scholarly journals An Indicator of the Multiple Equilibria Regime of the Atlantic Meridional Overturning Circulation

2010 ◽  
Vol 40 (3) ◽  
pp. 551-567 ◽  
Author(s):  
Selma E. Huisman ◽  
Matthijs den Toom ◽  
Henk A. Dijkstra ◽  
Sybren Drijfhout
Keyword(s):  
Ocean Circulation ◽  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Multiple Equilibrium ◽  
Global Ocean ◽  
Overturning Circulation ◽  
Freshwater Transport ◽  
Freshwater Budget ◽  
Meridional Overturning

Abstract Recent model results have suggested that there may be a scalar indicator Σ monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity Σ is based on the net freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased freshwater input in the northern North Atlantic. This paper addresses the issue of why the sign of Σ is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state freshwater budget and hence to Σ. The sign of Σ precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.

scholarly journals Intrinsic Variability of the Atlantic Meridional Overturning Circulation at Interannual-to-Multidecadal Time Scales

2015 ◽  
Vol 45 (7) ◽  
pp. 1929-1946 ◽  
Author(s):  
Sandy Grégorio ◽  
Thierry Penduff ◽  
Guillaume Sérazin ◽  
Jean-Marc Molines ◽  
Bernard Barnier ◽  
...  
Keyword(s):  
Time Scales ◽  
Gulf Stream ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Intrinsic Variability ◽  
Meridional Heat Transport ◽  
Overturning Circulation ◽  
Spectral Peaks ◽  
Meridional Overturning

AbstractThe low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, ¼°, and ° global ocean–sea ice simulations, with a specific focus on its internally generated (i.e., “intrinsic”) component. A 327-yr climatological ¼° simulation, driven by a repeated seasonal cycle (i.e., a forcing devoid of interannual time scales), is shown to spontaneously generate a significant fraction R of the interannual-to-decadal AMOC variance obtained in a 50-yr “fully forced” hindcast (with reanalyzed atmospheric forcing including interannual time scales). This intrinsic variance fraction R slightly depends on whether AMOCs are computed in geopotential or density coordinates, and on the period considered in the climatological simulation, but the following features are quite robust when mesoscale eddies are simulated (at both ¼° and ° resolutions); R barely exceeds 5%–10% in the subpolar gyre but reaches 30%–50% at 34°S, up to 20%–40% near 25°N, and 40%–60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 34°S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr ¼° climatological simulation, further indicate that spectral peaks of intrinsic AMOC variability (i) are found at specific frequencies ranging from interannual to multidecadal, (ii) often extend over the whole meridional scale of gyres, (iii) stochastically change throughout these 327 yr, and (iv) sometimes match the spectral peaks found in the fully forced hindcast in the North Atlantic. Intrinsic AMOC variability is also detected at multidecadal time scales, with a marked meridional coherence between 35°S and 25°N (15–30 yr periods) and throughout the whole basin (50–90-yr periods).

scholarly journals On the Near-Inertial Resonance of the Atlantic Meridional Overturning Circulation

2013 ◽  
Vol 43 (12) ◽  
pp. 2661-2672 ◽  
Author(s):  
Florian Sévellec ◽  
Joël J.-M. Hirschi ◽  
Adam T. Blaker
Keyword(s):  
General Circulation ◽  
Numerical Models ◽  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Resonance Phenomenon ◽  
Dimensionless Number ◽  
Overturning Circulation ◽  
Wide Range ◽  
Meridional Overturning

Abstract The Atlantic meridional overturning circulation (AMOC) is a crucial component of the global climate system. It is responsible for around a quarter of the global northward heat transport and contributes to the mild European climate. Observations and numerical models suggest a wide range of AMOC variability. Recent results from an ocean general circulation model (OGCM) in a high-resolution configuration (¼°) suggest the existence of superinertial variability of the AMOC. In this study, the validity of this result in a theoretical framework is tested. At a low Rossby number and in the presence of Rayleigh friction, it is demonstrated that, unlike a typical forced damped oscillator (which shows subinertial resonance), the AMOC undergoes both super- and subinertial resonances (except at low latitudes and for high friction). A dimensionless number Sr, measuring the ratio of ageo- to geostrophic forcing (i.e., the zonal versus meridional pressure gradients), indicates which of these resonances dominates. If Sr ≪ 1, the AMOC variability is mainly driven by geostrophic forcing and shows subinertial resonance. Alternatively and consistent with the recently published ¼° OGCM experiments, if Sr ≫ 1, the AMOC variability is mainly driven by the ageostrophic forcing and shows superinertial resonance. In both regimes, a forcing of ±1 K induces an AMOC variability of ±10 Sv (1 Sv ≡ 106 m3 s−1) through these near-inertial resonance phenomena. It is also shown that, as expected from numerical simulations, the spatial structure of the near-inertial AMOC variability corresponds to equatorward-propagating waves equivalent to baroclinic Poincaré waves. The long-time average of this resonance phenomenon, raising and depressing the pycnocline, could contribute to the mixing of the ocean stratification.

