Record-breaking La Niña events - An analysis of the La Niña life cycle and the impacts and significance of the 2010-11 and 2011-12 La Niña events ...
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Record-breaking La Niña events An analysis of the La Niña life cycle and the impacts and significance of the 2010–11 and 2011–12 La Niña events in Australia
Contents Introduction At a glance: the impact of these La Niña events in Australia 1 Background Story What are El Niño and La Niña events? 2 From El Niño to La Niña 3 When do El Niño and La Niña events occur? 4 From one La Niña to the next 5 The three phases of El Niño–Southern Oscillation 6 The Southern Oscillation Index 8 Record high Southern Oscillation Index values 9 El Niño–Southern Oscillation impacts – rainfall 10 Record rainfall and widespread flooding across Australia 11 El Niño–Southern Oscillation impacts – temperature 12 Record sea surface temperatures 14 Warmer nights and cooler days 15 Other El Niño–Southern Oscillation impacts 16 Tropical cyclone activity during 2010–11 and 2011–12 17 The Indian Ocean Dipole 18 Negative Indian Ocean Dipole increases rain 19 The Southern Annular Mode 18 Positive Southern Annular Mode and why 19 Western Australia missed out on rain A timeline of events 20 Stay informed References and acknowledgements 24 Published by the Bureau of Meteorology, July 2012 About this publication GPO Box 1289 Melbourne VIC 3001 La Niña events greatly influence Australia’s climate. Tel: (03) 9669 4000 The 2010–12 La Niñas were two of the most significant events Website: www.bom.gov.au in Australia’s recorded meteorological history. This publication ISBN: 978 0 642 70621 8 explores these extraordinary events and their effect on the weather and climate of Australia during 2010–12. © Commonwealth of Australia 2012 This work is copyright. Apart from any use as permitted under It provides an overview of how La Niña events occur and the the Copyright Act 1968, no part may be reproduced without prior extreme impacts that these events can have on Australia’s climate. written permission from the Bureau of Meteorology. Requests and The publication is structured according to two interrelated, inquiries concerning reproduction and rights should be addressed parallel streams to assist the reader – ‘background’ and ‘story’. to the Publishing Unit, Bureau of Meteorology, GPO Box 1289, The ‘background’ stream explains some of the major factors that Melbourne 3001. Requests for reproduction of material from drive La Niña events and how they influence Australia’s climate. the Bureau website should be addressed to AMDISS, Bureau The ‘story’ stream illustrates the significance and widespread of Meteorology, at the same address. impacts of the 2010–11 and 2011–12 La Niña events. Cover photograph: Steve Davis Unless otherwise indicated, all temperature and rainfall anomalies A heavy rain shower crosses Cleveland Bay, off Townsville, (i.e. departures from average) in this publication are calculated Queensland, as the first storms of the Wet Season brew with respect to the 1961–1990 average, as recommended by the (Bureau of Meteorology Weather Calendar, April 2011) United Nations World Meteorological Organization.
Introduction At a glance: the impact of these La Niña events in Australia The successive La Niña events spanning 2010–12 were associated with record rainfall over much of Australia and some of the biggest floods in living memory. This followed years of severe drought in many parts of the country, and while it brought relief to many Australians, it also brought devastation to others. Some facts about the 2010–11 and 2011–12 La Niña events 2010–11 La Niña The 2010–11 La Niña event was In October and December 2010, one of the strongest on record, and February and March 2011, comparable in strength with the Southern Oscillation Index the La Niña events of 1917–18, values (a measure of a La Niña’s 1955–56 and 1975–76 strength) were the highest recorded for each month since records commenced in 1876 2011 was Australia’s coolest 2010 was Australia’s third-wettest year in a decade (2001–2011) calendar year on record The Murray–Darling Basin 2011 was Australia’s second-wettest experienced its wettest calendar calendar year (with the wettest year on record in 2010 and year since national rainfall records Western Australia experienced began in 1900 being 1974 – also its wettest year on record in 2011 a La Niña year) Ocean temperatures to the April 2010 to March 2012 north of Australia were was Australia's wettest highest on record in 2010 two-year period on record Widespread flooding See pages 20–23 for a full occured in many parts timeline of the La Niña events of Australia associated 1 with the record rainfalls
Background What are El Niño and La Niña events? El Niño and La Niña events are a natural part of the global climate The El Niño–Southern system. They occur when the Pacific Ocean and the atmosphere Oscillation (ENSO) is described in more above it change from their neutral (‘normal’) state for several seasons. detail on pages 6–7. El Niño events are associated with a warming of the central and eastern tropical Pacific, while La Niña events are the reverse, with a sustained cooling of these same areas. Coupled ? These changes in the Pacific Ocean Even in a neutral state, temperatures Because ENSO involves and its overlying atmosphere occur in a in the Pacific Ocean vary from east to interaction between the ocean cycle known as the El Niño–Southern west – for example, the western Pacific and the atmosphere – both of Oscillation (ENSO). The atmosphere ‘warm pool’ in the tropical Pacific has which play a role in reinforcing and ocean interact, reinforcing each some of the warmest large-scale ocean changes in each other – it is other and creating a ‘feedback loop’ temperatures in the world. During known as a coupled ocean– which amplifies small changes in the an ENSO event, ocean temperatures atmosphere phenomenon. state of the ocean into an ENSO event. become warmer than usual or cooler When it is clear that the ocean and than usual at different locations, which atmosphere are fully coupled an ENSO are reflected in ocean temperature Thermocline ? event is considered established. gradients. The most important driver of ENSO is these temperature gradients Thermocline comes from the across the Pacific, both at the surface Greek for ‘heat slope’ and is the and below the surface, particularly name for the region separating at the thermocline. warm, well-mixed surface water from cool, deep ocean water. Typically water temperatures 60°N above the thermocline are more than 25°C while those below the 40°N thermocline are 15°C or less. Tropic of Cancer 20°N Central and Eastern Equator Equator Pacific Ocean Tropic of Capricorn Warmer water El Niño 20°S Cooler water La Niña 40°S Neither warm Neutral nor cool 100°E 140°E 180°E 140°W 100°W 60°W dateline Date Line Pacific Ocean – even in neutral state the Western Pacific is warm Why are they called El Niño and La Niña? The term El Niño translates from Spanish as ‘the boy-child’. Peruvian fishermen originally used the term to describe the appearance, around Christmas, of a warm ocean current off the South American coast. It is now the commonly accepted term to describe the warming of the central and eastern tropical Pacific Ocean. La Niña translates as ‘girl-child’ and is the opposite ENSO phase to El Niño. 2
