Response of the Southern Ocean to the Southern Annular Mode: Interannual Variability and Multidecadal Trend

2010 ◽  
Vol 40 (7) ◽  
pp. 1659-1668 ◽  
Author(s):  
A. M. Treguier ◽  
J. Le Sommer ◽  
J. M. Molines ◽  
B. de Cuevas
Keyword(s):  
Kinetic Energy ◽  
Southern Ocean ◽  
Interannual Variability ◽  
Time Scales ◽  
Meridional Circulation ◽  
Southern Annular Mode ◽  
Eddy Kinetic Energy ◽  
Overturning Circulation ◽  
Annular Mode ◽  
Meridional Overturning

Abstract The authors evaluate the response of the Southern Ocean to the variability and multidecadal trend of the southern annular mode (SAM) from 1972 to 2001 in a global eddy-permitting model of the DRAKKAR project. The transport of the Antarctic Circumpolar Current (ACC) is correlated with the SAM at interannual time scales but exhibits a drift because of the thermodynamic adjustment of the model (the ACC transport decreases because of a low renewal rate of dense waters around Antarctica). The interannual variability of the eddy kinetic energy (EKE) and the ACC transport are uncorrelated, but the EKE decreases like the ACC transport over the three decades, even though meridional eddy fluxes of heat and buoyancy remain stable. The contribution of oceanic eddies to meridional transports is an important issue because a growth of the poleward eddy transport could, in theory, oppose the increase of the mean overturning circulation forced by the SAM. In the authors’ model, the total meridional circulation at 50°S is well correlated with the SAM index (and the Ekman transport) at interannual time scales, and both increase over three decades between 1972 and 2001. However, given the long-term drift, no SAM-linked trend in the eddy contribution to the meridional overturning circulation is detectable. The increase of the meridional overturning is due to the time-mean component and is compensated by an increased buoyancy gain at the surface. The authors emphasize that the meridional circulation does not vary in a simple relationship with the zonal circulation. The model solution points out that the zonal circulation and the eddy kinetic energy are governed by different mechanisms according to the time scale considered (interannual or decadal).

scholarly journals Influence of the Southern Annular Mode on Projected Weakening of the Atlantic Meridional Overturning Circulation

2013 ◽  
Vol 26 (20) ◽  
pp. 8017-8036 ◽  
Author(s):  
Peter T. Spooner ◽  
Helen L. Johnson ◽  
Tim J. Woollings
Keyword(s):  
Climate Change ◽  
Southern Ocean ◽  
Greenhouse Gas ◽  
Climate Models ◽  
Atlantic Meridional Overturning Circulation ◽  
Southern Annular Mode ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Annular Mode ◽  
Meridional Overturning

Abstract Coupled climate models predict density-driven weakening of the Atlantic meridional overturning circulation (AMOC) under greenhouse gas forcing, with considerable spread in the response between models. There is also a large spread in the predicted increase of the southern annular mode (SAM) index across these models. Regression analysis across model space using 11 non-eddy-resolving models suggests that up to 35% of the intermodel spread in the AMOC response may be associated with uncertainty in the magnitude of the increase in the SAM. Models with a large, positive SAM index response generally display a smaller weakening of the AMOC under greenhouse gas forcing. The initial AMOC strength is also a major cause of intermodel spread in its response to climate change. The increase in the SAM acts to reduce the weakening of the AMOC over the next century by around ⅓, through increases in wind stress over the Southern Ocean, northward Ekman transport, and upwelling around Antarctica. The SAM response is also related to an increase in the northward salt flux across 30°S and to salinity anomalies in the high-latitude North Atlantic. These provide a positive feedback by further reinforcement of the AMOC. The results suggest that, compared with the real ocean where eddies oppose wind-driven changes in Southern Ocean circulation, climate models underestimate the effects of anthropogenic climate change on the AMOC.

