Constraining the North Atlantic circulation between 4.5 S and 39.5 N with transient tracer observations

[1] A 1 horizontal resolution North Atlantic general circulation model (GCM) in the latitude band 4.5 S to 39.5 N is compared to, and then combined with, chlorofluorocarbon and tritium transient tracer data. The method of Lagrange multipliers (adjoint) is used. Tracer distribution within the model ocean interior is primarily sensitive to the flux through the open northern boundary. This flux is determined by the estimation method and is found to deviate from the initial estimate significantly for all tracers. An attempt to carry the model domain to 78.5 N showed the GCM inadequacy, believed to be primarily a problem of spatial resolution, to compute the convective injection of tracer into the deep ocean. Any such errors persist throughout the tracer integration and corrupt all further tracer concentrations. In general, and consistent with earlier much simpler computations, uncertainties in surface and northern boundary conditions dominate the computed tracer concentrations. The final results here are improved estimates of the three-dimensional time histories of the tritium and CFCs and their boundary conditions over the model domain, with very little information available to constrain the GCM itself in the latitude band.