Vol 24, No 1, March 2011 - Enable

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‘La Niña – How AreAustralia’s RegionsFaring – From DroughtTo Flood’By Roger Stone,University of Southern QueenslandLa Niña – What Exactly Is It AndHow Does It Work?Who knew Australia has the highest year-to-year climatevariability on earth? <strong>No</strong> wonder much of Australia lurches fromdrought to flood – then back again. El Niño and La Niña aremostly to blame for this – but, then again, who has heard of LaNiña before? Many of us have heard of the opposite pattern –‘El Niño – mainly because the El Niño climate pattern has beenthe main underlying cause of Australia’s droughts over the past10 to 30 years and for many of our droughts in history.While ‘El Niño’ translates from the Spanish for ‘the boy-child’,La Niña translates from the Spanish for ‘the girl child’ – butwhy the Spanish names? It’s because these climate patterns tendto originate or were initially known to have a lot of their mainimpacts over in South American countries located on or nearthe Equator. However, these days we know these impacts canspread globally, including, of course, to Australia.What Does A La Niña ActuallyLook Like?A La Niña really is a massive cooling of the sea water in thecentral and eastern Pacific Ocean in equatorial regions fromjust west of the International Dateline, often right across tothe South American coast. You can look at satellite imagesof the sea-surface temperature patterns of La Niña on manyinternational web-sites (for example, STUDENT ACTIVITIES1. Describe the difference between the two maps showing seasurface temperatures.2. Describe the differences in temperature between the waters off theSouth American coastline and the seas surrounding Cape York.3. Estimate by how much temperatures were cooler than normal orwarmer than normal for the 7 days centred on December 29 2010at approximately 0˚, 170˚ W and 15˚ S, 165˚ E,At the same time, around the Australian and Indonesian coasts,sea temperatures can become much warmer than normal byabout two or more degrees Celsius. It is this contrast in seatemperature that interacts with the atmosphere above andthen results in the big shifts in rainfall (and temperature) overAustralia and in many other world regions – as we have indeedwitnessed during 2010/11.Interestingly, because of the strong easterly winds thataccompany these sea-surface temperatures during a La Niña, thesea level along the Australian coast may rise more than 20cmabove normal with associated increased concerns regardingbeach erosion, storm surges and coastal inundation, especiallywith the more frequent tropical cyclones that can then alsoaccompany La Niña.You can actually track the progress of a La Niña as it starts todevelop around May–June–July and then also often subsidesaround the following southern hemisphere autumn. Trackingthese patterns very carefully may give you some warningof the next La Niña or the break-down of this most recentdevastating La Niña.What About The SouthernOscillation Index (Soi) – How DoesThat Provide An Index Of La Niña?If you were to measure the air pressure just above the surfaceof the ocean (just using a simple barometer) and comparevalues in the eastern equatorial Pacific with those in the westernequatorial Pacific you would notice a see-saw in pressurevalues operating ‘hand-in-glove’ with changes in sea-surfacetemperatures and also with the strength and direction of thewinds just above the sea surface.This see-saw in pressure values is known as the SouthernOscillation. The Southern Oscillation Index or SOI gives us auseful measure of its strength so that a consistently stronglypositive value of the SOI (above about +8.0) generally coincideswith a La Niña pattern – and which occurred in 2010/11 – withsharply rising SOI values often occurring before-hand duringautumn or winter. Knowing this can form a useful forecastingtool if you want to track for yourself the beginning stages ofthese dramatic climate patterns. Table 1 gives you a completerecord of monthly SOI values since 1950.STUDENT ACTIVITIES4. Why would warmer sea temperatures result in wetter seasons?5. What is the basic physics behind sea temperatures and sea level rise?6. Why do strongly positive SOI readings in autumn and winterindicate a wetter austral summer in Queensland?7. Use two coloured highlighter pens to identify the La Nina and ElNino seasons, remembering that these temperature oscillations donot occur over the entire calendar year.8. Which years had the most marked El Nino and La Nina events?~ 2 ~
Figure 1: Recent sea-surface temperatures across the equatorial Pacific Ocean associated with the La Nina pattern. The toppicture shows the sea-surface temperatures while the lower picture shows the sea-surface temperature anomalies– how much the sea temperatures varied from normal. <strong>No</strong>te the warmer than normal temperatures off the northernand eastern Australian coast (shaded orange) contrasting with the cooler than normal temperatures off the SouthAmerican Coast and along the Equator (shaded blue).Source: Image courtesy CPC/NOAA~ 3 ~
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