Abstract

Climate models consistently project a robust weakening in the Atlantic Meridional Overturning Circulation (AMOC) and the Indonesian Throughflow (ITF) during the 21st century in response to greenhouse gas forcing. Here we elucidate the transient components of the global ocean overturning circulation and propose a transient conveyor belt, in which the AMOC is dynamically linked to the ITF on centennial timescales. Using a hierarchy of ocean and climate models, we show that there is a transient overturning compensation between the Atlantic and Indo-Pacific basins. In a warming climate, the AMOC weakens, but the Indo-Pacific develops an anomalous overturning circulation that opposes the Atlantic changes. The Indo-Pacific circulation is characterized by a northward transport anomaly close to surface. When considering the Indian and Pacific basins separately, the northward surface transport anomaly is largely confined to the Indian Ocean due to a basin-scale vorticity balance that constrains the surface transport in the Pacific. This surface transport response increases the sea surface height in the Indian Ocean and leads to a weakened ITF . We illustrate these dynamics using a 1.5-layer reduced gravity model and show that this idealized model agrees well with the response in multiple comprehensive general circulation models. Our results highlight the importance of transient inter-basin exchanges, especially on decadal to centennial time scales, in regulating the global ocean circulation in a changing climate.