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Paleoclimate proxy data suggest that the Atlantic Meridional Overturning Circulation (AMOC) was shallower at the Last Glacial Maximum (LGM) than its preindustrial (PI) depth. Previous studies have suggested that this shoaling necessarily accompanies Antarctic sea ice expansion at the LGM. Here the influence of Southern Ocean surface forcing on the AMOC depth is investigated using ocean-only simulations from a state-of-the-art climate model with surface forcing specified from the output of previous coupled PI and LGM simulations. In contrast to previous expectations, we find that applying LGM surface forcing in the Southern Ocean and PI surface forcing elsewhere causes the AMOC to shoal only about half as much as when LGM surface forcing is applied globally. We show that this occurs because diapycnal mixing renders the Southern Ocean overturning circulation more diabatic than previously assumed, which diminishes the influence of Southern Ocean surface buoyancy forcing on the depth of the AMOC.

Climate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate. Plain Language Summary: The Indonesian Throughflow (ITF) is a key component of the global ocean circulation. By exchanging water between the low‐latitude Indian Ocean and Pacific Ocean, the ITF has been suggested to play an important role in shaping global warming patterns in response to greenhouse gas forcing. Climate models consistently project the ITF strength to decline in the 21st century. Traditionally, changes in the strength of the ITF have been attributed to local processes, such as changes in precipitation and atmospheric winds. Here, we suggest that remote processes can also have a significant impact on ITF variability. In particular, we show that the projected weakening in the ITF during the 21st century could be tied to changes in the Atlantic meridional overturning circulation (AMOC). Through this transient version of the ocean's conveyor belt circulation, changes in the high‐latitude North Atlantic (e.g., Arctic sea ice melt) can affect the climate in the low‐latitude Indo‐Pacific Ocean. An intriguing corollary is the potential to use the ITF to monitor or interpret long‐term trends in the AMOC. Key Points: Basin‐scale transient responses of the global ocean overturning circulation are explored with a hierarchy of modelsChanges in AMOC strength can produce a response in ITF volume transport on centennial timescalesITF transport time series may assist in monitoring and interpreting long‐term trends in the AMOC [ABSTRACT FROM AUTHOR]

Surface buoyancy fluxes in the Southern and North Atlantic Oceans are presumed to disproportionately influence the ocean's residual global overturning circulation (GOC) with respect to those in the Indo‐Pacific. Here, this assumption is challenged through an assessment of global buoyancy transport in the Community Earth System Model 1.0, which reveals that the steady state GOC is equally constrained by surface buoyancy flux everywhere. Further, an unacknowledged aspect of the GOC is demonstrated: it transports buoyancy from where it is gained at the surface, predominately in the Indo‐Pacific, to where it is lost, predominately in the Atlantic and Southern Oceans. This global buoyancy transport requires zonal structure in the GOC, linking the Atlantic and Indo‐Pacific within the Southern Ocean, asymmetry and interbasin coupling absent from many conceptual descriptions of overturning dynamics. These results compel a more nuanced appreciation for an Indo‐Pacific influence in GOC evolution. Plain Language Summary: The great conveyor belt of global currents that transit the global ocean—the global overturning circulation—carry with them heat, freshwater, and dissolved gases that are essential to Earth's climate. Changes in heating and cooling of the ocean surface, or changes to patterns of precipitation, are known to influence this global‐scale circulation, but a full understanding of the response remains incomplete. In this study, we develop a new technique to study the global overturning circulation. Our methods use the spatial patterns of ocean surface processes, for example, heating and cooling, precipitation and evaporation, to infer circulation behaviors deep in the ocean's interior. Historically, deep ocean behaviors were thought to be influenced by surface processes occurring at the high latitudes. Here, using this new technique, we find that surface processes at the low latitudes of the Indo‐Pacific Oceans are equally essential in determining the strength and structure of the global overturning circulation. As a consequence, the climate that we experience at Earth's surface may depend far more heavily on the processes occurring at the equatorial Indo‐Pacific Ocean surface than often assumed. Key Points: Constraints on overturning pathways, based on global surface buoyancy forcing, are tested in comprehensive climate modelIndo‐Pacific air‐sea fluxes require zonal asymmetries in the global overturning circulationActive low‐latitude, Indo‐Pacific surface controls on global overturning dynamics are proposed [ABSTRACT FROM AUTHOR]