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. Around 50-34 million years ago, the Southern Hemisphere witnessed a major reorganization of continents. This led to the opening and deepening of two Southern Ocean gateways (OGs)-the Tasmanian Gateway between Australia and Antarctica, and Drake Passage between Cape Horn and the Antarctica Peninsula. During this period Earth's climate went through a major climate transition, from a hot world ("Greenhouse") to a cold world ("Icehouse"). One hypothesis to explain this dramatic climate transition is that the opening of these ocean gateways led to a major transition in the ocean's overturning circulation (i.e., its vertical circulation) with important consequences for the ocean's capability to store heat and carbon. In this study we use an ocean model to understand how the opening of an OG affects the ocean's overturning circulation. We show that it is small-scale processes, and their ability to transport heat southward and downward, which lead to a sudden increase of the ocean's overturning circulation as soon as the OG opens. Further deepening of the ocean gateways then leads to a decrease in the overturning circulation. This study therefore highlights the crucial role of small-scale processes in changing Earth's climate. The shallow opening of an ocean gateway leads to an abrupt onset of a deep-reaching overturning circulationThe deep-reaching overturning circulation is a consequence of standing meanders allowing for full-depth vertical heat transportFurther deepening of the gateway leads to a weaker overturning due to a decrease in heat transport toward southern convection regions