In 1939, Iselin noted that the properties of the ocean thermocline match those of the late-winter mixed layer, as opposed to a year-round average (Iselin, 1939) . Some 40 year later, Stommel (1979) suggested that this arises from the seasonality of the mixed layer depth, which effectively re-entrains water subducted at any time except late-winter. Stommel likened the process to that of "Maxwell's Demon" in thermodynamics, with the Demon here operating a trapdoor at the base of the mixed layer,

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... llowing access to the ocean interior only for water moving away fast enough to escape re-entrainment. Williams et al. (1995) subsequently used tracers in a numerical simulation to show that this process does indeed operate as Stommel described. The process, which has since become known as "Stommel's Demon," simplifies models of the ocean thermocline, whereby the late-winter mixed layer base can be adopted as the upper boundary and mixed layer seasonality effectively ignored (