Dynamics of western boundary currents in simple models of low-latitude circulations.

摘要

The dynamics of low-latitude western boundary currents and their connection to equatorial currents are studied in numerical models. In the first section, a western boundary current-equatorial current system is maintained in a 1{dollar}{lcub}1over 2{rcub}{dollar} layer model between a midlatitude mass source and a sponge layer along the eastern boundary. Potential vorticity analysis emphasizes that the dynamics of the connection region are similar to barotropic models of mid-latitude western boundary current extensions. Excess potential vorticity advected into the equatorial region by the western boundary current is dissipated by meandering and possibly eddy formation depending on the Reynolds number. Unstable cases radiate Yanai waves eastward. The effects of different boundary conditions (no-slip vs. free-slip) are also investigated.; The second section explores how the boundary's tilt away from meridional alignment affects the connection region. With a no-slip condition and a positive tilt, the current meanders. With a no-slip condition and a negative tilt or a free- slip condition and positive tilt, permanent gyres pairs elongate and intensify with increasing tilt angles up to {dollar}pi{dollar}/3. Meandering and recirculation are weak in the remaining case. Tilt also affects eddy variability. As the angle increases, eddy energy in the connection region increases with a no-slip boundary condition but decreases with a free-slip condition. In the latter case high eddy energy along the western boundary is due to a poleward-propagating train of eddies, whose properties are nearly independent of coastline tilt.; In the third section, a 2{dollar}{lcub}1over 2{rcub}{dollar} layer model is forced by wind stress and specified diapycnal velocities to form a subtropical cell, which consists of equatorial upwelling near the eastern boundary, poleward surface flow, subtropical subduction, and subsurface, equatorward flow in a western boundary current. When detrainment is symmetric, the zonal momentum balance along the equator is between the zonal gradient of kinetic energy density and horizontal momentum diffusion. Horizontal recirculation accounts for one-third of transport of the undercurrent. When the detrainment occurs in one hemisphere only, the undercurrent meanders in the western part of the basin, and is nearly linear in the eastern part. The model circulation and its equatorial momentum balance are contrasted with those associated with tropical detrainment.