Dynamics of Deep Ocean Eastern Boundary Current

Abstract:

Poleward ocean currents between two and four kilometers along the eastern boundaries of the South Atlantic, Pacific and Indian Oceans have been repeatedly observed. These deep eastern boundary currents (DEBCs) are important branches of the meridional overturning circulation, with considerable contributions 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 regional 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 ranges. A southeast Atlantic configuration also includes such a DEBC, again consistent with observations. The vorticity budget of the simulated DEBCs in the southeast Pacific and Atlantic Oceans is an interior-like balance over most of the width of the DEBC, with strong compensating signals of stretching and friction found very close to the eastern boundary.

Idealized MITgcm configurations with simplified bathymetry profiles for both southeast Pacific and Atlantic Oceans, and a Gaussian-shaped inflow/outflow boundary condition from the north and south, produce DEBCs with a similar vorticity budget. An idealized semi-analytical 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-like vorticity balance with vertical velocity induced by horizontal temperature mixing. 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 vorticity model is forced with a trench or a slope bathymetry, and gives rise to a poleward DEBC with the same dominant balance in the vorticity budget as the MITgcm. The simple model also shows that both stratification and bathymetry are important to trigger the vertical stretching term that dominates the vorticity balance in the MITgcm simulations.