Measuring the tail of Stommel’s demon

Seminar by Francois Primeau from University of California, Irvine

17 September 2019
10:00 - 11:00

The Seminar is being held in Room 1010 (Jasmin) - Intergrated mechanical engineering building. Click here for the campus map.

 It was noted long ago that the T-S properties of the main thermocline match those of the winter mixed layer rather than those of the annual mean state (Islin, 1939). Using kinematic arguments, Stommel (1979) illustrated how the subduction process in the presence of the seasonal migration of isopycnal outcrops leads to a preferential transfer of end-of-winter mixed layer properties into the main thermocline.á By analogy with Maxwell’s demon in thermodynamics, he suggested that a “demon” operates to select which T-S properties are allowed to pass from the mixed layer into the permanent pycnocline. It was later shown that eddies tend to spread the timing of subduction, but that Stommel’s mixed-layer demon still operates to impart a seasonal bias to the water mass properties of the main thermocline (Williams, 1995).á Here we use Green function methods applied to a climatological ocean state estimate to quantify the action of Stommel’s demon in the presence of eddy-diffusivity. á We show how the Green function for the adjoint circulation can be used to quantify where and during which month water in the interior of the ocean was last in the surface mixed layer.á Because of eddy-diffusion the Green function of a global ocean model has a very long tail and one might expect that lengthy tracer simulations would be needed to compute it.á However, áwe show how the moments of a cyclo-stationary Green function can be computed efficiently without expensive transient tracer simulations. We use the new computational framework and the data-constrained ocean state estimate to effectively measure the diffusive tail of Stommel’s demon.