How El Nino events differ
Every El Niño is different and, interestingly, the strongest El Niños aren’t necessarily the ones with the largest impacts in Australia.
This is particularly true for the east coast. While some El Niño events such as 2002 caused drought right across eastern Australia, in other El Niños such as 2006 and 2009 the east coast emerged relatively unscathed. Meanwhile, while the strong 2010 La Nina caused the wettest spring on record for Australia, along the east coast rainfall was barely above average.
Recent research published by UNSW and the Bureau of Meteorology is working to understand how different El Niño events impact the east coast. And the answer appears to lie in the Indian Ocean.
The Indian Ocean Dipole (IOD) can be considered the El Niño of the Indian Ocean. During a positive IOD year, the westerly systems and cold fronts that produce most of southern Australia’s winter rain decrease, and years with both El Niño and positive IOD tend to cause the most severe droughts in southeast Australia.
But the east coast is different – even during the winter, most of the rain on the east coast falls associated with easterly winds. This means that positive IOD (less westerly winds) increases rain on the east coast, at the same time as it decreases rain everywhere else. So when El Niño and positive IOD occur together, the result is average rain. And when a positive IOD occurs on its own, such as in 2007, we can see stark rainfall contrasts on either side of the Great Dividing Range:
Figure 1. Number of years between 1958 and 2012 with above normal rainfall, where there is an IOD event and no corresponding ENSO event.
By increasing our understanding of how ENSO and IOD interact differently in different parts of Australia, we have a better ability to distinguish whether an El Niño will hit the east coast, or whether the coast will escape, improving our chances to prepare in advance.
For more information on how El Niño, La Niño and the IOD influence Australian rainfall, visit this Bureau of Meteorology page.