scholarly journals A stable Atlantic Meridional Overturning Circulation in a changing North Atlantic Ocean since the 1990s

2020 ◽  
Vol 6 (48) ◽  
pp. eabc7836
Author(s):  
Yao Fu ◽  
Feili Li ◽  
Johannes Karstensen ◽  
Chunzai Wang
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
North Atlantic Ocean ◽  
Global Climate ◽  
Global Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The Past ◽  
Meridional Overturning

The Atlantic Meridional Overturning Circulation (AMOC) is crucially important to global climate. Model simulations suggest that the AMOC may have been weakening over decades. However, existing array-based AMOC observations are not long enough to capture multidecadal changes. Here, we use repeated hydrographic sections in the subtropical and subpolar North Atlantic, combined with an inverse model constrained using satellite altimetry, to jointly analyze AMOC and hydrographic changes over the past three decades. We show that the AMOC state in the past decade is not distinctly different from that in the 1990s in the North Atlantic, with a remarkably stable partition of the subpolar overturning occurring prominently in the eastern basins rather than in the Labrador Sea. In contrast, profound hydrographic and oxygen changes, particularly in the subpolar North Atlantic, are observed over the same period, suggesting a much higher decoupling between the AMOC and ocean interior property fields than previously thought.

Observed Evidence of a Stable Atlantic Meridional Overturning Circulation since the 1990s

2020 ◽  
Author(s):  
Yao Fu ◽  
Feili Li ◽  
Johannes Karstensen ◽  
N. Penny Holliday ◽  
Chunzai Wang
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Distribution Model ◽  
Overturning Circulation ◽  
State Estimate ◽  
The Past ◽  
Stable Partition ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulation (AMOC) is crucially important in the global climate system due to its role in the meridional heat and freshwater distribution. Model simulations and constructed AMOC indices suggest that the AMOC may have been weakening for decades. However, direct AMOC observations, introduced in 2004 in the subtropics (the RAPID program) and in 2014 in the subpolar North Atlantic (the OSNAP program), are not sufficiently long to capture changes dating back to previous periods. Here we use repeated hydrographic sections in the subtropical and subpolar North Atlantic through the early 1990s to the mid-2010s, combined with a box inverse model that is constrained using satellite altimetry, to analyze hydrographic changes and the AMOC. In combination with a state-of-the-art ocean state estimate, GECCO2, we show that despite dramatic hydrographic changes in the subtropical and subpolar North Atlantic over the past two and half decades, the AMOC has not significantly weakened over the same period. Our hydrography-based estimates also illustrate a remarkably stable partition of the subpolar overturning between the Labrador basin and the eastern subpolar basins on decadal timescales since the 1990s.</p>

scholarly journals Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate

2020 ◽  
Vol 6 (26) ◽  
pp. eaaz4876 ◽  
Author(s):  
Wei Liu ◽  
Alexey V. Fedorov ◽  
Shang-Ping Xie ◽  
Shineng Hu
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Arctic Sea Ice ◽  
Climate Impacts ◽  
Meridional Overturning Circulation ◽  
Future Climate Change ◽  
Boreal Summer ◽  
Overturning Circulation ◽  
Meridional Overturning

While the Atlantic Meridional Overturning Circulation (AMOC) is projected to slow down under anthropogenic warming, the exact role of the AMOC in future climate change has not been fully quantified. Here, we present a method to stabilize the AMOC intensity in anthropogenic warming experiments by removing fresh water from the subpolar North Atlantic. This method enables us to isolate the AMOC climatic impacts in experiments with a full-physics climate model. Our results show that a weakened AMOC can explain ocean cooling south of Greenland that resembles the North Atlantic warming hole and a reduced Arctic sea ice loss in all seasons with a delay of about 6 years in the emergence of an ice-free Arctic in boreal summer. In the troposphere, a weakened AMOC causes an anomalous cooling band stretching from the lower levels in high latitudes to the upper levels in the tropics and displaces the Northern Hemisphere midlatitude jets poleward.

