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1
Deshayes, Julie, and Claude Frankignoul. "Simulated Variability of the Circulation in the North Atlantic from 1953 to 2003." Journal of Climate 21, no. 19 (October 1, 2008): 4919–33. http://dx.doi.org/10.1175/2008jcli1882.1.
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Abstract The variability of the circulation in the North Atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003. The interannual-to-decadal variability of the subpolar gyre circulation and the Meridional Overturning Circulation (MOC) is mostly influenced by the North Atlantic Oscillation (NAO). Both circulations intensified from the early 1970s to the mid-1990s and then decreased. The monthly variability of both circulations reflects the fast barotropic adjustment to NAO-related Ekman pumping anomalies, while the interannual-to-decadal variability is due to the baroclinic adjustment to Ekman pumping, buoyancy forcing, and dense water formation, consistent with previous studies. An original characteristic of the oceanic response to NAO is presented that relates to the spatial patterns of buoyancy and wind forcing over the North Atlantic. Anomalous Ekman pumping associated with a positive NAO phase first induces a decrease of the southern subpolar gyre strength and an intensification of the northern subpolar gyre. The latter is reinforced by buoyancy loss and dense water formation in the Irminger Sea, where the cyclonic circulation increases 1–2 yr after the positive NAO phase. Increased buoyancy loss also occurs in the Labrador Sea, but because of the early decrease of the southern subpolar gyre strength, the intensification of the cyclonic circulation is delayed. Hence the subpolar gyre and the MOC start increasing in the Irminger Sea, while in the Labrador Sea the circulation at depth leads its surface counterpart. In this simulation where the transport of dense water through the North Atlantic sills is underestimated, the MOC variability is well represented by a simple integrator of convection in the Irminger Sea, which fits better than a direct integration of NAO forcing.
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Deshayes, Julie, Ruth Curry, and Rym Msadek. "CMIP5 Model Intercomparison of Freshwater Budget and Circulation in the North Atlantic." Journal of Climate 27, no. 9 (April 23, 2014): 3298–317. http://dx.doi.org/10.1175/jcli-d-12-00700.1.
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Abstract The subpolar North Atlantic is a center of variability of ocean properties, wind stress curl, and air–sea exchanges. Observations and hindcast simulations suggest that from the early 1970s to the mid-1990s the subpolar gyre became fresher while the gyre and meridional circulations intensified. This is opposite to the relationship of freshening causing a weakened circulation, most often reproduced by climate models. The authors hypothesize that both these configurations exist but dominate on different time scales: a fresher subpolar gyre when the circulation is more intense, at interannual frequencies (configuration A), and a saltier subpolar gyre when the circulation is more intense, at longer periods (configuration B). Rather than going into the detail of the mechanisms sustaining each configuration, the authors’ objective is to identify which configuration dominates and to test whether this depends on frequency, in preindustrial control runs of five climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). To this end, the authors have developed a novel intercomparison method that enables analysis of freshwater budget and circulation changes in a physical perspective that overcomes model specificities. Lag correlations and a cross-spectral analysis between freshwater content changes and circulation indices validate the authors’ hypothesis, as configuration A is only visible at interannual frequencies while configuration B is mostly visible at decadal and longer periods, suggesting that the driving role of salinity on the circulation depends on frequency. Overall, this analysis underscores the large differences among state-of-the-art climate models in their representations of the North Atlantic freshwater budget.
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Barrier, Nicolas, Christophe Cassou, Julie Deshayes, and Anne-Marie Treguier. "Response of North Atlantic Ocean Circulation to Atmospheric Weather Regimes." Journal of Physical Oceanography 44, no. 1 (January 1, 2014): 179–201. http://dx.doi.org/10.1175/jpo-d-12-0217.1.
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Abstract A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO−), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO− induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO− conditions. These responses are driven by changes in deep water formation in the Labrador Sea.
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Wills, Robert C. J., Kyle C. Armour, David S. Battisti, and Dennis L. Hartmann. "Ocean–Atmosphere Dynamical Coupling Fundamental to the Atlantic Multidecadal Oscillation." Journal of Climate 32, no. 1 (December 17, 2018): 251–72. http://dx.doi.org/10.1175/jcli-d-18-0269.1.
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Abstract The North Atlantic has shown large multidecadal temperature shifts during the twentieth century. There is ongoing debate about whether this variability arises primarily through the influence of atmospheric internal variability, through changes in ocean circulation, or as a response to anthropogenic forcing. This study isolates the mechanisms driving Atlantic sea surface temperature variability on multidecadal time scales by using low-frequency component analysis (LFCA) to separate the influences of high-frequency variability, multidecadal variability, and long-term global warming. This analysis objectively identifies the North Atlantic subpolar gyre as the dominant region of Atlantic multidecadal variability. In unforced control runs of coupled climate models, warm subpolar temperatures are associated with a strengthened Atlantic meridional overturning circulation (AMOC) and anomalous local heat fluxes from the ocean into the atmosphere. Atmospheric variability plays a role in the intensification and subsequent weakening of ocean overturning and helps to communicate warming into the tropical Atlantic. These findings suggest that dynamical coupling between atmospheric and oceanic circulations is fundamental to the Atlantic multidecadal oscillation (AMO) and motivate approaching decadal prediction with a focus on ocean circulation.
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Le Bras, Isabela, Fiamma Straneo, Morven Muilwijk, Lars H. Smedsrud, Feili Li, M. Susan Lozier, and N. Penny Holliday. "How Much Arctic Fresh Water Participates in the Subpolar Overturning Circulation?" Journal of Physical Oceanography 51, no. 3 (March 2021): 955–73. http://dx.doi.org/10.1175/jpo-d-20-0240.1.
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AbstractFresh Arctic waters flowing into the Atlantic are thought to have two primary fates. They may be mixed into the deep ocean as part of the overturning circulation, or flow alongside regions of deep water formation without impacting overturning. Climate models suggest that as increasing amounts of freshwater enter the Atlantic, the overturning circulation will be disrupted, yet we lack an understanding of how much freshwater is mixed into the overturning circulation’s deep limb in the present day. To constrain these freshwater pathways, we build steady-state volume, salt, and heat budgets east of Greenland that are initialized with observations and closed using inverse methods. Freshwater sources are split into oceanic Polar Waters from the Arctic and surface freshwater fluxes, which include net precipitation, runoff, and ice melt, to examine how they imprint the circulation differently. We find that 65 mSv (1 Sv ≡ 106 m3 s−1) of the total 110 mSv of surface freshwater fluxes that enter our domain participate in the overturning circulation, as do 0.6 Sv of the total 1.2 Sv of Polar Waters that flow through Fram Strait. Based on these results, we hypothesize that the overturning circulation is more sensitive to future changes in Arctic freshwater outflow and precipitation, while Greenland runoff and iceberg melt are more likely to stay along the coast of Greenland.
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Yeager, Stephen. "Topographic Coupling of the Atlantic Overturning and Gyre Circulations." Journal of Physical Oceanography 45, no. 5 (May 2015): 1258–84. http://dx.doi.org/10.1175/jpo-d-14-0100.1.
