President's Column - The strongest La Niña on record?
Fri, Jan 7 2011
Are we experiencing the strongest La Niña event on record?
The second half of 2010, and the start of 2011, has seen devastating floods across much of northern and eastern Australia, a pattern typical of a strong La Niña event. After the 2009 El Niño event, the world moved into a La Niña around April 2010. But how strong is this event?
Climate scientists usually examine sea surface temperatures in the central and eastern equatorial Pacific (the so-called NINO indices) to determine when we are in an El Niño or La Niña episode. Temperatures in this region are usually lower than normal during a La Niña episode. But these temperatures have several failings, if we want to use them to rank La Niña episodes. Firstly, they are not readily available back more than about 60 years, so it is difficult to use them, for instance, to compare 2010 with events early in the 20th century. Secondly, the general ocean warming we have seen over the past 50 or so years, due to anthropogenic enhancement of atmospheric greenhouse gases, confounds the use of these temperatures to compare a recent episode of cool temperatures with cool temperatures earlier in the record – the global warming may have offset some cooling associated with the strong, recent La Niña. This would bias any comparison between the 2010 event and earlier events, prior to the strong global warming of the second half of the 20th century.
But we can use the Southern Oscillation Index, or SOI to compare the strength of La Niña episodes across time. The SOI is the standardised difference in surface atmospheric pressure between Tahiti and Darwin. Monthly SOI values are available at www.bom.gov.au/climate/current/soihtm1.shtml. Positive values of the SOI (low pressures at Darwin and high pressures at Tahiti) indicate a La Niña event. There is no a priori reason to expect that global warming has necessarily led to long-term SOI changes that would confound our results if we use the SOI to compare historical and recent La Niña events. And values of the SOI are available from the end of the 19th century.
The SOI values confirm that we are in the middle of either the strongest La Niña event on record, or the second strongest. The SOI values for October 2010 and December 2010 were each the largest positive values on record for those months, as was the three-month average October-December 2010. If we take a longer perspective (July-December) then 1917 was stronger than 2010, but 2010 was still the second strongest in the historical record. Using either the October-December or the longer July-December periods, the strong La Niña events on 1973 and 1975 were both ranked as weaker than the 2010 event.
So, it is not surprising that we are seeing a great deal of rain and floods across much of Australia. It is worth pointing out that the Bureau of Meteorology seasonal forecast issued on 23 September indicated a strong chance of wet conditions for the period October-December across northeast Australia. These forecasts are based, partly, on the current state of the El Niño – Southern Oscillation, and it was clear months ago that we were in a La Niña. So an increased likelihood of wet conditions was certainly on the cards.
Of course we cannot make a definitive statement about the relative strength of the 2010 La Niña until the event concludes. This will, presumably, be sometime in 2011. But on the data available up to now, there is no doubt that this has been a super strong La Niña. And since La Niña events are generally followed, some months later, by global cooling, we can expect cool temperatures for quite some time.