Mean circulation and transports in the South Atlantic Ocean: Combining model and drifter data

Abstract

Abstract. Numerical experiments with a medium-resolution primitive equation model of the South Atlantic mean circulation are described. The results from the standard model realization indicate that the model succeeds in representing the large-scale transport and circulation features. However, a comparison with a velocity field derived from surface drifter data reveals discrepancies of the modeled velocities from the observations in magnitude as well as direction of the flow field. In order to diminish the model deviations from the data, an attempt is made to couple the model to the observations through a simple data assimilation technique. The assimilated model succeeds in improving the subtropical gyre circulation. Only a minor effect on the basin-scale integrated quantities is observed. However, the density field may be deformed as a response to the assimilation of velocity data without simultaneously adapting a corresponding density structure. The influence of the disturbance of the density structure is most prominent at the edges of the observed data set, which does not cover the entire model domain, and is confined to the upper ocean and balanced above the thermocline. We calculated a meridional heat transport that is generally in accordance with estimates from other sources. The analysis of heat and salt fluxes suggests that the model features both the so-called "warm water path" and "cold water path" in closing the global thermohaline circulation. While heat is mainly imported in surface and thermocline waters with the Agulhas Current around South Africa, it is the Antarctic Intermediate Water that compensates for more than 50% of the salt loss by the outflowing North Atlantic Deep Water.

A diagnostic stabilized finite-element ocean circulation model

“…These figures lie well within the limits of the corresponding estimates found in literature (e.g. Reid, 1989;Stutser and Krauss, 1998). The model also captures well such delicate feature as the latitude of the off-shore separation of the Brazil Current at 40°S near the coast of South America.…”

An Ensemble Smoother with Error Estimates

“…It is interesting to compare the smoother results with what can be expected from other methods. A nudging technique has been use by Holland et al (1991), Cai (1994), Stutzer and Kraus (1998), Florenchie and Verron (1998), and Zhang and Marotzke (1999) in this area of the world ocean. Florenchie and Verron (1998) use an iterative procedure to obtain a time mean field that is consistent with the TOPEX/Poseidon altimeter data.…”

Adiabatically Correcting an Eddy-Permitting Model Using Large-Scale Hydrographic Data: Application to the Gulf Stream and the North Atlantic Current

“…We want to stress that the success of the method, especially in improving the watermass characteristics, is by no means trivial since the tracer equations are unchanged in the semiprognostic model; that is, no artificial sources or sinks of tracers are introduced (nudging; Sarmiento and Bryan 1982). Note that the method is also different from ''momentum nudging,'' as used by, for example, Woodgate and Killworth (1997) and Stutzer and Krauss (1998). In the semiprognostic method there are no artificial Newtonian relaxation terms added to the primitive equations.…”