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Abstract Deep-reaching warming along the boundary of the Antarctic Circumpolar Current and the subtropical gyre is a consistent feature of multidecadal observational estimates and projections of future climate. In the Indian basin, the maximum ocean heat content change is collocated with the powerful Agulhas Return Current (ARC) in the west and Subantarctic Front (SAF) in the east, forming a southeastward band we denote as the ARC–SAF. We find that this jet-confined warming is linked to a poleward shift of these strong currents via the thermal wind relation. Using a suite of idealized ocean-only and partially coupled climate model experiments, we show that strong global buoyancy flux anomalies consistently drive a poleward shift of the ARC–SAF circulation and the associated heat content change maximum. To better understand how buoyancy addition modifies this circulation in the absence of wind stress change, we next apply buoyancy perturbations only to certain regions. Buoyancy addition across the Indian and Pacific Oceans (including the ARC–SAF) gives rise to a strong baroclinic circulation response and modest poleward shift. In contrast, buoyancy addition in the North Atlantic drives a vertically coherent poleward shift of the ARC–SAF, which we suggest is associated with an ocean heat content perturbation communicated to the Southern Ocean via planetary waves and advected eastward along the ARC–SAF. Whereas poleward-shifting circulation and banded warming under climate change have been previously attributed to poleward-shifting winds in the Southern Ocean, we show that buoyancy addition can drive this circulation change in the Indian sector independent of changing wind stress. Significance StatementThis research aims to identify which changes at the atmosphere–ocean interface cause ocean warming localized within strong Southern Ocean currents under climate change. Whereas previous regional studies have emphasized the role of changes in Southern Hemisphere winds, we show that these currents are also sensitive to additional heat and freshwater input into the ocean—even in the faraway North Atlantic. Adding heat and freshwater shifts the currents southward, which is dynamically tied to the “band” of ocean warming seen in both long-term observations and climate change projections. We demonstrate that the warming climate will modify ocean circulation in unexpected ways; the consequences for the ocean’s ability to continue removing anthropogenic heat and carbon from the atmosphere remain poorly understood. 

Abstract Positive feedbacks in climate processes can make it difficult to identify the primary drivers of climate phenomena. Some recent global climate model (GCM) studies address this issue by controlling the wind stress felt by the surface ocean such that the atmosphere and ocean become mechanically decoupled. Most mechanical decoupling studies have chosen to override wind stress with an annual climatology. In this study we introduce an alternative method of interannually varying overriding which maintains higher frequency momentum forcing of the surface ocean. Using a GCM (NCAR CESM1), we then assess the size of the biases associated with these two methods of overriding by comparing with a freely evolving control integration. We find that overriding with a climatology creates sea surface temperature (SST) biases throughout the global oceans on the order of ±1°C. This is substantially larger than the biases introduced by interannually varying overriding, especially in the tropical Pacific. We attribute the climatological overriding SST biases to a lack of synoptic and subseasonal variability, which causes the mixed layer to be too shallow throughout the global surface ocean. This shoaling of the mixed layer reduces the effective heat capacity of the surface ocean such that SST biases excite atmospheric feedbacks. These results have implications for the reinterpretation of past climatological wind stress overriding studies: past climate signals attributed to momentum coupling may in fact be spurious responses to SST biases. 

Abstract Cross-equatorial ocean heat transport (OHT) changes have been found to damp meridional shifts of the intertropical convergence zone (ITCZ) induced by hemispheric asymmetries in radiative forcing. Zonal-mean energy transport theories and idealized model simulations have suggested that these OHT changes occur primarily due to wind-driven changes in the Indo-Pacific’s shallow subtropical cells (STCs) and buoyancy-driven changes in the deep Atlantic meridional overturning circulation (AMOC). In this study we explore the partitioning between buoyancy and momentum forcing in the ocean’s response. We adjust the top-of-atmosphere solar forcing to cool the Northern Hemisphere (NH) extratropics in a novel set of comprehensive climate model simulations designed to isolate buoyancy-forced and momentum-forced changes. In this case of NH high-latitude forcing, we confirm that buoyancy-driven changes in the AMOC dominate in the Atlantic. However, in contrast with prior expectations, buoyancy-driven changes in the STCs are the primary driver of the heat transport changes in the Indo-Pacific. We find that buoyancy-forced Indo-Pacific STC changes transport nearly 4 times the amount of heat across the equator as the shallower wind-driven STC changes. This buoyancy-forced STC response arises from extratropical density perturbations that are amplified by the low cloud feedback and communicated to the tropics by the ventilated thermocline. While the ocean’s specific response is dependent on the forcing scheme, our results suggest that partitioning the ocean’s total response to energy perturbations into buoyancy and momentum forcing provides basin-specific insight into key aspects of how the ocean damps ITCZ migrations that previous zonal-mean frameworks omit.