The Dynamics of Deep Ocean Eastern Boundary Currents

The presence of poleward currents between two and four kilometers along the eastern boundaries of the South Atlantic/Pacific/Indian Oceans has been repeatedly revealed by observations. The deep eastern boundary currents (DEBCs) are important branches of the meridional overturning circulation, with considerable contribution to mass, heat, and tracer transports. Yet their dynamics have been a puzzle, and state-of-the-art models do not necessarily represent such currents satisfactorily without being constrained by observations.

We run realistic southeast Pacific configuration MITgcm simulations, forced by boundary conditions from SOSE (Southern Ocean State Estimate) and find a poleward DEBC at the observed depth and latitudinal range. A similar configuration for the southeast Atlantic also includes a concentrated poleward current near the eastern boundary, consistent with observations. The vorticity budget of the simulated DEBCs in both simulations is an interior balance (stretching v.s beta term) over most of the longitudinal range of the DEBC, with strong compensating signals of stretching and friction very close to the eastern boundary, over a limited longitudinal range of the flow.

Idealized MITgcm configurations with simplified bathymetry profiles for both southeast Pacific and Atlantic Oceans, and a Gaussian-shaped inflow/outflow boundary condition in the north and south, produce DEBCs with a similar vorticity budget. An idealized semi-analytic vorticity model is developed to show that there are solutions that decay away from both the western and eastern boundaries. But the two solutions decaying from the eastern boundaries differ from the classic western boundary solutions in that one of the decaying scales is much larger than the other, implying that DEBCs are mostly in interior vorticity balance with vertical velocity induced by horizontal heat diffusion. The short scale signal is only seen very close to the eastern boundary and serves to satisfy the no parallel-flow boundary condition. A vertically-integrated version of the simple model is forced with a trench or a slope, and produces a poleward DEBC with the same vorticity budget as the GCM. The simple model also shows that both stratification and bathymetry are important to trigger the vorticity balance structure observed in GCM simulations.