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The ocean transports vast amounts of heat around the planet, helping to regulate regional climate. One important component of this heat transport is the movement of warm water from equatorial regions toward the poles, with colder water flowing in return. Here, we introduce a framework relating meridional heat transport to the diabatic processes of surface forcing and turbulent mixing that move heat across temperature classes. Applied to a (1/4)° global ocean model the framework highlights the role of the tropical Indo‐Pacific in the global ocean heat transport. A large fraction of the northward heat transport in the Atlantic is ultimately sourced from heat uptake in the eastern tropical Pacific. Turbulent mixing moves heat from the warm, shallow Indo‐Pacific circulation to the cold deeper‐reaching Atlantic circulation. Our results underscore a renewed focus on the tropical oceans and their role in global circulation pathways.

The spatial and temporal variability of upwelling into the mixed layer in the Southern Ocean is studied using a 1/10° ocean general circulation model. Virtual drifters are released in a regularly-spaced pattern across the Southern Ocean at depths of 250, 500, and 1000 m during both summer and winter months. The drifters are advected along isopycnals for a period of four years, unless they outcrop into the mixed layer, where lateral advection and a parameterization of vertical mixing is applied. The focus of this study is on the discrete exchange between the model mixed layer and the interior. Localization of interior-mixed layer exchange occurs downstream of major topographic features across the Indian and Pacific basins, creating “hotspots” of outcropping. Minimal outcropping occurs in the Atlantic basin, while 59% of drifters outcrop in the Pacific sector and in Drake Passage (the region from 140° W to 40° W), a disproportionately large amount even when considering the relative basin sizes. Due to spatial and temporal variations in mixed layer depth, the Lagrangian trajectories provide a statistical measure of mixed layer residence times. For each exchange into the mixed layer, the residence time has a Rayleigh distribution with a mean of 30 days; the cumulative residence time of the drifters is 261 ± 194 days, over a period of four years. These results suggest that certain oceanic gas concentrations, such as CO_2 and ^(14)C, will likely not reach equilibrium with the atmosphere before being re-subducted.

Within Drake Passage, the southern flank of the Antarctic Circumpolar Current (ACC) hosts the ventilation of deep water, the injection of Antarctic shelf waters and interactions between westward and eastward boundary currents. This exchange is explored through the trajectories of forty surface drifters released in January 2012 in the northwestern Weddell Sea. The drifters detail Lagrangian transport pathways between the eastern Antarctic Peninsula and sites of elevated chlorophyll in the Scotia Sea. ACC frontal currents, in particular the Southern ACC Front, act as dynamical transport barriers to the drifters and influence surface chlorophyll distributions, indicating that ACC fronts partition Weddell source waters in the Scotia Sea. Interannual fluctuations in surface chlorophyll in the south Scotia Sea and the northern Weddell Sea covary. This suggests that Scotia Sea ecosystem dynamics are linked to water properties injected from the tip of the Antarctic Peninsula and respond to Weddell Gyre circulation changes.

We investigate the sensitivity of the deep meridional overturning circulation (MOC) in the Southern Ocean to changes in the easterly wind stress over the Antarctic continental slope. The deep MOC is driven by export of dense Antarctic Bottom Water from the Antarctic continental shelf, and exerts a strong influence on climate by ventilating the deep ocean with oxygen and storing carbon dioxide. The possibility that inter-climatic modifications of the deep overturning may have been driven by changes in the atmospheric circulation has motivated the recent interest in evaluating the sensitivity of the Southern Ocean MOC to modifications of the mid-latitude westerlies. Using a high-resolution eddy-resolving model of a sector of the Southern Ocean, we show that the deep cell of the MOC is highly sensitive to the strength of the polar easterlies, and relatively insensitive to the strength of the mid-latitude westerlies. Our results highlight that determining the deep ocean ventilation in past and future climates demands an accurate evaluation of the concurrent surface wind stress throughout the Southern Ocean.

The formation of persistent multiple fronts is an established feature of the Antarctic Circumpolar Current (ACC). Front strength and location are closely linked to eddy properties and therefore have important implications for the eddy-driven closure of the Southern Ocean meridional overturning circulation. ACC front structure is analyzed here by calculating regional probability density functions (PDFs) of potential vorticity diagnosed in an eddy-resolving ocean general circulation model. Rapid spatial transitions in the number of fronts and in the density classes over which they occur are found. Front transitions are associated with the major topographic obstacles Kerguelen Island, Campbell Plateau and Drake Passage; multiple fronts are preferentially found downstream of these features. These findings highlight the significant departure from zonal symmetry of the ACC front structure and emphasize the importance of local dynamics on large-scale Southern Ocean properties.

The ocean moderates the Earth's climate due to its vast capacity to store and transport heat; the influence of the large-scale ocean circulation on changes in climate is considered in this chapter. The ocean experiences both buoyancy forcing (through heating/cooling and evaporation/precipitation) and wind forcing. Almost all ocean forcing occurs at the surface, but these changes are communicated throughout the entire depth of the ocean through the meridional overturning circulation (MOC). In a few localized regions, water become sufficiently dense to penetrate thousands of meters deep, where it spreads, providing a continuous source of deep dense water to the entire ocean. Dense water returns to the surface and thus closes the MOC, either through density modification due to diapycnal mixing or by upwelling along sloping isopycnals across the Southern Ocean. Determination of the relative contributions of these two processes in the MOC remains an active area of research. Observations obtained primarily from isotopic compositions in ocean sediments provide substantial evidence that the structure of the MOC has changed significantly in the past. Indeed, large and abrupt changes to the Earth's climate during the past 120,000 years can be linked to either a reorganization or a complete collapse of the MOC. Two of the more dramatic instances of abrupt change include Dansgaard-Oeschger events, abrupt warmings that could exceed 10°C over a period as short as a few decades, and Heinrich events, which are associated with massive freshwater fluxes due to rapid iceberg discharges into the North Atlantic. Numerical models of varying complexity that have captured these abrupt transitions all underscore that the MOC is a highly nonlinear system with feedback loops, multiple equilibria, and hysteresis effects. Prediction of future abrupt shifts in the MOC or “tipping points” remains uncertain. However, the inferred behavior of the MOC during glacial climates suggests that significant modifications to the present circulation are possible and that any change is likely to have a large effect on the Earth's climate.