Our current understanding of the global meridional overturning circulation (GMOC) is revisited using a surface‐forced ocean model simulation constrained by global hydrographic data. The derived GMOC ...is qualitatively consistent with previous observation‐based studies and further provides enhanced spatial details in the sources, transformations, and transports of major global water masses including in poorly observed regions. Several important but relatively underexplored aspects of the GMOC are highlighted, including complex but vigorous heavy‐to‐light water mass transformation that occurs in the Indo‐Pacific and Southern Oceans, and the role of the equatorial Pacific upwelling in closing the GMOC circuit. These and other key aspects of the GMOC are poorly captured in a surface‐forced ocean model simulation without the temperature and salinity corrections, suggesting that current climate models do not realistically simulate the GMOC and the associated global heat, salt, and carbon balances.
Plain Language Summary
Ocean tracers such as heat, salt, and carbon are perpetually carried by the global meridional overturning circulation (GMOC) and redistributed between hemispheres and across ocean basins from their source regions. The GMOC is therefore a crucial component of the global heat, salt, and carbon balances. In order to better understand the GMOC, here we carry out a global ocean model simulation with its temperature and salinity corrected toward observations. The derived GMOC is presented, validated against observations, and summarized in a schematic, which highlights several important but relatively underexplored aspects of the GMOC, including the pathway through which the heaviest water mass formed around the Antarctica is brought to the surface. This and other key aspects of the GMOC are poorly captured in a model run without the temperature and salinity corrections, suggesting that current climate models do not reproduce realistic paths of the GMOC and the associated global heat, salt, and carbon balances.
Key Points
The global meridional overturning circulation is revisited using an ocean model with its T&S corrected and validated against observations
Several underexplored aspects are highlighted, including complex but vigorous heavy‐to‐light water conversion in the Indo‐Pacific Ocean
Key processes including deep water formation & heavy‐to‐light water conversion are poorly captured in a model without T&S corrections
The Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the ...Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004–2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream and altered patterns of heat content and sea surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air‐sea fluxes close to the western boundary reveal that the changes in ocean heat transport and sea surface temperature have altered the pattern of ocean‐atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal‐scale variability of North Atlantic climate.
Key Points
New data from the RAPID 26°N array show that the AMOC has been in a state of reduced overturning since mid‐2008
Observations of heat content and SSH indicate that the impact of the reduction in the AMOC is similar to that predicted by climate models
The results indicate that changes in ocean heat transport have altered ocean‐atmosphere heat exchange over the North Atlantic
The overflow of cold water through the Faroe Bank Channel (FBC) is the densest water crossing the Greenland‐Scotland Ridge and the densest source for the Atlantic Meridional Overturning Circulation ...(AMOC). Here, we show that the overflow volume transport remained stable from 1996 to 2022, but that the bottom water warmed at an average rate of 0.1°C per decade, mainly caused by warming of deep waters upstream. The salinity of the overflow water has increased as a lagged and reduced response to the salinity increase seen in the upper‐layer source waters. Therefore, the potential density of the bottom water over the FBC sill shows no statistically significant trend. After entrainment of warmer ambient waters downstream of the FBC, the nonlinear density dependence upon temperature implies, however, that the overflow contributed water of reduced density to the local overturning and the deep limb of the AMOC.
Plain Language Summary
As part of the Atlantic Meridional Overturning Circulation—often called AMOC—dense water is formed north of the subsea ridge between Greenland and Scotland. On its route southward at depth, the dense water has to cross this subsea ridge called the Greenland‐Scotland Ridge. The densest water flows through the deepest trench, which is the Faroe Bank Channel with a sill depth of 840 m. Here, the strength of the flow has been monitored since 1995 together with its temperature and salinity characteristics. While the strength of the flow has been stable throughout the monitoring period, the dense water in the Faroe Bank Channel has warmed and also become more saline. Despite the warming, the density of the water, which is determined by the combination of temperature and salinity, has remained nearly unchanged. After leaving the Faroe Bank Channel, the dense water intensively entrains and mixes with warmer surrounding water and forms a larger volume of water, termed Iceland‐Scotland Overflow Water, that feeds the AMOC. The density of this large volume is sensitive to warming. The observed warming of the dense water may, therefore, have implications for the AMOC and thereby on the regional as well as global climate.