An Adjoint Analysis of the Meridional Overturning Circulation in a Hybrid Coupled Model

2006 ◽  
Vol 19 (15) ◽  
pp. 3751-3767 ◽  
Author(s):  
Véronique Bugnion ◽  
Chris Hill ◽  
Peter H. Stone
Keyword(s):  
Boundary Conditions ◽  
Wind Stress ◽  
Ocean Circulation ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Model Parameters ◽  
Overturning Circulation ◽  
Near Surface ◽  
Meridional Overturning

Abstract Multicentury sensitivities in a realistic geometry global ocean general circulation model are analyzed using an adjoint technique. This paper takes advantage of the adjoint model’s ability to generate maps of the sensitivity of a diagnostic (i.e., the meridional overturning’s strength) to all model parameters. This property of adjoints is used to review several theories, which have been elaborated to explain the strength of the North Atlantic’s meridional overturning. This paper demonstrates the profound impact of boundary conditions in permitting or suppressing mechanisms within a realistic model of the contemporary ocean circulation. For example, the so-called Drake Passage Effect in which wind stress in the Southern Ocean acts as the main driver of the overturning’s strength, is shown to be an artifact of boundary conditions that restore the ocean’s surface temperature and salinity toward prescribed climatologies. Advective transports from the Indian and Pacific basins play an important role in setting the strength of the overturning circulation under “mixed” boundary conditions, in which a flux of freshwater is specified at the ocean’s surface. The most “realistic” regime couples an atmospheric energy and moisture balance model to the ocean. In this configuration, inspection of the global maps of sensitivity to wind stress and diapycnal mixing suggests a significant role for near-surface Ekman processes in the Tropics. Buoyancy also plays an important role in setting the overturning’s strength, through direct thermal forcing near the sites of convection, or through the advection of salinity anomalies in the Atlantic basin.

scholarly journals Regional Patterns of Sea Level Change Related to Interannual Variability and Multidecadal Trends in the Atlantic Meridional Overturning Circulation*

2010 ◽  
Vol 23 (15) ◽  
pp. 4243-4254 ◽  
Author(s):  
K. Lorbacher ◽  
J. Dengg ◽  
C. W. Böning ◽  
A. Biastoch
Keyword(s):  
Sea Level ◽  
Time Scales ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
Regional Patterns ◽  
Model Experiments ◽  
Basin Scale ◽  
Meridional Overturning

Abstract Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.

scholarly journals Two Decades of the Atlantic Meridional Overturning Circulation: Anatomy, Variations, Extremes, Prediction, and Overcoming Its Limitations

2013 ◽  
Vol 26 (18) ◽  
pp. 7167-7186 ◽  
Author(s):  
Carl Wunsch ◽  
Patrick Heimbach
Keyword(s):  
Ocean Circulation ◽  
Extreme Values ◽  
Statistical Significance ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Gaussian Stochastic Process ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The zonally integrated meridional volume transport in the North Atlantic [Atlantic meridional overturning circulation (AMOC)] is described in a 19-yr-long ocean-state estimate, one consistent with a diverse global dataset. Apart from a weak increasing trend at high northern latitudes, the AMOC appears statistically stable over the last 19 yr with fluctuations indistinguishable from those of a stationary Gaussian stochastic process. This characterization makes it possible to study (using highly developed tools) extreme values, predictability, and the statistical significance of apparent trends. Gaussian behavior is consistent with the central limit theorem for a process arising from numerous independent disturbances. In this case, generators include internal instabilities, changes in wind and buoyancy forcing fields, boundary waves, the Gulf Stream and deep western boundary current transports, the interior fraction in Sverdrup balance, and all similar phenomena arriving as summation effects from long distances and times. As a zonal integral through the sum of the large variety of physical processes in the three-dimensional ocean circulation, understanding of the AMOC, if it is of central climate importance, requires breaking it down into its unintegrated components over the entire basin.

