Interactions of the northern and southern branches of the thermohaline circulation
McDermott, David A., 1962-
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Coarse resolution models are used to investigate the influence of Southern Hemisphere processes on the Northern branch of the thermohaline circulation. The link between the zonal wind stress at the latitude of Drake Passage and the production of deep water in the Northern Hemisphere is explored. A nearly linear response to wind stress at the tip of South America is seen in northern deep water production rates in both one- and two-basin configurations. Transient studies are conducted that illustrate the transmission of the wind generated signal from the Southern Hemisphere to the northern sinking region.Mixed-boundary condition experiments are conducted in a number of model configurations. Deep-decoupling oscillations are produced in a one-hemisphere configuration, but can be suppressed by representing the influence of AABW by restoring the bottom temperatures to appropriate values. In two-hemisphere numerical experiments, the continuous production of AABW is seen to prevent the destabilization of the high northern latitudes, and deep-decoupling oscillations do not occur. In a two-basin configuration, the export of fresh water from the Atlantic basin during a colder climate allows century-scale deep-decoupling oscillations.The results of the OGCM mixed boundary condition experiments are investigated using simple box models. One-hemisphere circulations are generated that are consistent with the 3-D model, and the influence of deep restoring is explored. Deep-decoupling oscillations can only be produced in a two-hemisphere box model if boundary and initial conditions do not allow AABW production. Multiple equilibria are found in the overturning circulation.Two scenarios are presented to produce oscillations that are similar to deep-decoupling oscillations in the presence of continued AABW production. First, the high northern latitudes are subjected to a melt pulse/retreat pattern of freshening. These perturbations can cause transitions from one stable mode to another, as well as overturning flushes that are followed by a return to the original stable overturning mode. Second, stochastic forcing is applied to the high northern latitude surface fresh water flux. The variations in surface forcing are capable of producing transitions between overturning states that are similar to deep-decoupling oscillations. The stochastic forcing acts to overcome the stabilizing effect of the AABW. In two-basin mixed boundary condition experiments, cooling the climate is seen to result in deep-decoupling type oscillations under stochastic forcing that produced no such variability in a warmer climate.
- Atmospheric sciences