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

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.

scholarly journals A Study of Capturing AMOC Regime Transition through Observation-Constrained Model Parameters

2021 ◽  
Author(s):  
Zhao Liu ◽  
Shaoqing Zhang ◽  
Yang Shen ◽  
Yuping Guan ◽  
Xiong Deng
Keyword(s):  
Parameter Estimation ◽  
General Circulation ◽  
Multiple Equilibria ◽  
Coupled Model ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Model Parameters ◽  
Model Errors ◽  
Model Study ◽  
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.

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

2013 ◽  
Vol 26 (21) ◽  
pp. 8323-8340 ◽  
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.

Effect of the Atlantic Meridional Overturning Circulation on atmospheric pCO2

2021 ◽  
Author(s):  
Daan Boot ◽  
Henk Dijkstra
Keyword(s):  
Carbon Cycle ◽  
Multiple Equilibria ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Stable States ◽  
Overturning Circulation ◽  
Dimensional Parameter ◽  
Meridional Overturning ◽  
Project Model ◽  
Atmospheric Pco2

<p>The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in regulating the climate of the Northern Hemisphere. Several studies have shown that the AMOC can be in two stable states under equal forcing. This bistability, and associated tipping behavior, has been suggested as a mechanism for climate transitions in the past such as the Dansgaard-Oescher events. The relationship between AMOC variability and that in atmospheric pCO2 concentration is still unclear since different studies provide  contradictory results. Here, we investigate this  relationship using the Simple Carbon Project Model v1.0 (SCP-M), which we extended to represent a suite of nonlinear  carbon cycle feedbacks. By implementing SCP-M in the continuation and bifurcation software AUTO-07p, we can efficiently explore the multi-dimensional parameter space to address the AMOC - pCO2 relationship while varying the strengths of the carbon cycle feedbacks. We do not find multiple equilibria in the carbon-cycle dynamics, with fixed AMOC, but there are  intrinsic oscillations due to Hopf bifurcations with multi-millennial periods. The mechanisms of  this variability,  related to biological production and to calcium carbonate compensation, will be presented and their relevance  is addressed. </p>

The Impact of Wind Stress Feedback on the Stability of the Atlantic Meridional Overturning Circulation

2011 ◽  
Vol 24 (7) ◽  
pp. 1965-1984 ◽  
Author(s):  
Olivier Arzel ◽  
Matthew H. England ◽  
Oleg A. Saenko
Keyword(s):  
Wind Stress ◽  
Multiple Equilibria ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Atlantic Basin ◽  
Meridional Overturning ◽  
The Impact ◽  
Glacial Climate

Abstract Recent results based on models using prescribed surface wind stress forcing have suggested that the net freshwater transport Σ by the Atlantic meridional overturning circulation (MOC) into the Atlantic basin is a good indicator of the multiple-equilibria regime. By means of a coupled climate model of intermediate complexity, this study shows that this scalar Σ cannot capture the connection between the properties of the steady state and the impact of the wind stress feedback on the evolution of perturbations. This implies that, when interpreting the observed value of Σ, the position of the present-day climate is systematically biased toward the multiple-equilibria regime. The results show, however, that the stabilizing influence of the wind stress feedback on the MOC is restricted to a narrow window of freshwater fluxes, located in the vicinity of the state characterized by a zero freshwater flux divergence over the Atlantic basin. If the position of the present-day climate is farther away from this state, then wind stress feedbacks are unable to exert a persistent effect on the modern MOC. This is because the stabilizing influence of the shallow reverse cell situated south of the equator during the off state rapidly dominates over the destabilizing influence of the wind stress feedback when the freshwater forcing gets stronger. Under glacial climate conditions by contrast, a weaker sensitivity with an opposite effect is found. This is ultimately due to the relatively large sea ice extent of the glacial climate, which implies that, during the off state, the horizontal redistribution of fresh waters by the subpolar gyre does not favor the development of a thermally direct MOC as opposed to the modern case.

scholarly journals Sensitivity of Climate Change Induced by the Weakening of the Atlantic Meridional Overturning Circulation to Cloud Feedback

2010 ◽  
Vol 23 (2) ◽  
pp. 378-389 ◽  
Author(s):  
Rong Zhang ◽  
Sarah M. Kang ◽  
Isaac M. Held
Keyword(s):  
Climate Change ◽  
Coupled Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Global Scale ◽  
Cloud Feedback ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The Pacific ◽  
Low Cloud ◽  
Meridional Overturning

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.

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

2007 ◽  
Vol 20 (24) ◽  
pp. 5912-5928 ◽  
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.

scholarly journals High frequency variability of the Atlantic meridional overturning circulation

2011 ◽  
Vol 8 (1) ◽  
pp. 219-246 ◽  
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.

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

2011 ◽  
Vol 7 (3) ◽  
pp. 935-940 ◽  
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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