A commentary on the Atlantic meridional overturning circulation stability in climate models

This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate.

Download Full-text

Mid-pliocene Atlantic Meridional Overturning Circulation not unlike modern

Climate of the Past ◽

10.5194/cp-9-1495-2013 ◽

2013 ◽

Vol 9 (4) ◽

pp. 1495-1504 ◽

Cited By ~ 42

Author(s):

Z.-S. Zhang ◽

K. H. Nisancioglu ◽

M. A. Chandler ◽

A. M. Haywood ◽

B. L. Otto-Bliesner ◽

...

Keyword(s):

Heat Transport ◽

Climate Models ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Ocean Heat Transport ◽

Overturning Circulation ◽

Consistent Change ◽

Intercomparison Project ◽

Meridional Overturning ◽

Consistent Increase

Abstract. In the Pliocene Model Intercomparison Project (PlioMIP), eight state-of-the-art coupled climate models have simulated the mid-Pliocene warm period (mPWP, 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), northward ocean heat transport and ocean stratification simulated with these models. None of the models participating in PlioMIP simulates a strong mid-Pliocene AMOC as suggested by earlier proxy studies. Rather, there is no consistent increase in AMOC maximum among the PlioMIP models. The only consistent change in AMOC is a shoaling of the overturning cell in the Atlantic, and a reduced influence of North Atlantic Deep Water (NADW) at depth in the basin. Furthermore, the simulated mid-Pliocene Atlantic northward heat transport is similar to the pre-industrial. These simulations demonstrate that the reconstructed high-latitude mid-Pliocene warming can not be explained as a direct response to an intensification of AMOC and concomitant increase in northward ocean heat transport by the Atlantic.

Download Full-text

Detectability of changes to the Atlantic meridional overturning circulation in the Hadley Centre Climate Models

Climate Dynamics ◽

10.1007/s00382-012-1306-3 ◽

2012 ◽

Vol 39 (9-10) ◽

pp. 2533-2546 ◽

Cited By ~ 4

Author(s):

Christopher D. Roberts ◽

Matthew D. Palmer

Keyword(s):

Climate Models ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning ◽

Hadley Centre

Download Full-text

Mid-pliocene Atlantic meridional overturning circulation not unlike modern?

Climate of the Past Discussions ◽

10.5194/cpd-9-1297-2013 ◽

2013 ◽

Vol 9 (2) ◽

pp. 1297-1319 ◽

Cited By ~ 4

Author(s):

Z.-S. Zhang ◽

K. H. Nisancioglu ◽

M. A. Chandler ◽

A. M. Haywood ◽

B. L. Otto-Bliesner ◽

...

Keyword(s):

Heat Transport ◽

Climate Models ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Ocean Heat Transport ◽

Overturning Circulation ◽

Consistent Change ◽

Intercomparison Project ◽

Meridional Overturning ◽

Consistent Increase

Abstract. In the Pliocene Model Intercomparison Project (PlioMIP), eight state-of-the-art coupled climate models have simulated the mid-Pliocene warm period (mPWP, 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), northward ocean heat transport and ocean stratification simulated with these models. None of the models participating in the PlioMIP simulates a strong mid-Pliocene AMOC as suggested by earlier proxy studies. Rather, there is no consistent increase in AMOC maximum among the PlioMIP models. The only consistent change in AMOC is a shoaling of the overturning cell in the Atlantic, and a reduced influence of North Atlantic Deep Water (NADW) at depth in the basin. Furthermore, the simulated mid-Pliocene Atlantic northward heat transport is similar to the pre-industrial. These simulations demonstrate that the reconstructed high latitude mid-Pliocene warming can not be explained as a direct response to an intensification of AMOC and concomitant increase in northward ocean heat transport by the Atlantic.

