Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability
A critical question in the global warming debate concerns the causes of the observed trends of the Southern Hemisphere (SH) atmospheric circulation over recent decades. Secular trends have been identified in the frequency of occurrence of circulation regimes, namely the positive phase of the Souther...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2015-09-01
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| Series: | Nonlinear Processes in Geophysics |
| Online Access: | http://www.nonlin-processes-geophys.net/22/513/2015/npg-22-513-2015.pdf |
| _version_ | 1818174130695962624 |
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| author | C. L. E. Franzke T. J. O'Kane D. P. Monselesan J. S. Risbey I. Horenko |
| author_facet | C. L. E. Franzke T. J. O'Kane D. P. Monselesan J. S. Risbey I. Horenko |
| author_sort | C. L. E. Franzke |
| collection | DOAJ |
| description | A critical question in the global warming debate concerns the causes of the
observed trends of the Southern Hemisphere (SH) atmospheric circulation over
recent decades. Secular trends have been identified in the frequency of
occurrence of circulation regimes, namely the positive phase of the Southern
Annular Mode (SAM) and the hemispheric wave-3 pattern which is associated
with blocking. Previous studies into the causes of these secular trends have
either been purely model based, have not included observational forcing data
or have mixed external forcing with indices of internal climate variability
impeding a systematic and unbiased attribution of the causes of the secular
trends. Most model studies also focused mainly on the austral summer season.
However, the changes to the storm tracks have occurred in all seasons and
particularly in the austral winter and early spring when midlatitude
blocking is most active and stratospheric ozone should not play a role. Here
we systematically attribute the secular trends over the recent decades using
a non-stationary clustering method applied to both reanalysis and
observational forcing data from all seasons. While most previous studies
emphasized the importance of stratospheric ozone depletion in causing austral
summer SH circulation trends, we show observational evidence that
anthropogenic greenhouse gas concentrations have been the major driver of
these secular trends in the SAM and blocking when all seasons are considered.
Our results suggest that the recovery of the ozone hole might delay the
signal of global warming less strongly than previously thought and that
effects from all seasons are likely crucial in understanding the causes of
the secular trends. |
| first_indexed | 2024-12-11T19:39:31Z |
| format | Article |
| id | doaj.art-53470a1d131247348f8d5721855f65f9 |
| institution | Directory Open Access Journal |
| issn | 1023-5809 1607-7946 |
| language | English |
| last_indexed | 2024-12-11T19:39:31Z |
| publishDate | 2015-09-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Nonlinear Processes in Geophysics |
| spelling | doaj.art-53470a1d131247348f8d5721855f65f92022-12-22T00:53:03ZengCopernicus PublicationsNonlinear Processes in Geophysics1023-58091607-79462015-09-0122551352510.5194/npg-22-513-2015Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variabilityC. L. E. Franzke0T. J. O'Kane1D. P. Monselesan2J. S. Risbey3I. Horenko4Meteorological Institute and Center for Earth System Research and Sustainability, University of Hamburg, Hamburg, GermanyCSIRO Oceans and Atmosphere, Hobart, AustraliaCSIRO Oceans and Atmosphere, Hobart, AustraliaCSIRO Oceans and Atmosphere, Hobart, AustraliaUniversità della Svizzera Italiana, Lugano, SwitzerlandA critical question in the global warming debate concerns the causes of the observed trends of the Southern Hemisphere (SH) atmospheric circulation over recent decades. Secular trends have been identified in the frequency of occurrence of circulation regimes, namely the positive phase of the Southern Annular Mode (SAM) and the hemispheric wave-3 pattern which is associated with blocking. Previous studies into the causes of these secular trends have either been purely model based, have not included observational forcing data or have mixed external forcing with indices of internal climate variability impeding a systematic and unbiased attribution of the causes of the secular trends. Most model studies also focused mainly on the austral summer season. However, the changes to the storm tracks have occurred in all seasons and particularly in the austral winter and early spring when midlatitude blocking is most active and stratospheric ozone should not play a role. Here we systematically attribute the secular trends over the recent decades using a non-stationary clustering method applied to both reanalysis and observational forcing data from all seasons. While most previous studies emphasized the importance of stratospheric ozone depletion in causing austral summer SH circulation trends, we show observational evidence that anthropogenic greenhouse gas concentrations have been the major driver of these secular trends in the SAM and blocking when all seasons are considered. Our results suggest that the recovery of the ozone hole might delay the signal of global warming less strongly than previously thought and that effects from all seasons are likely crucial in understanding the causes of the secular trends.http://www.nonlin-processes-geophys.net/22/513/2015/npg-22-513-2015.pdf |
| spellingShingle | C. L. E. Franzke T. J. O'Kane D. P. Monselesan J. S. Risbey I. Horenko Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability Nonlinear Processes in Geophysics |
| title | Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability |
| title_full | Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability |
| title_fullStr | Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability |
| title_full_unstemmed | Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability |
| title_short | Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability |
| title_sort | systematic attribution of observed southern hemisphere circulation trends to external forcing and internal variability |
| url | http://www.nonlin-processes-geophys.net/22/513/2015/npg-22-513-2015.pdf |
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