The impact of horizontal resolution of density field on the calculation of the Atlantic meridional overturning circulation at 34°S

Abstract Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

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Noise-induced transitions of the Atlantic Meridional Overturning Circulation in CMIP5 models

Scientific Reports ◽

10.1038/s41598-020-76930-5 ◽

2020 ◽

Vol 10 (1) ◽

Author(s):

Daniele Castellana ◽

Henk A. Dijkstra

Keyword(s):

Transition Probabilities ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Multiple Equilibrium ◽

Cmip5 Models ◽

Freshwater Flux ◽

Overturning Circulation ◽

Southern Boundary ◽

Meridional Overturning ◽

The Impact

AbstractBy studying transition probabilities of the Atlantic Meridional Overturning Circulation (AMOC) in an ensemble of CMIP5 climate models, we revisit one of the stability indicators of the AMOC, i.e. the freshwater transport carried by the AMOC at the southern boundary of the Atlantic basin. A correction to this indicator, based on the transition probabilities, is suggested to measure whether an AMOC state is in a multiple equilibrium regime or not. As a consequence, the AMOC of all CMIP5 models considered is in a multiple equilibrium regime and hence, in principle, a collapsed AMOC state should exist in each of these models. The results further demonstrate the dependence of the Atlantic surface freshwater flux on the AMOC and the impact of extreme events in the AMOC on temperatures in the North Atlantic region.

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

Journal of Climate ◽

10.1175/2010jcli3137.1 ◽

2011 ◽

Vol 24 (7) ◽

pp. 1965-1984 ◽

Cited By ~ 4

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.

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The Atlantic Meridional Overturning Circulation and the Cabbeling Effect

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0085.1 ◽

2020 ◽

Vol 50 (9) ◽

pp. 2561-2572

Author(s):

Fabian Schloesser

Keyword(s):

North Atlantic ◽

Hydrological Cycle ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Density Gradients ◽

Overturning Circulation ◽

Mean Temperature ◽

Freshwater Forcing ◽

Meridional Overturning ◽

The Impact

AbstractNorth Atlantic meridional density gradients have been identified as a main driver of the Atlantic meridional overturning circulation (AMOC). Due to the cabbeling effect, these density gradients are increasingly dominated by temperature gradients in a warming ocean, and a direct link exists between North Atlantic mean temperature and AMOC strength. This paper quantifies the impact of this mechanism in the Stommel and Gnanadesikan models. Owing to different feedback mechanisms being included, a 1°C warming of North Atlantic mean ocean temperature strengthens the AMOC by 3% in the Gnanadesikan model and 8% in the Stommel model. In the Gnanadesikan model that increase is equivalent to a 4% strengthening of Southern Hemisphere winds and can compensate for a 14% increase in the hydrological cycle. Furthermore, mean temperature strongly controls a freshwater forcing threshold for the strong AMOC state, suggesting that the cabbeling effect needs to be considered to explain past and future AMOC variability.

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Effect of Atmospheric Feedbacks on the Stability of the Atlantic Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-11-00467.1 ◽

2012 ◽

Vol 25 (12) ◽

pp. 4081-4096 ◽

Cited By ~ 14

Author(s):

Matthijs den Toom ◽

Henk A. Dijkstra ◽

Andrea A. Cimatoribus ◽

Sybren S. Drijfhout

Keyword(s):

Atlantic Meridional Overturning Circulation ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Freshwater Flux ◽

Overturning Circulation ◽

Freshwater Fluxes ◽

Atmospheric Feedbacks ◽

Meridional Overturning ◽

The Impact

Abstract The impact of atmospheric feedbacks on the multiple equilibria (ME) regime of the Atlantic meridional overturning circulation (MOC) is investigated using a fully implicit hybrid coupled model (HCM). The HCM consists of a global ocean model coupled to an empirical atmosphere model that is based on linear regressions of the heat, net evaporative, and momentum fluxes generated by a fully coupled climate model onto local as well as Northern Hemisphere averaged sea surface temperatures. Using numerical continuation techniques, bifurcation diagrams are constructed for the HCM with the strength of an anomalous freshwater flux as the bifurcation parameter, which allows for an efficient first-order estimation of the effect of interactive surface fluxes on the MOC stability. The different components of the atmospheric fluxes are first considered individually and then combined. Heat feedbacks act to destabilize the present-day state of the MOC and to stabilize the collapsed state, thus leaving the size of the ME regime almost unaffected. In contrast, interactive freshwater fluxes cause a destabilization of both the present-day and collapsed states of the MOC. Wind feedbacks are found to have a minor impact. The joint effect of the three interactive fluxes is to narrow the range of ME. The shift of the saddle-node bifurcation that terminates the present-day state of the ocean is further investigated by adjoint sensitivity analysis of the overturning rate to surface fluxes. It is found that heat feedbacks primarily affect the MOC stability when they change the heat fluxes over the North Atlantic subpolar gyre, whereas interactive freshwater fluxes have an effect everywhere in the Atlantic basin.

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The impact of multidecadal Atlantic meridional overturning circulation variations on the Southern Ocean

Climate Dynamics ◽

10.1007/s00382-016-3190-8 ◽

2016 ◽

Vol 48 (5-6) ◽

pp. 2065-2085 ◽

Cited By ~ 9

Author(s):

Liping Zhang ◽

Thomas L. Delworth ◽

Fanrong Zeng

Keyword(s):

Southern Ocean ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning ◽

The Impact

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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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Interaction of Atlantic Meridional Overturning Circulation and Sub-Polar Gyre on decadal timescale

10.5194/egusphere-egu21-5333 ◽

2021 ◽

Author(s):

Stephen Ogungbenro ◽

Leonard Borchert ◽

Sebastian Brune ◽

Vimal Koul ◽

Levke Caesar ◽

...

Keyword(s):

North Atlantic ◽

Large Scale ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Max Planck ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning ◽

The North Atlantic ◽

The Impact

<p>North Atlantic climate variability is dominated by two important subsystems, the Atlantic Meridional Overturning Circulation (AMOC) and the Sub-Polar Gyre (SPG). While the AMOC is responsible for the transport of mass and heat into higher latitudes, SPG has been linked with large-scale changes in the subpolar marine environment. The changes in strength, intensity and positions of the constituent currents of the SPG impose variabilities in the distribution of heat and salt in the North Atlantic Ocean. Consequently, the predictability on decadal scales of the two subsystems is of huge importance for the understanding of variability in the North Atlantic.</p><p>Our contribution investigates the decadal and multi-decadal predictability of these subsystems within the Max Planck Institute for Meteorology Earth System Model (MPI-ESM). We analyse the model&#8217;s capability to predict these subsystems as well as the dependence of the two subsystems on each other. These investigations open new opportunities for a better understanding of the impact of the North Atlantic onto important marine ecosystems and its changes in the upcoming decade.</p>

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