scholarly journals Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation

2020 ◽  
Vol 1 (1) ◽  
Author(s):  
Tatsuya Hayashi ◽  
Toshiro Yamanaka ◽  
Yuki Hikasa ◽  
Masahiko Sato ◽  
Yoshihiro Kuwahara ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
North Atlantic ◽  
Deep Water ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
North Atlantic Deep Water ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Early Stages ◽  
Meridional Overturning

Abstract The global climate has been dominated by glacial–interglacial variations since the intensification of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic meridional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning circulation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly increased after 2.7 million years ago and was actively maintained even in early stages of individual glacials, in contrast with late stages when it drastically decreased because of iceberg melting. Probably, the active Nordic Seas overturning during the early stages of glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations.

scholarly journals Links between the Southern Annular Mode and the Atlantic Meridional Overturning Circulation in a Climate Model

2011 ◽  
Vol 24 (3) ◽  
pp. 624-640 ◽  
Author(s):  
Camille Marini ◽  
Claude Frankignoul ◽  
Juliette Mignot
Keyword(s):  
Time Scales ◽  
North Atlantic ◽  
Time Scale ◽  
Climate Model ◽  
Drake Passage ◽  
Atlantic Meridional Overturning Circulation ◽  
Southern Annular Mode ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning

Abstract The links between the atmospheric southern annular mode (SAM), the Southern Ocean, and the Atlantic meridional overturning circulation (AMOC) at interannual to multidecadal time scales are investigated in a 500-yr control integration of the L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL CM4) climate model. The Antarctic Circumpolar Current, as described by its transport through the Drake Passage, is well correlated with the SAM at the yearly time scale, reflecting that an intensification of the westerlies south of 45°S leads to its acceleration. Also in phase with a positive SAM, the global meridional overturning circulation is modified in the Southern Hemisphere, primarily reflecting a forced barotropic response. In the model, the AMOC and the SAM are linked at several time scales. An intensification of the AMOC lags a positive SAM by about 8 yr. This is due to a correlation between the SAM and the atmospheric circulation in the northern North Atlantic that reflects a symmetric ENSO influence on the two hemispheres, as well as an independent, delayed interhemispheric link driven by the SAM. Both effects lead to an intensification of the subpolar gyre and, by salinity advection, increased deep convection and a stronger AMOC. A slower oceanic link between the SAM and the AMOC is found at a multidecadal time scale. Salinity anomalies generated by the SAM enter the South Atlantic from the Drake Passage and, more importantly, the Indian Ocean; they propagate northward, eventually reaching the northern North Atlantic where, for a positive SAM, they decrease the vertical stratification and thus increase the AMOC.

Impact of Arctic gateways closure on the Atlantic Meridional Overturning Circulation in the Pliocene

2021 ◽  
Author(s):  
Julia Weiffenbach ◽  
Michiel Baatsen ◽  
Anna von der Heydt
Keyword(s):  
Boundary Conditions ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
The Arctic ◽  
Bering Strait ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning

<p>The mid-Pliocene climate is the most recent geological period with a greenhouse gas concentration of approximately 400 ppmv, similar to the present day. Proxy reconstructions indicate enhanced warming in the high North Atlantic in the mid-Pliocene, which has been suggested to be a response to a stronger Atlantic Meridional Overturning Circulation (AMOC). PlioMIP2 ensemble results show a stronger AMOC and simulated North Atlantic sea surface temperatures (SSTs) match reconstructions better than PlioMIP1. A major difference between PlioMIP1 and PlioMIP2 is the closure of the Bering Strait and Canadian Archipelago in the Pliocene. Previous studies have shown that closure of these Arctic gateways leads to an enhanced AMOC due to altered freshwater fluxes in the Arctic.</p><p>Analysis of our Community Earth System Model (CESM1) simulations shows that the simulated increase in North Atlantic SSTs and strengthened AMOC in the Pliocene is a result of Pliocene boundary conditions rather than CO<sub>2</sub> concentration increase. Here we compare results from two runs with pre-industrial boundary conditions and 280 and 560 ppmv CO<sub>2</sub> concentrations and three runs with PlioMIP2 boundary conditions and 280, 400 and 560 ppmv CO<sub>2</sub> concentrations. Results show a 10-15% stronger AMOC in the Pliocene simulations as well as enhanced warming and saltening of the North Atlantic sea surface. While there is a stronger AMOC, the Atlantic northward ocean heat transport (OHT) in the Pliocene simulations only increases 0-3% with respect to the pre-industrial. Analysis indicates there is an altered relationship between the AMOC and OHT in the Pliocene, pointing to fundamentally different behavior of the AMOC in the Pliocene simulations. This is supported by a specific spatial pattern of deep water formation (DWF) areas in the Pliocene simulations that is significantly different from that of the pre-industrial. In the Pliocene simulations, DWF areas adjacent to south Greenland disappear and new DWF areas appear further southwards in the Labrador Sea off the coast of Newfounland. These results indicate that insight into the effect of the palaeogeographic boundary conditions is crucial to understanding the Pliocene climate and its potential as a geological equivalent to a future greenhouse climate.</p>

