The Modulation Mechanism Of The Interannual And Interdecadal Variability Of The Southern Annular Mode And Their Corresponding Impacts On The Supergyre Circulation

The positive phase of the El Nino-Southern Oscillation (ENSO) increases globalmean temperature, and contributes to a negative phase of the Southern Annular Mode(SAM), the dominant mode of climate variability in the Southern Hemisphere. Thisinterannual relationship of a high global mean temperature associated with a negativeSAM, however, is opposite to the relationship between their trends under greenhousewarming.We show that over much of the20th century this relationship undergoes multidecadal fluctuations depending on the intensity of ENSO. During the period1925–1955, subdued ENSO activities weakened the relationship. However, a similarweakening has occurred since the late1970s despite the strong ENSO. Wedemonstrate that this recent weakening is induced by climate change in the SouthernHemisphere. Our result highlights a rare situation in which climate change signalsemerge against an opposing property of interannual variability, underscoring therobustness of the recent climate change.During the late20thcentury, Antarctic ozone depletion and increasing greenhousegases (GHGs) conspired to generate conspicuous atmospheric circulation trends in theSouthern Hemisphere (SH), contributing to a poleward intensification of the oceanicSupergyre Circulation. Forcing of Antarctic ozone depletion dominated the observedtrends during the depletion period (1961-2005), but Antarctic ozone is projected torecover by the middle of the21stcentury. The recovery provides a mechanism foroffsetting the impact from increasing GHG emissions. To what extent will therecovery of ozone mitigate SH atmosphere and ocean circulation trends expectedfrom increasing GHGs? We examine climate model outputs from the RepresentativeConcentration Pathways4.5and8.5(RCP4.5and RCP8.5, respectively) emissionscenario experiments, submitted to the Coupled Model Intercomparison Project phase5. Both scenarios are subject to the effect of ozone recovery. We show that during therecovery period (2006-2045), there is little trend of the poleward shift of thesuper-gyre circulation under either RCP scenario in austral summer, due to thedominance of ozone recovery. Further, under RCP8.5the trend in winter, a season inwhich ozone recovery has little impact, is greater (more poleward) than in summer,opposite to the seasonality of trends during the depletion period. Under RCP4.5, withthe contribution from ozone recovery, the summer poleward shift is projected tostabilise into the post-recovery decades, whereas under RCP8.5, the summer polewardshift accelerates in the post-recovery period, presenting a vastly different oceancirculation future.