A Diagnostic Indicator of the Stability of the Atlantic Meridional Overturning Circulation in CCSM3

2013 ◽  
Vol 26 (6) ◽  
pp. 1926-1938 ◽  
Author(s):  
Wei Liu ◽  
Zhengyu Liu
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Overturning Circulation ◽  
Atlantic Basin ◽  
Climate System Model ◽  
Freshwater Transport ◽  
Meridional Overturning ◽  
The Stability ◽  
Diagnostic Indicator

Abstract A diagnostic indicator ΔMov is proposed in this paper to monitor the stability of the Atlantic meridional overturning circulation (AMOC). The ΔMov is a diagnostic for a basinwide salt-advection feedback and defined as the difference between the freshwater transport induced by the AMOC across the southern border of the Atlantic Ocean and the overturning liquid freshwater transport from the Arctic Ocean to the North Atlantic. As validated in the Community Climate System Model, version 3 (CCSM3), for an AMOC in the conveyor state, a positive ΔMov (freshwater convergence) in the Atlantic basin indicates a monostable AMOC and a negative ΔMov (freshwater divergence) indicates a bistable AMOC. Based on ΔMov, the authors investigate the AMOC stability in the Last Glacial Maximum (LGM) and analyze the modulation of the AMOC stability by an open/closed Bering Strait. Moreover, the authors estimate that the real AMOC is likely to be bistable in the present day, since some observations suggest a negative ΔMov (freshwater divergence) is currently in the Atlantic basin. However, this estimation is very sensitive to the choice of the observational data.

scholarly journals Sediment <sup>231</sup>Pa/<sup>230</sup>Th as a recorder of the rate of the Atlantic meridional overturning circulation: insights from a 2-D model

2009 ◽  
Vol 6 (3) ◽  
pp. 2755-2829 ◽  
Author(s):  
Y. Luo ◽  
R. Francois ◽  
S. E. Allen
Keyword(s):  
Circulation Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Equatorial Atlantic ◽  
Overturning Circulation ◽  
Data Set ◽  
The North ◽  
Meridional Overturning ◽  
The Relationship ◽  
D Model

Abstract. A two dimensional scavenging-circulation model is used to investigate the patterns of sediment 231Pa/230Th generated by the Atlantic Meridional Overturning Circulation (AMOC) and further advance the application of this proxy for ocean paleocirculation studies. The scavenging parameters and the geometry of the overturning circulation cell have been chosen so that the model generates meridional sections of dissolved 230Th and 231Pa consistent with published water column profiles and an additional 12 previously unpublished profiles measured in the North and Equatorial Atlantic. The processes that generate the meridional sections of dissolved and particulate 230Th, dissolved and particulate 231Pa, dissolved and particulate 231Pa/230Th, and sediment 231Pa/230Th are discussed in detail. The results indicate that the relationship between sediment 231Pa/230Th at any given site and the overturning circulation is very complex. They clearly show that constraining past changes in the strength and geometry of the AMOC requires an extensive data set and they suggest strategies to maximize information from a limited number of samples.

scholarly journals An abrupt slowdown of Atlantic Meridional Overturning Circulation during 1915–1935 induced by solar forcing in a coupled GCM

2014 ◽  
Vol 10 (3) ◽  
pp. 2519-2546 ◽  
Author(s):  
P. Lin ◽  
Y. Song ◽  
Y. Yu ◽  
H. Liu
Keyword(s):  
Abrupt Change ◽  
Polar Vortex ◽  
Atlantic Meridional Overturning Circulation ◽  
Total Solar Irradiance ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
High Latitudes ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract. In this study, we explore an abrupt change of Atlantic Meridional Overturning Circulation (AMOC) apparent in the historical run simulated by the second version of the Flexible Global Ocean–Atmosphere–Land System model – Spectral Version 2 (FGOALS-s2). The abrupt change is noted during the period from 1915 to 1935, in which the maximal AMOC value is weakened beyond 6 Sv (1 Sv = 106 m3 s−1). The abrupt signal first occurs at high latitudes (north of 46° N), then shifts gradually to middle latitudes (∼35° N) three to seven years later. The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS) during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO) phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC. Moreover, due to increased TIS after 1914, the enhanced Atlantic northward ocean heat transport from low to high latitudes induces an abrupt warming of sea surface temperature or upper ocean temperature in mid–high latitudes, which can also weaken the AMOC. The abrupt change of AMOC also appears in the PiControl run, which is associated with the lasting negative NAO phases due to natural variability.

scholarly journals Larger Role for Shallow Intermediate Waters in Ocean Circulation

Eos ◽  
2020 ◽  
Vol 101 ◽  
Author(s):  
Sara Pratt
Keyword(s):  
Ocean Circulation ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Intermediate Waters

Water masses formed off southeastern Greenland may contribute more than previously thought to the variability of the Atlantic Meridional Overturning Circulation, which strongly influences global climate.

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

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.

Sign in / Sign up

Export Citation Format

Share Document