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A fourth-order, fully two-way-coupled dual-domain Pacific Ocean Model (DUPOM) is developed to simulate the regional circulation in Asian Marginal Seas based on the whole North Pacific Ocean simulation framework. The model uses 1/4° and 1/8° horizontal resolutions for the eastern and western Pacific Ocean domains, respectively. The fully two-way-coupled approach shows seamless coupling behavior without any sponge layer. We validate the Kuroshio Current System and the regional circulation in the Asian Marginal Sea, including circulation patterns, volume transport variation and vertical current structure across several cross-sections. A single strong current core exists east of Taiwan and extends to the East China Sea. A cold dome near the northeast of Taiwan is a semi-permanent feature in the model results as observed. South of Japan (e.g. PCM5 line), the Kuroshio shows strong transport along the coast with a countercurrent offshore, reflecting a recirculation gyre. The transient of the three typical paths of the Kuroshio meander is also studied. Our results suggest that the path variation, particularly the Large Meander path, may be triggered by the baroclinic instability associated with westward propagating eddies.

The conveyor of the source water that feeds into the Taiwan Strait (TS), particularly from the south, is investigated using historical CTD (Conductivity-Temperature-Depth) data and a North Pacific Ocean and East Asian seas coupled model. The modeled currents and drifter trajectories suggest that the Kuroshio Branch Water (KBW) rarely flows directly into the TS from the Luzon Strait (LS) in winter; instead the massive westward movement of the Kuroshio conveys high salinity water to the southeastern TS through a loop-like route. In summer, the modeled flow fields suggest that the Kuroshio surface currents hardly intrude into the TS directly from the LS. Observations and model results show that the monsoon-driven northeastwardflowing currents in the northern South China Sea transport relatively low salinity water through the entire TS.

The DieCAST ocean model is applied to a study of the circulation in the Black Sea, using 1/12° horizontal resolution and with 20 vertical layers. Boundary forcings are monthly wind stress, evaporation minus precipitation, air–sea heat flux, freshwater influx from 11 rivers and exchange with the Mediterranean Sea through the Bosphorus Strait. The model reproduces fundamental physical features of the Black Sea: seasonal fluctuations in the quasi-permanent cyclonic Rim Current, numerous anticyclonic meanders and eddies lying between the Rim Current and the coast, Rossby waves propagating westward across the basin, coastally trapped waves, and the annual cycle of vertical mixing. Model results shed light on the mechanisms affecting such features. These include interactions of the Rim Current with coastal bathymetry abutments, leading to recirculations that pinch off vortices as in island wakes, and possible baroclinic instability of the Rim Current; these are modulated by the large annual stratification cycle above a relatively shallow and strong pycnocline, as is the Rim Current itself. The resulting wake eddies often merge into major coastal circulation features such as the seasonal Batumi and Sevastopol eddies. These anticyclonic eddies play a fundamental role in coastal and open-sea exchange processes. Hydrographic data from sampling cruises and recent Topex–Poseiden (T/P) altimeter data strongly supports our analysis.

Low-pass-filtered velocities obtained from surface drifters and surface geostrophic velocities estimated from TOPEX/Poseidon altimeter data have recently revealed a clear and robust seasonal cycle in the surface eddy kinetic energy (EKE) in the California Current (CC) [Kelly et al., 1998; Strub and James, 2000]. The seasonal cycle begins in spring when a surface-intensified baroclinic equatorward jet develops next to the coast in response to strong upwelling favorable winds. This jet, and a developing eddy field, then moves offshore during summer and fall. The EKE maximum associated with the jet progresses only as far as 127°W, beyond which it decreases rapidly. This is a robust characteristic of the seasonal cycle that has been previously attributed only to an unspecified dissipation process. To investigate this aspect of the surface EKE, a multiyear simulation of the CC is carried out using the Dietrich/Center for Air-Sea Technology primitive equation regional ocean model [Dietrich, 1997]. The simulation accurately reproduces many aspects of the observed annual cycle, including the offshore propagation of the EKE at the surface. The model results indicate that the decrease of surface EKE west of 127°W in the simulation is not due to dissipation but rather is caused by the vertical redistribution of EKE to the deep ocean. This redistribution occurs through the transformation of kinetic energy from the vertical shear flow to the vertical mean flow. The transformation is a nonlinear process inherently associated with the life cycle of baroclinically unstable waves, and in the CC, it effectively energizes the deeper ocean at the expense of the upper ocean. The process is also known to be important in the atmosphere [Wiin-Nielsen, 1962]. Taken together, the recent California Current observations and the new model results strongly suggest that the CC regularly supplies EKE to the deep waters of the eastern North Pacific.

The results of an intercomparison experiment performed with five numerical ocean models of different architecture are presented. While all models are able to simulate the large-scale characteristics of the North Atlantic circulation with a fair degree of realism, they also exhibit differences that can be attributed to the choices made in vertical coordinates, domain size, and boundary conditions.