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AbstractThe vorticity dynamics associated with the mean and time-varying gyre and overturning circulations of the Atlantic Ocean are examined in a realistic ocean model hindcast simulation of the late twentieth century. Abyssal flow interaction with sloping bottom bathymetry gives rise to the bottom pressure torque (BPT) term of the vertically integrated vorticity equation. The dominance of this term in the closure of the barotropic gyre circulation noted in previous studies is corroborated here for both non-eddy-resolving and eddy-resolving versions of the Parallel Ocean Program (POP) model. This study shows that BPT is also a dominant term in the vorticity balance of the Atlantic meridional overturning circulation (AMOC) and therefore represents a key dynamical link between the overturning and gyre streamfunctions. The interannual variability of the Atlantic circulation over the last several decades, viewed in terms of time-varying integral vorticity balances, demonstrates the fundamental role played by BPT in coupling the large-scale barotropic and baroclinic flows. Forcing perturbation experiments show how flow–bathymetry interactions mediate buoyancy-driven changes in the gyre circulation and momentum-driven changes in the AMOC. Examples of topographic coupling of the overturning and gyre circulations that this analysis elucidates include the covariation of the high-latitude AMOC and subpolar gyre flows on decadal time scales, buoyancy-forced variance of the Gulf Stream, and large wind-driven variations in AMOC at subtropical latitudes.
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d’Orgeville, Marc, and W. Richard Peltier. "Implications of Both Statistical Equilibrium and Global Warming Simulations with CCSM3. Part II: On the Multidecadal Variability in the North Atlantic Basin." Journal of Climate 22, no. 20 (October 15, 2009): 5298–318. http://dx.doi.org/10.1175/2009jcli2775.1.
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Abstract The nature of the multidecadal variability in the North Atlantic basin is investigated through detailed analysis of multicentury integrations performed using the low-resolution version of the Community Climate System Model, version 3 (CCSM3), a modern atmosphere–ocean coupled general circulation model. Specifically, the results of control simulations under both preindustrial and present-day perpetual seasonal cycle conditions are compared to each other and also to the results of five simulations with increasing CO2 concentration scenarios. In the absence of greenhouse gas–induced warming, the meridional overturning circulation (MOC) variability is shown to be dependent on the details of the simulation. In the present-day control simulation, the MOC is characterized by a broad spectrum of low frequencies, whereas, in preindustrial control simulations, MOC variability is characterized either by a well-defined periodicity of 60 yr or by a broad spectrum of low frequencies. In all the control simulations, the MOC appears to respond with a delay of 10 yr to synchronous temperature and salinity anomalies in the deep water formation sites located in the subpolar gyre, but salinity dominates the density anomalies. The explanation of the modeled MOC periodicity is therefore sought in the creation of these density anomalies. The influence of increased sea ice coverage under cold/preindustrial conditions is shown to modify the salinity variability, but it is not a sufficient condition for the support of the MOC periodicity. Instead, its source appears to be a modified subpolar gyre circulation resulting from interaction with the bottom bathymetry, which is able to sustain strong coupling between the horizontal and overturning circulations. Based on the global warming analyses, for the simulations initialized from the cold/preindustrial statistical equilibrium run, the North Atlantic variability continues to be dominated by strong coupling between the horizontal and overturning circulations if the imposed forcing is weak. More generally, the delayed response of the MOC to surface density anomalies in the deep water formation regions is preserved under weak forcing.
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Wu, Yang, Xiaoming Zhai, and Zhaomin Wang. "Impact of Synoptic Atmospheric Forcing on the Mean Ocean Circulation." Journal of Climate 29, no. 16 (July 27, 2016): 5709–24. http://dx.doi.org/10.1175/jcli-d-15-0819.1.
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Abstract The impact of synoptic atmospheric forcing on the mean ocean circulation is investigated by comparing simulations of a global eddy-permitting ocean–sea ice model forced with and without synoptic atmospheric phenomena. Consistent with previous studies, transient atmospheric motions such as weather systems are found to contribute significantly to the time-mean wind stress and surface heat loss at mid- and high latitudes owing to the nonlinear nature of air–sea turbulent fluxes. Including synoptic atmospheric forcing in the model has led to a number of significant changes. For example, wind power input to the ocean increases by about 50%, which subsequently leads to a similar percentage increase in global eddy kinetic energy. The wind-driven subtropical gyre circulations are strengthened by about 10%–15%, whereas even greater increases in gyre strength are found in the subpolar oceans. Deep convection in the northern North Atlantic becomes significantly more vigorous, which in turn leads to an increase in the Atlantic meridional overturning circulation (AMOC) by as much as 55%. As a result of the strengthened horizontal gyre circulations and the AMOC, the maximum global northward heat transport increases by almost 50%. Results from this study show that synoptic atmospheric phenomena such as weather systems play a vital role in driving the global ocean circulation and heat transport, and therefore should be properly accounted for in paleo- and future climate studies.
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Wilson, Earle A., Andrew F. Thompson, Andrew L. Stewart, and Shantong Sun. "Bathymetric Control of Subpolar Gyres and the Overturning Circulation in the Southern Ocean." Journal of Physical Oceanography 52, no. 2 (February 2022): 205–23. http://dx.doi.org/10.1175/jpo-d-21-0136.1.
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Abstract The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre–ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing. Significance Statement This study explores the impact of seafloor bathymetry on the dynamics of subpolar gyres in the Southern Ocean. The subpolar gyres are major circulation features that connect the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. This work provides deeper insight for how the submarine ridges that exist along the northern periphery of these gyres shape the vertical distribution of tracers and overturning circulation in these regions. These findings highlight an underappreciated yet fundamentally important topographical constraint on the three-dimensional cycling of heat and carbon in the Southern Ocean—processes that have far-reaching implications for the global climate. Future work should explore how the presence of these ridges affect the time-evolving response of the Southern Ocean to changes in surface conditions.
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Czaja, Arnaud. "Atmospheric Control on the Thermohaline Circulation." Journal of Physical Oceanography 39, no. 1 (January 1, 2009): 234–47. http://dx.doi.org/10.1175/2008jpo3897.1.
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Abstract In an attempt to elucidate the role of atmospheric and oceanic processes in setting a vigorous ocean overturning circulation in the North Atlantic but not in the North Pacific, a comparison of the observed atmospheric circulation and net surface freshwater fluxes over the North Atlantic and Pacific basins is conducted. It is proposed that the more erratic meridional displacements of the atmospheric jet stream over the North Atlantic sector is instrumental in maintaining high surface salinities in its subpolar gyre. In addition, it is suggested that the spatial pattern of the net freshwater flux at the sea surface favors higher subpolar Atlantic salinity, because the geographical line separating net precipitation from net evaporation is found well south of the time-mean gyre separation in the North Pacific, whereas the two lines tend to coincide in the North Atlantic. Numerical experiments with an idealized two-gyre system confirm that these differences impact the salinity budget of the subpolar gyre. Further analysis of a coupled climate model in which the Atlantic meridional overturning cell has been artificially weakened suggests that the more erratic jet fluctuations in the Atlantic and the shift of the zero [net evaporation minus precipitation (E − P)] line are likely explained by features independent of the state of the thermohaline circulation. It is thus proposed that the atmospheric circulation helps “locking” high surface salinities and an active coupling between upper and deep ocean layers in the North Atlantic rather than in the North Pacific basin.
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Wen, Qin, and Haijun Yang. "Investigating the Role of the Tibetan Plateau in the Formation of Pacific Meridional Overturning Circulation." Journal of Climate 33, no. 9 (May 1, 2020): 3603–17. http://dx.doi.org/10.1175/jcli-d-19-0206.1.
