The subtropical recirculation of Mode Waters
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A “Mode Water” is a particular type of water mass characterized by its vertical homogeneity. There are two general varieties in the world ocean: Subtropical Mode Waters and Subpolar Mode Waters. The vertical homogeneity of a Mode Water is acquired by the process of deep vertical convection in winter. The low vertical density gradient imparted to the water column by convection persists laterally as the general circulation carries the Mode Water away from the formation zone. This lateral persistence is a consequence of the principle of conservation of potential vorticity. In this paper examples of Mode Waters from throughout the world ocean are illustrated using an approximation to the potential vorticity: the product of the Coriolis parameter and the hydrostatic stability. Relative vorticity is neglected. The Mode Waters are detectable to quite low latitudes within the subtropical gyres of the world ocean as pycnostads-layers of low vertical density gradient and therefore low potential vorticity. This subtropical influence occurs in two fashions: direct, via the interior anticyclonic recirculation of the subtropical gyres; and indirect, via equatorward flowing western boundary currents. The “thick” nature of Mode Waters is reflected in their large volume contribution to the Central Waters of the subtropical gyres of the world ocean. While the image of advection along isopycnals from the convective source regions is particularly vivid using potential vorticity, it is noted that mixing does take place. Following likely advection paths, potential vorticity increases, potential temperature and salinity change, oxygen decreases and there is some evidence that potential density increases. The last observation may indicate a nonisopycnal character for the mixing processes.