The Southern Annular Mode: Variability, trends, and climate impacts across the Southern Hemisphere

Abstract The Southern Hemisphere annular mode (SAM) is the dominant mode of climate variability in the extratropical Southern Hemisphere. Representing variations in pressure and the corresponding changes to the circumpolar zonal flow, it is typically thought of as an “annular” or ringlike structure. However, on seasonal time scales the zonal symmetry observed in the SAM in monthly or annual mean data is much less marked. This study further examines the seasonal changes in the SAM structure and explores temperature signals across the Southern Hemisphere that are strongly tied to the asymmetric SAM structure. The SAM asymmetries are most marked in the Pacific sector and in austral winter and spring, related to changes in the jet entrance and exit regions poleward of 30°S. Depending on the season, the asymmetric SAM structure explains over 25% of the variance in the overall SAM structure and has strong connections with ENSO or zonal wavenumber 3. In austral summer and autumn the SAM has been becoming more zonally symmetric, especially after 1980, perhaps tied to changes in anthropogenic forcing. Across the Pacific sector, including the Antarctic Peninsula, temperature variations are strongly tied to the asymmetric SAM structure, while temperatures across East Antarctica are more strongly tied to the zonally symmetric SAM structure. The results suggest that studies examining the climate impacts of the SAM across the Southern Hemisphere need to consider the seasonal variations in the SAM structure as well as varying impacts between its positive and negative polarity to adequately describe the underlying relationships.

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Regional climate impacts of the Southern Annular Mode

Geophysical Research Letters ◽

10.1029/2006gl027721 ◽

2006 ◽

Vol 33 (23) ◽

Cited By ~ 252

Author(s):

N. P. Gillett ◽

T. D. Kell ◽

P. D. Jones

Keyword(s):

Regional Climate ◽

Southern Annular Mode ◽

Climate Impacts ◽

Annular Mode

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Continuation of tropical Pacific Ocean temperature trend may weaken extreme El Niño and its linkage to the Southern Annular Mode

Scientific Reports ◽

10.1038/s41598-019-53371-3 ◽

2019 ◽

Vol 9 (1) ◽

Cited By ~ 6

Author(s):

Eun-Pa Lim ◽

Harry H. Hendon ◽

Pandora Hope ◽

Christine Chung ◽

Francois Delage ◽

...

Keyword(s):

El Niño ◽

El Nino ◽

Southern Annular Mode ◽

Climate Impacts ◽

Ocean Temperature ◽

Annular Mode ◽

Temperature Trends ◽

Surface Climate ◽

Mean State ◽

Extreme El Niño

AbstractObservational records show that occurrences of the negative polarity of the Southern Annular Mode (low SAM) is significantly linked to El Niño during austral spring and summer, potentially providing long-lead predictability of the SAM and its associated surface climate conditions. In this study, we explore how this linkage may change under a scenario of a continuation of the ocean temperature trends that have been observed over the past 60 years, which are plausibly forced by increasing greenhouse gas concentrations. We generated coupled model seasonal forecasts for three recent extreme El Niño events by initialising the forecasts with observed ocean anomalies of 1 September 1982, 1997 and 2015 added into (1) the current ocean mean state and into (2) the ocean mean state updated to include double the recent ocean temperature trends. We show that the strength of extreme El Niño is reduced with the warmer ocean mean state as a result of reduced thermocline feedback and weakened rainfall-wind-sea surface temperature coupling over the tropical eastern Pacific. The El Niño-low SAM relationship also weakens, implying the possibility of reduced long-lead predictability of the SAM and associated surface climate impacts in the future.

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The impact of the Southern Annular Mode on future changes in Southern Hemisphere rainfall

Geophysical Research Letters ◽

10.1002/2016gl069453 ◽

2016 ◽

Vol 43 (13) ◽

pp. 7160-7167 ◽

Cited By ~ 42

Author(s):

Eun-Pa Lim ◽

Harry H. Hendon ◽

Julie M. Arblaster ◽

Francois Delage ◽

Hanh Nguyen ◽

...

Keyword(s):

Southern Hemisphere ◽

Southern Annular Mode ◽

Annular Mode ◽

The Impact

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Southern Hemisphere Annular Mode Variability and the Role of Optimal Nonmodal Growth

Journal of the Atmospheric Sciences ◽

10.1175/jas3444.1 ◽

2005 ◽

Vol 62 (6) ◽

pp. 1947-1961 ◽

Cited By ~ 9

Author(s):

Harun A. Rashid ◽

Ian Simmonds

Keyword(s):

Southern Hemisphere ◽

Zonal Wind ◽

Positive Feedback ◽

General Model ◽

Initial Perturbation ◽

Low Frequency ◽

Southern Annular Mode ◽

Feedback Mechanism ◽

Annular Mode ◽

Positive Feedback Mechanism

Abstract The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.

