Eddy-Mean Flow Interaction in an Idealized Poleward Eastern Boundary Current

Previous studies have shown that poleward currents along the eastern boundaries
of the oceans are driven by the large-scale equator-to-pole buoyancy gradient.
These currents appear at the surface (e.g. Leuween Current, the Iberian
Poleward Current) or at intermediate depths (e.g. the California Undercurrent).
The factors responsible for trapping these currents near the boundary are as of
yet uncertain. Previous studies have attributed the trapping to bottom slope or
unrealistically high diapycnal diffusion. In this study, a poleward buoyancy
forced current is simulated in an eddy-resolving model with a flat bottom and
low diapycnal diffusivity. The budgets of momentum, buoyancy and potential
vorticity are analyzed in the residual-mean framework to determine the role
played by eddies in boundary trapping.

An isopycnal coordinate model in an idealized domain, forced by restoring the
sea surface temperature to a predefined meridional profile, is integrated to a
statistically steady state. A narrow poleward eastern boundary current at the
surface and an equatorward undercurrent at intermediate depths are observed. The
width of these currents scales as the Rossby deformation radius. The surface
current is fed by a slow zonal inflow arising from the large-scale meridional
pressure gradient.

This narrow boundary current, which transports warm water northward, impinges on
successively colder outcropping isopycnal surfaces. Water mass transformation
due to interior mixing and surface forcing is small, so the northward buoyancy
flux in the current is balanced by the shedding of anticyclonic warm core
eddies. As a result, the zonal residual-mean velocity is westward (reversed
relative to the Eulerian mean flow) within the top 200m from the surface where
these eddies are at their strongest.

The primary meridional momentum balance in the interior is a balance between the
gradient of Montgomery potential (the pressure gradient in isopycnal
coordinates), eddy form drag and the Coriolis force. Near the boundary, the form
drag as well as the Coriolis force vanish, but the gradient of Montgomery
potential does not. The difference is made up by the divergence of Reynolds
stress, which acts to decelerate the meridional velocity. The core of the
meridional boundary current coincides with the maximum zonal gradient of the
from drag, which ramps up away from the boundary as the eddies get free of the
boundary inhibition.