The role of the Indo-Pacific Ocean in mediating the transient response of the Atlantic Meridional Overturning Circulation

As the climate warms, coupled climate models project both a weaker Atlantic Meridional Overturning Circulation (AMOC) and stronger Southern Hemisphere (SH) westerly winds. The weaker AMOC appears to contradict previous studies that link strengthening SH westerlies to an intensification of the Southern Ocean upper overturning circulation cell and hence a stronger AMOC. Here, we propose a resolution of this issue that involves the Indo-Pacific component of the global ocean overturning circulation. On timescales shorter than 1000 years, perturbations to the AMOC strength lead to substantial changes in the stratification and overturning circulation of the Indo-Pacific Ocean, whereas the Southern Ocean remains relatively unchanged. In effect, the Indo-Pacific Ocean provides the first response to AMOC variations, which allows the Southern Ocean overturning circulation to be largely decoupled from the AMOC. We investigate this transient inter-basin compensation using an ocean-only general circulation model in which the AMOC strength is perturbed by modifying the idealized surface forcing fields in the North Atlantic, and we are able to reproduce the key features of this simulated Indo-Pacific response in a 1.5-layer reduced gravity model. We find that the response to perturbations in the North Atlantic Deep Water formation rate involves processes on two distinct timescales that are related to wave propagation (decadal timescale) and Southern Ocean eddies (millennial timescale). The results suggest the importance of inter-basin exchange in the response of the global ocean overturning circulation to a changing climate on decadal to centennial timescales.