scholarly journals Low-frequency oscillations of the Atlantic Ocean meridional overturning circulation in a coupled climate model

2007 ◽  
Vol 3 (1) ◽  
pp. 97-107 ◽  
Author(s):  
M. Schulz ◽  
M. Prange ◽  
A. Klocker
Keyword(s):  
Atlantic Ocean ◽  
Deep Water ◽  
Climate Model ◽  
Low Frequency ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
3 Dimensional ◽  
Low Frequency Oscillations ◽  
Water Formation

Abstract. Using a 3-dimensional climate model of intermediate complexity we show that the overturning circulation of the Atlantic Ocean can vary at multicentennial-to-millennial timescales for modern boundary conditions. A continuous freshwater perturbation in the Labrador Sea pushes the overturning circulation of the Atlantic Ocean into a bi-stable regime, characterized by phases of active and inactive deep-water formation in the Labrador Sea. In contrast, deep-water formation in the Nordic Seas is active during all phases of the oscillations. The actual timing of the transitions between the two circulation states occurs randomly. The oscillations constitute a 3-dimensional phenomenon and have to be distinguished from low-frequency oscillations seen previously in 2-dimensional models of the ocean. A conceptual model provides further insight into the essential dynamics underlying the oscillations of the large-scale ocean circulation. The model experiments indicate that the coupled climate system can exhibit unforced climate variability at multicentennial-to-millennial timescales that may be of relevance for Holocene climate variations.

scholarly journals Low-frequency oscillations of the Atlantic Ocean meridional overturning circulation in a coupled climate model

2006 ◽  
Vol 2 (5) ◽  
pp. 801-830 ◽  
Author(s):  
M. Schulz ◽  
M. Prange ◽  
A. Klocker
Keyword(s):  
Atlantic Ocean ◽  
Deep Water ◽  
Climate Model ◽  
Low Frequency ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
3 Dimensional ◽  
Low Frequency Oscillations ◽  
Water Formation

Abstract. Using a 3-dimensional climate model of intermediate complexity we show that the overturning circulation of the Atlantic Ocean can vary at multicentennial-to-millennial timescales for present-day boundary conditions. A weak and continuous freshwater input into the Labrador Sea pushes the overturning circulation of the Atlantic Ocean into a bi-stable regime, characterized by phases of active and inactive deep-water formation in the Labrador Sea. In contrast, deep-water formation in the Nordic Seas is active during all phases of the oscillations. The actual timing of the transitions between the two circulation states occurs randomly. The oscillations constitute a 3-dimensional phenomenon and have to be distinguished from low-frequency oscillations seen previously in 2-dimensional models of the ocean. A conceptual model provides further insight into the essential dynamics underlying the oscillations of the large-scale ocean circulation. The model experiments indicate that the coupled climate system can exhibit unforced climate variability at multicentennial-to-millennial timescales that may be of relevance for Holocene and future climate variations.

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.

Assessing the stability of the AMOC during past warm climates

2021 ◽  
Author(s):  
Megan Murphy O' Connor ◽  
Christophe Colin ◽  
Audrey Morley
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
Nordic Seas ◽  
Water Formation ◽  
Rockall Trough ◽  
Eastern North Atlantic ◽  
Warm Climates

<p>There is emergent evidence that abrupt shifts of the Atlantic Meridional Overturning Circulation (AMOC) have occurred during interglacial periods, with recent observations and model simulations showing that we may have over-estimated its stability during warm climates. In this study, we present a multi-proxy reconstruction of deep-water characteristics from the Rockall Trough in the Eastern North Atlantic to assess the variability of Nordic seas and Labrador Sea deep-water formation during past interglacial periods MIS 1, 5, 11, and 19. To test the warm climate stability hypothesis and to constrain the variability of deep-water formation for past warm climates, we performed geochemical analysis on planktic (Nd isotopes) and benthic foraminifera (δ<sup>18</sup>O and δ<sup>13</sup>C) along with sedimentological analysis. This approach allows us to reconstruct paleocurrent flow strength, as well as the origin and contribution of different water masses to one of the deep-water components of the AMOC in the Rockall Trough. We found that deep-water properties varied considerably during each of our chosen periods. For example during the Holocene εNd variability is smaller (1.8 per mil) when compared to variability during MIS 19 (3.3 per mil), an interglacial that experienced very similar orbital boundary conditions. Our results confirm that deep-water variability in the eastern North Atlantic basin was more variable in previous interglacial periods when compared to our current Holocene and provide new insight into the relative contribution of Nordic Seas Deep Water and Labrador Sea Water in the Rockall trough.</p>

Neoglacial Ice Advance and Sediment Provenance Changes in Southwest Greenland and its Marginal Seas Traced by Radiogenic Isotopes

2020 ◽  
Author(s):  
Lina Madaj ◽  
Claude Hillaire-Marcel ◽  
Friedrich Lucassen ◽  
Simone Kasemann
Keyword(s):  
Marine Sediments ◽  
Deep Water ◽  
Radiogenic Isotopes ◽  
Labrador Sea ◽  
Coastal Current ◽  
Deep Water Formation ◽  
The West ◽  
The Holocene ◽  
Water Formation ◽  
Southwest Greenland

