Dr David Jones, Head of Climate Analysis at the Australian Bureau of Meteorology, has claimed that over the past 11 years Melbourne’s rainfall has been about 20 percent below the long-term average.
It is common to refer to “the long-term average” when discussing climate data, but if the climate along the East Coast of Australia tends to be dominated by either El Nino or La Nina conditions, how meaningful is an average?
According to Associate Professor Stewart Franks, School of Engineering, University of Newcastle, when calculating a long-term average it is important to include an equal number of La Nina and also El Nino dominated periods.
Professor Franks is a hydrologist with an interest in understanding the risk of flooding. He has explained that if you take an annual maxima flood series for a northern New South Wales catchment, which is typical for the East Coast of Australia over the last 100 years, there have been two periods of El Nino conditions and a single La Nina.
So when a long-term average is calculated from this data it probably underestimates the real risk of a big flood event. In other words, if anything government policy and planning has underprepared us for big flood events.
In the opinion piece by Dr Jones entitled ‘Our hot, dry future’ published by Melbourne’s The Age newspaper recently, he claimed the below average rainfall in Melbourne was due to global warming and that there was worse to come.
But if the climate along the East Coast of Australia is a two state process dominated by El Nino or La Nina, then while Melbourne has experienced relatively dry conditions during the past 11 years, the expectation would be that at some point we will move back into a La Nina dominated phase. According to Professor Franks, it is probably a bit messier than that with periods that might not be dominated by either.
Nevertheless, it is reasonable to assume the longer the El Nino dominance continues, the likelier it is to end.
So, rather than preparing for more drought as Dr Jones suggests, perhaps we should prepare for more floods? Indeed climate always changes and floods and droughts are a natural hazard.
For more information:
D. C Verdon & S. W. Franks, 2006. Long-term behaviour of ENSO: Interactions with the PDO over the past 400 years inferred from paleoclimate records, Geophysical Research Letters, 33.