Thermocline Forced by Annual and Decadal Surface Temperature Variation
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A two-layer thermocline model is modified by adding an essentially passive mixed layer above it. The surface temperature variation is simulated by a moving outcrop line. It is found that, in contrast to a surface wind stress, a surface temperature variation causes strong variability in the ventilated zone through subducted water, while it affects the shadow zone little. Two types of buoyancy-forced solution are found. When the outcrop line moves slowly, the solutions are nonentrainment solutions. For these solutions, the surface heat flux is mainly balanced by the horizontal advection in the permanent thermocline. The mixed layer never entrains. The time-mean thermocline is close to the steady thermocline with the time-mean outcrop line. When the outcrop line moves southward rapidly during the cooling season, the solutions become entrainment solutions. Now, deep vertical convection must occur because the horizontal advection in the permanent thermocline is no longer strong enough to balance the surface cooling. The time-mean thermocline resembles the steady thermocline with the early spring mixed layer, as suggested by Stommel. The local variability in the permanent thermocline is most efficiently produced by decadal forcings.