scholarly journals Spectral Characteristics of the Response of the Meridional Overturning Circulation to Deep-Water Formation

2005 ◽  
Vol 35 (10) ◽  
pp. 1813-1825 ◽  
Author(s):  
Julie Deshayes ◽  
Claude Frankignoul
Keyword(s):  
High Frequency ◽  
Spectral Characteristics ◽  
Low Frequency ◽  
Meridional Overturning Circulation ◽  
Thermocline Depth ◽  
Deep Water Formation ◽  
Meridional Transport ◽  
Overturning Circulation ◽  
Water Formation ◽  
Meridional Overturning

Abstract The response of the upper limb of the meridional overturning circulation to the variability of deep-water formation is investigated analytically with a linear, reduced-gravity model in basins of simple geometry. The spectral characteristics of the model response are first derived by prescribing white-noise fluctuations in the meridional transport at the northern boundary. Although low-frequency basin modes are solutions to the eigenproblem, they are too dissipative to be significantly excited by the boundary forcing, and the thermocline depth response has a red spectrum with no prevailing time scale other than that of a high-frequency equatorial mode, only flattening at the millennial time scale because of vertical diffusivity. The meridional transport is asymmetric about the equator because the northern part of the basin is directly influenced by the boundary forcing while the southern part is mostly set in motion by long Rossby waves. This results in the equator acting as a low-pass filter for the Southern Hemisphere, which clarifies the so-called buffering effect of the equator. In a basin connected by a southern circumpolar channel, the thermocline depth and the transport spectra are redder than in the forced basin and, when a somewhat more realistic stochastic forcing derived from general circulation model simulations is considered, the variability is strongly reduced at high frequency. The linear model qualitatively explains several features of the low-frequency variability of the meridional overturning circulation in climate models, such as its red spectrum and its larger intensity in the North Atlantic Ocean.

scholarly journals Arctic–North Atlantic Interactions and Multidecadal Variability of the Meridional Overturning Circulation

2005 ◽  
Vol 18 (19) ◽  
pp. 4013-4031 ◽  
Author(s):  
Johann H. Jungclaus ◽  
Helmuth Haak ◽  
Mojib Latif ◽  
Uwe Mikolajewicz
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Multidecadal Variability ◽  
Water Formation ◽  
Meridional Overturning

Abstract Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation.

scholarly journals Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

2021 ◽  
Vol 17 (2) ◽  
pp. 615-632
Author(s):  
Frerk Pöppelmeier ◽  
Jeemijn Scheen ◽  
Aurich Jeltsch-Thömmes ◽  
Thomas F. Stocker
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Bering Strait ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
The North ◽  
Water Formation ◽  
Meridional Overturning ◽  
The Last Glacial Maximum ◽  
The Last Glacial

Abstract. The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations and can, thus, provide important constraints on the stability of the AMOC. However, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice sheets during the deglaciation, as well as its response to these perturbations remain debated. Here, we investigate the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait and the effect of increased tidal dissipation as a result of the altered bathymetry due to the lower glacial sea level stand. The vigorous and deep AMOC under these glacial boundary conditions, consistent with previous simulations with different models, reacts more strongly to North Atlantic freshwater forcings than under preindustrial conditions. This increased sensitivity is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thereby facilitating faster accumulation of freshwater in the North Atlantic and halting deep-water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, which is in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM. Simulations with reduced North Atlantic deep-water formation, as a consequence of potentially increased continental runoff from ice sheet melt and imposed changes in the hydrological cycle, more closely resemble the overturning circulation inferred from proxies. These circulation states also show bistable behavior, where the AMOC does not recover after North Atlantic freshwater hosing. However, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed based on paleoceanographic data.

scholarly journals Intensified Atlantic vs. weakened Pacific meridional overturning circulations in response to Tibetan Plateau uplift

2017 ◽  
Author(s):  
Baohuang Su ◽  
Dabang Jiang ◽  
Ran Zhang ◽  
Pierre Sepulchre ◽  
Gilles Ramstein
Keyword(s):  
Heat Flux ◽  
Tibetan Plateau ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
The North ◽  
Water Formation ◽  
Meridional Overturning

Abstract. The role of the Tibetan Plateau (TP) in maintaining large-scale overturning circulation in the Atlantic and Pacific is investigated using a coupled atmosphere–ocean model. For the present day with a realistic topography, model simulation shows a strong Atlantic meridional overturning circulation (AMOC) but a near absence of a Pacific meridional overturning circulation (PMOC), which is in good agreement with present observations. In contrast, the simulation without the TP depicts a collapsed AMOC and a strong PMOC that dominates deep water formation. The switch in deep water formation between the two basins results from changes in the large-scale atmospheric circulation and atmosphere–ocean feedback in the Atlantic and Pacific. The intensified westerly winds and increased freshwater flux over the North Atlantic cause an initial slowdown of the AMOC, but the weakened East Asian monsoon circulation and associated decreased freshwater flux over the North Pacific enhance initial intensification of the PMOC. The further decreased heat flux and the associated increase in sea-ice fraction promote the final AMOC collapse over the Atlantic, while the further increased heat flux leads to the final PMOC establishment over the Pacific. Although the simulations were done in a cold world, it still importantly implicates that the uplift of the TP alone could have been a potential driver for the reorganization of PMOC–AMOC between the Late Eocene and Early Oligocene.

