The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate Sann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, Sann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer. Geostrophic velocity fields, computed from the Levitus climatology assuming a level of no motion at 2.5 km, are used; the vertical velocity at the base of the mixed layer is deduced from observed surface Ekman pumping velocities and linear vorticity balance. A plausible pattern of Sann is obtained with subduction rates over the subtropical gyre approaching 100 m/yr—twice the maximum rate of Ekman pumping.
The subduction period 𝒯eff is found by viewing subduction as a transformation process converting mixed-layer fluid into stratified thermocline fluid. The effective period is that period of time during the shallowing of the mixed layer in which sufficient buoyancy is delivered to permit irreversible transfer of fluid into the main thermocline at the rate Sann. Typically 𝒯eff is found to be 1 to 2 months over the major part of the subtropical gyre, rising to 4 months in the tropics.
Finally, the heat budget of a column of fluid, extending from the surface down to the base of the seasonal thermocline is discussed, following it over an annual cycle. We are able to relate the buoyancy delivered to the mixed layer during the subduction period to the annual-mean buoyancy forcing through the sea surface plus the warming due to the convergence of Ekman heat fluxes. The relative importance of surface fluxes (heat and freshwater) and Ekman fluxes in supplying buoyancy to support subduction is examined using the climatologist observations of Isemer and Hasse, Schmitt et al., and Levitus. The pumping down of fluid from the warm summer Ekman layer into the thermocline makes a crucial contribution and, over the subtropical gyre, is the dominant term in the thermodynamics of subduction.