Mode waters and subduction rates in a high-resolution South Atlantic Simulation

Water mass production and destruction in the subtropical South Atlantic gyre is studied. A high resolution numerical model is used to examine regional mode water formation and estimate the associated instantaneous and mean subduction rates. Primitive equation dynamics expressed in depth following (sigma) coordinates are employed. The main hydrographic and kinematic features of the South Atlantic are faithfully reproduced by the model. In particular, the principle current systems appear and the model exhibits a sequence of ventilated potential vorticity minima on density surfaces coinciding with those of observed South Atlantic mode waters. The formation sequence within the model of these mode waters is described. Net formation rates are estimated using a pseudo-Lagrangian method and by diagnosing the time history of subsurface water mass volumes. Maximum formation rates occur in the density bands of the mode waters. It is argued that the roots of the model mode waters are found along open ocean late winter outcrops, rather than in the waters entering the gyre from the Brazil Current/Malvinas Current Confluence region. Eddies generate interannual variability in mode water formation and precondition the waters in the outcrop regions for convection. On the other hand, the eddy kinetic energy of the Confluence region is too intense to permit a direct connection between deep convection cells in the western boundary current and those in the open South Atlantic that directly form mode water.