Story From El Niño to La Niña The 2009–10 El Niño Monthly sea surface temperature anomalies (differences commenced in May 2009, from normal) in the Pacific Ocean indicate where the ocean reaching its peak in late is warmer than usual (red) and cooler than usual (blue). December 2009 before breaking down in the first 4.0 °C quarter of 2010. The Pacific 3.0 Ocean returned to neutral 2.0 by late April 2010, but 1.0 0.5 continued to cool rapidly –0.5 during autumn. –1.0 As early as April 2010, a number of –2.0 Australia –3.0 climate models from meteorological agencies around the world suggested –4.0 °C a La Niña event could commence December 2009 – peak of the 2009–10 El Niño; warmer than normal sea surface temperatures later in 2010. Subsequently, the first in the central and eastern Pacific observed signals of a potential La Niña became apparent in the tropical Pacific Ocean during the following month. 4.0 °C As sea surface temperatures 3.0 approached values (or ‘thresholds’) 2.0 associated with a La Niña in July 1.0 2010, and as long-range outlooks 0.5 became more consistent, the Bureau –0.5 of Meteorology announced that a –1.0 La Niña event was more likely than –2.0 not to persist for the rest of the year. Australia –3.0 By October 2010, and with the –4.0 °C event showing parallels to the La Niña May 2010 – between the end of El Niño and the start of the 2010–11 La Niña; relatively neutral events of the early 1970s, seasonal state of the ocean outlooks were increasingly suggesting wet conditions for northern and eastern Australia. As a result, the Bureau began briefing key federal 4.0 °C and state government agencies of 3.0 increased flood and tropical cyclone 2.0 risk, and decreased bushfire potential, 1.0 over the summer period. 0.5 –0.5 The La Niña strengthened further –1.0 during spring and into summer, –2.0 peaking around January 2011, before Australia –3.0 weakening during autumn 2011. The –4.0 °C 2010–11 La Niña drew to a close in May 2011, with both Pacific Ocean January 2011 – peak of the 2010–11 La Niña; cooler than normal sea surface temperatures and atmospheric indicators returning in the central and eastern Pacific to neutral levels by mid-year. 3
Background When do El Niño and La Niña events occur? The tropical Pacific Ocean and atmosphere swings, or oscillates, Climate models ? between warm, cool and neutral phases on a timescale of a few years. Climate models come A typical El Niño or La Niña event may In general, El Niño events tend to in two forms: show its first signs of development only last for a single cycle (i.e. one during the southern hemisphere year from autumn to autumn), but 1 Statistical climate models are autumn and strengthen over winter it is not uncommon for multi-year based on what has happened and spring. It will normally start to La Niña events to occur. For example, in the past – that is, they use decay in the mid to late southern the 1998–2001 La Niña affected three historical patterns to estimate what is likely to happen in summer, and finally dissipate in the consecutive years from autumn 1998 the future. subsequent autumn. ENSO events to autumn 2001. typically decay during autumn, as this 2 Dynamical climate models are is the time of year when the tropical based on physics – that is, they Pacific Ocean naturally evens out the model the physical processes temperature difference between the driving the current climate east and west. This annual weakening situation forward in time to of the temperature gradient across the predict what is likely to happen. Pacific also means the weather patterns Data are collected from a wide which help reinforce a La Niña or El Niño variety of sources – including ease, allowing ENSO to return to neutral. satellites, buoys (moored, drifting and expendable), sea level analysis and meteorological surface observations – all drawn from a network of national and international observing systems. Watching out for events The first signs of an emerging El Niño or La Niña event are often observed in the ocean. The Bureau of Meteorology monitors and reports on a range of ENSO indicators, including: • short-term bursts of tropical • atmospheric air pressure rainfall activity • cloudiness – measuring the amount • water temperatures at the sea of cloud in tropical regions surface and at depth • the strength of the trade winds • ocean heat content – and winds higher in the atmosphere measuring the amount of • ocean currents. energy stored in the ocean • the Southern Oscillation Index (see page 8) The Southern These climate indicators provide information about current ENSO conditions, Oscillation Index (SOI) and are inputs into climate models that are used to predict conditions for the is described in more months ahead. detail on page 8. 4
Story From one La Niña to the next The central Pacific began to Monthly sea surface temperature anomalies (difference cool again during winter 2011, from normal) in the Pacific Ocean indicate where the ocean and from September 2011 is warmer than usual (red) and cooler than usual (blue). models and observations indicated a re-emergence 4.0 °C of the La Niña during spring 3.0 was likely. 2.0 1.0 The 2011–12 La Niña was relatively 0.5 late forming, with most indicators –0.5 only reaching La Niña thresholds by –1.0 mid-October 2011. This event was –2.0 not forecast to be as strong as the Australia –3.0 2010–11 La Niña, as reflected in –4.0 °C seasonal climate outlooks issued at the time. Outlooks indicated a wetter September 2011 – cooling in the central Pacific as the La Niña re-forms than average spring and early summer for much of northern and eastern Australia, but were not as dramatic 4.0 °C as they were during the previous year. 3.0 The La Niña consolidated over spring 2.0 2011, with many indicators only 1.0 strengthening slightly during October 0.5 and early November. The event –0.5 reached its peak during December –1.0 2011 when climate models forecast a –2.0 Australia return to a neutral ENSO phase during –3.0 autumn 2012. The 2011–12 La Niña –4.0 °C gradually declined over late summer, with some atmospheric indicators December 2011 – peak of the 2011–12 La Niña; cooler than normal central and eastern Pacific, but not as cool as at the 2010–11 La Niña peak continuing to show a La Niña signal during March 2012, while oceanic indicators were generally faster to return to neutral levels. The 2011–12 4.0 °C La Niña concluded in late March 2012. 