Bistability of the atlantic meridional overturning circulation

Bistability of the atlantic meridional overturning circulation

While none of the models incorporated the additional effects of ice sheet melting, they all projected very similar behaviour during the 21st century. The results support previous assessments that it is very unlikely that the MOC will undergo an abrupt change to an off state as a consequence of global warming. American Geophysical Union. All Rights Reserved. Skip to main content.

User menu Cart Login. Search form Search. Weaver, A. Eby, M. Alexander, K. Kawamiy, M. Matsumoto, K. Steinacher, M. Tachiiri, K. Tokos, K. Yoshimori, Masakazu Zickfeld, K. Alley R. A Report by the U. A report by the U.

What is the Atlantic Meridional Overturning Circulation?

Climate Change Science Program and the Subcommittee on Global Change Researchde Vries Pedro, The Atlantic freshwater budget as a diagnostic for the existence of a stable shut down of the meridional overturning circulation Bibliographic reference Weaver, A.As warm water flows northwards it cools and some evaporation occurs, which increases the amount of salt.

Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southwards, several kilometres below the surface. This, in turn, contributes to the climate we experience today.

Oceanographers have been measuring the AMOC continuously since The measurements have shown that the AMOC varies from year to year, and it is likely that these variations have an impact on the weather in the UK. However it is too early to say for sure whether there are any long term trends.

Before the AMOC was only measured a few times, and to go back further into the past we need to look at indirect evidence for example from sediments on the sea floor. Climate models suggest that the AMOC will weaken over the 21 st Century as greenhouse gases increase.

This is because as the atmosphere warms, the surface ocean beneath it retains more of its heat. Meanwhile increases in rainfall and ice melt mean it gets fresher too. A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of the greenhouse gases over western Europe.

For the gradual weakening that is likely over the 21 st Century, the overall effect is still a warming. A waving front or frontal wave is a disturbance that runs along a cold front and slows its clearance, of….

What is the Atlantic Meridional Overturning Circulation? How does the AMOC work? You might also like. Help us improve our website Take our short survey.Yeager, Daniel E. Amrhein, Christopher M. Reviews of Geophysics. DOI: This paper provides a comprehensive review of the linkage between multidecadal Atlantic Meridional Overturning Circulation AMOC variability and Atlantic Multidecadal Variability AMV and associated climate impacts, by synthesizing recent studies that employed a wide range of approaches modern observations, paleo reconstructions, and climate model simulations.

The AMOC, which includes a northward flow of warm salty water in the upper Atlantic and a southward flow of the transformed cold fresh North Atlantic Deep Water in the deep Atlantic, transports a huge amount of heat northwards in the Atlantic. There is strong observational and modeling evidence that multidecadal AMOC variability is a crucial driver of the observed AMV and associated climate impacts, and an important source of enhanced decadal predictability and prediction skill.

This synthesis also points to a leading role of the AMOC in a range of AMV-related climate phenomena having enormous societal and economic implications, e. Paleoclimate evidence indicates that a similar linkage between multidecadal AMOC variability and AMV and many associated climate impacts may also have existed in the preindustrial era; that AMV has enhanced multidecadal power significantly above a red noise background; and that AMV is not primarily driven by external forcing.

The role of the AMOC in AMV and associated climate impacts has been underestimated in most state-of-the-art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts.

On the stability of the Atlantic meridional overturning circulation

July 18th, Key Findings There is strong evidence that multidecadal AMOC variability is a crucial driver of AMV and associated climate impacts, and a significant source for their enhanced decadal predictability and prediction skill. Paleo evidence indicates that a similar linkage between the multidecadal AMOC variability and AMV may also have existed in the preindustrial era, and that AMV has enhanced multidecadal power significantly above the red noise background and is not dominated by external forcing.

The role of the AMOC in AMV and many associated climate impacts has been underestimated in most state-of-the-art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts. The role of the AMOC in AMV and associated climate impacts has been underestimated in most state-of-the-art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts Observed AMV index and sea surface temperature SST pattern associated with AMV.

The residual SST anomaly at each grid point is computed by removing the local component regressed on the global mean SST anomaly.

Sorry, your blog cannot share posts by email.Using a generalized stability indicator Lwe explore the stability of the Atlantic meridional overturning circulation AMOC during the last deglaciation based on a paleoclimate simulation. From the last glacial maximum, as forced by various external climate forcings, notably the meltwater forcing, the AMOC experiences a collapse and a subsequent rapid recovery in the early stage of deglaciation.

This change of the AMOC induces an anomalous freshwater divergence and later convergence across the Atlantic and therefore leads to a positive Lsuggesting a negative basin-scale salinity advection feedback and, in turn, a mono-stable deglacial AMOC.

Our study has important implications to the deglacial simulations by climate models. Both terms appear positive in model. However, the former is likely to be distorted towards positive, as associated with a common bias existing over the South Atlantic in climate models.

