The shallow winter overturning circulation in the Red Sea is studied using a 50 year high-resolution MITgcm (MIT general circulation model) simulation with realistic atmospheric forcing. The overturning circulation for a typical year, represented by 1980, and the climatological mean are analyzed using model output to delineate the three-dimensional structure and to investigate the underlying dynamical mechanisms. The horizontal model circulation in the winter of 1980 is dominated by energetic eddies. The climatological model mean results suggest that the surface inflow intensifies in a western boundary current in the southern Red Sea that switches to an eastern boundary current north of 24°N. The overturning is accomplished through a cyclonic recirculation and a cross-basin overturning circulation in the northern Red Sea, with major sinking occurring along a narrow band of width about 20 km along the eastern boundary and weaker upwelling along the western boundary. The northward pressure gradient force, strong vertical mixing, and horizontal mixing near the boundary are the essential dynamical components in the model's winter overturning circulation. The simulated water exchange is not hydraulically controlled in the Strait of Bab el Mandeb; instead, the exchange is limited by bottom and lateral boundary friction and, to a lesser extent, by interfacial friction due to the vertical viscosity at the interface between the inflow and the outflow.

Recent measurements over the sill in the Yucatan Channel indicate that the deepest flows between the Caribbean Sea and the Gulf of Mexico, those that take place below the sill level at the Florida Straits, have zero mean net mass transport but carry significant amounts of heat and oxygen. The heat flux associated with the mean exchange exports approximately 150 GW from the deep Gulf toward the Caribbean and may be related to the formation of the Yucatan Undercurrent. The eddy heat transfer is also significantly different from zero and exports on average an additional 60 GW. This eddy transfer is attributable mostly to events that last from a few days to about 1.5 months, during which colder water from deeper levels in the Caribbean (beneath 2000 m) flows over the sill within a bottom boundary layer close to 200 m thick. The colder water is also very rich in oxygen, and the deep exchange sustains the near-bottom oxygen maximum in the Gulf of Mexico, whence that cold water must slide down the northern slope of the Yucatan Sill. Estimates of oxygen transport by diffusion from the deep water into the overlying intermediate water (∼50 m3 s−1) and the oxygen consumption reported in the literature (∼100 m3 s−1) are balanced by the rates of mean and eddy transfers over the sill (∼150 m3 s−1). The near-bottom mass transport [∼0.32 Sv (1 Sv ≡ 106 m3 s−1)] measured across the deepest portion of the central Yucatan Channel suggests a residence time for the deep waters of the Gulf of about 250 yr.

Regional ocean circulation along the Oregon coast is studied numerically for forcing fields derived from year 2005 and climatological-mean conditions. The primary object is to study directly the Lagrangian pathways by which fluid arrives in the Oregon upwelling zone. Roughly half of the upwelling fluid is found to arrive in the regional domain from alongshelf source points, primarily north but also south of the Oregon upwelling zone, while the other half ultimately arrives from points offshore and west of the zone. For both the year 2005 and the climatological simulations, different regimes of dominant alongshelf source waters are found, with Cape Blanco being a dividing point for northern versus southern sources, and with the water parcels coming primarily from depths below 100 m. For the offshore sources, most upwelling fluid originates from depths between 150 and 250 m during 2005, but from within the upper 150 m for the climatological simulations. In both cases there are specific regions along the shelf of enhanced vertical motion, which appear to be associated with topographic features such as submarine banks and canyons. A perhaps surprising result is the apparently small role played by the poleward undercurrent as a direct, immediate source of upwelling fluid; on the seasonal time scale considered here, most trajectories are found to move southward over the slope and shelf, with weaker northward motion only farther offshore, in the deep interior. In 2005 the water parcels cover distances longer than in the climatology, and the meridional exchange along their paths is more vigorous. These differences are likely associated in part with the presence of mesoscale vortical structures seaward of the shelf, which are largely absent in the climatological simulation, perhaps because their formation may depend on the large-amplitude, short-time-scale wind variations and reversals that occur in the year 2005 wind field but not in the multiyear mean climatological wind field.