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.

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.

scholarly journals Southern Annular Mode drives multicentury wildfire activity in southern South America

2017 ◽  
Vol 114 (36) ◽  
pp. 9552-9557 ◽  
Author(s):  
Andrés Holz ◽  
Juan Paritsis ◽  
Ignacio A. Mundo ◽  
Thomas T. Veblen ◽  
Thomas Kitzberger ◽  
...  
Keyword(s):  
South America ◽  
Southern Hemisphere ◽  
Time Scales ◽  
Climate Modeling ◽  
Stratospheric Ozone ◽  
Southern Annular Mode ◽  
Southern South America ◽  
Annular Mode ◽  
Climate Conditions ◽  
Fire Activity

The Southern Annular Mode (SAM) is the main driver of climate variability at mid to high latitudes in the Southern Hemisphere, affecting wildfire activity, which in turn pollutes the air and contributes to human health problems and mortality, and potentially provides strong feedback to the climate system through emissions and land cover changes. Here we report the largest Southern Hemisphere network of annually resolved tree ring fire histories, consisting of 1,767 fire-scarred trees from 97 sites (from 22 °S to 54 °S) in southern South America (SAS), to quantify the coupling of SAM and regional wildfire variability using recently created multicentury proxy indices of SAM for the years 1531–2010 AD. We show that at interannual time scales, as well as at multidecadal time scales across 37–54 °S, latitudinal gradient elevated wildfire activity is synchronous with positive phases of the SAM over the years 1665–1995. Positive phases of the SAM are associated primarily with warm conditions in these biomass-rich forests, in which widespread fire activity depends on fuel desiccation. Climate modeling studies indicate that greenhouse gases will force SAM into its positive phase even if stratospheric ozone returns to normal levels, so that climate conditions conducive to widespread fire activity in SAS will continue throughout the 21st century.

scholarly journals Southern Hemisphere Synoptic Behavior in Extreme Phases of SAM, ENSO, Sea Ice Extent, and Southern Australia Rainfall

2008 ◽  
Vol 21 (21) ◽  
pp. 5566-5584 ◽  
Author(s):  
Alexandre Bernardes Pezza ◽  
Tom Durrant ◽  
Ian Simmonds ◽  
Ian Smith
Keyword(s):  
Sea Ice ◽  
Southern Hemisphere ◽  
High Latitude ◽  
Large Scale ◽  
Southern Oscillation ◽  
Southern Annular Mode ◽  
Sea Ice Extent ◽  
Antarctic Sea Ice ◽  
Rainfall Anomalies ◽  
Western Side

Abstract The association between Southern Hemisphere cyclones and anticyclones and the El Niño–Southern Oscillation (ENSO), southern annular mode (SAM), Antarctic sea ice extent (SIE), and rainfall in Perth and Melbourne is explored. Those cities are, respectively, located in the southwestern and southeastern corners of Australia, where substantial decreasing rainfall trends have been observed over the last decades. The need for a more unified understanding of large-scale anomalies in storm indicators associated with the climate features itemized above has motivated this study. The main aim is to identify cyclone-anomalous areas that are potentially important in characterizing continental rainfall anomalies from a hemispheric perspective, focusing on midlatitude Australia. The study covers the “satellite era” from 1979 to 2003 and was conducted for the southern winter when midlatitude rainfall is predominantly baroclinic. The results indicate a well-organized hemispheric cyclone pattern associated with ENSO, SAM, SIE, and rainfall anomalies. There is a moderate large-scale, high-latitude resemblance between La Niña, negative SAM, and reduced SIE in some sectors. In particular, there is a suggestion that SIE anomalies over the Indian Ocean and Western Australia sectors are associated with a large-scale pattern of cyclone/anticyclone anomalies that is more pronounced over the longitudes of Australia and New Zealand. Spatial correlation analysis suggests a robust link between cyclone density over the sectors mentioned above and rainfall in Perth and Melbourne. Statistical analyses of rainfall and SIE show modest correlations for Perth and weak correlations for Melbourne, generally corroborating the above. It is proposed that SAM and SIE are part of a complex physical system that is best understood as a coupled mechanism, and that their impacts on the circulation can be seen as partially independent of ENSO. While SAM and SIE have greater influence on the circulation affecting rainfall in the western side of Australia, ENSO is the dominant influence on the eastern half of the country. A contraction of the sea ice seems to be accompanied by a southward shift of high-latitude cyclones, which is also hypothesized to increase downstream cyclone density at midlatitudes via conservation of mass, similarly to what is observed during the extreme positive phase of the SAM. These associations build on previous developments in the literature. They bring a more unified view on high-latitude climate features, and may also help to explain the declining trends in Australian rainfall.

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.

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.

Differential Credibility of Climate Modes in CMIP6

2021 ◽  
pp. 1
Author(s):  
Jacob Coburn ◽  
S.C. Pryor
Keyword(s):  
Southern Oscillation ◽  
Probability Distributions ◽  
Critical Role ◽  
Regional Scale ◽  
Southern Annular Mode ◽  
Climate Projections ◽  
Annular Mode ◽  
Climate Modes ◽  
First Order ◽  
Skill Scores

AbstractThis work quantitatively evaluates the fidelity with which the Northern Annular Mode (NAM), Southern Annular Mode (SAM), Pacific-North American pattern (PNA), El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) and the first-order mode interactions are represented in Earth System Model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climate modes. Substantial variability in skill scores manifests across realizations from individual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM-PNA first-order interaction and underestimate the NAM-AMO connection. These results suggest that the choice of ESM and ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near-term.

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