Mediterranean Overflow Water (MOW) simulation using a coupled multiple‐grid Mediterranean Sea/North Atlantic Ocean model

Abstract

A z‐level, 4th‐order‐accurate ocean model is applied in six two‐way‐coupled grids spanning the Mediterranean Sea and North Atlantic Ocean (MEDiNA). Resolutions vary from 1/4° in central North Atlantic to 1/24° in Strait of Gibraltar region. This allows the MEDiNA model to efficiently resolve small features (e.g., Strait of Gibraltar) in a multibasin, multiscale model. Such small features affect all scales because of nonlinearity and low dissipation. The grid coupling using one coarse grid overlap is nearly seamless without intergrid sponge layers. No instant convective adjustment or other highly diffusive process is used. The deep water in the 1/8° Mediterranean Sea grid is formed by the resolved flows that emulate subgrid‐scale processes directly. Downslope migration of Mediterranean Overflow Water (MOW) water involves dense water flowing away from the bottom laterally over bottom stairsteps in the z‐level model, thus flowing over less dense underlying water. Without excessively water mass‐diluting process, the advection dominates the downslope migration of thin, dense MOW in the simulation. The model results show realistic MOW migration to the observed equilibrium depth, followed by lateral spreading near that depth. The results are also consistent with the climatology along 43°N, where the MOW hugs a steep shelfslope centered at ∼1 km depth and then spreads westward, with the salinity core (S > 35.7) reaching 18°W. This study clearly restores z‐level models to a competitive status doing density current simulations.