Paul Vaughan writes in suggestions:
It’s the wind.
Rial (2012) drew my attention to a fundamental correction that’s underway in oceanography (more notes forthcoming on this later) ….
Lozier, Susan (2010). Deconstructing the conveyor belt. Science 328, 1507-1511.
Though appealing in its simplicity, the ocean conveyor-belt paradigm has lost luster over the years […] the ocean’s eddy field, unaccounted for just decades ago […] figures prominently in the dismantling of the conveyor-belt paradigm. Another player in this dismantling is the ocean’s wind field. The traditional assignation of surface ocean gyres to wind-forcing and overturning to buoyancy forcing has ignored the vital impact of winds on overturning pathways and mechanics. […] the conveyor-belt model no longer serves the community well […] because it ignores crucial structure and mechanics of the ocean’s intricate global overturning.
[…] wind forcing, rather than buoyancy forcing, can play a dominant role in changing the transport of the overturning […]
Although many past studies have invoked buoyancy forcing at high latitudes as the driving mechanism for the overturning […] wind forcing―by creating surface mass fluxes and/or by providing the mixing needed to return deep waters to the surface―is instead the dominant mechanism (14). […] 14. C. Wunsch, Science 298, 1179 (2002).
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Wunsch, Carl (2002). What is the Thermohaline Circulation? Science 298, 1179-1181.
[…] the deep ocean is in a near-equilibrium state, and it is not possible, without an intricate calculation, to determine if the density/pressure differences drive the flow field, or the reverse. Some authors claim to be able to separate the fraction of the flow derived from density field gradients from that caused by the wind field (definition 6). But the density gradients are set up primarily by the wind.
The only possible sources of this work are tidal stirring and the wind field […] a convective mode of motion cannot generate the turbulence required to carry the MOC across the stable stratification.
The conclusion from this and other lines of evidence is that the ocean’s mass flux is sustained primarily by the wind, and secondarily by tidal forcing. Both in models and the real ocean, surface buoyancy boundary conditions strongly influence the transport of heat and salt, because the fluid must become dense enough to sink, but these boundary conditions do not actually drive the circulation.
The ocean is thus best viewed as a mechanically driven fluid engine, capable of importing, exporting, and transporting vast quantities of heat and freshwater. Although of very great climate influence, this transport is a nearly passive consequence of the mechanical machinery. When Stommel (10) first introduced the term “thermohaline circulation” in a box model, he explicitly provided a source of mechanical energy in the form of mixing devices. These devices disappeared in subsequent discussions and extensions of this influential model.
For past or future climates, the quantity of first-order importance is the nature of the wind field. It not only shifts the near-surface wind-driven components of the mass flux, but also changes the turbulence at depth; this turbulence appears to control the deep stratification. The wind field will also, in large part, determine the regions of convective sinking and of the resulting 3D water properties. Fluxes and net exports of properties such as heat and carbon are determined by both the mass flux and spatial distribution of the property, and not by either alone.
The term “thermohaline circulation” should be reserved for the separate circulations of heat and salt, and not conflated into one vague circulation with unknown or impossible energetics. No shortcut exists for determining property fluxes from the mass circulation without knowledge of the corresponding property distribution.