3.0 2.0 1.0 Want more? 0.5 The Bureau publishes a regular –0.5 ENSO Wrap-Up and Model –1.0 Summary on its website –2.0 Australia –3.0 http://www.bom.gov.au/ climate/enso/ –4.0 °C These reports contain information March 2012 – 2011–12 La Nina has declined; ocean temperatures in the Pacific approach neutral about current conditions across the tropical Pacific Ocean, as well as a summary of predictions for ENSO conditions several months ahead from a number of international dynamical climate models, including the Bureau’s own model, POAMA (Predictive Ocean Atmosphere Model for Australia). 5
Background The three phases of El Niño–Southern Oscillation The neutral phase Convection ? In the neutral state (neither El Niño Warm sea surface temperatures in Convection is the process nor La Niña) trade winds blow east the western Pacific pump heat and generally associated with warm to west across the surface of the moisture into the atmosphere above. rising air and the formation of cloud. tropical Pacific Ocean, bringing In a process known as atmospheric warm moist air and warmer convection, this warm air rises high surface waters towards the western into the atmosphere and, if the air Pacific and keeping the central is moist enough, causes towering Walker Circulation ? Pacific Ocean relatively cool. The cumulonimbus clouds and rain. This thermocline is deeper in the west now-drier air then travels east before The Walker Circulation than the east. descending over the cooler eastern is named after Sir Gilbert tropical Pacific. The pattern of air rising Walker, the scientist who first in the west and falling in the east with recognised a semi-regular westward moving air at the surface is pattern of high and low rainfall referred to as the Walker Circulation. (and hence feast and famine) over India. Neutral Upwelling ? Upwelling is a vertical motion of water. When wind or currents displace water at the surface of the ocean, water from deeper in the ocean is drawn up to replace the displaced water. Deep water is generally cooler and richer in nutrients than surface water so upwelling is important in supporting productivity in the oceans of the world. El Niño During an El Niño event, Sea surface temperatures around trade winds weaken or may northern Australia are cooler than normal even reverse, allowing the area and the focus of convection migrates of warmer than normal water away from Australia eastward towards to move into the central and the central tropical Pacific Ocean. This eastern tropical Pacific Ocean. results in increased rainfall for nations such as Kiribati and Peru, but less rainfall These warmer than normal ocean over Australia. The greatest impacts are temperatures are associated with a usually felt over inland eastern Australia, deepening of the thermocline in the while effects for regions such as central to eastern Pacific. A weaker southwest Western Australia and coastal upwelling of cooler ocean waters New South Wales can vary from event from below also contributes to warmer to event, and in western Tasmania the sea surface temperatures. 6 effects are generally weak.
La Niña Below the surface During a La Niña event, the Walker Convection and hence cloudiness over Circulation intensifies with greater the region north of Australia increases ENSO events are typically led convection over the western Pacific as stronger winds provide more and sustained by changes in moisture to the overlying atmosphere the amount of heat held in the and stronger trade winds. waters below the surface of the and the Walker Circulation intensifies. As the trade winds strengthen, the tropical Pacific Ocean. This strengthens the Australian pool of warmer water is confined to the monsoon and, if the conditions are The deeper ocean is important far western tropical Pacific, resulting in warmer than usual sea surface right, directs increased humidity and in gauging the strength, and temperatures in the region north of rainfall inland over Australia. La Niña hence potential longevity, of Australia. Sea surface temperatures events are associated with increased an event. These large stores across the central and eastern tropical rainfall over much of northern and of heat (El Niño) or lack of heat Pacific Ocean become cooler than eastern Australia. Parts of northern (La Niña) act like a flywheel and usual and the thermocline moves closer and central Australia tend to feel the ensure that an event will not to the surface – cool waters from the impacts of La Niña more than they dissipate rapidly. For example, deep ocean are drawn to the surface as feel the impacts of El Niño. during the 1997–98 El Niño upwelling strengthens. event – which many consider the El Niño of the century – surface La Niña temperatures were around 3.5°C warmer than normal in the eastern tropical Pacific, but temperatures at 150 m below the surface were up to 8°C above average. Conversely, during the 2010–11 La Niña, eastern Pacific Ocean surface temperatures were up to 2 °C cooler than normal, but the subsurface temperatures were almost 7 °C below average. Such large changes in the deeper ocean ensure that the surface waters stay warm or cool even when the atmosphere above might try to push the system back towards neutral, and so can sustain or lead an El Niño ENSO event. 7
Background The Southern Oscillation Index The Southern Oscillation Index (SOI) is a measure of La Niña events over time the intensity or strength of the Walker Circulation. It is one of the key atmospheric indices As La Niña events recur on a two to seven-year cycle, there for gauging the strength of have been many over the last century, varying in strength El Niño and La Niña events and impacts. The SOI and sea surface temperatures can be and their potential impacts used to compare the intensity of La Niña events. on the Australian region. (See graph below for more details.) The SOI measures the difference Atmospheric and oceanic intensity of La Niña events since 1900. Intensity in surface air pressure between ranked by SOI values for atmosphere, while oceanic intensity is ranked by Tahiti and Darwin. The index is best sea surface temperature indicators (only available reliably since mid-century). represented by monthly (or longer) Some multi-year events have two or three La Niña peaks. averages as daily or weekly SOI values can fluctuate markedly due to short-lived, day-to-day weather patterns, particularly if a tropical cyclone is present. Sustained positive SOI values above about +8 indicate a La Niña event while sustained negative values below about –8 indicate an El Niño. 8
