Frontogenesis, subduction, and cross-front exchange at upper ocean fronts
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The subduction of water at upper ocean fronts driven by internal instabilities is investigated. A simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel. Parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation, while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation. A nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front. Frontogenesis generates vertical velocities of O(30 m d(-1)) and ageostrophic cross front flows of 3 to 5 cm s(-1). The subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front. The subducted parcels quickly develop a circulation through balanced adjustment, anticyclonic for deep surface layers and cyclonic for shallow surface layers. The characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices. The horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer. The total permanent subduction rate is estimated to be 20 m yr(-1) for frontal subduction driven by frontogenesis, although local (and temporary) subduction can be much larger. The subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices, thus forming a baroclinic dipole that propagates at approximately 3.5 cm s(-1) and is capable of transporting the anomalous water properties far from the region of formation. Similarities with observations in the North Atlantic Subtropical Convergence Zone and near the Gulf Stream are discussed.