scholarly journals Chaotic variability of the meridional overturning circulation on subannual to interannual timescales

2012 ◽  
Vol 9 (5) ◽  
pp. 3191-3238 ◽  
Author(s):  
J. J.-M. Hirschi ◽  
A. T. Blaker ◽  
B. Sinha ◽  
A. Coward ◽  
B. de Cuevas ◽  
...  
Keyword(s):  
Initial Conditions ◽  
Large Fraction ◽  
Mesoscale Eddy ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
Ocean Eddies ◽  
Mesoscale Ocean Eddies ◽  
Meridional Overturning

Abstract. Observations and numerical simulations have shown that the meridional overturning circulation (MOC) exhibits substantial variability on sub- to interannual timescales. This variability is not fully understood. In particular it is not known what fraction of the MOC variability is caused by processes such as mesoscale ocean eddies and internal waves which are ubiquitous in the ocean. Here we analyse twin experiments performed with a global ocean model at eddying (1/4°) and non-eddying (1°) resolutions. The twin experiments are forced with the same surface fluxes for the 1958 to 2001 period but start from different initial conditions. Our results show that on subannual to interannual timescales a large fraction of MOC variability directly reflects variability in the surface forcing. Nevertheless, in the eddy-permitting case there is an initial condition dependent MOC variability (hereinafter referred to as "chaotic" variability) of several Sv (1 Sv = 106 m3 s−1) in the Atlantic and the Indo-Pacific. In the Atlantic the chaotic MOC variability represents up to 30% of the total variability at the depths where the maximum MOC occurs. In comparison the chaotic MOC variability is only 5–10% in the non-eddying case. The surface forcing being identical in the twin experiments suggests that mesoscale ocean eddies are the most likely cause for the increased chaotic MOC variability in the eddying case. The exact formation time of eddies is determined by the initial conditions which are different in the two accordance with and as a consequence the mesoscale eddy field is decorrelated in the twin experiments. In regions where eddy activity is high in the eddy-permitting model, the correlation of sea surface height variability in the twin runs is close to zero. In the non-eddying case in contrast, we find high correlations (0.9 or higher) over most regions. Looking at the sub- and interannual MOC components separately reveals that despite the amplitude of the chaotic variability being larger on subannual than on interannual timescales, the ratio of chaotic to total MOC variability is larger on interannual than on subannual timescales.

scholarly journals Chaotic variability of the meridional overturning circulation on subannual to interannual timescales

Ocean Science ◽  
2013 ◽  
Vol 9 (5) ◽  
pp. 805-823 ◽  
Author(s):  
J. J.-M. Hirschi ◽  
A. T. Blaker ◽  
B. Sinha ◽  
A. Coward ◽  
B. de Cuevas ◽  
...  
Keyword(s):  
Initial Conditions ◽  
Large Fraction ◽  
Mesoscale Eddy ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
Ocean Eddies ◽  
Mesoscale Ocean Eddies ◽  
Meridional Overturning

Abstract. Observations and numerical simulations have shown that the meridional overturning circulation (MOC) exhibits substantial variability on sub- to interannual timescales. This variability is not fully understood. In particular it is not known what fraction of the MOC variability is caused by processes such as mesoscale ocean eddies and waves which are ubiquitous in the ocean. Here we analyse twin experiments performed with a global ocean model at eddying (1/4°) and non-eddying (1°) resolutions. The twin experiments are forced with the same surface fluxes for the 1958 to 2001 period but start from different initial conditions. Our results show that on subannual to interannual timescales a large fraction of MOC variability directly reflects variability in the surface forcing. Nevertheless, in the eddy-permitting case there is an initial-condition-dependent MOC variability (hereinafter referred to as "chaotic" variability) of several Sv (1Sv = 106 m3 s−1) in the Atlantic and the Indo-Pacific. In the Atlantic the chaotic MOC variability represents up to 30% of the total variability at the depths where the maximum MOC occurs. In comparison the chaotic MOC variability is only 5–10% in the non-eddying case. The surface forcing being almost identical in the twin experiments suggests that mesoscale ocean eddies are the most likely cause for the increased chaotic MOC variability in the eddying case. The exact formation time of eddies is determined by the initial conditions which are different in the two model passes, and as a consequence the mesoscale eddy field is decorrelated in the twin experiments. In regions where eddy activity is high in the eddy-permitting model, the correlation of sea surface height variability in the twin runs is close to zero. In the non-eddying case in contrast, we find high correlations (0.9 or higher) over most regions. Looking at the sub- and interannual MOC components separately reveals that most of the chaotic MOC variability is found on subannual timescales for the eddy-permitting model. On interannual timescales the amplitude of the chaotic MOC variability is much smaller and the amplitudes are comparable for both the eddy-permitting and non-eddy-permitting model resolutions. Whereas the chaotic MOC variability on interannual timescales only accounts for a small fraction of the total chaotic MOC variability in the eddy-permitting case, it is the main contributor to the chaotic variability in the non-eddying case away from the Equator.

