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Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability


Franzke, CLE and O'Kane, TJ and Monselesan, DP and Risbey, JS and Horenko, I, Systematic attribution of observed Southern Hemisphere circulation trends to external forcing and internal variability, Nonlinear Processes in Geophysics, 22, (5) pp. 513-525. ISSN 1023-5809 (2015) [Refereed Article]


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Copyright 2015 The Authors Licensed under Creative Commons Attribution 3.0 Unported (CC BY 3.0)

DOI: doi:10.5194/npg-22-513-2015


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.

Item Details

Item Type:Refereed Article
Keywords:Southern Hemisphere, external forcing, internal variability, systematic attribution, circulation trends
Research Division:Earth Sciences
Research Group:Climate change science
Research Field:Climate change processes
Objective Division:Environmental Management
Objective Group:Air quality, atmosphere and weather
Objective Field:Atmospheric processes and dynamics
UTAS Author:O'Kane, TJ (Dr Terry O'Kane)
UTAS Author:Risbey, JS (Dr James Risbey)
ID Code:118473
Year Published:2015
Web of Science® Times Cited:15
Deposited By:Directorate
Deposited On:2017-07-12
Last Modified:2017-08-09
Downloads:132 View Download Statistics

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