Therefore, the AMOC is potentially biased towards mono-stability in most paleoclimate simulations. This is a preview of subscription content, log in to check access.

bistability of the atlantic meridional overturning circulation

Rent this article via DeepDyve. Palaeogeogr Palaeoclimatol Palaeoecol — Science — Quat Sci Rev 19 1—5 — Nature — Geophys Res Lett L Google Scholar. Bryan F High-latitude salinity effects and interhemispheric thermohaline circulations. Quat Sci Rev — J Clim — Clim Dyn — Quat Sci Rev 21 1—3 :1—7. J Geophys Res C12 — Nat Geosci 5 4 — J Geophys Res — Dijkstra H Characterization of the multiple equilibria regime in a global ocean model.

Atlantic meridional overturning circulation

Tellus 59A— Dong B, Sutton RT Adjustment of the coupled ocean—atmosphere system to a sudden change in the thermohaline circulation. Geophys Res Lett J Phys Oceanogr —Liu, W. However, this estimation is very sensitive to the choice of the observational data. The Atlantic meridional overturning circulation AMOC has been suggested to play a key role in rapid climate changes in the past Clark et al.

The rapid change is associated with the nonlinear nature and, in turn, multiple equilibria of the AMOC. For example, Stouffer et al.

bistability of the atlantic meridional overturning circulation

To better study the stability of the AMOC, especially in a complex climate model and in the real world, it is highly desirable to have a diagnostic indicator. In a box model, in which the Atlantic and Arctic Oceans are combined into a single box, Rahmstorf found that the meridional freshwater transport associated with the AMOC F OT across the southern boundary of the Atlantic basin can be used as a diagnostic indicator for AMOC stability, with a positive F OT freshwater import and negative F OT freshwater export indicating a monostable regime and a bistable regime, respectively.

Weber et al. This is consistent with the hosing experiments of Stouffer et al. The latter will further lead to a basinwide freshening and in turn the suppression of deep convection in the North Atlantic and eventually the collapse of the AMOC.

In this argument, it is clear that a better indicator should be the net convergence of the freshwater transport associated with the AMOC, with a net convergence for the monostable regime and a net divergence for the bistable regime.

In general, the convergence indicator will not be the same as the transport indicator if the AMOC exchanges freshwater not only in the south with the Southern Ocean but also in the north with the Arctic. This convergence indicator has subsequently been shown valid in an OGCM coupled with an energy-balance atmosphere model Huisman et al. The remainder of the paper is structured as follows: Section 2 provides the information regarding the model and experimental design.

In section 3we propose and validate an improved AMOC stability indicator in the model. In section 4we explore the usage of the indicator for other applications.

Concluding remarks and further discussions are given in section 5. CCSM3 is a global, coupled ocean—atmosphere—sea ice—land surface climate model without flux adjustment Collins et al. Benefited from the x3ocn grid, the model resolution becomes significantly finer toward Greenland so that the model topography is well resolved in the Arctic and the North Atlantic. The Bering Strait is open, and the model resolution increases over the Canadian Archipelago so that it is possible to open a relatively realistic Northwest Passage between Baffin Bay and the Beaufort Sea.

By yearthe model has reached a quasi equilibrium, except for some very slow adjustment associated with the abyssal water Yeager et al. Thus, here, we start from year denoted here as new control year 0 and use the following yr model integration as the control experiment CTRL. A yr pulse of 1. The freshwater flux of 1. Here, it should be mentioned that a yr transient hosing is just a common routine for testing the AMOC stability in AOGCMs, which may not enable the model to reach a different steady state.

The experimental designs are shown in Fig. For the description of the experiments, see the main text. These two parts are defined as. However, as shown in Fig. The M az and M ov are calculated from the monthly output by Eqs. The zero line is drawn as a dashed line light gray; long dashed. The AMOC strength is defined as the maximum streamfunction value in the circulation below m within the North Atlantic basin. It is calculated from the annual mean output and shown as a decadal average.To browse Academia.

Skip to main content. Log In Sign Up. Download Free PDF. Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport Ed Hawkins. Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport. Hawkins1, R. Smith1, L. Allison1, J.

Gregory1,2, T. Woollings1, H. Pohlmann2 and B. Atlantic paleoclimate nized. However, up to now, no Bond et al. Here we demonstrate AMOC et al. The results also support magnitude of the change [Meehl et al. Deter- of bistability. We also present new estimates for this net mining the risk of such a rapid change occurring in the future critically depends on whether the AMOC exhibits bistable freshwater transport by the AMOC from a range of ocean behaviour and if so, whether the real ocean is currently close reanalyses which suggest that the Atlantic AMOC is cur- to a critical threshold for collapse [Knutti and Stocker ].

bistability of the atlantic meridional overturning circulation

However, with a correction and addition of extra unpub- such a collapse of the AMOC in the near future is considered lished results to Fig. A possible explanation for this increased stability is that the presence of a dynamical atmosphere in 1. An alternative ex- is an important component of the climate system. It trans- planation considers the net northwards freshwater transport ports heat and salt northwards from the tropics via the near- by the AMOC at the southern boundary of the Atlantic as a surface Gulf Stream and North Atlantic Current.

