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

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 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 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.

scholarly journals Response of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode Conditions

2020 ◽  
Vol 47 (22) ◽  
Author(s):  
K. D. Stewart ◽  
A.McC. Hogg ◽  
M. H. England ◽  
D. W. Waugh
Keyword(s):  
Southern Ocean ◽  
Southern Annular Mode ◽  
Overturning Circulation ◽  
Annular Mode

scholarly journals Supplementary material to "The Southern Annular Mode (SAM) influences phytoplankton communities in the seasonal ice zone of the Southern Ocean"

2019 ◽  
Author(s):  
Bruce L. Greaves ◽  
Andrew T. Davidson ◽  
Alexander D. Fraser ◽  
John P. McKinlay ◽  
Andrew Martin ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Annular Mode ◽  
Annular Mode ◽  
Phytoplankton Communities ◽  
Supplementary Material ◽  
Seasonal Ice Zone

scholarly journals Southern Annular Mode Influence on Wintertime Ventilation of the Southern Ocean Detected in Atmospheric O 2 and CO 2 Measurements

2020 ◽  
Vol 47 (4) ◽  
Author(s):  
Cynthia D. Nevison ◽  
David R. Munro ◽  
Nicole S. Lovenduski ◽  
Ralph F. Keeling ◽  
Manfredi Manizza ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Annular Mode ◽  
Annular Mode

scholarly journals The Southern Annular Mode (SAM) influences phytoplankton communities in the seasonal ice zone of the Southern Ocean

2019 ◽  
Author(s):  
Bruce L. Greaves ◽  
Andrew T. Davidson ◽  
Alexander D. Fraser ◽  
John P. McKinlay ◽  
Andrew Martin ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Relative Abundance ◽  
Phytoplankton Community ◽  
Southern Annular Mode ◽  
Taxonomic Composition ◽  
Austral Spring ◽  
Annular Mode ◽  
Phytoplankton Communities ◽  
Phytoplankton Community Succession ◽  
Seasonal Ice Zone

Abstract. Ozone depletion and climate change are causing the Southern Annular Mode (SAM) to become increasingly positive, driving stronger winds southward in the Southern Ocean (SO), with likely effects on phytoplankton habitat due to changes in ocean mixing, nutrient upwelling, and sea ice. This study examined the effect of the SAM and other environmental variables on the abundance of siliceous and calcareous phytoplankton in the seasonal ice zone (SIZ) of the SO. Samples were collected during repeat transects between Hobart, Australia, and Dumont d'Urville, Antarctica, centred around longitude 142° E, over 11 consecutive austral spring-summers (2002–2012). Twenty-two taxa, comprised of species, genera or higher taxonomic groups, were analysed using CAP analysis, cluster analysis and correlation. The SAM significantly affected phytoplankton community composition, with the greatest influence exerted by a SAM index averaged across 57 days centred on 11th March in the preceding autumn, explaining 13.3 % of the variance of taxa composition during the following spring–summer, and showing correlation with the relative abundance of 12 of the 22 taxa resolved. The day through the spring-summer that a sample was collected exerted the greatest influence on phytoplankton community structure (15.4 % of variance explained), reflecting the extreme seasonal variation in the physical environment in the SIZ that drives phytoplankton community succession. The response of different species of Fragilariopsis spp. and Chaetoceros spp. differed over the spring–summer and with the SAM, indicating the importance of species-level observation in detecting subtle changes in pelagic ecosystems. This study indicated that higher SAM favoured increases in the relative-abundance of large Chaetoceros spp. that predominated later in the spring–summer and reductions in small diatom taxa and siliceous and calcareous flagellates that predominated earlier in the spring–summer. Such changes in the taxonomic composition of phytoplankton, the pasture of the SO and principal energy source for Antarctic life, may alter both carbon sequestration and composition of higher tropic levels of the SIZ region of the SO.

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.

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