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

2012 ◽  
Vol 25 (12) ◽  
pp. 4081-4096 ◽  
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.

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).

Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic

2021 ◽  
Author(s):  
Claus W. Böning ◽  
Arne Biastoch ◽  
Klaus Getzlaff ◽  
Patrick Wagner ◽  
Siren Rühs ◽  
...  
Keyword(s):  
High Resolution ◽  
North Atlantic ◽  
Deep Convection ◽  
Alternative Interpretation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Labrador Sea ◽  
Overturning Circulation ◽  
First Results ◽  
Meridional Overturning

<p>A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20°) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4°-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by “sinking” (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.</p>

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.

14C Ventilation ages suggest a brief reversal of ocean Meridional Overturning Circulation during deglacial ‘Heinrich Stadial 1’

2020 ◽  
Author(s):  
Michael Sarnthein ◽  
Pieter M. Grootes
Keyword(s):  
North Pacific ◽  
Model Simulation ◽  
Ice Cores ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Age Difference ◽  
Overturning Circulation ◽  
Deep Waters ◽  
Global Correlation ◽  
Meridional Overturning

<p>Changes in the geometry of ocean Meridional Overturning Circulation (MOC) are crucial in controlling changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes in the MOC of different ocean basins still present a major challenge. The suite of jumps and plateaus in the record of past atmospheric radiocarbon (<sup>14</sup>C) concentrations offers a unique opportunity of age control and global correlation. The upper and lower boundaries of atmospheric <sup>14</sup>C plateaus in the <sup>14</sup>C records of both tree rings and Lake Suigetsu (age calibrated on the basis of Hulu U/Th model ages)­ provide a detailed stratigraphic ’rung ladder’ of ~30 age tie points from 29 to 10 ka that can be used for dating of planktic <sup>14</sup>C records and an age correlation, by now employed to ~20 sediment cores obtained from key locations of MOC all over the global ocean. The age difference between paired planktic and benthic <sup>14</sup>C ages provides an estimate of the ventilation age of deep waters since their last contact with the atmosphere. <sup>14</sup>C ventilation ages of Last Glacial Maximum (LGM) deep waters reveal coeval opposed geometries of Atlantic and Pacific MOC. Similar to today, LGM Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. Vice versa, low <sup>14</sup>C ventilation ages of N.E. Pacific deep waters suggest a reversed, anti-estuarine MOC during early Heinrich Stadial 1 with a ~1500 year-long flushing of the deep North Pacific up to the South China Sea, when the North Atlantic was marked by an estuarine circulation geometry, gradually starting near 19 ka. Elevated <sup>14</sup>C ventilation ages of LGM deep waters reflect a major drawdown of atmospheric carbon. Subsequent massive age drops accompanying changes in MOC reflect major events of carbon release to the atmosphere as recorded in Antarctic ice cores. These contemporaneous features of the MOC and the carbon cycle offer a great test case for comparison with model simulation.</p>

scholarly journals Optimal Surface Salinity Perturbations of the Meridional Overturning and Heat Transport in a Global Ocean General Circulation Model

2008 ◽  
Vol 38 (12) ◽  
pp. 2739-2754 ◽  
Author(s):  
Florian Sévellec ◽  
Thierry Huck ◽  
Mahdi Ben Jelloul ◽  
Nicolas Grima ◽  
Jérôme Vialard ◽  
...  
Keyword(s):  
Heat Transport ◽  
General Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Transient Growth ◽  
Surface Salinity ◽  
Meridional Heat Transport ◽  
Overturning Circulation ◽  
Optimal Surface ◽  
Meridional Overturning