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AbstractThe effects of the Tibetan Plateau (TP) on the Pacific Ocean circulation are investigated using a fully coupled climate model. Sensitivity experiments are designed to demonstrate that the presence of the TP is the reason for the lack of strong deep water formation in the subpolar North Pacific, because removing the TP in the model would enable the establishment of the Pacific meridional overturning circulation (PMOC). The processes involved are described in detail as follows. Removing the TP in the model would excite an anomalous high pressure over the subpolar North Pacific, causing anomalous Ekman downwelling that enhances surface water subduction north of 40°N. Removing the TP would also lead to less freshwater flux into the western Pacific, increasing sea surface salinity over the region. The high-salinity surface water can then be advected northward and eastward by the Kuroshio and its extension, subducting along the 26–27σθ isopycnal surfaces to the deeper ocean, which enables the formation of deep water in the North Pacific and the setup of the PMOC. Afterward, more high-salinity warm water would be transported from the tropics to the extratropics by the Kuroshio, leading to the establishment of the PMOC. The role of the Rocky Mountains is also examined in this study. We conclude that the Rocky Mountains may play a trivial role in modulating the meridional overturning circulations in both the Pacific and Atlantic Oceans.
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MacGilchrist, Graeme A., Alberto C. Naveira Garabato, Peter J. Brown, Loïc Jullion, Sheldon Bacon, Dorothee C. E. Bakker, Mario Hoppema, Michael P. Meredith, and Sinhué Torres-Valdés. "Reframing the carbon cycle of the subpolar Southern Ocean." Science Advances 5, no. 8 (August 2019): eaav6410. http://dx.doi.org/10.1126/sciadv.aav6410.
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Global climate is critically sensitive to physical and biogeochemical dynamics in the subpolar Southern Ocean, since it is here that deep, carbon-rich layers of the world ocean outcrop and exchange carbon with the atmosphere. Here, we present evidence that the conventional framework for the subpolar Southern Ocean carbon cycle, which attributes a dominant role to the vertical overturning circulation and shelf-sea processes, fundamentally misrepresents the drivers of regional carbon uptake. Observations in the Weddell Gyre—a key representative region of the subpolar Southern Ocean—show that the rate of carbon uptake is set by an interplay between the Gyre’s horizontal circulation and the remineralization at mid-depths of organic carbon sourced from biological production in the central gyre. These results demonstrate that reframing the carbon cycle of the subpolar Southern Ocean is an essential step to better define its role in past and future climate change.
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Li, Feili, M. Susan Lozier, and William E. Johns. "Calculating the Meridional Volume, Heat, and Freshwater Transports from an Observing System in the Subpolar North Atlantic: Observing System Simulation Experiment." Journal of Atmospheric and Oceanic Technology 34, no. 7 (July 2017): 1483–500. http://dx.doi.org/10.1175/jtech-d-16-0247.1.
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AbstractA transbasin monitoring array from Labrador to Scotland was deployed in the summer of 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). The aim of the observing system is to provide a multiyear continuous measure of the Atlantic meridional overturning circulation (AMOC) and the associated meridional heat and freshwater transports in the subpolar North Atlantic. Results from the array are expected to improve the understanding of the variability of the subpolar transports and the nature and degree of the AMOC’s latitudinal dependence. In this present work, the measurements of the OSNAP array are described and a suite of observing system simulation experiments in an eddy-permitting numerical model are used to assess how well these measurements will estimate the fluxes across the OSNAP section. The simulation experiments indicate that the OSNAP array and calculation methods will adequately capture the mean and temporal variability of the overturning circulation and of the heat and freshwater transports across the subpolar North Atlantic.
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Born, Andreas, and Thomas F. Stocker. "Two Stable Equilibria of the Atlantic Subpolar Gyre." Journal of Physical Oceanography 44, no. 1 (January 1, 2014): 246–64. http://dx.doi.org/10.1175/jpo-d-13-073.1.
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Abstract The cyclonic circulation of the Atlantic subpolar gyre is a key mechanism for North Atlantic climate variability on a wide range of time scales. It is generally accepted that it is driven by both cyclonic winds and buoyancy forcing, yet the individual importance and dynamical interactions of the two contributions remain unclear. The authors propose a simplified four-box model representing the convective basin of the Labrador Sea and its shallow and deep boundary current system, the western subpolar gyre. Convective heat loss drives a baroclinic flow of relatively light water around the dense center. Eddy salt flux from the boundary current to the center increases with a stronger circulation, favors the formation of dense waters, and thereby sustains a strong baroclinic flow, approximately 10%–25% of the total. In contrast, when the baroclinic flow is not active, surface waters may be too fresh to convect, and a buoyancy-driven circulation cannot develop. This situation corresponds to a second stable circulation mode. A hysteresis is found for variations in surface freshwater flux and the salinity of the near-surface boundary current. An analytical solution is presented and analyzed.
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Solman, Silvina A., and Isidoro Orlanski. "Subpolar High Anomaly Preconditioning Precipitation over South America." Journal of the Atmospheric Sciences 67, no. 5 (May 1, 2010): 1526–42. http://dx.doi.org/10.1175/2009jas3309.1.
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Abstract The mechanisms associated with the intraseasonal variability of precipitation over South America during the spring season are investigated with emphasis on the influence of a quasi-stationary anomalous circulation over the southeastern South Pacific Ocean (SEP). A spectral analysis performed to the bandpass-filtered time series of daily precipitation anomalies for the La Plata Basin (LPB) and the South Atlantic convergence zone (SACZ) regions revealed several statistically relevant peaks corresponding to periods of roughly 23 days and 14–16 days—with the lower (higher) frequency peaks more prevalent for the SACZ (LPB). The large-scale circulation patterns preconditioning precipitation variability over both regions were explored by means of a regression analysis performed on the daily 500-hPa geopotential anomaly field provided by the NCEP–NCAR reanalysis dataset. The most prominent feature of the regression fields is the presence of a quasi-stationary anomalous anticyclonic (cyclonic) circulation over the southeastern South Pacific Ocean associated with positive rainfall anomalies over the LPB (SACZ) and, emanating from that high (low), an external Rossby wave propagating northeastward toward the South American continent. The synoptic-scale activity, quantified in terms of a frontal activity index, showed a strong influence on precipitation over the LPB and to a lesser extent over the SACZ. Moreover, the frontal activity is actually modulated by the anomalous high circulation over the SEP region. The behavior of this anomalous circulation may be supported by a positive feedback mechanism that can enhance the response of the high anomaly itself, which in turns reinforces the Rossby wave train propagating toward the South American continent.
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Zhao, Bowen, Thomas Reichler, Courtenay Strong, and Cecile Penland. "Simultaneous Evolution of Gyre and Atlantic Meridional Overturning Circulation Anomalies as an Eigenmode of the North Atlantic System." Journal of Climate 30, no. 17 (September 2017): 6737–55. http://dx.doi.org/10.1175/jcli-d-16-0751.1.
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The authors identify an interdecadal oscillatory mode of the North Atlantic atmosphere–ocean system in a general circulation model (GFDL CM2.1) via a linear inverse model (LIM). The oscillation mechanism is mostly embedded in the subpolar gyre: anomalous advection generates density anomalies in the eastern subpolar gyre, which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later, when associated anomalous advection of the opposite sign starts the other half cycle. As density anomalies reach the Labrador Sea deep convection region, Atlantic meridional overturning circulation (AMOC) anomalies are also induced. Both the gyre and AMOC anomalies then propagate equatorward slowly, following the advection of density anomalies. The oscillation is further demonstrated to be more likely an ocean-only mode while excited by the atmospheric forcing; in particular, it can be approximated as a linearly driven damped oscillator that is partly excited by the North Atlantic Oscillation (NAO). The slowly evolving interdecadal oscillation significantly improves and prolongs the LIM’s prediction skill of sea surface temperature (SST) evolution.