Key Points
Observed Faroe Bank Channel overflow strength has been stable from 1996 to 2022, but the overflow waters warmed in the recent two decades
Concurrent salinity increase prevented the bottom water density from decreasing significantly from the effect of warming
After entrainment downstream, the overflow contributed water of reduced density to global overturning due to non‐linear effects of warming
The meridional overturning circulation (MOC) decreases rapidly in subpolar and Nordic regions where the warm upper layer loses its buoyancy due to intense heat loss, sinks, and flows south. The major ...volume loss of the upper limb of the MOC, ~9.6 Sv out of 18.4 ± 3.4 Sv, occurs as subduction across the Iceland Basin and Irminger Sea while the major heat loss, 273 TW out of 395 ± 74 TW is associated with the MOC branch that continues into the Nordic Seas where North Atlantic deep overflow water is produced. The 122 ± 79 TW heat flux convergence in the subpolar gyre appears to be significantly larger than various estimates of heat loss to the atmosphere. Much of the 0.09 ± 0.02 Sv freshwater divergence is presumably balanced by runoff from the Greenland shelf. These estimates suggest that the Nordic Seas, not the Labrador Sea, are key to the state of the MOC.
Plain language summary
The meridional overturning circulation is a two‐dimensional view of the flow north of upper‐ocean warm water and its return south as cold deep and intermediate water. But the actual pathways of warm‐to‐cold conversion are several and remarkably diverse: One branch continues into the Nordic Seas where very dense water is produced and eventually spills back into the deep North Atlantic, another branch weaves its way around the entire subpolar basin and the southern tip of Greenland to the Labrador Sea where intermediate water is formed, and the third branch is an overturning that takes place within the subpolar waters between Greenland and Scotland. Volumetrically, this is the largest branch, but in terms of heat loss the Nordic Seas, branch surrenders far more heat to the atmosphere than the other two combined. It thus plays the key role in maintaining a strong meridional overturning circulation.
Key Points
Great heat loss and production of dense water overflow water in Nordic Seas key to the state of the MOC not the Labrador Sea
Heat flux convergence greater than climatological estimates of subpolar gyre heat loss
Subpolar fresh water divergence balanced by freshwater loss from Greenland shelf
By synthesizing recent studies employing a wide range of approaches (modern observations, paleo reconstructions, and climate model simulations), this paper provides a comprehensive review of the ...linkage between multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability and Atlantic Multidecadal Variability (AMV) and associated climate impacts. There is strong observational and modeling evidence that multidecadal AMOC variability is a crucial driver of the observed AMV and associated climate impacts and an important source of enhanced decadal predictability and prediction skill. The AMOC‐AMV linkage is consistent with observed key elements of AMV. Furthermore, this synthesis also points to a leading role of the AMOC in a range of AMV‐related climate phenomena having enormous societal and economic implications, for example, Intertropical Convergence Zone shifts; Sahel and Indian monsoons; Atlantic hurricanes; El Niño–Southern Oscillation; Pacific Decadal Variability; North Atlantic Oscillation; climate over Europe, North America, and Asia; Arctic sea ice and surface air temperature; and hemispheric‐scale surface temperature. Paleoclimate evidence indicates that a similar linkage between multidecadal AMOC variability and AMV and many associated climate impacts may also have existed in the preindustrial era, that AMV has enhanced multidecadal power significantly above a red noise background, and that AMV is not primarily driven by external forcing. The role of the AMOC in AMV and associated climate impacts has been underestimated in most state‐of‐the‐art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts.
Key Points
There is strong evidence that multidecadal variability of the AMOC is a key driver of AMV and associated climate impacts
The AMOC‐AMV linkage is consistent with observed key elements of AMV, and it is important to use multivariate metrics to understand AMV
AMOC‐induced Atlantic meridional heat transport and net surface heat flux anomalies are crucial for many AMV‐related climate impacts
Six years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin‐wide ...meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning ~20 months during 2009–2010 and ~4 years during 2013–2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak‐to‐peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year‐to‐year variations.
