Oceanic Ventilation in the Eastern North Atlantic

Abstract

Ventilation of the eastern North Atlantic by subpolar mode water is illustrated by recent hydrographic section and analyzed using a large-scale stationary thermocline model including a surface mixed layer. The distributions of potential vorticity along the hydrographic lines and the geostrophic transports across them confirm the circulation pattern of the mode water that was previously deduced from older data, that is, arrival from the west at intergyre latitudes and southward subduction of the lightest part at about 42°N. The model approach emphasizes the large-scale advective-thermodynarnic equilibrium, which controls the shape of the thick (>500 m) winter mixed layer observed in the eastern basin. Following recent theories, the process of ventilation is interpreted in terms of exchanges between the mixed layer and the stratified thermocline: Mode water is formed where thermocline water is entrained into a thick mixed layer and is subducted where mixed layer water is injected into the thermocline. The density ranges involved in these exchanges are correctly reproduced, as is the density distribution at the sea surface, which exhibits the observed northwest-southeast orientation of the isopyonals. The subduction rates show realistic magnitudes. yet are probably underestimated in the mode water subduction region. The inability of the stationary model to reproduce the real processes occurring there is pointed out. Despite this and a few other limitations inherent to the large-scale and purely advective characteristics of the model, its application to a real oceanic configuration demonstrates the capability of first-order dynamics to reproduce at basin scale the essential features of the ventilation mechanism.