Observed and simulated inter-decadal changes in the structure of Southern Hemisphere large-scale circulation

Ana C V Freitas, Jorgen S Frederiksen, Jennifer Whelan, Terence J. O’Kane, Tércio Ambrizzi

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11 Citations (Scopus)

Abstract

Several studies have identified that, in the mid-1970s to early 1980s, a major shift occurred in the structure of the large-scale circulation in both hemispheres. This work employs the CSIRO Mk3L general circulation model in ensemble simulations with observed sea surface temperatures (SSTs) and historical time-evolving carbon dioxide (CO2) concentrations to investigate the inter-decadal changes found observationally in the jet streams, temperature, Hadley circulation, mean sea level pressure and precipitation. First, the performance of the model in simulating these changes for the mean July climate fields of 1949–1968 and 1975–1994, in comparison with the corresponding observations (NCEP/NCAR Reanalysis I and the Twentieth Century Reanalysis V2), is investigated. We find that the model is quite skilful in reproducing the broad features of the important inter-decadal changes that occurred in the mid-1970s. The model simulations and the NCEP/NCAR and twentieth century reanalyses agree in the eastern hemisphere; whereas in the western hemisphere the reanalyses show differences, and the simulations combine aspects of these two datasets. The role of the direct radiative forcing due to CO2in driving the inter-decadal changes is also examined. Results indicate that, in comparison with the indirect effect of CO2carried by the changing SSTs, there is little additional impact of the direct radiative forcing due to CO2on the changes in the latter period. However, our simulations with fixed CO2concentration have shown clearly that the atmospheric simulations with historical time-evolving CO2concentrations are more skilful in reproducing the inter-decadal changes. The sensitivity of the ensemble results to employing the same or different time evolving sea ice boundary conditions in the ensemble members is also studied. The contributions of internal and external variability are discussed.

Original languageEnglish
Pages (from-to)2993-3017
Number of pages25
JournalClimate Dynamics
Volume45
Issue number11-12
DOIs
Publication statusPublished - 1 Dec 2015
Externally publishedYes

Keywords

  • Hadley circulation
  • Jet streams
  • Mean sea level pressure
  • Precipitation
  • Temperature

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