<p>Marine sediments from the West Greenland margin represent high-resolution archives of Holocene climate history, past ice sheet dynamics, changes in meltwater discharge and coastal current intensities. We investigate potential changes of sediment provenances using strontium (Sr) and neodymium (Nd) radiogenic isotopes as tracers for the origin and pathways of the silicate detrital fraction in marine sediments. Meltwater discharge and coastal currents are the most important transport pathways for detrital sediments into (northeast) Labrador Sea, which is an important pathway for freshwater from the Arctic Ocean and meltwater from the Greenland Ice Sheet to enter the North Atlantic, where deep water formation takes place. Variations in freshwater supply into Labrador Sea may influence deep water formation and therefore further circulation and climate patterns on a global scale.</p><p>The marine sediment record collected in Nuuk Trough, southwest Greenland, displays uniform isotopic compositions throughout most of the Holocene, indicating well mixed detrital material from local sources through meltwater discharge and distal sources transported via the West Greenland Current. From around 4 ka BP to present the composition of Nd isotopes reveals a steep (εNd: -29 to -35) and the Sr isotope composition a slight (<sup>87</sup>Sr/<sup>86</sup>Sr: 0.723 to 0.728) but pronounced shift. This time interval coincides with the transition into the Neoglacial time period [1], which is characterized by a significant drop in atmospheric temperatures [2], and the onset of the modern Labrador Sea circulation pattern (e.g. [3]). We suggest that the shift in Nd and Sr isotopes indicates a change towards less distal and more local sediment sources, possibly caused by enhanced erosion of the local bedrock during Neoglacial ice advance [4], along with a decrease in meltwater discharge [5] and coastal current strength, leading to a sediment delivery shift.</p><p>[1] Funder & Fredskild (1989) Quaternary geology of Canada and Greenland, 775–783. [2] Seidenkrantz et al. (2007) The Holocene 17, 387-401. [3] Fagel et al. (2004) Paleoceanography 19, PA3002. [4] Funder et al. (2011) Developments in Quaternary Sciences 15, 699-713, (and references therein). [5] Møller et al. (2006) The Holocene 16, 685-695.</p>

Absence of deep-water formation in the Labrador Sea during the last interglacial period

Nature ◽  
2001 ◽  
Vol 410 (6832) ◽  
pp. 1073-1077 ◽  
Author(s):  
C. Hillaire-Marcel ◽  
A. de Vernal ◽  
G. Bilodeau ◽  
A. J. Weaver
Keyword(s):  
Deep Water ◽  
Labrador Sea ◽  
Interglacial Period ◽  
Deep Water Formation ◽  
Last Interglacial ◽  
Last Interglacial Period ◽  
Water Formation

scholarly journals Surface and subsurface Labrador Shelf water mass conditions during the last 6000 years

2020 ◽  
Vol 16 (4) ◽  
pp. 1127-1143
Author(s):  
Annalena A. Lochte ◽  
Ralph Schneider ◽  
Markus Kienast ◽  
Janne Repschläger ◽  
Thomas Blanz ◽  
...  
Keyword(s):  
Deep Water ◽  
Late Holocene ◽  
Thermohaline Circulation ◽  
Sea Water ◽  
Labrador Sea ◽  
Circulation System ◽  
Deep Water Formation ◽  
Holocene Thermal Maximum ◽  
Water Formation

Abstract. The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

scholarly journals Early Eocene vigorous ocean overturning and its contribution to a warm Southern Ocean

2020 ◽  
Author(s):  
Yurui Zhang ◽  
Thierry Huck ◽  
Camille Lique ◽  
Yannick Donnadieu ◽  
Jean-Baptiste Ladant ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Heat Transport ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Deep Water ◽  
Atmospheric Co2 ◽  
Early Eocene ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
Water Formation

Abstract. The early Eocene (~ 55 Ma) is the warmest period, and most likely characterized by the highest atmospheric CO2 concentrations, of the Cenozoic era. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 coupled climate model set up with paleogeographic reconstructions of this period from the DeepMIP project, with different levels of atmospheric CO2, and compare them with simulations of the modern conditions. This allows us to explore the changes of the ocean circulation and the resulting ocean meridional heat transport. At a CO2 level of 840 ppm, the Early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60º S), fed by deep water formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the build-up of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with the subpolar gyres, sustains the poleward advection of saline subtropical water to the convective region in the Southern Ocean, maintaining deep-water formation. This salt-advection feedback mechanism works similarly in the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55 Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3–0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7 PW southward at 20° S, and contributing to maintain the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO2 levels show that the ocean circulation and heat transport are relatively insensitive to CO2-doubling.

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.

Deep water formation in the North Atlantic Ocean during the last ice age

Nature ◽  
1980 ◽  
Vol 286 (5772) ◽  
pp. 479-482 ◽  
Author(s):  
Jean-Claude Duplessy ◽  
J. Moyes ◽  
C. Pujol
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
North Atlantic Ocean ◽  
Ice Age ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
Last Ice Age ◽  
The North Atlantic
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