Story Record high Southern Oscillation Index values During the 2010–12 La Niña events, record and near-record high Southern Oscillation Index values occurred in many individual months and multi-month periods. The surface air pressures near Tahiti typical weather patterns over were consistently high throughout the South Pacific, which relate to The El Niño–Southern both the 2010–11 and 2011–12 La Niña the strong trade winds experienced Oscillation (ENSO) events, and they were particularly high over the same period and an is described in more during the first event. These high SOI enhanced Walker Circulation. detail on pages 6–7. values reflect large changes in the Monthly SOI values for the 2010–11 and 2011–12 La Niña events – numerous record and near-record values for that particular month are indicated Comparison to past La Niña events The 2010–11 La Niña was one of the strongest on record, event, bringing significantly above-average comparable in strength to the events of 1917–18, 1955–56 and rainfall across the north of Australia, although most parts of southeastern Australia 1975–76. Widespread impacts were experienced across a large received below-average rainfall, consistent part of Australia, including record rainfall and severe flooding. with persistent drought conditions in place While the intensity of atmospheric indicators The previous two La Niña episodes for the region since at least 2000. during the event was exceptional, tropical were also multi-year events, lasting from The 1998–2001 La Niña persisted for Pacific Ocean indicators did not reach record September 2007 to March 2009 and from three years with three distinct peaks over cool levels. This may have been partly due May 1998 to March 2001, respectively. the summers of 1998–99, 1999–2000, to the general warming trend in the Pacific and 2000–01. The event was generally The 2007–08 and 2008–09 La Niña events Ocean, which has warmed around 0.5°C moderate in strength, with widespread were weak to moderate, with relatively since 1950. above-average rainfall and flooding, minor impacts across Australia. Although The 2011–12 La Niña was a weaker the 2007–08 event brought the typical particularly in New South Wales and event, but still of moderate strength by heavy rainfall to most of northern Australia Queensland. While much of the north both atmospheric and oceanic measures. and the eastern tropics, the southern received record high falls, parts of the Despite flooding in a number of areas during half of the Murray–Darling Basin did not southeast and Tasmania missed out on the summer, the impact of this second event receive the above-average rainfall and heavy rainfall, and hence did not get the relief upon Australia's climate was generally less cool temperatures typical of past La Niña from the dry conditions that had started significant than during the previous event. events. The 2008–09 La Niña was a short around late 1996. 9
Background El Niño–Southern Oscillation impacts – rainfall The complex interactions between the ocean, Rainfall patterns during La Niña and El Niño events atmosphere and adjacent landmasses across the Pacific Australian rainfall data for 13 of the strongest ‘classic’ or ‘canonical’ mean that ENSO events have events (having the typical autumn to autumn pattern of evolution and impacts on weather in areas decay) since 1900 have been combined to form a composite of average outside the tropical Pacific impacts of La Niña and El Niño events upon rainfall across Australia. region. El Niño and La Niña Each map shows mean rainfall deciles, where green to blue tones indicate events are associated with above-average to very much above-average rainfall totals and yellow to red distinct climatic conditions tones indicate below-average to very much below-average rainfall. Note that around the Pacific. the rainfall patterns can vary significantly from one event to the next. La Niña events are associated with greater convection over the El Niño La Niña warmer ocean to Australia’s north. Typically this leads to higher than El Niño is typically associated La Niña is typically associated average rainfall across much of with reduced rainfall in northern with increased rainfall in northern Australia, particularly inland eastern and eastern Australia and eastern Australia and northern regions, sometimes causing floods. During El Niño events, the ocean near Australia is cooler than usual, bringing lower than average winter–spring rainfall over eastern and northern Australia. Although most major Australian droughts Deciles Deciles have been associated with El Niño events, widespread drought is 1 2 3 4 5/6 7 8 9 10 1 2 3 4 5/6 7 8 9 10 certainly not guaranteed when Winter/spring rainfall – below average Winter/spring rainfall – above average an El Niño is present. across eastern Australia across most of eastern and northern Australia Deciles ? Deciles are calculated by (1) taking all available data (say, annual Australian rainfall from 1901 to 2000), (2) ordering them from lowest to highest, and (3) dividing them into Deciles Deciles 10 separate groups of equal size. Each group is called a 1 2 3 4 5/6 7 8 9 10 1 2 3 4 5/6 7 8 9 10 decile – the lowest 10 per cent Summer rainfall – mostly near Summer rainfall – above average of historical values is decile 1, average in eastern and northern Australia the next lowest 10 per cent will be decile 2, and so on, up to the The onset years for the 13 strongest The onset years for the 13 strongest highest 10 per cent of historical 'classic' El Niño events used are 1905, 'classic' La Niña events used are 1906, values which lie in decile 10. This 1914, 1940, 1941, 1946, 1965, 1972, 1910, 1916, 1917, 1950, 1955, 1956, 1971, creates a scale against which we 1977, 1982, 1991, 1994, 1997 and 2002. 1973, 1975, 1988, 1998 and 2010. can rank the amount or intensity of a measurement or event. 10
