Simulation of the Atlantic meridional overturning circulation in an atmosphere–ocean global coupled model. Part I: a mechanism governing the variability of ocean convection in a preindustrial experiment

Abstract A variety of observational and modeling studies show that changes in the Atlantic meridional overturning circulation (AMOC) can induce rapid global-scale climate change. In particular, a substantially weakened AMOC leads to a southward shift of the intertropical convergence zone (ITCZ) in both the Atlantic and the Pacific Oceans. However, the simulated amplitudes of the AMOC-induced tropical climate change differ substantially among different models. In this paper, the sensitivity to cloud feedback of the climate response to a change in the AMOC is studied using a coupled ocean–atmosphere model [the GFDL Coupled Model, version 2.1 (CM2.1)]. Without cloud feedback, the simulated AMOC-induced climate change in this model is weakened substantially. Low-cloud feedback has a strong amplifying impact on the tropical ITCZ shift in this model, whereas the effects of high-cloud feedback are weaker. It is concluded that cloud feedback is an important contributor to the uncertainty in the global response to AMOC changes.

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Effects of Salt Compensation on the Climate Model Response in Simulations of Large Changes of the Atlantic Meridional Overturning Circulation*

Journal of Climate ◽

10.1175/2007jcli1662.1 ◽

2007 ◽

Vol 20 (24) ◽

pp. 5912-5928 ◽

Cited By ~ 34

Author(s):

Thomas F. Stocker ◽

Axel Timmermann ◽

Manuel Renold ◽

Oliver Timm

Keyword(s):

Southern Ocean ◽

Climate Model ◽

Temperature Response ◽

Coupled Model ◽

Atlantic Meridional Overturning Circulation ◽

Ocean Model ◽

Meridional Overturning Circulation ◽

Salt Flux ◽

Overturning Circulation ◽

Meridional Overturning

Abstract Freshwater hosing experiments with a comprehensive coupled climate model and a coupled model of intermediate complexity are performed with and without global salt compensation in order to investigate the robustness of the bipolar seesaw. In both cases, a strong reduction of the Atlantic meridional overturning circulation is induced, and a warming in the South Atlantic results. When a globally uniform salt flux is applied at the surface in order to keep the global mean salinity constant, this causes additional widespread warming in the Southern Ocean. It is shown that this warming is mainly due to heat transport anomalies that are induced by the specific parameterization in ocean models to represent eddy mixing. Surface salt fluxes tend to move outcropping isopycnals equatorward. As the density perturbation originates at the surface, changes in isopycnal slopes are generated that lead to anomalies in the bolus velocity field. The associated bolus heat flux convergence creates a warming enhancing the bipolar seesaw response, particularly in the Southern Ocean. The importance of this mechanism is illustrated in coupled model simulations in which this parameterization in the ocean model component is switched on or off. Additional experiments in which the same total amount of freshwater is delivered at rates 10 times smaller show that the effect of the global salt compensation is not important in this case, but that the eddy-mixing parameterization is still responsible for a substantial temperature response in the Southern Ocean.

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High frequency variability of the Atlantic meridional overturning circulation

Ocean Science Discussions ◽

10.5194/osd-8-219-2011 ◽

2011 ◽

Vol 8 (1) ◽

pp. 219-246 ◽

Cited By ~ 2

Author(s):

B. Balan Sarojini ◽

J. M. Gregory ◽

R. Tailleux ◽

G. R. Bigg ◽

A. T. Blaker ◽

...

Keyword(s):

Interannual Variability ◽

Climate Models ◽

Coupled Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

The North ◽

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. 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 amplitude of the simulated annual cycle is similar to observations. 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. 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 is not well-correlated with the AMOC at 26° N.

