Uniqueness and variability of the ocean's thermohaline circulation

A numerical modelling approach is chosen to study equilibrium and time-dependent aspects of the ocean's thermohaline circulation. In the first part, the roles of basin geometry and surface buoyancy forcing in determining the asymmetry of the present-day thermohaline circulation are considered. An idealized flat-bottomed two-basin model of the Atlantic and Pacific is found to favour equilibria with sinking in the southern hemisphere only (Southern Sinking) or also in the North Atlantic (Conveyor), even under a freshwater flux forcing field with more precipitation over the North Atlantic than over the North Pacific.Another new result is the range of Conveyor equilibria found under mixed boundary conditions. Rare cases with North Pacific sinking are characterized by a very fresh halocline in the Southern Ocean and a reversed pole-to-pole surface density contrast. A more quantitative investigation leads to an approximately linear relationship between the Atlantic overturning and the meridional gradient of zonally-averaged depth-integrated steric height from the northern boundary of the ocean to the southern tip of Africa; on the other hand, the local linear relationships postulated in most two-dimensional plane models of the overturning circulation could not be validated.In the second part, the climatology of a global ocean model is presented, and the importance in the model of the warm water route of the Conveyor through the Indian Ocean relative to the cold water route through Drake Passage is noted. The implied ocean heat and freshwater transports from the Canadian Climate Centre second generation atmospheric general circulation model are then presented, and are shown to be incompatible with the present-day thermohaline circulation.Finally, in the third part, a simple new parameterization of the sea surface temperature-evaporation feedback is developed as an extension of the traditional mixed boundary conditions. The positive sign of the feedback for the thermohaline circulation is demonstrated, and three examples featuring decadal, century and millennial timescale variability in one-hemisphere idealized basins are discussed. No fundamental alterations of the mechanisms under mixed boundary conditions are found, although the timescale is altered or the variability interrupted sooner in some cases