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Hakkinen, S. "Decline of Subpolar North Atlantic Circulation During the 1990s." Science 304, no. 5670 (April 23, 2004): 555–59. http://dx.doi.org/10.1126/science.1094917.
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Berk, J. van den, S. S. Drijfhout, and W. Hazeleger. "Circulation adjustment in the Arctic and Atlantic in response to Greenland and Antarctic mass loss." Climate Dynamics 57, no. 7-8 (July 21, 2021): 1689–707. http://dx.doi.org/10.1007/s00382-021-05755-3.
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AbstractFollowing a high-end projection for mass loss from the Greenland and Antarctic ice-sheets, a freshwater forcing was applied to the ocean surface in the coupled climate model EC-Earthv2.2 to study the response to meltwater release assuming an RCP8.5 emission scenario. The meltwater forcing results in an overall freshening of the Atlantic that is dominated by advective changes, strongly enhancing the freshening due to dilution by Greenland meltwater release. The strongest circulation change occurs in the western North Atlantic subpolar gyre and in the gyre in the Nordic Seas, leaving the North Atlantic subpolar gyre the region where most advective salt export occurs. Associated with counteracting changes in both gyre systems, the response of the Atlantic Meridional Overturning Circulation is rather weak over the 190 years of the experiment; it reduces with only 1 Sv ($$= 10^6$$ = 10 6 m $$^3$$ 3 s $$^{-1}$$ - 1 ), compared to changes in the subpolar gyre of 5 Sv. This relative insensitivity of the AMOC to the forcing is attributed to enhanced convection in the Nordic Seas and stronger overflows that compensate reduced convection in the Labrador and Irminger Seas, and lead to higher sea surface temperatures (SSTs) in the former and lower SSTs in the latter region. The weakened subpolar gyre in the west also shifts the North Atlantic Current and the subpolar-subtropical gyre boundary; with the subtropical gyre expanding, and the western subpolar gyre contracting. The SST changes are associated with obduction of Atlantic waters in the Nordic Seas that would otherwise obduct in the western subpolar gyre. The anomalous SSTs also induce a coupled ocean-atmosphere feedback that further strengthens the Nordic Seas circulation and weakens the western subpolar gyre. This occurs because the anomalous SST-gradient enhances the westerlies, especially between 65$$^{\circ }$$ ∘ N and 70$$^{\circ }$$ ∘ N; the associated increase in windstress curl consequently enhances the gyre in the Nordic Seas. This feedback is driven by the Greenland mass loss; Antarctic meltwater discharge causes a weaker, opposite response and more particularly affects the South Atlantic salinity budget through northward advection of low-salinity waters from the Southern Ocean. This effect, however, becomes visible only hundred years after the onset of Antarctic mass loss. We conclude that the response to freshwater forcing from both ice caps can lead to a complex response in the Atlantic circulation systems with opposing effects in different subbasins. The relative strength of the response is time-dependent and largely governed by internal feedbacks; the forcing acts mainly as a trigger and is decoupled from the response.
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Yang, Hu, Gerrit Lohmann, Xiaoxu Shi, and Chao Li. "Enhanced Mid-Latitude Meridional Heat Imbalance Induced by the Ocean." Atmosphere 10, no. 12 (November 27, 2019): 746. http://dx.doi.org/10.3390/atmos10120746.
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The heat imbalance is the fundamental driver for the atmospheric circulation. Therefore, it is crucially important to understand how it responds to global warming. In this study, the role of the ocean in reshaping the atmospheric meridional heat imbalance is explored based on observations and climate simulations. We found that ocean tends to strengthen the meridional heat imbalance over the mid-latitudes. This is primarily because of the uneven ocean heat uptake between the subtropical and subpolar oceans. Under global warming, the subtropical ocean absorbs relatively less heat as the water there is well stratified. In contrast, the subpolar ocean is the primary region where the ocean heat uptake takes place, because the subpolar ocean is dominated by upwelling, strong mixing, and overturning circulation. We propose that the enhanced meridional heat imbalance may potentially contribute to strengthening the water cycle, westerlies, jet stream, and mid-latitude storms.
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Lai, W. K. M., J. I. Robson, L. J. Wilcox, and N. Dunstone. "Mechanisms of Internal Atlantic Multidecadal Variability in HadGEM3-GC3.1 at Two Different Resolutions." Journal of Climate 35, no. 4 (February 15, 2022): 1365–83. http://dx.doi.org/10.1175/jcli-d-21-0281.1.
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Abstract This study broadly characterizes and compares the key processes governing internal Atlantic multidecadal variability (AMV) in two resolutions of HadGEM3-GC3.1: N216ORCA025, corresponding to ∼60 km in the atmosphere and 0.25° in the ocean, and N96ORCA1 (∼135 km in the atmosphere and 1° in the ocean). Both models simulate AMV with a time scale of 60–80 years, which is related to low-frequency ocean and atmosphere circulation changes. In both models, ocean heat transport convergence dominates polar and subpolar AMV, whereas surface heat fluxes associated with cloud changes drive subtropical AMV. However, details of the ocean circulation changes differ between the models. In N216 subpolar subsurface density anomalies propagate into the subtropics along the western boundary, consistent with the more coherent circulation changes and widespread development of SST anomalies. In contrast, N96 subsurface density anomalies persist in the subpolar latitudes for longer, so circulation anomalies and the development of SST anomalies are more centered there. The drivers of subsurface density anomalies also differ between models. In N216, the NAO is the dominant driver, while upper-ocean salinity-controlled density anomalies that originate from the Arctic appear to be the dominant driver in N96. These results further highlight that internal AMV mechanisms are model dependent and motivate further work to better understand and constrain the differences.
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Ortega, Pablo, Jon I. Robson, Matthew Menary, Rowan T. Sutton, Adam Blaker, Agathe Germe, Jöel J. M. Hirschi, Bablu Sinha, Leon Hermanson, and Stephen Yeager. "Labrador Sea subsurface density as a precursor of multidecadal variability in the North Atlantic: a multi-model study." Earth System Dynamics 12, no. 2 (April 26, 2021): 419–38. http://dx.doi.org/10.5194/esd-12-419-2021.
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Abstract. The subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large-scale ocean circulation changes. This article analyses the interrelationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the subpolar gyre (SPG) circulation, and the upper-ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked to the strength of the subpolar AMOC and gyre circulation, and it is also linked to the subtropical AMOC, although the strength of this relationship is model-dependent and affected by the inclusion of the Ekman component. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including the mean density stratification in the Labrador Sea, the strength and depth of the AMOC, and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link to the subtropical AMOC might be more realistic.
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Levang, Samuel J., and Raymond W. Schmitt. "What Causes the AMOC to Weaken in CMIP5?" Journal of Climate 33, no. 4 (February 15, 2020): 1535–45. http://dx.doi.org/10.1175/jcli-d-19-0547.1.