Story Record rainfall and widespread flooding across Australia In 2010, Australia experienced its third-wettest year since national Widespread flooding rainfall records began in 1900, with second place taken by 2011. Averaged across Australia, both years experienced rainfall well above The record-breaking rainfall the long-term average of 465 mm – 703 mm in 2010 and 708 mm in 2011. during the 2010–11 La Niña led to widespread flooding in many Only 1974, dominated by one of the as a whole. regions between September strongest La Niña events on record, was 2010 and March 2011. As well as wetter with 760 mm. 2010 was also the While the 2010–11 La Niña event was the severe flooding in southeast wettest year on record for the Murray– costly in an economic and social sense, Queensland, large areas of Darling Basin and Queensland, while it relieved one of the longest and most northern and western Victoria, 2011 was the wettest year on record for severe droughts across the Murray– New South Wales, northwestern Western Australia. Darling Basin in recorded history. Heavy Western Australia and eastern rain provided a significant boost During the 2010–11 La Niña, most to water storages in Queensland, Tasmania were subject to of mainland Australia experienced New South Wales and South Australia. significant flooding. There were significantly higher than average rainfall Nationally, water held in major publicly also some highly unseasonable over the nine months from July 2010 owned storages rose by more than rain events in the tropics during to March 2011. Parts of Tasmania also 20 per cent between May 2010 and what is typically its dry season. received heavy rainfall while southwest May 2011. Western Australia missed out, Flooding was also widespread experiencing its driest year on record. A During the 2011–12 La Niña, rainfall during the 2011–12 La Niña. number of new Australian rainfall records was above average for most of mainland Much of inland southern and were set: wettest September, December Australia for the six months from October far northern Queensland, most and March on record and second-wettest 2011 to March 2012, but not as much of New South Wales, northern October and February. May to October above average as for the 2010–11 event. Victoria, and central Australia 2010 was the wettest ‘dry’ season on Nevertheless, several rainfall records experienced flooding at least record in northern Australia, and July to were set: second-wettest November and once between late November December 2010 was the wettest second spring on record for Western Australia, 2011 and March 2012. half of the year on record for Australia as and second-wettest March for New a whole. South Wales. Several seasonal records were also set: wettest spring on record for Australia, Combined, the two events yielded See the timeline and all States except Victoria and Australia’s wettest 24-month period on pages 20–23 for Tasmania; wettest summer on record for on record (April 2010 to March 2012), more details on the Victoria; and second-wettest summer on and wettest two-calendar-year period widespread flooding record for Western Australia and Australia (2010–2012). The record rainfall of across Australia. 1411mm in 2010–2011 beat the previous Heavy rainfall during both La Niña events 2010–11 La Niña event 2011–12 La Niña The two events combined Extreme rainfall Well above-average rainfall Record-breaking rainfall 1 2–3 4–7 8–9 10 Deciles 1 2–3 4–7 8–9 10 Deciles 1 2–3 4–7 8–9 10 Deciles Lowest Very Below Average Above Very Highest Lowest Very Below Average Above Very Highest Lowest Very Below Average Above Very Highest on much average average much on on much average average much on on much average average much on record below above record record below above record record below above record average average average average average average July 2010 to March 2011 rainfall deciles October 2011 to March 2012 rainfall deciles July 2010 to March 2012 rainfall deciles (based on climatology of gridded monthly (based on climatology of gridded monthly (based on climatology of gridded monthly rainfall analyses from 1900) rainfall analyses from 1900) rainfall analyses from 1900) 11
Background El Niño–Southern Oscillation impacts – temperature The increased cloudiness and rainfall associated with La Niña periods Evaporative cooling ? typically reduces daytime temperatures and keeps nights warmer, particularly over northern and eastern Australia. In the north of Evaporation of surface water, Australia, the monsoon is typically enhanced, which can lead to such as lakes, or moisture in the soil, cools the environment both cooler days and nights during the summer monsoon season as surrounding it. This happens the higher rainfall allows for increased evaporative cooling, and because changing a substance increased onshore winds provide additional cooling in the same way from a liquid phase to a gaseous a sea-breeze brings relief from a hot summer day. phase (evaporation) requires energy. The energy required for a phase change is known as latent heat, and can be provided by absorption of solar radiation or by drawing heat energy from the air or other substances in contact with the liquid. Temperature patterns El Niño during ENSO events Average impacts of La Niña and El Niño events on maximum (daytime) and minimum (night-time) temperatures across Australia are shown Deciles Deciles below in composite maps combining temperature 1 2 3 4 5/6 7 8 9 10 1 2 3 4 5/6 7 8 9 10 data from 12 of the strongest Winter/spring daytime temperatures – Summer daytime temperatures – ‘classic’ events. above average across southern Australia slightly above average for most of eastern and northern Australia The maps show mean maximum and minimum temperature deciles, where La Niña blue tones indicate below- average temperatures and orange to red tones indicate above-average temperatures. Note that the temperature patterns can vary significantly from one event to the next. Deciles Deciles 1 2 3 4 5/6 7 8 9 10 1 2 3 4 5/6 7 8 9 10 Winter/spring daytime temperatures – Summer daytime temperatures – below average across southern Australia below average across most of northern and eastern Australia 12
In contrast, during El Niño events, The temperature effect of El Niño reduced cloudiness means daytime events are felt most strongly during During La Niña temperatures are typically warmer winter and spring, while the effects of increased cloudiness and than normal, exacerbating the effect of La Niña events tend to have the greatest rainfall can lead to cooler days. lower than normal rainfall by increasing impact between October and March. evaporation. Reduced cloudiness The effect of La Niña events also tends During El Niño reduced cloudiness can also means that nights can be cool, to be stronger than that for El Niño; lead to warmer days. sometimes leading to widespread and temperatures are generally further below severe frosts; Australia’s lowest recorded average during La Niña events than they temperature, –23.0 ˚C, was observed are above average during El Niño events. at Charlotte Pass on 29 June 1994, during the 1994–95 El Niño event. The onset years for the 12 strongest Deciles Deciles 'classic' El Niño events used are 1914, 1940, 1941, 1946, 1965, 1972, 1977, 1982, 1991, 1994, 1997 and 2002. 1 2 3 4 5/6 7 8 9 10 1 2 3 4 5/6 7 8 9 10 Winter/spring minimum temperatures – Summer minimum temperatures – above average for the southwest, below average above average across southern Australia for the northeast and parts of the east The onset years for the 12 strongest Deciles Deciles 'classic' La Niña events used are 1910, 1916, 1917, 1950, 1955, 1956, 1971, 1 2 3 4 5/6 7 8 9 10 1 2 3 4 5/6 7 8 9 10 1973, 1975, 1988, 1998 and 2010. Winter/spring minimum temperatures – Summer minimum temperatures – above average for much of northern Australia, below average for northern Australia, below average for most of the southwest above average for parts of the southeast 13
