Interior Transport Research Articles

Cross-Gyre Transports

What is the fate of surface Ekman transport entering a subtropical gyre through its zonal boundaries? This question is investigated by resolving interior transport of a deep surface layer into nonvortical (potential flow) and nondivergent (solenoidal) components. For an idealized 1½-layer model driven by zonal wind, explicit analytical solutions are readily found separately for the transport potential and the streamfunction of the solenoidal transport component. These, together with classical results on Sverdrup transport and the pressure field, reveal transport balances that the classical solutions tend to conceal. Application to the North Atlantic shows the necessity of massive cross-gyre transport by the western boundary current from the tropical region into the subtropical gyre, as well as from the latter to the subpolar gyre. These cross-gyre transports constitute a major conduit for the poleward flow of warm (25°C) surface water.

Baroclinic Flow and Transient-Tracer Fields in the Canary–Cape Verde Basin

Simulated transient-tracer distributions (tritium, 3H3, freons) on the isopycnal horizons σ0=26.5 and 26.8 kg m−3 are presented for the East Atlantic, 10° −40°N. Tracer transport is modeled by employing a baroclinic flow field based on empirical data in a kinematic isopycnal advection-diffusion numerical model, in which winter convection is taken as the mechanism of communication with the ocean surface layer, and the isopycnal diffusivity is a free parameter. Diapucnic transport is ignored. The simulations employ time-dependent tracer boundary conditions, which are constructed on the basis of available observations. Simulations are compared to data obtained on a meridional section in 1981 (F/S Meteor, cruise 56/5). Best simulations were obtained by means of a subjective optimization procedure. On both levels, the observed distributions and the best simulated distributions agree well. The fact that the surface boundary conditions and interior distributions of the tracers are distinctly different leads us to the conclusion that our model provides a consistent description of upper main-thermocline ventilation and interior transport Surface-water densities in February are found to represent adequately the winter outcrop boundaries with an uncertainty of about ±300 km across. The required isopycnal diffusivity south of 29°N is 1700 m2 s−1, and 2900 m2 s−1 further north (+70/−40%). Interior transport is found to be predominantly advective. Advective ventilation across 30.5°N east of 33°W amounts to only 12% and 40% for the 26.5 and 26.8 horizons of the total ventilation rates reported by Sarmiento. The North Atlantic/South Atlantic Central Water boundary near 15°N is found to be predominantly determined by advection.

The General Circulation in a Dissipative Ocean Basin with Longshore Wind Stresses

Solutions of the Stommel equation are presented which take the form of free waves in the interior of the ocean basin, driven by convergences and divergences in coastal transports brought about by the variation of the longshore wind stress around the coast. These waves have been termed “coastal waves” and result from the beta-effect in the presence of a uniform frictional process, such as the loss of momentum by the ocean to the atmosphere. The coastal waves which typically transport 5 Sv extend significantly into the interior of the ocean from all boundaries except the western boundary, and also drive a westward nonlinear current, appear to be an important feature in the general circulation. A good example of a quasi-steady wavefield induced by intermediate-scale coastline geography occurs in the Flinders Current off the south coast of Australia. The western boundary current, of course, compensates for imbalances in interior transport. Its structure results from forcing, both by this transport and the longshore wind stress on the western coast itself, which produces no net transport outside of the boundary layer.

Seasonal Heat Transport in a Forced Equatorial Baroclinic Model

Seasonal heat transport is examined in a simple, linear, shallow-water model on the equatorial beta plane. It is found in this model that meridional transport by the seasonally varying western boundary current is of the same magnitude but opposite phase to the seasonally varying interior transport and therefore tends to cancel.

Lateral friction in the oceans as an effect of potential vorticity mixing

Abstract Applying a mixing-length calculation to potential vorticity rather than to momentum a new type of lateral friction appears in the oceanic mass transport equations. This friction is evaluated for the special case of horizontally homogeneous, quasi-geostrophic turbulence. The main effect is a westward force arising from the so-called β-term. This produces an additional southward interior transport and a strengthening of the western boundary current. A turbulent exchange coefficient KH = 108 em2s−1 is sufficient to give a Gulf Stream transport twice that obtained by the classical Sverdrup model.

Horizontal transport in a rotating basin of parabolic depth profile

Horizontal transports in a rotating parabolic basin are found for three elementary steady stress fields. For a uniform stress, the circulation consists of two vortices so arranged that the inshore transport is more or less in the direction of the surface stress, with a return flow in the interior. For a stress with constant curl, the interior transport is a large symmetric vortex, with possibly a weak countercurrent near the shore; for a stress with constant divergence, a weak vortex occurs in the interior, with weak countercurrents occurring near shore. For a combined surface stress, the transport in the interior is dominated by the curl component; near shore the uniform stress component may be equally important.