final_program_abstracts[1]

11 IMSC Session Program
Recent unprecedented skewness towards more positive
Indian Ocean Dipole occurrences and its impact on regional
climate
Monday - Parallel Session 5
Tim Cowan and Wenju Cai
CSIRO Marine and Atmospheric Research, Aspendale, Australia
During 2006-2008 a rare realization of three consecutive positive Indian Ocean
Dipoles (pIODs) occurred, including an unusual occurrence with a La Niña in 2007
(Luo et al. 2008). These consecutive pIODs contributed to the prolonged drought
(termed the “Big Dry”) over southeastern Australia (SEA), preceding the devastating
“Black Saturday” bushfires near Melbourne that killed over 170 people and
devastated many communities. Is the recent high frequency of pIOD events a
consequence of global warming? Is there a linkage between recent major bushfires
over SEA and the recent skewness toward more frequent positive events?
From available observations and reanalyses, there has been a pIOD frequency
increase from about four per 30 years early in the 20th century to about 10 over the
last 30 years (Cai et al. 2009a). By contrast, the number of negative Indian Ocean
Dipole (nIOD) events (that are associated with above average rainfall in SEA) has
decreased from about 10 to two over the same periods. These changes in the IOD
account for about 40% of the observed austral spring rainfall reduction since 1950
across SEA. Out of the 16 pIODs since 1950, 11 have been followed by a major
bushfire event (Cai et al. 2009b), relating to the fact that pIODs lead to lower rainfall
totals and higher temperatures during the rainy season for SEA.
As a means of undertaking a statistical assessment into the rarity of such events, we
take 50 years from 19 climate model simulations submitted for the Intergovernmental
Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The simulations
without climate change produce only two occurrences of three-consecutive pIOD
events, whilst those with climate change the occurrence increases to 8, comparable to
the observed frequency of one per century (Cai et al. 2009c). Although we cannot
attribute the trigger of the observed events in 2006-2008, climate change is increasing
the frequency of pIODs. This could ultimately mean an increased bushfire risk for
much of SEA in the future as global warming persists.
Luo, J.-J., S. Behera, Y. Masumoto, H. Sakuma, and T. Yamagata (2008), Successful
prediction of the consecutive IOD in 2006 and 2007, Geophys. Res. Lett., 35, L14S02,
doi:10.1029/2007GL032793.
Cai, W., T. Cowan, and A. Sullivan (2009a), Recent unprecedented skewness towards
positive Indian Ocean Dipole occurrences and its impact on Australian rainfall,
Geophys. Res. Lett., 36, L11705, doi:10.1029/2009GL037604.
Cai, W., T. Cowan, and M. Raupach (2009b), Positive Indian Ocean Dipole events
precondition southeast Australia bushfires, Geophys. Res. Lett., 36, L19710,
doi:10.1029/2009GL039902.
Abstracts 83