scholarly journals Seasonal Zonal Asymmetries in the Southern Annular Mode and Their Impact on Regional Temperature Anomalies

2012 ◽  
Vol 25 (18) ◽  
pp. 6253-6270 ◽  
Author(s):  
Ryan L. Fogt ◽  
Julie M. Jones ◽  
James Renwick
Keyword(s):  
Southern Hemisphere ◽  
Austral Summer ◽  
Zonal Flow ◽  
Southern Annular Mode ◽  
Dominant Mode ◽  
Climate Impacts ◽  
Annular Mode ◽  
The Pacific ◽  
Zonal Wavenumber ◽  
Regional Temperature

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.

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

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.

Extratropical Southern Hemisphere Synchronous Pressure Variability in the Early Twentieth Century

2021 ◽  
Vol 34 (14) ◽  
pp. 5795-5811
Author(s):  
Ryan L. Fogt ◽  
Charlotte J. Connolly
Keyword(s):  
Twentieth Century ◽  
Southern Hemisphere ◽  
Austral Summer ◽  
Southern Annular Mode ◽  
Annular Mode ◽  
Tropical Variability ◽  
Early Twentieth Century ◽  
Zonal Wavenumber ◽  
Meteorological Observations ◽  
Statistical Relationships

Abstract Because continuous meteorological observations across Antarctica did not start until the middle of the twentieth century, little is known about the full spatial pattern of pressure variability across the extratropical Southern Hemisphere (SH) in the early twentieth century, defined here as the period from 1905 to 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 twentieth 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 twentieth 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 3 pattern. Although the reduced network of SH observations and Antarctic reconstruction captures the southern annular mode in the early twentieth century, reanalysis 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.

Multiple Timescales of the Southern Annular Mode

2021 ◽  
Author(s):  
Qiuyan Zhang ◽  
Yang Zhang ◽  
Zhaohua Wu
Keyword(s):  
Seasonal Variability ◽  
Low Frequency ◽  
Austral Summer ◽  
Zonal Flow ◽  
Southern Annular Mode ◽  
Multiple Timescales ◽  
Annular Mode ◽  
Flow Interactions ◽  
The Cross ◽  
Sst Anomalies

<p>Using the ensemble empirical mode decomposition (EEMD) method, this study systematically investigates the multiple timescales of the Southern Annular Mode (SAM) and identifies their relative contributions to the low-frequency persistence of SAM. Analyses show that the subseasonal sustaining of SAM mainly depends on the contribution of longer-timescale variabilities, especially the cross-seasonal variability. When subtracting the cross-seasonal variability from the SAM, the positive covariance between the eddy and zonal flow, which is suggested the positive eddy feedback in SAM, disappears. Composite analysis shows that only with strong cross-seasonal variability, the meridional shift of zonal wind, eddy momentum forcing and baroclinicity anomalies can be maintained for more than 20 days, mainly resulting from the longer-timescale (especially the cross-seasonal timescale) eddy-zonal flow interactions. This study further suggests that the dipolar sea surface temperature (SST) anomalies in the mid latitude of Southern Hemisphere (SH) is a possible cause for the cross-seasonal variability. Analysis shows that about half of the strong cross-seasonal timescale events are accompanied by evident dipolar SST anomalies, which mostly occur in austral summer. The cross-seasonal dependence of the eddy-zonal flow interactions suggests the longer-timescale (especially the cross-seasonal timescale) contribution cannot be neglected in subseasonal prediction of SAM.</p>

The Continuum of Wintertime Southern Hemisphere Atmospheric Teleconnection Patterns*,+

2015 ◽  
Vol 28 (24) ◽  
pp. 9507-9529 ◽  
Author(s):  
Chueh-Hsin Chang ◽  
Nathaniel C. Johnson
Keyword(s):  
Sea Ice ◽  
Southern Hemisphere ◽  
Time Scales ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Oscillatory Behavior ◽  
Annular Mode ◽  
Teleconnection Patterns ◽  
Zonal Wavenumber ◽  
The Continuum

Abstract This study uses the method of self-organizing maps (SOMs) to categorize the June–August atmospheric teleconnections in the 500-hPa geopotential height field of the Southern Hemisphere (SH) extratropics. This approach yields 12 SOM patterns that provide a discretized representation of the continuum of SH teleconnection patterns from 1979 to 2012. These 12 patterns are large in spatial scale, exhibiting a mix of annular mode characteristics and wave trains of zonal wavenumber varying from 2 to 4. All patterns vary with intrinsic time scales of about 5–10 days, but some patterns exhibit quasi-oscillatory behavior over a period of 20–30 days, whereas still others exhibit statistically significant enhanced and suppressed frequencies up to about four weeks in association with the Madden–Julian oscillation. Two patterns are significantly influenced by El Niño–Southern Oscillation (ENSO) on interannual time scales. All 12 patterns have strong influences on surface air temperature and sea ice concentrations, with the sea ice response occurring over a time scale of about 2–4 weeks. The austral winter has featured a positive frequency trend in patterns that project onto the negative phase of the southern annular mode (SAM) and a negative frequency trend in positive SAM-like patterns. Such atmospheric circulation trends over 34 yr may arise through atmospheric internal variability alone, and, unlike other seasons in the SH, it is not necessary to invoke external forcing as a dominant source of circulation trends.

