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 […]