scholarly journals Estimating trends of Atlantic meridional overturning circulation from long-term hydrographic data and model simulations

2008 ◽  
Vol 58 (2) ◽  
pp. 127-138 ◽  
Author(s):  
Gerrit Lohmann ◽  
Helmuth Haak ◽  
Johann H. Jungclaus
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Hydrographic Data ◽  
Overturning Circulation ◽  
Model Simulations ◽  
Meridional Overturning

scholarly journals Seasonal Variability of the Atlantic Meridional Overturning Circulation at 26.5°N

2010 ◽  
Vol 23 (21) ◽  
pp. 5678-5698 ◽  
Author(s):  
T. Kanzow ◽  
S. A. Cunningham ◽  
W. E. Johns ◽  
J. J-M. Hirschi ◽  
J. Marotzke ◽  
...  
Keyword(s):  
Seasonal Cycle ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Wind Stress Curl ◽  
Meridional Transport ◽  
Overturning Circulation ◽  
Long Time ◽  
Meridional Overturning

Abstract The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, 48-month-long time series of the vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008, the AMOC had a mean strength of 18.7 ± 2.1 Sv (1 Sv ≡ 106 m3 s−1) with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic midocean and Gulf Stream transports of 2.2 and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.

scholarly journals Horizontal circulation across density surfaces contributes substantially to the long-term mean northern Atlantic Meridional Overturning Circulation

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Rong Zhang ◽  
Matthew Thomas
Keyword(s):  
Deep Convection ◽  
Atlantic Meridional Overturning Circulation ◽  
Open Ocean ◽  
Meridional Overturning Circulation ◽  
Greenland Sea ◽  
Overturning Circulation ◽  
Horizontal Circulation ◽  
Meridional Overturning ◽  
The Mean

AbstractThe Greenland Sea is often viewed as the northern terminus of the Atlantic Meridional Overturning Circulation. It has also been proposed that the shutdown of open-ocean deep convection in the Labrador or Greenland Seas would substantially weaken the Atlantic Meridional Overturning Circulation. Here we analyze Robust Diagnostic Calculations conducted in a high-resolution global coupled climate model constrained by observed hydrographic climatology to provide a holistic picture of the long-term mean Atlantic Overturning Circulation at northern high latitudes. Our results suggest that the Arctic Ocean, not the Greenland Sea, is the northern terminus of the mean Atlantic Overturning Circulation; open-ocean deep convection, in either the Labrador or Greenland Seas, contributes minimally to the mean Atlantic Overturning Circulation, hence it would not necessarily be substantially weakened by a shutdown of open-ocean deep convection; horizontal circulation across sloping isopycnals contributes substantially (more than 40%) to the maximum mean northeastern subpolar Atlantic Overturning Circulation.

Transient and equilibrium responses of the Atlantic meridional overturning circulation to warming in coupled climate models

2021 ◽  
Author(s):  
David Bonan ◽  
Andrew Thompson ◽  
Emily Newsom ◽  
Shantong Sun ◽  
Maria Rugenstein
Keyword(s):  
Atmospheric Carbon Dioxide ◽  
Climate Models ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Atlantic Basin ◽  
Coupled Climate Models ◽  
Salinity Anomaly ◽  
Meridional Overturning

<p>The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic climate change remains poorly understood in part, due to the computational expenses associated with running fully-coupled climate models to equilibrium. Here, we use a collection of millennial-length simulations from multiple state-of-the-art climate models to examine the transient and equilibrium responses of the AMOC to an abrupt quadrupling of atmospheric carbon-dioxide. All climate models exhibit a weakening of the AMOC on centennial timescales, but they disagree on the recovery of the AMOC over next millennia, despite the same greenhouse-gas forcing. In some models, the AMOC recovers after approximately 200 years, while in others the AMOC does not fully recover even after approximately 1000 years. To explain the behavior of the AMOC we relate the overturning circulation in the North Atlantic to the meridional density difference between the basin interior and the region of deep-water formation. This scaling both reproduces the initial decline and gradual recovery of the AMOC, and explains the inter-model spread of the AMOC responses. The initial shoaling and weakening occurs on centennial timescales and is attributed to the warming of the northern convection region. We argue that the AMOC weakens on a timescale linked to a combination of its initial depth and the global surface heat flux sensitivity. The recovery of the AMOC results from a pile-up of salinity in the Atlantic basin, when the AMOC is weakened, that propagates northward and reinvigorates convection. A weaker AMOC recovery is associated with a smaller salinity anomaly. We further show through surface water mass transformation that Southern Ocean processes may impact the salinity anomaly in the Atlantic basin. These results highlight the importance of considering the evolution of the AMOC and ocean heat transport beyond the 21st century as short-term changes are not indicative of long-term changes.</p>

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).

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