Fiammetta Straneo

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Convection, Sinking and the Poleward Heat Transport

There is widespread belief that convective regions are regions where water is made dense and sinks, thus identifying convective locations with the sinking limb of the overturning circulation. Instead, both theory, high-resolution non-hydrostatic models and observations show that very little sinking occurs in the open-ocean deep convection regions, where the bulk of the dense water formation occurs, due to rotational constraints inhibiting large-scale vertical motion. The sinking can, on the other hand, occur in the boundary current. The boundary current sinking, however, is not equivalent to the amount of dense water formed. Instead only a fraction of the dense water sinks. From this, it also follows that the poleward heat transport related to convection regions is not all achieved by an overturning cell. In turn, this implies that one expects the variability in the overturning and in the poleward heat transport, due to dense water formation, to differ.

Schematic of a two layer model that represents dense water formation in a marginal sea. The interior is a region of net heat loss and of dense water formation - thus maintaing a reservoir of dense fluid. The boundary current that flows around it exchanges heat with the interior (via instability processes) and becomes progressively colder. A net sinking and associated barotropization of the flow are associated with the boundary current transformation.

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