The Origin of Deep Zonal Flows in the Brazil Basin

Abstract

Recent data from a deployment of Lagrangian floats in the Brazil Basin of the South Atlantic reveal a swift western boundary current and predominantly zonal flow in the interior at a depth of about 2500 m. Dynamical mechanisms for the deep interior flow are considered using two high-resolution models, a global and a regional one, together with a suite of sensitivity studies at low resolution. Outside the western boundary region, model energy levels are similar to observations. The models are able to reproduce, at somewhat reduced strength depending on resolution, much of the meridional structure of the observed deep zonal flows. Several candidates for generating such flows are examined, including nonlinear rectification, baroclinic instability, and thermohaline and wind forcing. A primary mechanism for the deep flow in the models is the response to the wind stress, as recently argued to be the case for a model of the Pacific Ocean. However, thermohaline forcing is significant, especially where density contrasts between basins generate strong currents in deep passages. The deep thermohaline flow appears to be linked to the depth of the midocean ridge. Baroclinic instability of the mean meridional flow, which is alone capable of generating nearly zonal currents of the observed scale, is a possible additional forcing but is not essential in the models investigated here. The meridional scale of the zonal flows in the models is extremely dependent on the horizontal resolution and horizontal mixing.