Atlantic Meridional Heat Transport Research Articles

Fine-Tuning of Sub-Annual Resolution Spectral Index Time Series from Eifel Maar Sediments, Western Germany, to the NGRIP δ18O Chronology, 26–60 ka

Recent technological advancements in spectral imaging core-scanning techniques have proved to be a promising tool to study lake sediments at extremely high resolution. We used this novel analytical approach to scan core AU3 of the Pleistocene Auel maar, Western Germany. The resulting ultra-high-resolution RABD670 spectral index, a proxy for the lake’s primary production, shows an almost complete succession of Greenland Interstadials of the NGRIP ice core chronology back to around 60,000 years. Using the ELSA-20 chronology and its anchor points to the NGRIP record as a stratigraphic basis, we were able to compare and fine-tune prominent climate signals occurring in both regions. This in-depth correlation yields strong evidence that the climates of Greenland and Central Europe were not only strongly coupled on timescales of stadials and interstadials but even on multidecadal scales, showing prominent climate cycles between 20 and 125 years. As climate changes in these regions were ultimately driven by variations in the North Atlantic meridional heat transport, their strong coupling becomes most apparent during cold and arid intervals. In contrast, longer-lasting warmer and more humid phases caused the activation of various regional feedback mechanisms (e.g., soil formation, forest growth), resulting in more complex patterns in the proxy records.

An ARGO and XBT Observing System for the Atlantic Meridional Overturning Circulation and Meridional Heat Transport (AXMOC) at 22.5°S

AbstractChanges in the Atlantic Meridional Overturning Circulation (AMOC) and associated Meridional Heat Transport (MHT) can affect climate and weather patterns, regional sea levels, and ecosystems. Direct observations of the AMOC are still limited, particularly in the South Atlantic. This study establishes a cost‐effective trans‐basin section to estimate for the first time the AMOC and MHT at 22.5°S, using only sustained ocean observations. For this, an optimal mapping method that minimizes the difference between surface in situ dynamic height and satellite altimetry was developed to retrieve monthly temperature and salinity profiles from Argo and XBT data along the 22.5°S section. The mean states, as well as the interannual and seasonal changes of the obtained AMOC and MHT were compared with other products. The mean AMOC and MHT for 22.5°S are 16.3 ± 3.2 Sv and 0.7 ± 0.2 PW, respectively, showing stronger transports during austral fall/winter and weaker in spring. The high‐density XBT data available at the western boundary were vital for capturing the highly variable Brazil Current (BC), whose mean and variability was improved compared to other products. At 22.5°S, the North Atlantic Deep Water is divided into two cores that flow along both the western and the eastern boundaries near 2,500 m depth. Our results (a) suggest a greater influence of the western boundary current system on the AMOC variability at 22.5°S, (b) highlight the importance of high‐density in situ data for AMOC estimates, and (c) contribute to a better understanding of the AMOC and MHT variability in the South Atlantic.

South Atlantic overturning and heat transport variations in ocean reanalyses and observation-based estimates

Abstract. The variability in the South Atlantic Meridional Overturning Circulation (MOC) and meridional heat transport measured across 34.5∘ S during 2013–2017 differs significantly between observational and ocean reanalysis estimates. Variability in an ocean reanalysis ensemble and an eddy-resolving reanalysis is similar to an altimeter-based estimate but smaller than energy-budget and mooring-based estimates. Over 1993–2020, there is no long-term trend in the ensemble-mean overturning and heat transport, although there are inter-model differences, whereas the altimeter-based and energy-budget estimate transports increase over this period. Time-mean overturning volume transport (and the depth of maximum overturning) across 34.5∘ S in the ensemble and observations are similar, whereas the corresponding mean heat transports differ by up to 0.3 PW. The seasonal cycle of these transports varies between estimates, due to differences in the methods for estimating the geostrophic flow and the sampling characteristics of the observational approaches. The baroclinic, barotropic, and Ekman MOC components tend to augment each other in mooring-based estimates, whereas in other estimates they tend to counteract each other, so the monthly-mean, interannual, and seasonal MOC anomalies have a greater magnitude in the mooring-based estimates. Thus, the mean and variation in real-world South Atlantic transports and the amplitude of their fluctuations are still uncertain. Ocean reanalyses are useful tools to identify and understand the source of these differences and the mechanisms that control volume and heat transport variability in the South Atlantic, a region critical for determining the global overturning pathways and inter-basin transports.