Download Full-text

Frequency Domain Multimodel Analysis of the Response of Atlantic Meridional Overturning Circulation to Surface Forcing

Journal of Climate ◽

10.1175/jcli-d-12-00717.1 ◽

2013 ◽

Vol 26 (21) ◽

pp. 8323-8340 ◽

Cited By ~ 18

Author(s):

Douglas G. MacMartin ◽

Eli Tziperman ◽

Laure Zanna

Keyword(s):

Frequency Domain ◽

Climate Models ◽

Transfer Functions ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Model Errors ◽

Overturning Circulation ◽

Model Version ◽

Spectral Peaks ◽

Meridional Overturning

Abstract The dynamics of the Atlantic meridional overturning circulation (AMOC) vary considerably among different climate models; for example, some models show clear peaks in their power spectra while others do not. To elucidate these model differences, transfer functions are used to estimate the frequency domain relationship between surface forcing fields, including sea surface temperature, salinity, and wind stress, and the resulting AMOC response. These are estimated from the outputs of the Coupled Model Intercomparison Project phase 5 (CMIP5) and phase 3 (CMIP3) control runs for eight different models, with a specific focus on Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1), and the Community Climate System Model, version 4 (CCSM4), which exhibit rather different spectral behavior. The transfer functions show very little agreement among models for any of the pairs of variables considered, suggesting the existence of systematic model errors and that considerable uncertainty in the simulation of AMOC in current climate models remains. However, a robust feature of the frequency domain analysis is that models with spectral peaks in their AMOC correspond to those in which AMOC variability is more strongly excited by high-latitude surface perturbations that have periods corresponding to the frequency of the spectral peaks. This explains why different models exhibit such different AMOC variability. These differences would not be evident without using a method that explicitly computes the frequency dependence rather than a priori assuming a particular functional form. Finally, transfer functions are used to evaluate two proposed physical mechanisms for model differences in AMOC variability: differences in Labrador Sea stratification and excitation by westward-propagating subsurface Rossby waves.

Download Full-text

Does the Atlantic Multidecadal Oscillation Get Its Predictability from the Atlantic Meridional Overturning Circulation?

Journal of Climate ◽

10.1175/jcli-d-16-0030.1 ◽

2016 ◽

Vol 29 (14) ◽

pp. 5267-5280 ◽

Cited By ~ 18

Author(s):

Laurie Trenary ◽

Timothy DelSole

Keyword(s):

Time Scales ◽

Autoregressive Model ◽

Climate Models ◽

Optimization Technique ◽

Atlantic Multidecadal Oscillation ◽

Atlantic Meridional Overturning Circulation ◽

Statistical Optimization ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning

Abstract This paper investigates the predictive relation between the Atlantic multidecadal oscillation (AMO) and Atlantic meridional overturning circulation across different climate models. Three overturning patterns that are significantly coupled to the AMO on interannual time scales across all climate models are identified using a statistical optimization technique. Including these structures in an autoregressive model extends AMO predictability by 2–9 years, relative to an autoregressive model without these structures.

Download Full-text

A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline

10.5194/os-2020-71 ◽

2020 ◽

Author(s):

Emma L. Worthington ◽

Ben I. Moat ◽

David A. Smeed ◽

Jennifer V. Mecking ◽

Robert Marsh ◽

...

Keyword(s):

Time Series ◽

Empirical Model ◽

Climate Models ◽

Linear Models ◽

Climate Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Deep Circulation ◽

Meridional Overturning

Abstract. A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID array with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this, however direct observations are sparse prior to 2004, giving only snapshots of the overturning circulation. Previous studies have used linear models based on upper layer temperature anomalies to extend AMOC estimates back in time, however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher fidelity empirical model of AMOC variability based on RAPID data, and associated physically with changes in thickness of the persistent upper, intermediate and deep water masses at 26° N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations, and that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate change-related AMOC decline.