Abstract Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the past few decades. As a step toward the estimation of the sensitivity of the thermohaline circulation to salinity anomalies, optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model [Océan Parallélisé (OPA)]; optimality is defined successively with respect to the meridional overturning circulation intensity and the meridional heat transport maximum. Although the system is asymptotically stable, the nonnormality of the dynamics is able to produce a transient growth through an initial stimulation. Optimal perturbations are calculated subject to three constraints: the perturbation applies to surface salinity; the perturbation conserves the global salt content; and the perturbation is normalized, to remove the degeneracy in the linear maximization problem. Maximization using Lagrangian multipliers leads to explicit solutions (rather than eigenvalue problems), involving the integration of the model adjoint for each value to maximize. The most efficient transient growth for the intensity of the meridional overturning circulation appears for a delay of 10.5 yr after the perturbation by the optimal surface salinity anomaly. This optimal growth is induced by an initial anomaly located north of 50°N. In the same way, the most efficient transient growth for the intensity of the meridional heat transport appears for a shorter delay of 2.2 yr after the perturbation by the optimal surface salinity anomaly. This initial optimal perturbation corresponds to a zonal salinity gradient around 24°N. The optimal surface salinity perturbations studied herein yield upper bounds on the intensity of the response in meridional overturning circulation and meridional heat transport. Using typical amplitudes of the Great Salinity Anomalies, the upper bounds for the associated variability are 0.8 Sv (1 Sv ≡ 106 m3 s−1) (11% of the mean circulation) and 0.03 PW (5% of the mean circulation), respectively.

A Theory of the Interhemispheric Meridional Overturning Circulation and Associated Stratification

2012 ◽  
Vol 42 (10) ◽  
pp. 1652-1667 ◽  
Author(s):  
Maxim Nikurashin ◽  
Geoffrey Vallis
Keyword(s):  
Deep Water ◽  
General Circulation ◽  
Mesoscale Eddy ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Minor Role ◽  
Diapycnal Mixing ◽  
Overturning Circulation ◽  
Northern Latitudes ◽  
Meridional Overturning

Abstract A quantitative theoretical model of the meridional overturning circulation and associated deep stratification in an interhemispheric, single-basin ocean with a circumpolar channel is presented. The theory includes the effects of wind, eddies, and diapycnal mixing and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other parameters of the problem. It relies on a matching among three regions: the circumpolar channel at high southern latitudes, a region of isopycnal outcrop at high northern latitudes, and the ocean basin between. The theory describes both the middepth and abyssal cells of a circulation representing North Atlantic Deep Water and Antarctic Bottom Water. It suggests that, although the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel, middepth stratification results from a balance between the wind-driven upwelling in the channel and deep-water formation at high northern latitudes. Diapycnal mixing in the ocean interior can lead to warming and upwelling of deep waters. However, for parameters most representative of the present ocean mixing seems to play a minor role for the middepth cell. In contrast, the abyssal cell is intrinsically diabatic and controlled by a balance between the deep mixing-driven upwelling and the residual of the wind-driven and eddy-induced circulations in the Southern Ocean. The theory makes explicit predictions about how the stratification and overturning circulation vary with the wind strength, diapycnal diffusivity, and mesoscale eddy effects. The predictions compare well with numerical results from a coarse-resolution general circulation model.

scholarly journals Fueling primary productivity: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation

2010 ◽  
Vol 7 (3) ◽  
pp. 4045-4088 ◽  
Author(s):  
J. B. Palter ◽  
J. L. Sarmiento ◽  
A. Gnanadesikan ◽  
J. Simeon ◽  
D. Slater
Keyword(s):  
Southern Ocean ◽  
Primary Productivity ◽  
Deep Ocean ◽  
Ocean Model ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Nutrient Return ◽  
Mode Water ◽  
Meridional Overturning

Abstract. In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be catastrophically reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global primary productivity between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, the high preformed nutrients subducted in the SAMW layer are converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

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