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A study of capturing Atlantic meridional overturning circulation (AMOC) regime transition through observation-constrained model parameters

Nonlinear Processes in Geophysics ◽

10.5194/npg-28-481-2021 ◽

2021 ◽

Vol 28 (4) ◽

pp. 481-500

Author(s):

Zhao Liu ◽

Shaoqing Zhang ◽

Yang Shen ◽

Yuping Guan ◽

Xiong Deng

Keyword(s):

Parameter Estimation ◽

Multiple Equilibria ◽

Coupled Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Model Parameters ◽

Model Errors ◽

Overturning Circulation ◽

Meridional Overturning ◽

Regime Transitions

Abstract. The multiple equilibria are an outstanding characteristic of the Atlantic meridional overturning circulation (AMOC) that has important impacts on the Earth climate system appearing as regime transitions. The AMOC can be simulated in different models, but the behavior deviates from the real world due to the existence of model errors. Here, we first combine a general AMOC model with an ensemble Kalman filter to form an ensemble coupled model data assimilation and parameter estimation (CDAPE) system and derive the general methodology to capture the observed AMOC regime transitions through utilization of observational information. Then we apply this methodology designed within a “twin” experiment framework with a simple conceptual model that simulates the transition phenomenon of AMOC multiple equilibria as well as a more physics-based MOC box model to reconstruct the “observed” AMOC multiple equilibria. The results show that the coupled model parameter estimation with observations can significantly mitigate the model deviations, thus capturing regime transitions of the AMOC. This simple model study serves as a guideline when a coupled general circulation model is used to incorporate observations to reconstruct the AMOC historical states and make multi-decadal climate predictions.

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Impact of North Atlantic – GIN Sea exchange on deglaciation evolution of the Atlantic Meridional Overturning Circulation

Climate of the Past ◽

10.5194/cp-7-935-2011 ◽

2011 ◽

Vol 7 (3) ◽

pp. 935-940 ◽

Cited By ~ 4

Author(s):

J. Cheng ◽

Z. Liu ◽

F. He ◽

B. L. Otto-Bliesner ◽

C. Colose

Keyword(s):

North Atlantic ◽

Coupled Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Last Deglaciation ◽

Overturning Circulation ◽

Key Factor ◽

Meridional Overturning ◽

Proxy Reconstructions ◽

The Impact

Abstract. In a transient simulation of the last deglaciation with a fully coupled model (TraCE-21000), an overshoot of the Atlantic Meridional Overturning Circulation (AMOC) is simulated and proposed as a key factor for the onset of the Bølling-Allerød (BA) warming event. There is collaborating evidence for an AMOC overshoot at the BA in various proxy reconstructions although the mechanism governing its behavior is not well understood. Here, we present two new sensitivity experiments to explicitly illustrate the impact of North Atlantic – GIN Sea exchange on the AMOC's deglacial evolution. Results show that this oceanic exchange dominates the convection restarting in the GIN Sea, the occurrence of the AMOC overshoot, and the full BA warming.

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On the Relationship between the North Atlantic Meridional Overturning Circulation and the Surface-Forced Overturning Streamfunction

Journal of Climate ◽

10.1175/2009jcli2574.1 ◽

2009 ◽

Vol 22 (19) ◽

pp. 4989-5002 ◽

Cited By ~ 29

Author(s):

Jeremy P. Grist ◽

Robert Marsh ◽

Simon A. Josey

Keyword(s):

Climate Models ◽

Coupled Model ◽

Surface Flux ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Transformation Theory ◽

Overturning Circulation ◽

Intergovernmental Panel ◽

The North ◽

Meridional Overturning

Abstract The influence of surface thermohaline forcing on the variability of the Atlantic meridional overturning circulation (MOC) at mid–high latitudes is investigated using output from three Intergovernmental Panel on Climate Change (IPCC) coupled climate models. The method employed is an extension of the surface-forced streamfunction approach, based on water mass transformation theory, used in an earlier study by Marsh (2000). The maximum value of the MOC at 48°N is found to have a significant lagged relationship with the maximum surface-forced streamfunction in the region north of 48°N with a surface density greater than σ0 = 27.5 kg m−3. This correlation peaks when the index of the surface-forced streamfunction leads the MOC by 2–4 yr, depending on the coupled model considered. A method for estimating the MOC variability solely from the surface forcing fields is developed and found to be in good agreement with the actual model MOC variability in all three of the models considered when a past averaging window of 10 yr is employed. This method is then applied with NCEP–NCAR reanalysis surface flux fields for the period 1949–2007 to reconstruct MOC strength over 1958–2007. The reconstructed MOC shows considerable multidecadal variability but no discernible trend over the modern observational era.

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