Reconstruction of Sea Ice Concentration in Northern Baffin Bay Using Deuterium Excess in a Coastal Ice Core From the Northwestern Greenland Ice Sheet

AbstractVariations in the conditions of sea ice in the northern part of Baffin Bay and North Open Water polynya influence human activity in northwestern Greenland through oceanic circulation and heat balance between air and sea. To evaluate the impact of variations in sea ice conditions on the surrounding environment, it is important to understand the mechanism of sea ice variations over long periods. In this study, we estimated the age of the SIGMA‐A ice core collected northwestern Greenland Ice Sheet and researched the relationship between annual or seasonal deuterium excess (d‐excess) and seasonal sea ice concentration. We found that a temporal variation in the spring d‐excess in the ice core negatively correlated significantly with that of sea ice concentration in February–April in northern Baffin Bay from 1979–2005 (r = −0.61,p < 0.001). Using this relationship, we reconstructed the temporal variations in sea ice concentrations for 100 years from the ice core drilled in the northwestern Greenland Ice Sheet. The sea ice concentration in the early twentieth century was lower than that in the present. The decrease in sea ice concentration was consistent with analytical results for marine sediments obtained from Baffin Bay. We also suggested that the sea ice concentration was controlled by atmospheric conditions from the 1920s to 1940s based on examinations of correlations with the North Atlantic Oscillation index and air temperature in Ilulissat and by oceanographic conditions from 1945–1955, 1959–1969, and 1982–1992 based on the Atlantic Multidecadal Oscillation index and meridional heat transport to western Greenland.

Synchronous and proportional deglacial changes in Atlantic meridional overturning and northeast Brazilian precipitation

AbstractChanges in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high‐resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near‐linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.

The North Atlantic Eddy Heat Transport and Its Relation with the Vertical Tilting of the Gulf Stream Axis

AbstractCorrelations between temperature and velocity fluctuations are a significant contribution to the North Atlantic meridional heat transport, especially at the northern boundary of the subtropical gyre. In satellite observations and in a numerical model at ⅞° resolution, a localized pattern of positive eddy heat flux is found northwest of the Gulf Stream, downstream of its separation at Cape Hatteras. It is confined to the upper 500 m. A simple kinematic model of a meandering jet can explain the surface eddy flux, taking into account a spatial shift between the maximum velocity of the jet and the maximum cross-jet temperature gradient. In the Gulf Stream such a spatial shift results from the nonlinear temperature profile and the vertical tilting of the velocity profile with depth. The numerical model suggests that the meandering of the Gulf Stream could account, at least in part, for the large eddy heat transport (of order 0.3 PW) near 36°N in the North Atlantic and for its compensation by the mean flow.

Changes in the seasonal cycle of the Atlantic meridional heat transport in a RCP 8.5 climate projection in MPI-ESM

Abstract. We investigate changes in the seasonal cycle of the Atlantic Ocean meridional heat transport (OHT) in a climate projection experiment with the Max Planck Institute Earth System Model (MPI-ESM) performed for the Coupled Model Intercomparison Project Phase 5 (CMIP5). Specifically, we compare a Representative Concentration Pathway (RCP) RCP 8.5 climate change scenario, covering the simulation period from 2005 to 2300, to a historical simulation, covering the simulation period from 1850 to 2005. In RCP 8.5, the OHT declines by 30–50 % in comparison to the historical simulation in the North Atlantic by the end of the 23rd century. The decline in the OHT is accompanied by a change in the seasonal cycle of the total OHT and its components. We decompose the OHT into overturning and gyre component. For the OHT seasonal cycle, we find a northward shift of 5° and latitude-dependent shifts between 1 and 6 months that are mainly associated with changes in the meridional velocity field. We find that the changes in the OHT seasonal cycle predominantly result from changes in the wind-driven surface circulation, which projects onto the overturning component of the OHT in the tropical and subtropical North Atlantic. This leads in turn to latitude-dependent shifts between 1 and 6 months in the overturning component. In comparison to the historical simulation, in the subpolar North Atlantic, in RCP 8.5 we find a reduction of the North Atlantic Deep Water formation and changes in the gyre heat transport result in a strongly weakened seasonal cycle with a weakened amplitude by the end of the 23rd century.