The Atlantic Meridional Overturning Circulation (AMOC) simulated during interglacials and its impact on climate

2021 ◽  
Author(s):  
Zhiyi Jiang ◽  
Chris Brierley ◽  
David Thornalley ◽  
Sophie Sax
Keyword(s):  
Spatial Structure ◽  
Surface Temperature ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Last Interglacial ◽  
Overturning Circulation ◽  
Poleward Heat Transport ◽  
Meridional Overturning ◽  
Proxy Reconstructions

<p>The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of poleward heat transport and an important part of the global climate system. How it responded to past changes inforcing, such as experienced during Quaternary interglacials, is an intriguing and open question. Previous modelling studies suggest an enhanced AMOC in the mid-Holocene compared to the pre-industrial period. In previous simulations from the Palaeoclimate Modelling Intercomparison Project (PMIP), this arose from feedbacks between sea ice and AMOC changes, which also depended on resolution. Here I present aninitial analysis of the recently available PMIP4 simulations. This shows the overall strength of the AMOC does not markedly change between the mid-Holocene and piControl experiments (at least looking at the maximum of the mean meridional mass overturning streamfunction below 500m at 30<sup>o</sup>N and 50<sup>o</sup>N). This is not inconsistent with the proxy reconstructions using sortable silt and Pa/Th for the mid-Holocene. Here we analyse changes in the spatial structure of the meridional overturning circulation, along with their fingerprints on the surface temperature (computed through regression). We then estimate the percentage of the simulated surface temperature changes between the mid-Holocene and pre-industrial period that can be explained by AMOC. Furthermore, the analysis for the changes in the AMOC spatial structure has been extended to see if the same patterns of change hold for the last interglacial. The simulations will be compared to existing proxy reconstructions, as well as new palaeoceanographic reconstructions.</p>

scholarly journals An improved parameterization of tidal mixing for ocean models

2014 ◽  
Vol 7 (1) ◽  
pp. 211-224 ◽  
Author(s):  
A. Schmittner ◽  
G. D. Egbert
Keyword(s):  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Tidal Mixing ◽  
Overturning Circulation ◽  
Microstructure Measurements ◽  
High Resolution Bathymetry ◽  
Diurnal Tides

Abstract. Two modifications to an existing scheme of tidal mixing are implemented in the coarse resolution ocean component of a global climate model. First, the vertical distribution of energy flux out of the barotropic tide is determined using high resolution bathymetry. This shifts the levels of mixing higher up in the water column and leads to a stronger mid-depth meridional overturning circulation in the model. Second, the local dissipation efficiency for diurnal tides is assumed to be larger than that for the semi-diurnal tides poleward of 30°. Both modifications are shown to improve agreement with observational estimates of diapycnal diffusivities based on microstructure measurements and circulation indices. We also assess impacts of different spatial distributions of the barotropic energy loss. Estimates based on satellite altimetry lead to larger diffusivities in the deep ocean and hence a stronger deep overturning circulation in our climate model that is in better agreement with observation based estimates compared to those based on a tidal model.

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