Abstract
In a subtropical gyre, the convergent Ekman layer forces water downward into the geostrophic flow below. The properties and depth of the mixed layer vary considerably during the course of the year, but this variability does not penetrate into the geostrophic region. Evidently there is some process at work that selects only late winter water for actual net downward pumping. It is a process much like that performed by Maxwell's Demon. Arguing from a particular example in a North Atlantic region where Sverdrup dynamics is presumed to prevail, an elementary description is given as to how the Demon works and a sample set of profiles by season is computed. Present day mixed layer models involve a local vertical mixing process. Thermocline theories involve adiabatic flow from high latitudes. There is an intermediate depth range at which the two processes operate alternatively, according to season.
Keywords: subtropical gyre, Ekman pumping, water mass origins
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