Deepening of Southern Ocean gateway leads to abrupt onset of a deep-reaching meridional overturning circulation

During the Eocene and the Eocene-Oligocene transition, the lower cell of the meridional overturning circulation (MOC), associated with bottom water formation, underwent changes associated with the geological evolution of Southern Ocean gateways. These are important for the Cenozoic climate transition from Greenhouse to Icehouse, but their dynamics still remain elusive. We demonstrate, using an idealized eddying ocean model, that the opening of a gateway leads to the abrupt onset of a vigorous, deep-reaching, MOC. This MOC has a maximum transport for a shallow gateway, and decreases with further deepening of the gateway. This abrupt change in the MOC can be explained through the ability with which standing meanders—turbulent features located downstream of the gateway—can induce deep vertical heat transport at high latitudes where bottom waters are produced. Our results demonstrate the crucial role of turbulent processes in setting the strength of the global ocean's deep-reaching MOC.

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