Inferring the Subduction Rate and Period over the North Atlantic

Abstract

Abstract 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 ...