Record sea surface temperatures Following the 2009–10 El Niño, 2010 was globally (both land and ocean) –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 °C the warmest year on record, marginally warmer than 1998 (which immediately followed the strongest El Niño event of that century). Sea surface temperatures north of with the Australian monsoon arriving 10°N Australia were also at record-breaking earlier and being stronger than highs – October, November and normal during the 2010–11 northern December 2010 tropical sea surface wet season. Ultimately, the increase 10°S temperatures north of Australia broke in monsoonal activity and evaporated previous records by large margins and, water added to the potential for in contrast to temperatures over land, high rainfall over northern and 30°S ocean temperatures around Australia eastern Australia. were the highest on record during 2010. Sea surface temperatures around the This contributed to the strength of the northern coasts of Australia were also 70°E 110°E 150°E 170°W 2010–11 La Niña and its impacts on above average during the 2011–2012 90°E 130°E 170°E 150°W Australia. The very high sea surface La Niña event, particularly between temperatures contributed to an increase December 2011 and February 2012, Sea surface temperature anomalies (°C) in evaporation and high (and at times though were not at the record-breaking in the Australian region, for the period record) humidity levels over Australia. levels seen in 2010. This would suggest May 2010 to April 2011 The increase in humidity was associated a weaker influence on Australian rainfall. Cool down under, warm globally 2011 was Australia’s coolest year in a decade (2001–2011). Eight Global temperatures are also of the last nine years with sustained La Niña conditions recorded influenced by La Niña and El Niño events due to the exchange of heat a cooler than normal Australian average temperature, except 2008, between the atmosphere and oceans. which was Australia's warmest year on record commencing with The end of an El Niño is typically a La Niña. associated with higher than average global air temperatures. However, years commencing with a La Niña in place are cooler than average. The World Meteorological Organization ranked 2011 as the equal-tenth- warmest year on record. 2011 was the warmest La Niña year on record globally, considerably warmer than the most recent moderate-to-strong La Niña years (2008, 2000, and 1989). The ten-year average for 2002–2011 was the equal-warmest ten-year period on record both for Australia and globally. Australian (top) and global (bottom) annual mean temperature anomalies (difference from normal) – in this instance La Niña years are defined as those where central Pacific sea surface temperature anomalies were below –1 °C for a sustained period leading into the start of the year 14
Story Warmer nights and cooler days Record high sea surface temperatures in the tropics from June to December 2010 also contributed to high minimum temperatures 2010–11 La Niña in this region, as sea surface temperatures moderate minimum The 2010–11 La Niña was air temperatures over coastal and island regions. associated with much cooler than average daytime For instance, at Horn and Coconut High rainfall and associated cloud temperatures and much islands in the Torres Strait, the previous cover generally kept Australian warmer than average Queensland August record high maximum temperatures below average night-time temperatures. minimum temperature of 25.4 °C was during both La Niña events. Conversely, surpassed on 24 separate occasions increased cloud cover and damp soils 2011–12 La Niña during 2010, peaking at 26.8 °C at meant less heat was lost during the Horn Island on 19 August. Similarly, nights, resulting in above-average The 2011–12 La Niña was record warm ocean waters off Western minimum temperatures for most of associated with cooler than Australia's west coast led to the hottest Australia during the 2010–11 event, average daytime temperatures year on record for southwest Australia. and for southern Australia during the in some areas, but the deviation 2011–12 event. from normal was not as marked. The maps below show mean maximum (daytime) and minimum (night- time) temperature deciles, where blue tones indicate below-average temperatures and orange tones indicate above-average temperatures. 2010–11 La Niña 2011–12 La Niña 2010 Record warm ocean waters off the west coast of Western Australia was associated with the hottest year on record for southwest Australia. 1 2–3 4–7 8–9 10 Deciles 1 2–3 4–7 8–9 10 Deciles Lowest Very Below Average Above Very Highest Lowest Very Below Average Above Very Highest on much average average much on on much average average much on record below above record record below above record average average average average Daytime temperatures (July 2010 to March Daytime temperatures (October 2011 to 2011) – below to very much below average March 2012) – below average in some parts across most of inland Australia of inland Australia, above average in some coastal regions 1 2–3 4–7 8–9 10 Deciles 1 2–3 4–7 8–9 10 Deciles Lowest Very Below Average Above Very Highest Lowest Very Below Average Above Very Highest on much average average much on on much average average much on record below above record record below above record average average average average Night-time temperatures (July 2010 to Night-time temperatures (October 2011 to March March 2011) – above average to highest 2012) – above average across parts of southern on record for large parts of the country Australia and the north, below average inland 15
