Seminars

Seminar Abstract

Have Australian rainfall and cloudiness increased due to the remote effects of Asian anthropogenic aerosols?

There is ample evidence that anthropogenic aerosols have important effects on climate in the Northern Hemisphere, but there is little such evidence in the less polluted Southern Hemisphere. To investigate possible effects of aerosols on the Australian hydrological cycle, 20th Century transient simulations performed with a new low-resolution version of the CSIRO global climate model have been analyzed. This version of the model ("Mk3A") includes an interactive treatment of aerosols from both natural and anthropogenic sources.

Observations of Australian rainfall and cloudiness since 1950 show increases over much of the continent, and especially in the northwest. We found that when anthropogenic aerosol changes were included in the model, it gave increasing rainfall and cloudiness over Australia during 1951-1996. The pattern of increasing rainfall was strongest over northwestern Australia, in agreement with the observed trends. When anthropogenic aerosol changes were omitted, the model gave decreasing rainfall and cloudiness over Australia during 1951-1996.

The strong impact of aerosols was predominantly due to the massive Asian aerosol haze, as confirmed by a sensitivity test in which only Asian anthropogenic aerosols were included. The Asian haze altered the north-south temperature and pressure gradients over the tropical Indian Ocean, thereby increasing the tendency of monsoonal winds to flow towards Australia. Inclusion of anthropogenic aerosols also made the simulated pattern of surface-temperature change in the tropical Pacific more like La Nina, since they induced a cooling of the surface waters in the extratropical North Pacific, which were then transported to the tropical eastern Pacific several decades later via the deep ocean. This effect was related to aerosols from regions other than Asia.

Transient climate model simulations forced only by increased greenhouse gases have generally not reproduced the observed rainfall increase over northwestern and central Australia. Our results suggest that a likely reason for this failure was the omission of forcing by Asian aerosols, and that inclusion of this forcing is essential in future modeling of Australian climate change.

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CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
To schedule a seminar, contact:
Karen Wild-Allen, (Oceanographic seminars) CSIRO Marine and Atmospheric Research (03) 6232 5010
Piers Dunstan, (Biological seminars) CSIRO Marine and Atmospheric Research (03) 6232 5382
Annabel Ozimec (seminar administrator) CSIRO Marine and Atmospheric Research (03) 6232 5462
Sandra Zicus, Antarctic Climate and Ecosystems CRC (03) 6226 7888 & Institute of Antarctic and Southern Ocean Studies (IASOS) University of Tasmania (03) 6226 2509