Middepth equatorial tracer tongues in a model of the Atlantic Ocean

Observational estimates of middepth tracer tongues in the equatorial Atlantic are reviewed and are compared with results from several eddy-resolving model simulations. Local maxima of chlorofluorocarbon (CFC) concentrations along the equator at around 1500 m depth are related to mean eastward jet structures in the models at similar depth ranges and can also be identified in several simulated tracer distributions. Similar to the observations, strong eastward jets are located in the simulations 1 degrees-2 degrees north and south of the equator. The model simulations show, in addition, consistent with the CFC observations, weaker jets at around 4 degrees-6 degrees N/S and 8 degrees-10 degrees N/S, suggestive of a large-scale alternating eastward/westward current system in the western tropical Atlantic in this depth range. Lagrangian transport estimates in the model using float diagnostics show a transport of 1-3 Sv in each of the eastward jets 1 degrees-2 degrees N/S off the equator compared to 3-12 Sv throughflow into the South Atlantic, with no seasonal cycle apparent in the transport fractioning. Comparing different model solutions reveals the choice of the subgrid-scale mixing parameterization as important for the amplitudes of the jets. Enhanced (reduced) diapycnal mixing is related to stronger (weaker) jets.