Variability and trend of the north west Australia rainfall: observations and coupled climate modeling

Shi, Ge and Cai, Wenju and Cowan, Tim and Ribbe, Joachim and Rotstayn, Leon and Dix, Martin (2008) Variability and trend of the north west Australia rainfall: observations and coupled climate modeling. Journal of Climate, 21 (12). pp. 2938-2959. ISSN 0894-8755

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Since 1950, there has been an increase in rainfall over North West Australia (NWA), occurring mainly during the Southern Hemisphere (SH) summer season. A recent study
using 20th century multi-member ensemble simulations in a global climate model forced with and without increasing anthropogenic aerosols suggests that the rainfall increase is attributable to increasing Northern Hemisphere aerosols. The present study investigates the dynamics of the observed trend toward increased rainfall and compares the observed trend with that generated in the model forced with
increasing aerosols.
We find that the observed positive trend in rainfall is projected onto two modes of variability. The first mode is associated with an anomalously low mean sea level pressure (MSLP) off NWA instigated by the enhanced sea surface temperature (SST) gradients towards the coast. The associated cyclonic flows bring high moisture air to northern Australia, leading to an increase in rainfall. The second mode is associated with an anomalously high MSLP over much of the Australian continent; the anticyclonic circulation pattern with northwesterly flows west of 130°E
and generally opposite flows in northeastern Australia, determine that when rainfall is anomalously high, west of 130oE, rainfall is anomalously low east of this longitude. The sum of the upward trends in these two modes compares well to the observed increasing trend pattern. The modeled rainfall trend, however, is generated by a different process. The model suffers from an equatorial cold-tongue bias: the tongue of anomalies associated with El Niño-Southern Oscillation extends too far west into the eastern Indian Ocean. Consequently, there is an unrealistic
relationship in the SH summer between Australian rainfall and eastern Indian Ocean SST: the rise in SST is associated with an increasing rainfall over NWA. In the presence of increasing aerosols, a significant SST increase occurs in the eastern tropical Indian Ocean. As a result, the modeled rainfall increase in the presence of aerosol forcing is
accounted for by these unrealistic relationships. It is not clear whether, in a model without such defects, the observed trend can be generated by increasing aerosols. Thus, the impact of aerosols on Australian rainfall remains an open question.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Deposited in accordance with the copyright policy of the publisher.
Faculty / Department / School: Historic - Faculty of Sciences - Department of Biological and Physical Sciences
Date Deposited: 18 Mar 2008 05:51
Last Modified: 25 Jul 2014 05:45
Uncontrolled Keywords: Australia; climate; Australian climate; climate variability; Indian Ocean dipole; El Nino Southern Oscillation; North West Australia; rainfall; climate modelling; drought; aerosols
Fields of Research : 09 Engineering > 0907 Environmental Engineering > 090702 Environmental Engineering Modelling
04 Earth Sciences > 0406 Physical Geography and Environmental Geoscience > 040605 Palaeoclimatology
04 Earth Sciences > 0401 Atmospheric Sciences > 040105 Climatology (excl.Climate Change Processes)
Socio-Economic Objective: D Environment > 96 Environment > 9603 Climate and Climate Change > 960303 Climate Change Models
Identification Number or DOI: 10.1175/2007JCLI1908.1

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