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AbstractIn a transient warming scenario, the North Atlantic is influenced by a complex pattern of surface buoyancy flux changes that ultimately weaken the Atlantic meridional overturning circulation (AMOC). Here we study the AMOC response in the CMIP5 experiment, using the near-geostrophic balance of the AMOC on interannual time scales to identify the role of temperature and salinity changes in altering the circulation. The thermal wind relationship is used to quantify changes in the zonal density gradients that control the strength of the flow. At 40°N, where the overturning cell is at its strongest, weakening of the AMOC is largely driven by warming between 1000- and 2000-m depth along the western margin. Despite significant subpolar surface freshening, salinity changes are small in the deep branch of the circulation. This is likely due to the influence of anomalously salty water in the subpolar intermediate layers, which is carried northward from the subtropics in the upper limb of the AMOC. In the upper 1000 m at 40°N, salty anomalies due to increased evaporation largely cancel the buoyancy increase due to warming. Therefore, in CMIP5, temperature dynamics are responsible for AMOC weakening, while freshwater forcing instead acts to strengthen the circulation in the net. These results indicate that past modeling studies of AMOC weakening, which rely on freshwater hosing in the subpolar gyre, may not be directly applicable to a more complex warming scenario.
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Rhein, Monika, Dagmar Kieke, Sabine Hüttl-Kabus, Achim Roessler, Christian Mertens, Robert Meissner, Birgit Klein, Claus W. Böning, and Igor Yashayaev. "Deep water formation, the subpolar gyre, and the meridional overturning circulation in the subpolar North Atlantic." Deep Sea Research Part II: Topical Studies in Oceanography 58, no. 17-18 (September 2011): 1819–32. http://dx.doi.org/10.1016/j.dsr2.2010.10.061.
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Meccia, Virna L., Doroteaciro Iovino, and Alessio Bellucci. "North Atlantic gyre circulation in PRIMAVERA models." Climate Dynamics 56, no. 11-12 (February 14, 2021): 4075–90. http://dx.doi.org/10.1007/s00382-021-05686-z.
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AbstractWe study the impact of horizontal resolution in setting the North Atlantic gyre circulation and representing the ocean–atmosphere interactions that modulate the low-frequency variability in the region. Simulations from five state-of-the-art climate models performed at standard and high-resolution as part of the High-Resolution Model Inter-comparison Project (HighResMIP) were analysed. In some models, the resolution is enhanced in the atmospheric and oceanic components whereas, in some other models, the resolution is increased only in the atmosphere. Enhancing the horizontal resolution from non-eddy to eddy-permitting ocean produces stronger barotropic mass transports inside the subpolar and subtropical gyres. The first mode of inter-annual variability is associated with the North Atlantic Oscillation (NAO) in all the cases. The rapid ocean response to it consists of a shift in the position of the inter-gyre zone and it is better captured by the non-eddy models. The delayed ocean response consists of an intensification of the subpolar gyre (SPG) after around 3 years of a positive phase of NAO and it is better represented by the eddy-permitting oceans. A lagged relationship between the intensity of the SPG and the Atlantic Meridional Overturning Circulation (AMOC) is stronger in the cases of the non-eddy ocean. Then, the SPG is more tightly coupled to the AMOC in low-resolution models.
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Fu, Yao, Feili Li, Johannes Karstensen, and Chunzai Wang. "A stable Atlantic Meridional Overturning Circulation in a changing North Atlantic Ocean since the 1990s." Science Advances 6, no. 48 (November 2020): eabc7836. http://dx.doi.org/10.1126/sciadv.abc7836.
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The Atlantic Meridional Overturning Circulation (AMOC) is crucially important to global climate. Model simulations suggest that the AMOC may have been weakening over decades. However, existing array-based AMOC observations are not long enough to capture multidecadal changes. Here, we use repeated hydrographic sections in the subtropical and subpolar North Atlantic, combined with an inverse model constrained using satellite altimetry, to jointly analyze AMOC and hydrographic changes over the past three decades. We show that the AMOC state in the past decade is not distinctly different from that in the 1990s in the North Atlantic, with a remarkably stable partition of the subpolar overturning occurring prominently in the eastern basins rather than in the Labrador Sea. In contrast, profound hydrographic and oxygen changes, particularly in the subpolar North Atlantic, are observed over the same period, suggesting a much higher decoupling between the AMOC and ocean interior property fields than previously thought.
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Koman, G., W. E. Johns, A. Houk, L. Houpert, and F. Li. "Circulation and overturning in the eastern North Atlantic subpolar gyre." Progress in Oceanography 208 (November 2022): 102884. http://dx.doi.org/10.1016/j.pocean.2022.102884.
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Hatun, H. "Influence of the Atlantic Subpolar Gyre on the Thermohaline Circulation." Science 309, no. 5742 (September 16, 2005): 1841–44. http://dx.doi.org/10.1126/science.1114777.
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Palter, Jaime B., Charles‐André Caron, Kara Lavender Law, Joshua K. Willis, David S. Trossman, Igor M. Yashayaev, and Denis Gilbert. "Variability of the directly observed, middepth subpolar North Atlantic circulation." Geophysical Research Letters 43, no. 6 (March 16, 2016): 2700–2708. http://dx.doi.org/10.1002/2015gl067235.
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Lozier, M. S., F. Li, S. Bacon, F. Bahr, A. S. Bower, S. A. Cunningham, M. F. de Jong, et al. "A sea change in our view of overturning in the subpolar North Atlantic." Science 363, no. 6426 (January 31, 2019): 516–21. http://dx.doi.org/10.1126/science.aau6592.
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To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.
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McManus, Jerry F., Delia W. Oppo, Lloyd D. Keigwin, James L. Cullen, and Gerard C. Bond. "Thermohaline Circulation and Prolonged Interglacial Warmth in the North Atlantic." Quaternary Research 58, no. 1 (July 2002): 17–21. http://dx.doi.org/10.1006/qres.2002.2367.
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AbstractDeep-sea sediment cores provide spatially coherent evidence for the climatic and hydrographic conditions in the subpolar North Atlantic during the last interglaciation. Taken together with similarly high-resolution terrestrial sequences, these records indicate a regional climatic progression, beginning with the extreme and variable climate late in the penultimate glaciation, continuing through a relatively stable climatic optimum during the interglaciation, and concluding with the reestablishment of the markedly variable regime that characterized the last 100,000-yr glaciation. Relatively mild conditions in much of the subpolar region significantly outlasted the minimum in global ice volume, despite declining summer insolation and the cooling influence of incipient proximal glaciers. These effects were partially offset by enhanced thermohaline circulation that paradoxically increased heat transport into the region while simultaneously providing the likely moisture source for the growth of large northern ice sheets. The inception of the last glacial cycle thus provides an example of the influence of ocean circulation on regional climate. In contrast to the apparent orbital pace of the ongoing ice-sheet growth, the subsequent deterioration of surface conditions was abrupt and dramatic.
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Gastineau, Guillaume, and Claude Frankignoul. "Influence of the North Atlantic SST Variability on the Atmospheric Circulation during the Twentieth Century." Journal of Climate 28, no. 4 (February 11, 2015): 1396–416. http://dx.doi.org/10.1175/jcli-d-14-00424.1.
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Abstract The ocean–atmosphere coupling in the North Atlantic is investigated during the twentieth century using maximum covariance analysis of sea surface temperature (SST) and 500-hPa geopotential height analyses and performing regressions on dynamical diagnostics such as Eady growth rate, wave activity flux, and velocity potential. The North Atlantic Oscillation (NAO) generates the so-called SST anomaly tripole. A rather similar SST anomaly tripole, with the subpolar anomaly displaced to the east and a more contracted subtropical anomaly, which is referred to as the North Atlantic horseshoe pattern, in turn influences the atmosphere. In the fall and early winter, the response is NAO like and primarily results from subpolar forcing centered over the Labrador Sea and off Newfoundland. In summer, the largest atmospheric response to SST resembles the east Atlantic pattern and results from a combination of subpolar and tropical forcing. To emphasize the interannual to multidecadal variability, the same analysis is repeated after low-pass filtering. The SST influence is dominated by the Atlantic multidecadal oscillation (AMO), which also has a horseshoe shape, but with larger amplitude in the subpolar basin. A warm AMO phase leads to an atmospheric warming limited to the lower troposphere in summer, while it leads to a negative phase of the NAO in winter. The winter influence of the AMO is suggested to be primarily forced by the Atlantic SSTs in the northern subtropics. Such influence of the AMO is found in winter instead of early winter because the winter SST anomalies have a larger persistence, presumably because of SST reemergence.