Decadal Modulations of Interhemispheric Global Atmospheric Circulations and Monsoons by the South Atlantic Meridional Overturning Circulation

Abstract This study presents a physical mechanism on how low-frequency variability of the South Atlantic meridional heat transport (SAMHT) may influence decadal variability of atmospheric circulation. A multicentury simulation of a coupled general circulation model is used as basis for the analysis. The highlight of the findings herein is that multidecadal variability of SAMHT plays a key role in modulating global atmospheric circulation via its influence on interhemispheric redistributions of momentum, heat, and moisture. Weaker SAMHT at 30°S produces anomalous ocean heat divergence over the South Atlantic, resulting in negative ocean heat content anomalies about 15–20 years later. This forces a thermally direct anomalous interhemispheric Hadley circulation, transporting anomalous atmospheric heat from the Northern Hemisphere (NH) to the Southern Hemisphere (SH) and moisture from the SH to the NH, thereby modulating global monsoons. Further analysis shows that anomalous atmospheric eddies transport heat northward in both hemispheres, producing eddy heat flux convergence (divergence) in the NH (SH) around 15°–30°, reinforcing the anomalous Hadley circulation. The effect of eddies on the NH (SH) poleward of 30° depicts heat flux divergence (convergence), which must be balanced by sinking (rising) motion, consistent with a poleward (equatorward) displacement of the jet stream. This study illustrates that decadal variations of SAMHT could modulate the strength of global monsoons with 15–20 years of lead time, suggesting that SAMHT is a potential predictor of global monsoon variability. A similar mechanistic link exists between the North Atlantic meridional heat transport (NAMHT) at 30°N and global monsoons.

On the evolution of Atlantic Meridional Overturning Circulation Fingerprint and implications for decadal predictability in the North Atlantic

AbstractIt has been suggested previously that the Atlantic Meridional Overturning Circulation (AMOC) anomaly associated with changes in the North Atlantic Deep Water formation propagates southward with an advection speed north of 34°N. In this study, using Geophysical Fluid Dynamics Laboratory Coupled Model version 2.1 (GFDL CM2.1), we show that this slow southward propagation of the AMOC anomaly is crucial for the evolution and the enhanced decadal predictability of the AMOC fingerprint—the leading mode of upper ocean heat content (UOHC) in the extratropical North Atlantic. A positive AMOC anomaly in northern high latitudes leads to a convergence/divergence of the Atlantic meridional heat transport (MHT) anomaly in the subpolar/Gulf Stream region, thus warming in the subpolar gyre (SPG) and cooling in the Gulf Stream region after several years. Recent decadal prediction studies successfully predicted the observed warm shift in the SPG in the mid‐1990s. Our results here provide the physical mechanism for the enhanced decadal prediction skills in the SPG UOHC.

The impact of historical biases on the XBT‐derived meridional overturning circulation estimates at 34°S

AbstractAn observational system based on high‐density expendable bathythermograph (XBT) data has provided the longest record of the South Atlantic meridional overturning and heat transport estimates across 34°S. Measurement biases are a point of concern for the capability of an XBT system to capture long‐term trends in volume and heat transports, and the impact of such biases on the meridional overturning estimates has never been quantified. In the present study, the sensitivity of the meridional overturning circulation and heat transport to uncertainties in XBT measurements is quantified under the framework of an eddy‐resolving model simulation. Results show that XBT measurement biases after 2010 can translate into small meridional overturning errors on the order of 3% or 0.38 Sv (1Sv = 106 m3 s−1), and 0.025 PW (1 PW = 1015 W) or 8% of the meridional heat transport in the model. Historical XBT‐derived trends in transport estimates across 34°S are stronger and statistically significant after the late 1990s, 0.3 Sv decade−1 and 0.02 PW decade−1. These trends are mostly due to the XBT linear depth bias, with smaller contributions associated with temperature and depth offsets from the historical record. Long‐term trends calculated from Simple Ocean Data Assimilation reanalysis, estimated as 0.1 Sv/decade and 0.006 PW/decade, are 3 times smaller than the XBT‐derived historical trends. Therefore, an adequate correction of historical XBT data is necessary for an early detection of trends in the meridional overturning circulation and heat transport.