Plain Language Summary
Changes in the meridional overturning circulation, characterized by north‐south flows throughout the Atlantic Ocean basin and vertical exchange between the surface and the deep ocean, are related to changes in important ocean‐atmosphere‐climate signals like precipitation patterns, sea level, and extreme weather (e.g., drought, heat waves, and hurricane intensification). This study presents, for the first time, a multiyear daily record of the meridional overturning circulation flow based on direct measurements in the South Atlantic Ocean at 34.5°S. The roughly six years of observations presented in this study provided the ability to study seasonal and interannual changes in these important flows with continuous daily data, and they demonstrated a complexity of the ocean circulation as compared to other latitudes where this flow has been studied in the past.
Key Points
The meridional overturning circulation at 34.5°S has significant variability at time scales ranging from daily to interannual
Seasonal variations of the meridional overturning circulation at 34.5°S are driven primarily by density variations at the eastern boundary
Interannual variations of the meridional overturning circulation at 34.5°S have significant baroclinic and barotropic contributions
The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate variability. Our understanding, based largely on climate models, is that the Labrador Sea has an important role in ...shaping the evolution of the AMOC. However, a recent high‐profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence the credibility of the AMOC representation in climate models. Here, we attempt to reconcile these viewpoints by making the first direct comparison between OSNAP and a coupled climate model. The model compares well to the observations, demonstrating a more prominent role for overturning in the eastern than western subpolar gyre. Density anomalies generated by surface forcing in the Irminger Sea propagate into the Labrador Sea, where they dominate the density variability. Thus, the Labrador Sea may not be the origin of AMOC variability despite correlations with densities there.
Key Points
A coupled climate model shows agreement with recent observations (OSNAP), displaying a prominent role for the eastern subpolar gyre
This model also demonstrates a link between AMOC and Labrador Sea densities, consistent with previous modeling‐based paradigms
In our model, this link arises as Labrador Sea density anomalies originate via surface forcing upstream in the Irminger Sea
The global ocean overturning circulation, critically dependent on the global density stratification, plays a central role in regulating climate evolution. While it is well known that the global ...stratification profile exhibits a strong dependence to Southern Ocean dynamics and in particular to wind and buoyancy forcing, we demonstrate here that the stratification is also acutely sensitive to the mesoscale eddy energy dissipation timescale. Within the context of a global ocean circulation model with an energy constrained mesoscale eddy parameterization, it is shown that modest variations in the eddy energy dissipation timescale lead to significant variations in key metrics relating to ocean circulation, namely the Antarctic Circumpolar Current transport, Atlantic Meridional Overturning Circulation strength, and global ocean heat content, over long timescales. The results highlight a need to constrain uncertainties associated with eddy energy dissipation for climate model projections over centennial timescales and also for paleoclimate simulations over millennial timescales.
Plain Language Summary
The ocean is populated by “eddies”, analogous to weather systems in the atmosphere, but occurring on much smaller scales of typically 10–100 km. Recent advances in our understanding of the circulation of the Southern Ocean, which connects all of the major ocean basins to the north, have revealed a crucial role for the energy balance of Southern Ocean eddies. In this paper, we show that the timescale over which energy is removed from Southern Ocean eddies has a dramatic impact on the following: (a) the strength of the Antarctic Circumpolar Current, the largest current in the global ocean; (b) the strength of the Atlantic meridional overturning circulation, responsible in part for the relatively mild climatic conditions over northwestern Europe; and (c) global ocean heat content, a key parameter in the global climate system. These results have significant implications for both past and future climates, and highlight the importance of combining observational, theoretical and modeling efforts to better understand and constrain the energy balance of ocean eddies.
Key Points
Key metrics of global ocean circulation (Antarctic Circumpolar Current transport, Atlantic Meridional Overturning Circulation strength, and Ocean Heat Content anomaly) acutely sensitive to an eddy energy dissipation timescale
Modest variations in the dissipation timescale has a comparable effect to significant variations in the Southern Ocean wind forcing
Constraints on the dissipation timescale critical to longtime integrations of ocean climate models such as paleoclimate scenarios
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