One mechanism due to computational constraints. Evidence from The pioneering work of Stommel [] first suggested that ocean reanalyses is used to suggest that the real ocean is cur- this density driven circulation in the Atlantic Ocean has two rently in a bistable regime in Section 5 and we conclude and discuss the results in Section 6. Top left panel: the control run mean. Other panels show time means of various portions of the transient increasing hosing simulation, labelled with the hosing value H at the end of the time period indicated.

Drijfhout, pers. The ocean component has a horizontal reso- Atlantic. The flux of freshwater is increased slowly from zero lution of 2. The hosing then becomes at over model years per wall-clock day, making it suitable negative, i. MOUS is in reasonable agreement with the observed climate Additional simulations keeping H fixed for at least several [Smith et al.

Atlantic Meridional Overturning Circulation

However, there is a winter cold bias in the hundred years were started from various points during both North Atlantic which may make the salinity contributions to the transient increase and decrease experiments. Deep water formation regions are found in the Irminger and Nordic Seas For all hosing experiments additional freshwater is added [Smith and Gregory ].

This hosing region is chosen ] and with recent observations at this latitude [Cunning- to allow a direct comparison with the previous study exam- ham et al.

Experimental design mation regions directly.Author contributions: M. One of the most important large-scale ocean current systems for Earth's climate is the Atlantic meridional overturning circulation AMOC. Here we review its stability properties and present new model simulations to study the AMOC's hysteresis response to freshwater perturbations.

We employ seven different versions of an Ocean General Circulation Model by using a highly accurate tracer advection scheme, which minimizes the problem of numerical diffusion. We find that a characteristic freshwater hysteresis also exists in the predominantly wind-driven, low-diffusion limit of the AMOC. However, the shape of the hysteresis changes, indicating that a convective instability rather than the advective Stommel feedback plays a dominant role.

We show that model errors in the mean climate can make the hysteresis disappear, and we investigate how model innovations over the past two decades, like new parameterizations and mixing schemes, affect the AMOC stability. Finally, we discuss evidence that current climate models systematically overestimate the stability of the AMOC. The water sinks in deep-water formation regions in the Nordic and Labrador Seas and returns to the south at depths of 2,—3, m.

Atlantic Meridional Overturning Circulation: Observed Transport and Variability

Paleoclimatic evidence points to instabilities in this current system and demonstrates that large and abrupt shifts in Atlantic ocean circulation have repeatedly occurred e. Global temperatures are projected to increase by up to 6. Moreover, higher sea surface temperatures will further decrease the density of waters in the Nordic Seas, inhibiting deep-water formation. The impacts of such a transition would likely be severe 6.

Research over the past decades, starting with Stommel's seminal paper 7has found interesting stability properties of the AMOC, reviewed, e. For increasing freshwater inflow to the Atlantic, this bistable regime passes a saddle-node bifurcation to a monostable regime where only the off state exists. For lesser freshwater inflow, there is another monostable regime where only the on state exists. This nonlinear behavior is explained by the positive salt-advection feedback first described by Stommel 7.

But it is the flow itself that maintains high salinity there in a region of net precipitation by bringing salty subtropical waters northward.

In the past decades, the AMOC was considered primarily thermohaline driven, which requires a strong role of turbulent mixing to bring buoyancy i. With recent observational estimates pointing to rather low turbulent mixing rates in the real ocean 910the alternative concept of a primarily wind-driven AMOC has arisen 11 These driving mechanisms have recently been reviewed by Kuhlbrodt et al.

These findings have caused a reexamination of the AMOC stability properties, and some authors have argued on theoretical grounds that a predominantly wind-driven AMOC would not show the freshwater hysteresis found in the relatively diffusive ocean models 14 — 16whereas others have argued that it would Progress in numerical techniques now allows us to test these hypotheses.

It was shown in ref. Here we present freshwater hysteresis studies with such an ocean model. We present these results in the context of a broader discussion of the realism of AMOC stability in models and the changes that have been brought by advances in coarse-resolution ocean and climate modeling over the past decades.

Investigating the equilibria and stability properties of the ocean circulation requires the integration of models over very long time spans. It takes several thousand years for the ocean circulation to approach a thermodynamic equilibrium state because of the slow mixing time scale in the ocean. In equilibrium, properties like the interior temperature and salinity fields are determined by a balance between the diffusive and advective transport processes the former representing turbulent mixing.


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