scholarly journals Southern Annular Mode Dynamics in Observations and Models. Part II: Eddy Feedbacks

2013 ◽  
Vol 26 (14) ◽  
pp. 5220-5241 ◽  
Author(s):  
Isla R. Simpson ◽  
Theodore G. Shepherd ◽  
Peter Hitchcock ◽  
John F. Scinocca
Keyword(s):  
Negative Feedback ◽  
Planetary Wave ◽  
Southern Annular Mode ◽  
Summer Season ◽  
Substantial Contribution ◽  
Mean Flow ◽  
Annular Mode ◽  
Planetary Scale ◽  
Zonal Wavenumber ◽  
Climatological Circulation

Abstract Many global climate models (GCMs) have trouble simulating southern annular mode (SAM) variability correctly, particularly in the Southern Hemisphere summer season where it tends to be too persistent. In this two-part study, a suite of experiments with the Canadian Middle Atmosphere Model (CMAM) is analyzed to improve the understanding of the dynamics of SAM variability and its deficiencies in GCMs. Here, an examination of the eddy–mean flow feedbacks is presented by quantification of the feedback strength as a function of zonal scale and season using a new methodology that accounts for intraseasonal forcing of the SAM. In the observed atmosphere, in the summer season, a strong negative feedback by planetary-scale waves, in particular zonal wavenumber 3, is found in a localized region in the southwest Pacific. It cancels a large proportion of the positive feedback by synoptic- and smaller-scale eddies in the zonal mean, resulting in a very weak overall eddy feedback on the SAM. CMAM is deficient in this negative feedback by planetary-scale waves, making a substantial contribution to its bias in summertime SAM persistence. Furthermore, this bias is not alleviated by artificially improving the climatological circulation, suggesting that climatological circulation biases are not the cause of the planetary wave feedback deficiency in the model. Analysis of the summertime eddy feedbacks in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) confirms that this is indeed a common problem among GCMs, suggesting that understanding this planetary wave feedback and the reason for its deficiency in GCMs is key to improving the fidelity of simulated SAM variability in the summer season.

Regional climate impacts of the Southern Annular Mode

2006 ◽  
Vol 33 (23) ◽  
Author(s):  
N. P. Gillett ◽  
T. D. Kell ◽  
P. D. Jones
Keyword(s):  
Regional Climate ◽  
Southern Annular Mode ◽  
Climate Impacts ◽  
Annular Mode

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

2019 ◽  
Vol 9 (1) ◽  
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.

scholarly journals The South Atlantic–South Indian Ocean Pattern: a Zonally Oriented Teleconnection along the Southern Hemisphere Westerly Jet in Austral Summer

Atmosphere ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 259 ◽  
Author(s):  
Zhongda Lin
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Southern Hemisphere ◽  
Austral Summer ◽  
South Atlantic ◽  
The South ◽  
South Indian Ocean ◽  
Westerly Jet ◽  
South Indian ◽  
Zonal Wavenumber

Extratropical teleconnections significantly affect the climate in subtropical and mid-latitude regions. Understanding the variability of atmospheric teleconnection in the Southern Hemisphere, however, is still limited in contrast with the well-documented counterpart in the Northern Hemisphere. This study investigates the interannual variability of mid-latitude circulation in the Southern Hemisphere in austral summer based on the ERA-Interim reanalysis dataset during 1980–2016. A stationary mid-latitude teleconnection is revealed along the strong Southern Hemisphere westerly jet over the South Atlantic and South Indian Ocean (SAIO). The zonally oriented SAIO pattern represents the first EOF mode of interannual variability of meridional winds at 200 hPa over the region, with a vertical barotropic structure and a zonal wavenumber of 4. It significantly modulates interannual climate variations in the subtropical Southern Hemisphere in austral summer, especially the opposite change in rainfall and surface air temperature between Northwest and Southeast Australia. The SAIO pattern can be efficiently triggered by divergences over mid-latitude South America and the southwest South Atlantic, near the entrance of the westerly jet, which is probably related to the zonal shift of the South Atlantic Convergence Zone. The triggered wave train is then trapped within the Southern Hemisphere westerly jet waveguide and propagates eastward until it diverts northeastward towards Australia at the jet exit, in addition to portion of which curving equatorward at approximately 50° E towards the southwest Indian Ocean.

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