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A critical question in the global warming debate concerns the causes of theobserved trends of the Southern Hemisphere (SH) atmospheric circulation overrecent decades. Secular trends have been identified in the frequency ofoccurrence of circulation regimes, namely the positive phase of the SouthernAnnular Mode (SAM) and the hemispheric wave-3 pattern which is associatedwith blocking. Previous studies into the causes of these secular trends haveeither been purely model based, have not included observational forcing dataor have mixed external forcing with indices of internal climate variabilityimpeding a systematic and unbiased attribution of the causes of the seculartrends. Most model studies also focused mainly on the austral summer season.However, the changes to the storm tracks have occurred in all seasons andparticularly in the austral winter and early spring when midlatitudeblocking is most active and stratospheric ozone should not play a role. Herewe systematically attribute the secular trends over the recent decades usinga non-stationary clustering method applied to both reanalysis andobservational forcing data from all seasons. While most previous studiesemphasized the importance of stratospheric ozone depletion in causing australsummer SH circulation trends, we show observational evidence thatanthropogenic greenhouse gas concentrations have been the major driver ofthese secular trends in the SAM and blocking when all seasons are considered.Our results suggest that the recovery of the ozone hole might delay thesignal of global warming less strongly than previously thought and thateffects from all seasons are likely crucial in understanding the causes ofthe secular trends

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