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Cessi, Paola. "The Effect of Northern Hemisphere Winds on the Meridional Overturning Circulation and Stratification." Journal of Physical Oceanography 48, no. 10 (October 2018): 2495–506. http://dx.doi.org/10.1175/jpo-d-18-0085.1.
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AbstractThe current paradigm for the meridional overturning cell and the associated middepth stratification is that the wind stress in the subpolar region of the Southern Ocean drives a northward Ekman flow, which, together with the global diapycnal mixing across the lower boundary of the middepth waters, feeds the upper branch of the interhemispheric overturning. The resulting mass transport proceeds to the Northern Hemisphere of the North Atlantic, where it sinks, to be eventually returned to the Southern Ocean at depth. Seemingly, the wind stress in the Atlantic basin plays no role. This asymmetry occurs because the Ekman transport in the Atlantic Ocean is assumed to return geostrophically at depths much shallower than those occupied by the interhemispheric overturning. However, this vertical separation fails in the North Atlantic subpolar gyre region. Using a conceptual model and an ocean general circulation model in an idealized geometry, we show that the westerly wind stress in the northern part of the Atlantic provides two opposing effects. Mechanically, the return of the Ekman transport in the North Atlantic opposes sinking in this region, reducing the total overturning and deepening the middepth stratification; thermodynamically, the subpolar gyre advects salt poleward, promoting Northern Hemisphere sinking. Depending on which mechanism prevails, increased westerly winds in the Northern Hemisphere can reduce or augment the overturning.
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Garuba, Oluwayemi A., and Barry A. Klinger. "The Role of Individual Surface Flux Components in the Passive and Active Ocean Heat Uptake." Journal of Climate 31, no. 15 (August 2018): 6157–73. http://dx.doi.org/10.1175/jcli-d-17-0452.1.
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Surface flux perturbations (heat, freshwater, and wind) due to an increase of atmospheric CO2 cause significant intermodel spread in ocean heat uptake; however, the mechanism underlying their impact is not very well understood. Here, we use ocean model experiments to isolate the impact of each perturbation on the ocean heat uptake components, focusing on surface heat flux anomalies caused directly by atmospheric CO2 increase (passive) and indirectly by ocean circulation change (active). Surface heat flux perturbations cause the passive heat uptake, while all the surface flux perturbations influence ocean heat uptake through the active component. While model results have implied that the active component increases ocean heat uptake because of the weakening of the Atlantic meridional overturning circulation (AMOC), we find that it depends more on the shallow circulation change patterns. Surface heat flux perturbation causes most of the AMOC weakening, yet it causes a net global active heat loss (12% of the total uptake), which occurs because the active heat loss in the tropical Pacific through the subtropical cell weakening is greater than the active heat gain in the subpolar Atlantic through AMOC weakening. Freshwater perturbation weakens the AMOC a little more, but increases the subpolar Atlantic heat uptake a great deal through a large weakening of the subpolar gyre, thereby causing a large global active heat gain (34% of the total uptake). Wind perturbation also causes an active heat loss largely through the poleward shift of the Southern Hemisphere subtropical cells.
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Williams, Richard G., Vassil Roussenov, Doug Smith, and M. Susan Lozier. "Decadal Evolution of Ocean Thermal Anomalies in the North Atlantic: The Effects of Ekman, Overturning, and Horizontal Transport." Journal of Climate 27, no. 2 (January 15, 2014): 698–719. http://dx.doi.org/10.1175/jcli-d-12-00234.1.
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Abstract Basin-scale thermal anomalies in the North Atlantic, extending to depths of 1–2 km, are more pronounced than the background warming over the last 60 years. A dynamical analysis based on reanalyses of historical data from 1965 to 2000 suggests that these thermal anomalies are formed by ocean heat convergences, augmented by the poorly known air–sea fluxes. The heat convergence is separated into contributions from the horizontal circulation and the meridional overturning circulation (MOC), the latter further separated into Ekman and MOC transport minus Ekman transport (MOC-Ekman) cells. The subtropical thermal anomalies are mainly controlled by wind-induced changes in the Ekman heat convergence, while the subpolar thermal anomalies are controlled by the MOC-Ekman heat convergence; the horizontal heat convergence is generally weaker, only becoming significant within the subpolar gyre. These thermal anomalies often have an opposing sign between the subtropical and subpolar gyres, associated with opposing changes in the meridional volume transport driving the Ekman and MOC-Ekman heat convergences. These changes in gyre-scale convergences in heat transport are probably induced by the winds, as they correlate with the zonal wind stress at gyre boundaries.
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Yeager, Stephen, and Gokhan Danabasoglu. "The Origins of Late-Twentieth-Century Variations in the Large-Scale North Atlantic Circulation." Journal of Climate 27, no. 9 (April 23, 2014): 3222–47. http://dx.doi.org/10.1175/jcli-d-13-00125.1.
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Abstract Surface forcing perturbation experiments are examined to identify the key forcing elements associated with late-twentieth-century interannual-to-decadal Atlantic circulation variability as simulated in an ocean–sea ice hindcast configuration of the Community Earth System Model, version 1 (CESM1). Buoyancy forcing accounts for most of the decadal variability in both the Atlantic meridional overturning circulation (AMOC) and the subpolar gyre circulation, and the key drivers of these basin-scale circulation changes are found to be the turbulent buoyancy fluxes: evaporation as well as the latent and sensible heat fluxes. These three fluxes account for almost all of the decadal AMOC variability in the North Atlantic, even when applied only over the Labrador Sea region. Year-to-year changes in surface momentum forcing explain most of the interannual AMOC variability at all latitudes as well as most of the decadal variability south of the equator. The observed strengthening of Southern Ocean westerly winds accounts for much of the simulated AMOC variability between 30°S and the equator but very little of the recent AMOC change in the North Atlantic. Ultimately, the strengthening of the North Atlantic overturning circulation between the 1970s and 1990s, which contributed to a pronounced SST increase at subpolar latitudes, is explained almost entirely by trends in the atmospheric surface state over the Labrador Sea.
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Larson, Sarah M., Martha W. Buckley, and Amy C. Clement. "Extracting the Buoyancy-Driven Atlantic Meridional Overturning Circulation." Journal of Climate 33, no. 11 (June 1, 2020): 4697–714. http://dx.doi.org/10.1175/jcli-d-19-0590.1.