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Impacts of a Midlatitude Oceanic Frontal Zone for the Baroclinic Annular Mode in the Southern Hemisphere

Journal of Climate ◽

10.1175/jcli-d-20-0359.1 ◽

2021 ◽

pp. 1-63

Author(s):

MORIO NAKAYAMA ◽

HISASHI NAKAMURA ◽

FUMIAKI OGAWA

Keyword(s):

Southern Hemisphere ◽

General Circulation ◽

Frontal Zone ◽

Circulation Model ◽

Southern Annular Mode ◽

Internal Dynamics ◽

Near Surface ◽

Annular Mode ◽

Major Mode ◽

Sst Gradient

AbstractAs a major mode of annular variability in the Southern Hemisphere, the baroclinic annular mode (BAM) represents the pulsing of extratropical eddy activity. Focusing mainly on sub-weekly disturbances, this study assesses the impacts of a midlatitude oceanic frontal zone on the BAM and its dynamics through a set of “aqua-planet” atmospheric general circulation model experiments with zonally uniform sea-surface temperature (SST) profiles prescribed. Though idealized, one experiment with realistic frontal SST gradient reasonably well reproduces observed BAM-associated anomalies as a manifestation of a typical lifecycle of migratory baroclinic disturbances. Qualitatively, these BAM features are also simulated in the other experiment where the frontal SST gradient is removed. However, the BAM-associated variability weakens markedly and shifts equatorward, in association with the corresponding modifications in the climatological-mean stormtrack activity. The midlatitude oceanic frontal zone amplifies and anchors the BAM variability by restoring near-surface baroclinicity through anomalous sensible heat supply from the ocean and moisture supply to cyclones, although the BAM is essentially a manifestation of atmospheric internal dynamics. Those experiments and observations further indicate that the BAM modulates momentum flux associated with transient disturbances to induce a modest but robust meridional shift of the polar-front jet, suggesting that the BAM can help maintain the southern annular mode. They also indicate that the quasi-periodic behavior of the BAM is likely to reflect internal dynamics in which atmospheric disturbances on both sub-weekly and longer time scales are involved.

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Climate Impacts of the Southern Annular Mode Simulated by the CMIP3 Models

Journal of Climate ◽

10.1175/2009jcli2788.1 ◽

2009 ◽

Vol 22 (13) ◽

pp. 3751-3768 ◽

Cited By ~ 29

Author(s):

Alexey Yu Karpechko ◽

Nathan P. Gillett ◽

Gareth J. Marshall ◽

James A. Screen

Keyword(s):

Sea Ice ◽

Climate Models ◽

Temperature Response ◽

Coupled Model ◽

Surface Air Temperature ◽

Southern Annular Mode ◽

Climate Impacts ◽

Sea Ice Concentration ◽

Annular Mode ◽

Simulated Precipitation

Abstract The southern annular mode (SAM) has a well-established impact on climate in the Southern Hemisphere. The strongest response in surface air temperature (SAT) is observed in the Antarctic, but the SAM’s area of influence extends much farther, with statistically significant effects on temperature and precipitation being detected as far north as 20°S. Here the authors quantify the ability of the Coupled Model Intercomparison Project, phase 3 (CMIP3) coupled climate models to simulate the observed SAT, total precipitation, sea surface temperature (SST), and sea ice concentration responses to the SAM. The models are able to simulate the spatial pattern of response in SAT reasonably well; however, all models underestimate the magnitude of the response over Antarctica, both at the surface and in the free troposphere. This underestimation of the temperature response has implications for prediction of the future temperature changes associated with expected changes in the SAM. The models possess reasonable skill in simulating patterns of precipitation and SST response; however, some considerable regional deviations exist. The simulated precipitation and SST responses are less constrained by the observations than the SAT response, particularly in magnitude, as significant discrepancies are detected between the responses in the reference datasets. The largest problems are identified in simulating the sea ice response to the SAM, with some models even simulating a response that is negatively correlated with that observed.

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Extratropical Southern Hemisphere Synchronous Pressure Variability in the Early 20th Century

Journal of Climate ◽

10.1175/jcli-d-20-0498 ◽

2021 ◽

pp. 1-41

Author(s):

Ryan L. Fogt ◽

Charlotte J. Connolly

Keyword(s):

Southern Hemisphere ◽

Historical Data ◽

Austral Summer ◽

Southern Annular Mode ◽

Early 20Th Century ◽

Annular Mode ◽

Tropical Variability ◽

Zonal Wavenumber ◽

Meteorological Observations ◽

Statistical Relationships

AbstractBecause continuous meteorological observations across Antarctica did not start until the middle of the 20th century, little is known about the full spatial pattern of pressure variability across the extratropical Southern Hemisphere (SH) in the early 20th century, defined here as the period from 1905-1956. To fill this gap, this study analyzes pressure observations across the SH in conjunction with seasonal pressure reconstructions across Antarctica, which are based on observed station-to-station statistical relationships between pressure over Antarctica and the southern midlatitudes. Using this newly generated dataset, it is found that the early 20th century is characterized by synchronous, but opposite signed pressure relationships between Antarctica and the SH midlatitudes, especially in austral summer and autumn. The synchronous pressure relationships are consistent with the Southern Annular Mode, extending its well-known influence on SH extratropical pressure since 1957 into the early 20th century. Apart from connections with the Southern Annular Mode, regional and shorter-duration pressure trends are found to be associated with influences from tropical variability and potentially the zonal wavenumber three pattern. Although the reduced network of SH observations and Antarctic reconstruction capture the Southern Annular Mode in the early 20th century, reanalyses products show varying skill in reproducing trends and variability, especially over the oceans and high southern latitudes prior to 1957, which stresses the importance of continual efforts of historical data rescue in data sparse regions to improve their quality.

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