows is a significant weakness of current global/climate models. Recent developments of the AGRIF nesting tool allow to embed in NEMO configurations multiple nests with refined grids and independent vertical coordinate systems. We are implementing two-way nests in a global eORCA12 configuration to asses their impact in key overflow regions, such as the Greenland-Scotland Ridge and Gibraltar Strait.