The Influence of Eddies on Tracer Transport in the Abyssal Ocean

Abstract

Tracer transport mechanisms in a deep western boundary current (DWBC) are explored using a three-layer, eddy-resolving quasigeostrophic model. The model is forced with a steady, sinusoidal wind stress. The effects of the wind stress, coupled with instabilities, create an unsteady, zonal surface jet at the boundary between the model subtropical and subpolar gyres. The bottom layer includes a DWBC created by specifying a mass inflow at the northern boundary and a mass outflow at the southern boundary. A numerical tracer is introduced into the DWBC with the mass source at the northern boundary. The surface jet creates an effective boundary to tracer transport in the DWBC. This barrier to meridional tracer transport causes approximately 70% of the tracer moved out of the northern part of the DWBC to be transported zonally into the interior with only 30% continuing southward in the DWBC. In model runs with a flat bottom, approximately 20% of the total meridional tracer flux is due to the eddy field. When the model is run with sloping topography along the western boundary, only 2% of the meridional tracer flux is due to the eddy field. The dependence of zonal tracer flux on eddies is highly latitude dependent, and in the region of interest (far from the northern boundary) the mean field is responsible for most of the zonal tracer flux. Eddies play a role in tracer flux in two ways: 1) the direct influence of the eddies in transporting tracer and 2) the effect of the eddies in driving the mean flow. A comparison between eddy-resolving runs and those with eddy diffusivity to parameterize the eddies suggests that the role of the eddy field in driving the mean flow field is of primary importance.