The thermodynamic processes attendant on the transfer of fluid between a surface mixed layer and a stratified thermocline beneath are discussed. For a parcel of fluid in the mixed layer to pass into the stratified thermocline—to subduct—it must be stratified by buoyancy input; this buoyancy can be supplied by local air–sea exchange and/or by lateral advective processes.
A series of experiments is described in which a mixed layer, coupled to an ideal-fluid thermocline, undergoes differing seasonal cycles: in one limit the mixed layer is held fixed in a steady, winter configuration; in the other the mixed layer is, more realistically, shallow over most of the year and deepens briefly in late winter. It is shown that the annual subduction rate Sann depends, to first order, only on late winter mixed layer properties. However the annual-mean air–sea buoyancy exchange is sensitive to the details of the seasonal cycle and becomes vanishingly small as the effective subduction period shortens. In this limit the buoyancy is provided through advective processes in the Ekman layer.
The authors conclude that in ocean models that do not explicitly represent a seasonal cycle it is necessary to parameterize the process through a prescription of the winter mixed layer density and depth. The buoyancy forcing diagnosed from such models must be interpreted as the combined contribution of the annual air–sea exchange and lateral advectivc processes in the summer Ekman layer.