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AbstractVariations in the Atlantic meridional overturning circulation (AMOC) driven by buoyancy forcing are typically characterized as having a low-frequency time scale, interhemispheric structure, cross-equatorial heat transport, and linkages to the strength of Northern Hemisphere gyre circulations and the Gulf Stream. This study first tests whether these attributes ascribed to the AMOC are reproduced in a coupled model that is mechanically decoupled and, hence, is only buoyancy coupled. Overall, the mechanically decoupled model reproduces these attributes, with the exception that in the subpolar gyre, buoyancy drives AMOC variations on interannual to multidecadal time scales, yet only the multidecadal variations penetrate into the subtropics. A stronger AMOC is associated with a strengthening of the Northern Hemisphere gyre circulations, Gulf Stream, and northward oceanic heat transport throughout the basin. We then determine whether the characteristics in the mechanically decoupled model can be recovered by low-pass filtering the AMOC in a fully coupled version of the same model, a common approach used to isolate the buoyancy-driven AMOC. A major conclusion is that low-pass filtering the AMOC in the fully coupled model reproduces the buoyancy-driven AMOC pattern and most of the associated attributes, but not the statistics of the temporal variability. The strength of the AMOC–Gulf Stream connection is also not reproduced. The analyses reveal caveats that must be considered when choosing indexes and filtering techniques to estimate the buoyancy-driven AMOC. Results also provide insight on the latitudinal dependence of time scales and drivers of ocean circulation variability in coupled models, with potential implications for measurement and detection of the buoyancy-driven AMOC in the real world.
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Lohmann, K., J. H. Jungclaus, D. Matei, J. Mignot, M. Menary, H. R. Langehaug, J. Ba, et al. "The role of subpolar deep water formation and Nordic Seas overflows in simulated multidecadal variability of the Atlantic meridional overturning circulation." Ocean Science 10, no. 2 (April 14, 2014): 227–41. http://dx.doi.org/10.5194/os-10-227-2014.
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Abstract. We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is partly done by analysing long (order of 1000 years) control simulations with five coupled climate models. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55 to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland–Scotland Ridge. The latter might, however, be underestimated, as the models in general do not realistically simulate the flow path of the Iceland–Scotland overflow water south of the Iceland–Scotland Ridge. The influence of variations in subpolar deep water formation is, on multimodel average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multimodel average, about half and one-third respectively of the decadal to multidecadal AMOC variance. Apart from analysing multimodel control simulations, we have performed sensitivity experiments with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. The sensitivity experiments indicate that variations in subpolar deep water formation and Nordic Seas overflows are not completely independent. We further conclude from these experiments that the decadal to multidecadal AMOC variability north of about 50° N is mainly related to variations in Nordic Seas overflows. At 45° N and south of this latitude, variations in both subpolar deep water formation and Nordic Seas overflows contribute to the AMOC variability, with neither of the processes being very dominant compared to the other.
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Martin, Torge, and Arne Biastoch. "On the ocean's response to enhanced Greenland runoff in model experiments: relevance of mesoscale dynamics and atmospheric coupling." Ocean Science 19, no. 1 (February 20, 2023): 141–67. http://dx.doi.org/10.5194/os-19-141-2023.
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Abstract. Increasing Greenland Ice Sheet melting is anticipated to impact water mass transformation in the subpolar North Atlantic and ultimately the meridional overturning circulation. Complex ocean and climate models are widely applied to estimate magnitude and timing of related impacts under global warming. We discuss the role of the ocean mean state, subpolar water mass transformation, mesoscale eddies, and atmospheric coupling in shaping the response of the subpolar North Atlantic Ocean to enhanced Greenland runoff. In a suite of eight dedicated 60- to 100-year-long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated and with regular and significantly enlarged Greenland runoff, we find (1) a major impact by the interactive atmosphere in enabling a compensating temperature feedback, (2) a non-negligible influence by the ocean mean state biased towards greater stability in the coupled simulations, both of which make the Atlantic meridional overturning circulation less susceptible to the freshwater perturbation applied, and (3) a more even spreading and deeper mixing of the runoff tracer in the subpolar North Atlantic and enhanced inter-gyre exchange with the subtropics in the strongly eddying simulations. Overall, our experiments demonstrate the important role of mesoscale ocean dynamics and atmosphere feedback in projections of the climate system response to enhanced Greenland Ice Sheet melting and hence underline the necessity to advance scale-aware eddy parameterizations for next-generation climate models.
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Li, Feili, Young-Heon Jo, Xiao-Hai Yan, and W. Timothy Liu. "Climate Signals in the Mid- to High-Latitude North Atlantic from Altimeter Observations." Journal of Climate 29, no. 13 (June 21, 2016): 4905–25. http://dx.doi.org/10.1175/jcli-d-12-00670.1.
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Abstract The variability of the sea surface height anomaly (SSHA) in the mid- to high-latitude North Atlantic for the period of 1993–2010 was investigated using the ensemble empirical mode decomposition to identify the dominant time scales. Sea level variations in the North Atlantic subpolar gyre (SPG) are dominated by the annual cycle and the long-term increasing trend. In comparison, the SSHA along the Gulf Stream (GS) is dominated by variability at intraseasonal and annual time scales. Moreover, the sea level rise in the SPG developed at a reduced rate in the 2000s compared to rates in the 1990s, which was accompanied by a rebound in SSHA variability following a period of lower variability in the system. These changes in both apparent trend and low-frequency SSHA oscillations reveal the importance of low-frequency variability in the SPG. To identify the possible contributing factors for these changes, the heat content balance (equivalent variations in the sea level) in the subpolar region was examined. The results indicate that horizontal circulations may primarily contribute to the interannual to decadal variations, while the air–sea heat flux is not negligible at annual time scale. Furthermore, the low-frequency variability in the SPG relates to the propagation of Atlantic meridional overturning circulation (AMOC) variations from the deep-water formation region to midlatitudes in the North Atlantic, which might have the implications for recent global surface warming hiatus.
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Van Nieuwenhove, Nicolas, Christof Pearce, Mads Faurschou Knudsen, Hans Røy, and Marit-Solveig Seidenkrantz. "Meltwater and seasonality influence on Subpolar Gyre circulation during the Holocene." Palaeogeography, Palaeoclimatology, Palaeoecology 502 (August 2018): 104–18. http://dx.doi.org/10.1016/j.palaeo.2018.05.002.
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Huang, Rui Xin. "Numerical Simulation of Wind-Driven Circulation in a Subtropical/Subpolar Basin." Journal of Physical Oceanography 16, no. 10 (October 1986): 1636–50. http://dx.doi.org/10.1175/1520-0485(1986)016<1636:nsowdc>2.0.co;2.
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Bersch, Manfred, Igor Yashayaev, and Klaus Peter Koltermann. "Recent changes of the thermohaline circulation in the subpolar North Atlantic." Ocean Dynamics 57, no. 3 (May 17, 2007): 223–35. http://dx.doi.org/10.1007/s10236-007-0104-7.
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Mertens, Christian, Monika Rhein, Maren Walter, Claus W. Böning, Erik Behrens, Dagmar Kieke, Reiner Steinfeldt, and Uwe Stöber. "Circulation and transports in the Newfoundland Basin, western subpolar North Atlantic." Journal of Geophysical Research: Oceans 119, no. 11 (November 2014): 7772–93. http://dx.doi.org/10.1002/2014jc010019.
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Zhong, Yafang, and Zhengyu Liu. "On the Mechanism of Pacific Multidecadal Climate Variability in CCSM3: The Role of the Subpolar North Pacific Ocean." Journal of Physical Oceanography 39, no. 9 (September 1, 2009): 2052–76. http://dx.doi.org/10.1175/2009jpo4097.1.
