Indian Ocean Climate Initiative - Western Australia

Project 2.1: Have Asian aerosols caused the rainfall increase in north west Australia? (Leon Rotstayn)

Australia's natural rainfall variability is substantially driven by natural oscillations or "modes" in the surrounding ocean basins, of which the El Nino Southern Oscillation (ENSO) is the best known. If a climate model is to be used to investigate the drivers of Australian rainfall changes, it is important that the model has a good simulation of these modes of variability. We have used a technique called empirical orthogonal teleconnections (EOTs) to evaluate the simulation of Australian rainfall variability in a current climate simulation of the CSIRO Mk3.6 climate model and several other models (see Rotstayn et al., 2009 for further details). The Mk3.6 model is to be used to study whether the increased rainfall in north-western and central Australia can be attributed to forcing from Asian aerosol pollution. The figure shows the leading mode of rainfall variability from observations (top left panel) and five climate models, including the Mk3.6 model and its predecessor (Mk3.5). The observations show an "ENSO-related" rainfall mode, centred over eastern Australia. This is well captured by the Mk3.6 model, whereas other models shown incorrectly locate this mode over northern WA or the Northern Territory. The improved simulation in Mk3.6 relative to Mk3.5 is important, because if the ENSO-related mode is located over northern WA, the response of the modelled rainfall there to a change of forcing (such as aerosols or greenhouse gases) is likely to be unrealistically dominated by the response of ENSO to that forcing.

Planned outcomes:

• Refine the high-resolution CSIRO climate model (Mk3.5A) so that it achieves a stable, high-resolution simulation of present global climate, including aerosols, and an evaluation of this simulation. (The inclusion of aerosols is non-trivial task.)
• Describe the observed climate changes, the modelled changes and their limitations; and then incorporate time-varying aerosol forcing into model with the aim of capturing the observed rainfall increases with increased confidence.
• Investigate the effects of aerosols and greenhouse gases on past and future rainfall trends in the North-West using the Mk3.5A model.

Figure 2.1.1: Leading mode of annual rainfall variability over Australia, from observations (Bureau of Meteorology), the CSIRO Mk3.6 and Mk3.5 climate models, and three leading international models: HadGEM1 (United Kingdom), GFDL CM2.1 (USA) and MIROC 3.2, medium resolution (Japan).

The observations show an "ENSO-related" rainfall mode, centred over eastern Australia. This is well captured by the Mk3.6 model, whereas other models shown incorrectly locate this mode over northern WA or the Northern Territory. The improved simulation in Mk3.6 relative to Mk3.5 is important, because if the ENSO-related mode is located over northern WA, the response of the modelled rainfall there to a change of forcing (such as aerosols or greenhouse gases) is likely to be unrealistically dominated by the response of ENSO to that forcing.

Reference

Rotstayn, L.D., M.A. Collier, Y. Feng, H.B. Gordon, S.P. O'Farrell, I.N. Smith, J. Syktus, 2009. Improved simulation of Australian climate and ENSO-related rainfall variability in a GCM with an interactive aerosol treatment. Submitted to Int. J. Climatol.