Background Other El Niño–Southern Oscillation impacts Winds and cyclones Cloudiness Other La Niña impacts Measurements of winds, both from Changes in the location of convection • stronger easterly trade winds tracking weather balloons and from over the Pacific mean that the location • increased risk of tropical cyclones satellite observations, also tell us about and level of cloudiness can be used around Australia ENSO. When trade winds blowing from to track the intensity and phase of • increased cloudiness over the east increase at the surface, and the Walker Circulation, much as the Australia. winds around 10 km above the ground SOI tracks changes in atmospheric strengthen in the opposite direction, it pressure. Radiation (‘heat’) released is clear that the Walker Circulation has into space acts as a good proxy (an Other El Niño impacts strengthened – typical of La Niña. If the indirect measure) for cloudiness as Walker Circulation shifts into an El Niño cloud tops are generally far cooler • weakened easterly trade winds phase, the surface easterlies weaken, than the earth’s surface, and hence • reduced risk of tropical cyclones or even reverse, and the westerly winds more long-wave radiation is released around Australia aloft do likewise. from cloud-free regions, and less in • reduced cloudiness over Australia. cloudy regions. This radiation can With a stronger Walker Circulation be easily and accurately detected by comes a shift westward in the typical satellites. Cloudiness along the equator area of the Pacific affected by tropical – particularly near the Date Line – cyclones. Hence the only wet seasons typically increases during an El Niño and in which Queensland has experienced decreases during a La Niña. Likewise, multiple tropical cyclones crossing its rainfall for equatorial countries near coast have been during La Niña periods. the Date Line, such as Kiribati, is lower during La Niña and higher during El Niño – the opposite of Australian impacts. Tropical depression ? No two events are the same A tropical depression is a low pressure system of clouds and Each El Niño and La Niña event is different from region to region, and thunderstorms with a defined event to event. Variations in the timing, location and magnitude of ocean circulation but does not usually temperature anomalies and wind patterns cause differences both in the have an eye or the spiral shape strength and extent of climate impacts. associated with cyclones or more powerful storms. In addition, a chaotic ‘weather’ factor is involved in any event. For instance, flooding in western Victoria during the 2010–11 La Niña event originated from two tropical cyclones (Anthony and Yasi) which decayed over central Australia The Indian Ocean before moving south. While both were ‘random’ weather events, more cyclones Dipole (IOD) and the and tropical depressions occur in the Australian region and more cross the Southern Annular eastern coast during La Niña events. ENSO also interacts with other drivers of climate variability, such as the Indian Ocean Dipole (IOD) and the Southern Mode (SAM) are Annular Mode (SAM), which increases the likelihood of each event differing described in more from the last. detail on pages 18–19. 16
Story Tropical cyclone activity during 2010–11 and 2011–12 Tropical cyclone activity in the 2010–11 season was, overall, near normal, with 10 tropical cyclones in the Australian region, while the 2011–12 season was below average, with 7 tropical cyclones. Usually, during La Niña events, tropical cyclone numbers around Australia’s north are higher than the long-term average (11) over the November to April tropical cyclone season. However, five of the tropical cyclones landfall in Queensland since 1918 during 2010–11 were in the severe (when two powerful tropical cyclones category, which is above average. At hit Innisfail and Mackay, also at the tail least 29 systems developed into tropical end of a La Niña), crossed the coast depressions, which is one level below between Cairns and Townsville on a tropical cyclone. This is well above 3 February 2011. the number of tropical depressions Tropical cyclone Yasi as it approached the Tropical cyclone Carlos brought observed in any tropical cyclone season Queensland coast just south of Cairns, very heavy rainfall to the greater Darwin captured by the Moderate Resolution Imaging in the Australian region since at least area between 15 and 17 February 2011, Spectroradiometer (MODIS) on NASA’s Aqua the mid-1990s (20 tropical depressions breaking numerous records, including satellite, 2 February 2011 developed in 2011–12, also above record high daily rainfall at Darwin average). In addition, three tropical Airport with 367.6 mm on 16 February cyclones ( Tasha, Yasi, and Anthony) strongest cyclone to cross the Western and a record three-day total of crossed the Queensland coast in the Australian coast since Laurence in 2009, 684.8 mm. The average February 2010–11 season, and two ex-tropical and brought significant rainfall to a monthly rainfall at Darwin Airport cyclones ( Grant and Jasmine) in broad area of western and central is 376.1 mm. 2011–12. Historically, multiple landfalls of Western Australia. severe tropical cyclones between Port Tropical cyclone Grant brought very A tropical depression made landfall Douglas and Ballina on the Queensland heavy falls to the Top End over 25 and in the Gascoyne region of Western coast have only occurred in a single 26 December 2011, resulting in flooding Australia, and while it never reached season during La Niña years. It north of Katherine, which caused official tropical cyclone status it resulted remains unclear why both seasons saw significant infrastructure damage, in 207.8 mm of rainfall at Carnarvon on numerous tropical depressions but an cutting roads and rail links, as well as 17 December 2010 and 255 mm for the unusually small number of these derailing a freight train. event. Carnarvon’s average December eventually formed into tropical cyclones. Tropical cyclone Lua made landfall rainfall is 5.6 mm and its annual average Severe tropical cyclone Yasi, possibly on the Pilbara coast as a severe tropical total rainfall is 231 mm. the most powerful cyclone to make cyclone on 17 March 2012. It was the Cloud cover 55 Wm -2 45 During the peak of the 2010–11 35 La Niña event, cloudiness over 25 Australia increased. This increase 15 in cloud can be correlated with 5 the increased rainfall (as seen –5 on page 10), reduced daytime –15 temperatures and increased night- –25 time temperatures (pages 12–13) –35 observed during the La Niña event. –45 –55 Wm -2 Outgoing long-wave radiation (OLR) anomalies over the greater Australasian region for summer 2010–11. Negative anomalies (blue and purple colours) indicate increased cloudiness and less heat lost to space, while positive anomalies (yellow and red colours) indicate reduced cloudiness. The widespread cloud over the Australia–Indonesia region during the summer is clearly shown. 17