An ocean response to typhoon Kai-Tak is simulated using an accurate fourth-order, basin-scale ocean model. The surface winds of typhoon Kai-Tak were obtained from QuikSCAT satellite images blended with the ECMWF wind fields. An intense nonlinear mesoscale eddy is generated in the northeast South China Sea (SCS) with a Rossby number of O (1) and on a 50-100 km horizontal scale. Inertial oscillation is clearly observed. Advection dominates as a strong wind shear drives the mixed layer flows outward, away from the typhoon center, thus forcing upwelling from deep levels with a high upwelling velocity (＞ 30m day^(-1)). A drop in sea surface temperature (SST) of more than 9℃ is found in both observation and simulation. We attribute this significant SST drop to the influence of the slow moving typhoon, initial stratification and bathymetry-induced upwelling in the northeast of the SCS where the typhoon hovered.

[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.

In this paper, we present the results from a 1/8° horizontal resolution numerical simulation of the Mediterranean Sea using an ocean model (DieCAST) that is stable with low general dissipation and that uses accurate control volume fourth-order numerics with reduced numerical dispersion. The ocean model is forced using climatological monthly mean winds and relaxation towards monthly climatological surface temperature and salinity. The variability of the circulation obtained is assessed by computing the volume transport through certain sections and straits where comparison with observations is possible. The seasonal variability of certain currents is reproduced in the model simulations. More important, an interannual variability, manifested by changes in currents and water mass properties, is also found in the results. This may indicate that the oceanic internal variability (not depending on external atmospheric forcing), is an important component of the total variability of the Mediterranean circulation; variability that seems to be very significant and well documented by in situ and satellite data recovered in the Mediterranean Sea during the last decade. © 2005 Elsevier Ltd. All rights reserved., This study is a contribution to the Projects MAR1998-0840 and REN2001-3982. Partial support from SOFT, EC funded project, EVK3-2000-00561 is also acknowledged. V. Fernández acknowledges a FPI grant from the Spanish Ministry for Science and Technology (PN1998 36125588). David E. Dietrich acknowledges a grant from the Spanish ministry for Education and Culture (SAB1999-0037)

[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.

[1] Numerical experiments varying lateral viscosity and diffusivity between 20 and 150 m2/s in a North Atlantic Ocean (NAO) model having 4th-order accurate numerics, in which the dense deep current system (DCS) from the northern seas and Arctic Ocean is simulated directly show that Gulf Stream (GS) separation is strongly affected by the dissipation of the DCS. This is true even though the separation is highly inertial with large Reynolds number for GS separation flow scales. We show that realistic NAO modeling requires less than 150 m2/s viscosity and diffusivity in order to maintain the DCS material current with enough intensity to get realistic GS separation near Cape Hatteras (CH). This also demands accurate, low dissipation numerics, because of the long transit time (1–10 years) of DCS material from its northern seas and Arctic Ocean source regions to the Cape Hatteras region and the small lateral and vertical scales of DCS.

A new model-based method of determining the surface fluxes of heat and freshwater that are needed to force ocean models is presented. In contrast to deriving the fluxes from a simulation with a restoring surface boundary condition, the new method determines the fluxes as a residual within the framework of physically realistic and natural boundary conditions on the sea surface temperature (SST) and sea surface salinity (SSS). The fluxes are computed (diagnosed) in such a way that an ensemble average of the model-simulated annual cycles of SST and SSS match the observed climatological annual cycles of SST and SSS, respectively. The surface boundary condition on the SST implicitly includes a net radiative flux (diagnosed) and a physically realistic heat exchange with the atmosphere (restoring flux), while the boundary condition on the SSS is the real freshwater flux (diagnosed) as proposed by Huang (J. Phys. Oceanogr., 33 (1993) 2428). Apart from being based on physically realistic surface boundary conditions, the advantage of the method is that it results in a realistic model simulation of the observed annual cycle of SST and SSS with no artificial damping of surface watermass fronts. The resulting heat fluxes and freshwater sources are realistic if the observed climatological data and model internal physics are accurate. The performance of the method is demonstrated using the DieCAST ocean model adapted to the Mediterranean Sea where the obtained model fluxes are compared with observations. © 2004 Elsevier B.V. All rights reserved., We acknowledge MCyT and FEDER support from project IMAGEN REN2001-0802-C02-01/MAR. D. E. Dietrich acknowledges a grant from the Spanish ministry for Education and Culture (SAB1999-0037). V. Fernández acknowledges a FPI grant from the Spanish Ministry for Science and Technology (PN1998 36125588)

To improve coastal and semi-enclosed seas modeling, a new DieCAST ocean Model has been developed. In application to the Gulf of Mexico (GOM), this model gives remarkably similar results to that obtained from the Sandia Ocean Model System (SOMS) and at a computational speed much faster than SOMS or other ocean models presently available. The results are supported by many GOM observations including GEOSAT rms sea-surface-height anomaly data; mean thermocline and empirical orthogonal functions; the evolution of vortex pairs in the western GOM; major Loop Current eddy shedding period; shed eddy size, phase speed, dispersion rate and vorticity; and detailed Loop Current and shed eddy structure. Finally, and a major advantage of DieCAST is its capability to be easily relocated to any geographical region of the globe. Preliminary applications in several regions such as the Mediterranean, South China Sea, Great Lakes, and Labrador Current are described.... DieCAST, Model, Relocatable, CAST, Ocean, Application, Evaluation, Coastal