Gulf Stream impacts of regional resolution refinement in the E3SM unstructured-mesh ocean model MPAS-Ocean

What are the advantages and limitations of adding refined coastal resolution within a half-degree ocean model mesh? Using the Energy Exascale Earth System Model’s (E3SM) unstructured ocean and sea ice models, we compare results from three different meshes: (1) base case global low-resolution 30-60 km mesh, (2) goal case global high-resolution 6-18 km mesh, and (3) experimental coastal-refined mesh with 8 km resolution within 400 km from the North American coastline and low-resolution everywhere else. Our analysis focuses on improving low-resolution biases in the Gulf Stream. Gulf Stream transport is too weak in both the low-resolution and the coastal-refined runs, but our findings indicate that coastal-refined results can likely be improved. Wind-driven transport over the coarse region of the North Atlantic basin in the coastal-refined model agrees well with the high-resolution model. Instead, suppressed deepwater formation in the North Atlantic produces a weak deep western boundary current which explains the missing Gulf Stream transport. This was a surprising result since the Labrador Sea deepwater formation region is within the refined coastal resolution, but we are investigating changes to the model configuration which may increase deepwater formation (and as a result, Gulf Stream strength) at little to no additional computational cost.