Response of the Southern Ocean overturning circulation to extreme Southern Annular Mode conditions

2020 ◽  
Author(s):  
Kial Douglas Stewart ◽  
Andrew McC. Hogg ◽  
Matthew H. England ◽  
Darryn W. Waugh
Keyword(s):  
Southern Ocean ◽  
Southern Annular Mode ◽  
Overturning Circulation ◽  
Annular Mode

scholarly journals Response of the Southern Ocean overturning circulation to extreme Southern Annular Mode conditions

2020 ◽  
Author(s):  
Kial Douglas Stewart ◽  
Andrew McC. Hogg ◽  
Matthew H. England ◽  
Darryn W. Waugh
Keyword(s):  
Southern Ocean ◽  
Southern Annular Mode ◽  
Overturning Circulation ◽  
Annular Mode

scholarly journals Standing and Transient Eddies in the Response of the Southern Ocean Meridional Overturning to the Southern Annular Mode

2012 ◽  
Vol 25 (20) ◽  
pp. 6958-6974 ◽  
Author(s):  
C. O. Dufour ◽  
J. Le Sommer ◽  
J. D. Zika ◽  
M. Gehlen ◽  
J. C. Orr ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Annular Mode ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Positive Trend ◽  
Meridional Transport ◽  
Annular Mode ◽  
Flux Parameterization ◽  
Meridional Overturning ◽  
Wind Events

Abstract To refine the understanding of how the Southern Ocean responds to recent intensification of the southern annular mode (SAM), a regional ocean model at two eddy-permitting resolutions was forced with two synthetic interannual forcings. The first forcing corresponds to homogeneously intensified winds, while the second concerns their poleward intensification, consistent with positive phases of the SAM. Resulting wind-driven responses differ greatly between the nearly insensitive Antarctic Circumpolar Current (ACC) and the more sensitive meridional overturning circulation (MOC). As expected, eddies mitigate the response of the ACC and MOC to poleward-intensified winds. However, transient eddies do not necessarily play an increasing role in meridional transport with increasing resolution. As winds and resolution increase, meridional transport from standing eddies becomes more efficient at balancing wind-enhanced overturning. These results question the current paradigms on the role of eddies and present new challenges for eddy flux parameterization. Results also indicate that spatial patterns of wind anomalies are at least as important as the overall change in intensity in influencing the Southern Ocean’s dynamic response to wind events. Poleward-intensified wind anomalies from the positive trend in the SAM are far more efficient in accelerating the ACC than homogeneous wind anomalies.

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.

Subannual, Seasonal, and Interannual Variability of the North Atlantic Meridional Overturning Circulation

2007 ◽  
Vol 37 (5) ◽  
pp. 1246-1265 ◽  
Author(s):  
Joël J-M. Hirschi ◽  
Peter D. Killworth ◽  
Jeffrey R. Blundell
Keyword(s):  
Interannual Variability ◽  
Time Scales ◽  
North Atlantic ◽  
Rossby Waves ◽  
Density Field ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract An eddy-permitting numerical ocean model is used to investigate the variability of the meridional overturning circulation (MOC). Both wind stress and fluctuations of the seawater density contribute to MOC changes on subannual and seasonal time scales, whereas the interannual variability mainly reflects changes in the density field. Even on subannual and seasonal time scales, a significant fraction of the total MOC variability is due to changes of the density field in the upper 1000 m of the ocean. These changes reflect perturbations of the isopycnal structure that travel westward as Rossby waves. Because of a temporally changing phase difference between the eastern and western boundaries, the Rossby waves affect the MOC by modifying the basinwide east–west density gradient. Both the numerical model used in this study and calculations based on Rossby wave theory suggest that this effect can account for an MOC variability of several Sverdrups (Sv ≡ 106 m3 s−1). These results have implications for the interpretation of variability signals inferred from hydrographic sections and might contribute to the understanding of the results obtained from the Rapid Climate Change (RAPID) monitoring array deployed at 26°N in the North Atlantic Ocean.

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).

Sign in / Sign up

Export Citation Format

Share Document