Deepening of Southern Ocean Gateway Leads to Abrupt Onset of a Deep-Reaching Meridional Overturning Circulation

Abstract

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.