Numerical simulation of the world ocean circulation and its climatic variability for 1948–2007 using the INMOM

The results of simulating global ocean circulation and its interannual variability in 1948–2007 using INM RAS ocean general circulation model INMOM (Institute of Numerical Mathematics Ocean Model) are presented. One of the INMOM versions is also used for the Black Sea dynamics simulation. The CORE datasets were used to set realistic atmospheric forcing. Sea ice area decrease by 2007 was reproduced in the Arctic Ocean that is in good agreement with observations. The interdecadal climatic variability was revealed with significant decrease of Atlantic thermohaline circulation (ATHC) and meridional heat transport (MHT) in North Atlantic (NA) since the late 1990’s. MHT presents decrease of heat transport from NA to the atmosphere since the mid-1990’s. Therefore the negative feedback is revealed in the Earth climate system that leads to reducing of climate warming caused primarily by anthropogenic factor for the last decades. Long-term variability (60 years) of ATHC is revealed as well which influences NA thermal state with 10 year delay. The assumption is argued that this mechanism can make a contribution in the ATHC own long-term variability.

The Atlantic Meridional Heat Transport at 26.5°N and Its Relationship with the MOC in the RAPID Array and the GFDL and NCAR Coupled Models

Abstract The link at 26.5°N between the Atlantic meridional heat transport (MHT) and the Atlantic meridional overturning circulation (MOC) is investigated in two climate models, the GFDL Climate Model version 2.1 (CM2.1) and the NCAR Community Climate System Model version 4 (CCSM4), and compared with the recent observational estimates from the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) array. Despite a stronger-than-observed MOC magnitude, both models underestimate the mean MHT at 26.5°N because of an overly diffuse thermocline. Biases result from errors in both overturning and gyre components of the MHT. The observed linear relationship between MHT and MOC at 26.5°N is realistically simulated by the two models and is mainly due to the overturning component of the MHT. Fluctuations in overturning MHT are dominated by Ekman transport variability in CM2.1 and CCSM4, whereas baroclinic geostrophic transport variability plays a larger role in RAPID. CCSM4, which has a parameterization of Nordic Sea overflows and thus a more realistic North Atlantic Deep Water (NADW) penetration, shows smaller biases in the overturning heat transport than CM2.1 owing to deeper NADW at colder temperatures. The horizontal gyre heat transport and its sensitivity to the MOC are poorly represented in both models. The wind-driven gyre heat transport is northward in observations at 26.5°N, whereas it is weakly southward in both models, reducing the total MHT. This study emphasizes model biases that are responsible for the too-weak MHT, particularly at the western boundary. The use of direct MHT observations through RAPID allows for identification of the source of the too-weak MHT in the two models, a bias shared by a number of Coupled Model Intercomparison Project phase 5 (CMIP5) coupled models.

Late Quaternary sedimentological and climate changes at Lake Bosumtwi Ghana: New constraints from laminae analysis and radiocarbon age modeling

Late Quaternary sedimentological and climate changes at Lake Bosumtwi Ghana: New constraints from laminae analysis and radiocarbon age modeling

Continuous observations of North Atlantic heat transport

The Atlantic meridional overturning circulation (AMOC), which transports warm water northward and cold water back southward, is important in transferring heat to the North Atlantic Ocean. Some models predict that AMOC will slow down as Earth's temperatures rise due to anthropogenic warming, which could have serious climate consequences for the Northern Hemisphere. However, the response of AMOC to global warming is uncertain—different models predict different rates of slowdown—and there have been few continuous observations of AMOC heat transport. Hobbs and Willis used temperature, salinity, and displacement data measured from foats in the Argo array, combined with sea surface heights measured by satellites, to estimate a continuous time series of Atlantic meridional heat transport from 2002 to 2010 at 41°N latitude. They found that the mean heat transport was about 0.5 petawatt. The authors note that this estimate is consistent with previous studies in similar latitudes based on atmospheric flux data but is lower than most hydrographic estimates. Heat transport varied on an annual cycle as well as on shorter time scales, with atmospheric variability explaining most of the short‐term variance. The researchers note that the period of study was too short to infer any long‐term trends, and they emphasize the need for continued monitoring of AMOC. (Journal of Geophysical Research‐Oceans, doi:10.1029/2011JC007039, 2012)

