Shallow Pycnoclines and Mode Water Subduction in the Eastern North Atlantic.
Résumé
A Lagrangian one-dimensional model is used to study the subduction of Subpolar Mode Water in the eastern North Atlantic and to analyze recently observed hydrological features related to this process. Considering a southward moving column of mode water, subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget. The seasonal pycnocline on top of the column can no longer be completely eroded in winter, and it is shown how its remnant forms a shallow “secondary” pycnocline at about 200 m depth, which isolates the lower part of the fluid column from the atmosphere. This mechanism for the subduction of Subpolar Mode Water induces a strong meridional gradient in the winter mixed layer depth. The various components of the buoyancy input are thoroughly studied. Horizontal advection in the shallow Ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column, with magnitudes comparable to the air–sea exchanges. The processes that control the temperature–salinity relation of the model fluid column are studied. It appears that if horizontal baroclinic advection can be held as negligible, a good correlation between the annual inputs of heat and salt to the winter mixed layer has to exist to account for the quasi-linear T–S relation prevailing in the Central Water. Vertical mixing by turbulent processes, including double diffusion, is seen to cause only a limited rearrangement of the relation defined at the surface. Finally, comparing the location of the different types of mode water in the subtropical North Atlantic, that of a climatological line of zero buoyancy flux assumed to drive subduction, and the general circulation pattern in the upper layers, shows a good consistency and supports the conclusions of this study.