Warming causes more extreme shifts of the Southern Hemisphere’s largest rain band
South Pacific countries will experience more extreme floods and droughts, in response to increasing greenhouse gas
emissions, according to a paper out today in the journal Nature.
The changes will result from the South Pacific rain band responding to greenhouse warming The South Pacific rain band is
largest and most persistent of the Southern Hemisphere spanning the Pacific from south of the Equator, south-eastward to
French Polynesia. Occasionally, the rain band moves northwards towards the Equator by 1000 kilometres, inducing extreme
climate events.
The international study, led by CSIRO oceanographer Dr Wenju Cai, focuses on how the frequency of such movement may
change in the future. The study finds the frequency will almost double in the next 100 years, with a corresponding
intensification of the rain band.
Dr Wenju and colleagues turned to the extensive archives of general circulation models submitted for the fourth and
fifth IPCC Assessments and found that increases in greenhouse gases are projected to enhance equatorial Pacific warming.
In turn, and in spite of disagreement about the future of El Niño events, this warming leads to the increased frequency
of extreme excursions of the rain band.
During moderate El Niño events with warming in the equatorial eastern Pacific, the rain band moves north-eastward by 300
kilometres. Countries located within the bands’ normal position such as Vanuatu, Samoa, and the southern Cook Islands
experience forest fires and droughts as well as increased frequency of tropical cyclones, whereas countries to which the
rain band moves experience extreme floods.
“During extreme El Niño events, such as 1982/83 and 1997/98, the band moved northward by up to 1000 kilometres. The
shift brings more severe extremes, including cyclones to regions such as French Polynesia that are not accustomed to
such events,” said Dr Cai, a scientist at the Wealth from Oceans Flagship.
A central issue for community adaptation in Australia and across the Pacific is understanding how the warming atmosphere
and oceans will influence the intensity and frequency of extreme events. The impact associated with the observed extreme
excursions includes massive droughts, severe food shortage, and coral reef mortality through thermally-induced coral
bleaching across the South Pacific.
“Understanding changes in the frequency of these events as the climate changes proceed is therefore of broad scientific
and socio-economic interest."
The paper, More extreme swings of the South Pacific Convergence Zone due to greenhouse warming, was co-authored by
Australian scientists Dr Simon Borlace, Mr Tim Cowan from CSIRO and Drs Scott Power and Jo Brown, two Bureau of
Meteorology scientists at the Centre for Australian Weather and Climate research, who were joined by French, US, UK, and
Cook Island scientists.
The research effort from Australian scientists was supported by the Australian Climate Change Science Program, the CSIRO
Office of Chief Executive Science Leader programme, and the Pacific-Australia Climate Change Science and Adaptation
Planning Program.
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