scholarly journals Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

2017 ◽  
Vol 98 (4) ◽  
pp. 737-752 ◽  
Author(s):  
M. Susan Lozier ◽  
Sheldon Bacon ◽  
Amy S. Bower ◽  
Stuart A. Cunningham ◽  
M. Femke de Jong ◽  
...  
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Deep Water ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Intergovernmental Panel ◽  
Water Formation ◽  
Meridional Overturning

Abstract For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

Atlantic Dominance of the Meridional Overturning Circulation

2008 ◽  
Vol 38 (2) ◽  
pp. 435-450 ◽  
Author(s):  
A. M. de Boer ◽  
J. R. Toggweiler ◽  
D. M. Sigman
Keyword(s):  
Deep Water ◽  
Hydrological Cycle ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Ocean Temperature ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Salinity Gradients ◽  
Water Formation ◽  
Meridional Overturning

Abstract North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy–driven system and the robustness of NA dominance is investigated within this framework. Using an idealized geometry ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are tested for their effect on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales.

scholarly journals Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
F. Li ◽  
M. S. Lozier ◽  
S. Bacon ◽  
A. S. Bower ◽  
S. A. Cunningham ◽  
...  
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Minimal Impact ◽  
Water Formation ◽  
Meridional Overturning

AbstractChanges in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.

Overturning Variations in the Subpolar North Atlantic in an Ocean Reanalyses Ensemble

2021 ◽  
Author(s):  
Jonathan Baker ◽  
Richard Renshaw ◽  
Laura Jackson ◽  
Clotilde Dubois ◽  
Dorotea Iovino ◽  
...  
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Relative Importance ◽  
Deep Water Formation ◽  
Meridional Transport ◽  
Ocean Reanalyses ◽  
Water Formation ◽  
Meridional Overturning

<p>The ocean’s Atlantic Meridional Overturning Circulation (AMOC) has a significant influence on global climate through its meridional transport of heat and carbon. Deep water formation occurring in the subpolar North Atlantic is an essential component the AMOC. Understanding the nature and causes of its multidecadal variation at these high latitudes is critical to more accurately predict future changes. We analyse the subpolar overturning in an ensemble of eddy permitting ¼ degree global ocean reanalyses, restrained by observations and historical forcings, over the period 1993-2018. This overturning transport is validated against the continuous measurements obtained along the Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array since 2014. The ability of each reanalysis to capture the observed changes in the overturning will be determined, providing confidence in their ability to simulate changes prior to the availability of OSNAP, and exposing their limitations. We analyse the eastern and western sections of the OSNAP array to determine the relative importance of the overturning along these sections and the temporal variability on various timescales. This research complements a previous study investigating changes in the subtropical Atlantic overturning using the same reanalyses ensemble which was shown to provide a good approximation to observations.</p>

scholarly journals Locally and Remotely Forced Subtropical AMOC Variability: A Matter of Time Scales

2020 ◽  
Vol 33 (12) ◽  
pp. 5155-5172
Author(s):  
Quentin Jamet ◽  
William K. Dewar ◽  
Nicolas Wienders ◽  
Bruno Deremble ◽  
Sally Close ◽  
...  
Keyword(s):  
Time Series ◽  
Time Scales ◽  
North Atlantic ◽  
Time Scale ◽  
Low Frequency ◽  
Meridional Overturning Circulation ◽  
Open Boundary ◽  
Overturning Circulation ◽  
Decadal Scale ◽  
Meridional Overturning

AbstractMechanisms driving the North Atlantic meridional overturning circulation (AMOC) variability at low frequency are of central interest for accurate climate predictions. Although the subpolar gyre region has been identified as a preferred place for generating climate time-scale signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array–Western Boundary Time Series (RAPID–MOCHA–WBTS) program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low-frequency AMOC variability. We analyze for this a set of four ensembles of a regional (20°S–55°N), eddy-resolving (1/12°) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals. Their analysis reveals the predominance of local, atmospherically forced signal at interannual time scales (2–10 years), whereas signals imposed by the boundaries are responsible for the decadal (10–30 years) part of the spectrum. Due to this marked time-scale separation, we show that, although the intergyre region exhibits peculiarities, most of the subtropical AMOC variability can be understood as a linear superposition of these two signals. Finally, we find that the decadal-scale, boundary-forced AMOC variability has both northern and southern origins, although the former dominates over the latter, including at the site of the RAPID array (26.5°N).

scholarly journals Variability of the Atlantic Meridional Overturning Circulation in CCSM4

2012 ◽  
Vol 25 (15) ◽  
pp. 5153-5172 ◽  
Author(s):  
Gokhan Danabasoglu ◽  
Steve G. Yeager ◽  
Young-Oh Kwon ◽  
Joseph J. Tribbia ◽  
Adam S. Phillips ◽  
...  
Keyword(s):  
Low Frequency ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Positive Density ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Control Simulation ◽  
Meridional Overturning

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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