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Abstract Previous analyses of the Community Climate System Model, version 3 (CCSM3) standard integration have revealed pronounced multidecadal variability in the Pacific climate system. The purpose of the present work is to investigate physical mechanism underlying this Pacific multidecadal variability (PMV). To better isolate the mechanism that selects the long multidecadal time scale for the PMV, a few specifically designed sensitivity experiments are carried out. When the propagating Rossby waves are blocked in the subtropics from the midbasin, the PMV remains outstanding. In contrast, when the Rossby waves are blocked beyond the subtropics across the entire North Pacific, the PMV is virtually suppressed. It suggests that the PMV relies on propagating Rossby waves in the subpolar Pacific, whereas those in the subtropics are not critical. A novel mechanism of PMV is advanced based on a more comprehensive analysis, which is characterized by a crucial role of the subpolar North Pacific Ocean. The multidecadal ocean temperature and salinity anomalies may originate from the subsurface of the subpolar North Pacific because of the wave adjustment to the preceding basin-scale wind curl forcing. The anomalies then ascend to the surface and are amplified through local temperature–salinity convective feedback. Along the southward Oyashio, these anomalies travel to the Kuroshio Extension (KOE) region and are further intensified through a similar convective feedback. The oceanic temperature anomaly in the KOE is able to feed back to the large-scale atmospheric circulation, inducing a wind curl anomaly over the subpolar North Pacific, which in turn generates anomalous oceanic circulation and causes temperature and salinity variability in the subpolar subsurface. Thereby, a closed loop of PMV is established in the form of an extratropical delayed oscillator. The phase transition of PMV is driven by the delayed negative feedback that resides in the wave adjustment of the subpolar North Pacific via propagating Rossby waves, whereas the convective positive feedback provides the growth mechanism. A significant role of salinity variability is unveiled in both the delayed negative feedback and convective positive feedback.
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Hodson, Daniel L. R., Jon I. Robson, and Rowan T. Sutton. "An Anatomy of the Cooling of the North Atlantic Ocean in the 1960s and 1970s." Journal of Climate 27, no. 21 (October 24, 2014): 8229–43. http://dx.doi.org/10.1175/jcli-d-14-00301.1.
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Abstract In the 1960s and early 1970s, sea surface temperatures in the North Atlantic Ocean cooled rapidly. There is still considerable uncertainty about the causes of this event, although various mechanisms have been proposed. In this observational study, it is demonstrated that the cooling proceeded in several distinct stages. Cool anomalies initially appeared in the mid-1960s in the Nordic Seas and Gulf Stream extension, before spreading to cover most of the subpolar gyre. Subsequently, cool anomalies spread into the tropical North Atlantic before retreating, in the late 1970s, back to the subpolar gyre. There is strong evidence that changes in atmospheric circulation, linked to a southward shift of the Atlantic ITCZ, played an important role in the event, particularly in the period 1972–76. Theories for the cooling event must account for its distinctive space–time evolution. The authors’ analysis suggests that the most likely drivers were 1) the “Great Salinity Anomaly” of the late 1960s; 2) an earlier warming of the subpolar North Atlantic, which may have led to a slowdown in the Atlantic meridional overturning circulation; and 3) an increase in anthropogenic sulfur dioxide emissions. Determining the relative importance of these factors is a key area for future work.
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Williams, Richard G., Vassil Roussenov, M. Susan Lozier, and Doug Smith. "Mechanisms of Heat Content and Thermocline Change in the Subtropical and Subpolar North Atlantic." Journal of Climate 28, no. 24 (December 15, 2015): 9803–15. http://dx.doi.org/10.1175/jcli-d-15-0097.1.
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Abstract In the North Atlantic, there are pronounced gyre-scale changes in ocean heat content on interannual-to-decadal time scales, which are associated with changes in both sea surface temperature and thermocline thickness; the subtropics are often warm with a thick thermocline when the subpolar gyre is cool with a thin thermocline, and vice versa. This climate variability is investigated using a semidiagnostic dynamical analysis of historical temperature and salinity data from 1962 to 2011 together with idealized isopycnic model experiments. On time scales of typically 5 yr, the tendencies in upper-ocean heat content are not simply explained by the area-averaged atmospheric forcing for each gyre but instead dominated by heat convergences associated with the meridional overturning circulation. In the subtropics, the most pronounced warming events are associated with an increased influx of tropical heat driven by stronger trade winds. In the subpolar gyre, the warming and cooling events are associated with changes in western boundary density, where increasing Labrador Sea density leads to an enhanced overturning and an influx of subtropical heat. Thus, upper-ocean heat content anomalies are formed in a different manner in the subtropical and subpolar gyres, with different components of the meridional overturning circulation probably excited by the local imprint of atmospheric forcing.
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Schleussner, C. F., and G. Feulner. "A volcanically triggered regime shift in the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age." Climate of the Past 9, no. 3 (June 25, 2013): 1321–30. http://dx.doi.org/10.5194/cp-9-1321-2013.
Full textAbstract:
Abstract. Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly succeeded by the Little Ice Age are of exceptional importance. The origin of these regional climate anomalies remains a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully coupled model of intermediate complexity Climber 3α forced with stochastically reconstructed wind-stress fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea ice–ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions in our model leads to a spin-up of the subpolar gyre and a weakened Atlantic meridional overturning circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for understanding the dynamics of past and future climate.
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Schleussner, C. F., and G. Feulner. "A volcanically triggered regime shift in the subpolar North Atlantic ocean as a possible origin of the Little Ice Age." Climate of the Past Discussions 8, no. 6 (December 18, 2012): 6199–219. http://dx.doi.org/10.5194/cpd-8-6199-2012.
Full textAbstract:
Abstract. Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly (MCA), succeeded by the Little Ice Age (LIA) are of exceptional importance. The origin of these regional climate anomalies remains however a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully-coupled model Climber 3α forced with stochastically reconstructed wind fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea-ice – ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions leads to a spin-up of the subpolar gyre (SPG) and a weakened Atlantic Meridional Overturning Circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for past and future climate.
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Gervais, Melissa, Jeffrey Shaman, and Yochanan Kushnir. "Mechanisms Governing the Development of the North Atlantic Warming Hole in the CESM-LE Future Climate Simulations." Journal of Climate 31, no. 15 (August 2018): 5927–46. http://dx.doi.org/10.1175/jcli-d-17-0635.1.
Full textAbstract:
A warming deficit in North Atlantic sea surface temperatures is a striking feature in global climate model future projections. This North Atlantic warming hole has been related to a slowing of the Atlantic meridional overturning circulation (AMOC); however, the detailed mechanisms involved in its generation remain an open question. An analysis of the Community Earth System Model Large Ensemble simulations is conducted to obtain further insight into the development of the warming hole and its relationship to the AMOC. It is shown that increasing freshwater fluxes through the Arctic gates lead to surface freshening and reduced Labrador Sea deep convection, which in turn act to cool Labrador Sea sea surface temperatures. Furthermore, the resulting changes in surface ocean circulation lead to enhanced transport of cooled Labrador Sea surface waters into the interior of the subpolar gyre and a more zonal orientation of the North Atlantic Current. As a result, there is an increase in ocean advective heat flux divergence within the center of the subpolar gyre, causing this warming deficit in North Atlantic sea surface temperatures. These local changes to the ocean circulation affect the AMOC and lead to its slowdown.
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Frankignoul, Claude, Guillaume Gastineau, and Young-Oh Kwon. "Wintertime Atmospheric Response to North Atlantic Ocean Circulation Variability in a Climate Model." Journal of Climate 28, no. 19 (September 29, 2015): 7659–77. http://dx.doi.org/10.1175/jcli-d-15-0007.1.
Full textAbstract:
Abstract Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean–atmosphere interactions are broadly realistic in CCSM4.