Interdecadal Changes in the Austral Winter Large-Scale Circulation

Abstract:

A major and well-documented shift occurred in the structure of large-scale circulation in both hemispheres in the mid-1970s to early 1980s. The CSIRO Mk3L general circulation model is employed in simulations with observed sea surface temperatures (SSTs) and historical time-evolving carbon dioxide concentrations to investigate the interdecadal changes found observationally in the jet streams, temperature, Hadley circulation, mean sea level pressure and precipitation. The model simulations are compared with the corresponding observations (NCEP/NCAR Reanalysis I and the Twentieth Century Reanalysis V2) for the austral winter between the periods 1949-68 and 1975-94. The results show that model is quite skilful in reproducing the broad features of the important interdecadal changes that occurred in the mid-1970s. The model simulations and the NCEP/NCAR and 20th Century reanalyses agree in the eastern hemisphere; whereas in the western hemisphere the reanalyses show differences, and the simulations combine aspects of these two dataset. The role of the time-varying external radiative forcing is clearly demonstrated, especially for the mean sea level pressure and precipitation fields.