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[1] 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.

[1] This paper presents simulations of the Adriatic Sea using the DieCAST model applied on a 1.2–min grid (about 2–km resolution). The simulations resolve the mesoscale variability because the grid size falls below the first baroclinic deformation radius (about 5–10 km) and DieCAST has very low horizontal dissipation. The model is initialized with seasonally averaged temperature and salinity data and forced with climatological winds and surface buoyancy fluxes (both heat flux and evaporation minus precipitation). River discharges are varied daily according to a perpetual year for every river, and the open-boundary conditions at Otranto Strait are obtained by nesting in two larger-scale models. The present simulations demonstrate that the DieCAST model allows mesoscale instabilities to develop at length scales of 5–20 km and over time scales of a few days. The simulated variability exhibits pronounced similarities with the actual mesoscale variability, in terms of location, nature and temporal evolution of the features. Meanders, swirls and eddies are noted along the relatively smooth Italian coast while offshore jets and filaments better describe the mesoscale activity along the more rugged coast of Croatia. In sum, DieCAST is highly suitable for the study of mesoscale variability in the Adriatic Sea. The present simulations also show that the seasonal hydrography of the Adriatic Sea is intrinsically unstable to mesoscale perturbations, and that the mesoscale variability along the Italian coast is the result of baroclinic instability of the Western Adriatic Current. It is shown how the properties of this instability are related to the local bottom topography.

[1] Ocean circulation off the west coast of the United States is driven by a variety of mechanisms, the most important of which are the seasonally varying local wind stress and coastal irregularities. Remote forcing is also important and expressed through the open boundary conditions. We use a nonhydrostatic, Dietrich/Center for Air-Sea Technology (DieCAST) ocean model to simulate the regional circulation in the vicinity of Monterey Bay, California. Satellite images often show a cyclonic eddy in the bay and an anticyclonic eddy outside the bay during spring and summer. We compare the simulation results with observed mooring and HF radar-derived velocity data. The coastal geometry plays an important role in the generation and movement of coastal eddies. Quantitative comparisons between hydrostatic and nonhydrostatic models are made to investigate the importance of the nonhydrostatic effects in coastal ocean simulation. The results show that both the Sur Ridge area and the Monterey Submarine Canyon contribute significantly to nonhydrostatic effects and small-scale features. The strong nonhydrostatic and small-scale features result from the California Undercurrent flows across sloping bathymetry and interactions with near-surface California Current in summer. Rapid changes in slope in the presence of strong flows cause vertical acceleration, which violates the hydrostatic approximation. Surface-trapped nonhydrostatic fronts also occur frequently in the shallow ocean during winter. These effects have seasonal variation and cannot be ignored in coastal ocean modeling with complex bathymetry.

We model the eddy shedding phenomenon in the Gulf of Mexico using the Sandia Ocean Modeling System. In the first part of the study a parameter sensitivity study is performed using a rectangular basin two-level model. In particular, the sensitivity of the model eddy shedding to inflow/outflow conditions, horizontal resolution (80, 40, and 20 km), and reduced gravity is examined. The results are interpreted in terms of the quasi-geostrophic vertical motion equation and vorticity conservation. In the second part of the paper we include realistic coastlines and bathymetry in the model. A horizontal resolution of 20 km is used together with 16 vertical levels. Various observed features of the eddy shedding are simulated by the model. A boundary current which follows topographic contours is generated around the Gulf because of the splitting of the flows associated with the loop current and the shed eddy. This current is likely to be important in the dissipative stages of the eddies in the western Gulf. In addition, our results suggest that higher-order baroclinic modes are important in the dissipation of loop current eddies.