Download Full-text

Surface Constraints on the Depth of the Atlantic Meridional Overturning Circulation: Southern Ocean versus North Atlantic

Journal of Climate ◽

10.1175/jcli-d-19-0546.1 ◽

2020 ◽

Vol 33 (8) ◽

pp. 3125-3149 ◽

Cited By ~ 2

Author(s):

Shantong Sun ◽

Ian Eisenman ◽

Laure Zanna ◽

Andrew L. Stewart

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Surface Density ◽

Climate Models ◽

Geometric Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning ◽

Surface Buoyancy

AbstractPaleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.

Download Full-text

A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline.

10.5194/egusphere-egu21-7817 ◽

2021 ◽

Author(s):

Emma Worthington ◽

Ben Moat ◽

David Smeed ◽

Jennifer Mecking ◽

Robert Marsh ◽

...

Keyword(s):

Time Series ◽

Empirical Model ◽

Climate Models ◽

Linear Models ◽

Climate Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Deep Circulation ◽

Meridional Overturning

<p>A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID array with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic re-search suggests that the AMOC may have been declining for decades or even centuries before this, however direct observations are sparse prior to 2004, giving only &#8216;snapshots&#8217; of the overturning circulation. Previous studies have used linear models based on upper layer temperature anomalies to extend AMOC estimates back in time, however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher fidelity empirical model of AMOC variability based on RAPID data, and associated physically with changes in thickness of the persistent upper, intermediate and deep water masses at 26&#176;N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC toapproximately annual shows multi-annual variability in agreement with RAPID observations, and that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline asindicated by other proxies and high resolution climate models. Our results reinforce that adequately capturing changes to thedeep circulation is key to detecting any anthropogenic climate change-related AMOC decline</p>

Download Full-text

High frequency variability of the Atlantic meridional overturning circulation

Ocean Science ◽

10.5194/os-7-471-2011 ◽

2011 ◽

Vol 7 (4) ◽

pp. 471-486 ◽

Cited By ~ 22

Author(s):

B. Balan Sarojini ◽

J. M. Gregory ◽

R. Tailleux ◽

G. R. Bigg ◽

A. T. Blaker ◽

...

Keyword(s):

Interannual Variability ◽

Annual Cycle ◽

Climate Models ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Overturning Circulation ◽

Wide Range ◽

Meridional Overturning ◽

Observational Estimate

Abstract. We compare the variability of the Atlantic meridional overturning circulation (AMOC) as simulated by the coupled climate models of the RAPID project, which cover a wide range of resolution and complexity, and observed by the RAPID/MOCHA array at about 26° N. We analyse variability on a range of timescales, from five-daily to interannual. In models of all resolutions there is substantial variability on timescales of a few days; in most AOGCMs the amplitude of the variability is of somewhat larger magnitude than that observed by the RAPID array, while the time-mean is within about 10 % of the observational estimate. The amplitude of the simulated annual cycle is similar to observations, but the shape of the annual cycle shows a spread among the models. A dynamical decomposition shows that in the models, as in observations, the AMOC is predominantly geostrophic (driven by pressure and sea-level gradients), with both geostrophic and Ekman contributions to variability, the latter being exaggerated and the former underrepresented in models. Other ageostrophic terms, neglected in the observational estimate, are small but not negligible. The time-mean of the western boundary current near the latitude of the RAPID/MOCHA array has a much wider model spread than the AMOC does, indicating large differences among models in the simulation of the wind-driven gyre circulation, and its variability is unrealistically small in the models. In many RAPID models and in models of the Coupled Model Intercomparison Project Phase 3 (CMIP3), interannual variability of the maximum of the AMOC wherever it lies, which is a commonly used model index, is similar to interannual variability in the AMOC at 26° N. Annual volume and heat transport timeseries at the same latitude are well-correlated within 15–45° N, indicating the climatic importance of the AMOC. In the RAPID and CMIP3 models, we show that the AMOC is correlated over considerable distances in latitude, but not the whole extent of the North Atlantic; consequently interannual variability of the AMOC at 50° N, where it is particularly relevant to European climate, is not well-correlated with that of the AMOC at 26° N, where it is monitored by the RAPID/MOCHA array.

Download Full-text