Midlatitude North Atlantic heat transport: A time series based on satellite and drifter data

Using temperature, salinity, and displacement data from Argo floats combined with satellite sea surface height, a time series of the Atlantic meridional heat transport from January 2002 to August 2010 has been estimated for 41°N. The calculation method is validated against hydrographic climatologies and output from the ECCO2 ocean data assimilation model, and the assumptions are shown to be reasonable; the greatest source of error is from the sparse distribution of Argo floats. The mean heat transport is 0.50 ± 0.1 PW, which is consistent with previous estimates made using surface flux data but is low compared estimates from hydrographic cruise data. Consistent with results from the RAPID array, the heat transport has a significant annual cycle and high degree of subannual variability, indicating that statistical uncertainty in previous calculations may have been underestimated. There is little evidence of a trend over the short period of available data. Correlations with sea surface temperature suggest clear physical relationships between heat transport and SST, even on the short time scales of available data.

Timescales and regions of the sensitivity of Atlantic meridional volume and heat transport: Toward observing system design

Timescales and regions of the sensitivity of Atlantic meridional volume and heat transport: Toward observing system design

Varied representation of the Atlantic Meridional Overturning across multidecadal ocean reanalyses

Varied representation of the Atlantic Meridional Overturning across multidecadal ocean reanalyses

Decadal variation of the North Atlantic meridional heat transport and its relation to atmospheric processes

The effects of the meridional heat transport in the North Atlantic Ocean (HTR) on the north hemispheric climate are studied using the results of the coupled model ECHAM5/MPI‐OM. Significant correlations exist between HTR and atmospheric processes over the Nordic Seas and the Eurasian continent only for low (periods longer than 40 years) and intermediate frequency variations (periods between 25 and 40 years). A positive HTR anomaly at 30°N is highly correlated with turbulent heat fluxes around 50°N. The transport through 70°N is directly related to the fluxes over the Nordic seas. From the correlation pattern with the atmospheric surface temperature and pressure one can conclude that the heat anomalies propagate along the cyclone tracks towards northeast over the Eurasian continent. The HRT anomalies are negatively correlated with the pressure over the Nordic seas and with the winter time anticyclone intensity over Siberia.

Changes in MOC and gyre‐induced Atlantic Ocean heat transport

Anthropogenic change of the Atlantic meridional ocean heat transport (AOHT) is diagnosed from a large ensemble of climate simulations over the period 1940–2080, as well as its relation to changes in the Meridional Overturning Circulation (MOC) and the gyre circulation in the Atlantic. Internal variability in AOHT is closely associated with MOC variability. The anthropogenic change in AOHT, however, does not follow the forced decrease in the MOC. Thermohaline changes in the intermediate and deep water masses that are formed in the subpolar gyre cause the vertical temperature gradient near the western boundary to increase. As a result, the heat transport by the baroclinic gyre component largely compensates the decreased heat transport by the MOC.

The dominant mechanisms of variability in Atlantic Ocean Heat Transport in a Coupled Ocean‐Atmosphere GCM

The variability of the Atlantic meridional ocean heat transport (OHT) has been diagnosed from a simulation of a coupled ocean‐atmosphere general circulation model (GCM), and the mechanisms responsible for this variability have been elucidated. Interannual variability is dominated by windstress‐driven Ekman fluctuations, which account for 50.3% of the OHT variance. By contrast, decadal and multidecadal variability in Atlantic OHT is dominated by a mixed thermohaline/gyre mode driven by variations in buoyancy fluxes and windstress curl. It accounts for 55.6% of low pass filtered OHT variance. The North Atlantic Oscillation (NAO) has a significant role in both the interannual mode and the low frequency mode, but it is not the only important driver. A notable feature of both modes is significant changes in the tropical atmosphere and ocean. We highlight a number of potential mechanisms involved in the tropical‐extratropical teleconnections.