Background The Indian Ocean Dipole The Indian Ocean Dipole (IOD) is defined by the difference in sea The Indian Ocean Dipole surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an Positive event eastern pole in the eastern Indian Ocean south of Indonesia. The IOD • Warmer sea surface affects the climate of Australia and other countries that surround temperatures in the western Indian Ocean relative to the east the Indian Ocean Basin, and is a significant contributor to rainfall • Easterly wind anomalies variability in this region. across the Indian Ocean and less cloudiness to Australia’s northwest Like ENSO, the change in temperature throughflow (the flow of warm tropical • Less rainfall over southern gradients across the Indian Ocean results ocean water from the Pacific into the Australia and the Top End. in changes in the preferred regions of Indian Ocean). Hence, positive IOD rising and descending moisture and air. events are often associated with Negative event El Niño and negative events with • Cooler sea surface temperatures In scientific terms, the IOD is a coupled La Niña. When the IOD and ENSO in the western Indian Ocean ocean and atmosphere phenomenon, are in phase the impacts of El Niño and relative to the east similar to ENSO but in the equatorial La Niña events are often most extreme • Winds become more westerly, Indian Ocean. It is thought that the IOD bringing increased cloudiness over Australia, while when they are has a link with ENSO events through to Australia’s northwest out of phase the impacts of El Niño an extension of the Walker Circulation • More rainfall in the Top End and La Niña events can be diminished. to the west and associated Indonesian and southern Australia. The Southern Annular Mode The Southern Annular Mode (SAM), also known as the Antarctic Southern Annular Mode Oscillation, describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes Positive phase of the southern hemisphere. • Band of westerly winds contracts toward Antarctica • Higher pressures over The changing position of the westerly During autumn and winter, a positive southern Australia wind belt influences the strength and SAM value can mean cold fronts and • Can relate to stable, dry position of cold fronts and mid-latitude storms are farther south, and hence conditions. storm systems, and is an important southern Australia generally misses driver of rainfall variability in southern out on rainfall. However, in spring Negative phase Australia. and summer, a strong positive SAM • Band of westerly winds can mean that southern Australia is expands towards the equator In a positive SAM event, the belt influenced by the northern half of high • More (or stronger) low of strong westerly winds contracts pressure systems, and hence there are pressure systems over towards Antarctica. This results in more easterly winds bringing moist air southern Australia weaker than normal westerly winds from the Tasman Sea. This increased • Can mean increased storms and higher pressures over southern moisture can turn to rain as the winds hit and rain. Australia, restricting the penetration the coast and the Great Dividing Range. of cold fronts inland. In recent years, a high positive SAM has Conversely, a negative SAM event dominated during autumn–winter, and reflects an expansion of the belt of strong has been a significant contributor to the westerly winds towards the equator. This 'big dry' observed in southern Australia shift in the westerly winds results in more from 1997 to 2010. (or stronger) storms and low pressure 18 systems over southern Australia.
Story Negative Indian Ocean Dipole increases rain The IOD was negative from late August to late November 2010 –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 °C encouraging clouds to form over the eastern Indian Ocean and enhancing the flow of moisture over Australia from the northwest. 10°N The 2010 IOD event was characterised across Australia which may partially by much warmer than normal ocean explain the strong rainfall response. surface temperatures in the eastern However, during the 2010–11 La Niña 10°S Indian Ocean, and slightly above- a strong positive Southern Annular average ocean surface temperatures Mode (SAM) partly offset at least some in the west. This differed from the of the effect of the negative IOD across 30°S classical pattern for a negative IOD, southern, and particularly southwestern, which has warmer than usual water Australia (see SAM below). in the east and cooler than usual water There was a weakly positive IOD 70°E 110°E 150°E 170°W in the west. At least part of the unusual event during spring 2011, coinciding 90°E 130°E 170°E 150°W behaviour of the IOD can be linked with the 2011–12 La Niña. The positive to global warming which is leading to IOD may have partially moderated the a rapid warming of the Indian Ocean October 2010 monthly sea surface temperature effect of the La Niña in southeastern anomalies (˚C) in the Indian Ocean show both in the east and west. Australia during these months, reducing much warmer than usual waters in the eastern The combination of a negative IOD rainfall from what might otherwise have end of the dipole near Australia. (Anomalies and La Niña conditions has historically been observed. are calculated with respect to the Reynolds increased the likelihood of heavy rainfall climatology for 1971–2000 from the National Climatic Data Center.) Positive Southern Annular Mode and why Western Australia missed out on rain The SAM was positive from March 2010 until February 2011. Record- high positive monthly values were observed in 2010 for June, July and SAM event November, while August and October were the second-highest on record. The SAM appears to have a weak but discernible relationship The strong positive SAM event during 2010 is likely to have with La Niña and El Niño in summer. Hence the positive SAM values contributed to the dry conditions in late 2010 may well have been enhanced by the La Niña event. in southwest Western Australia. For southwest Western Australia, 2010 The persistently strong SAM during was the driest year on record, continuing 2010 meant that high-pressure and worsening the long drying trend anomalies dominated parts of southern that has affected the southwest since Australia for most of the southern the late 1960s. At least partly as a hemisphere winter–spring. The result of climate change, there has dominance of the SAM over southern been a change in the dominant weather Australia is likely to have contributed systems over the region, including a to dry conditions in southwest Western weakening of storms and southwards Australia during winter and spring. movement of storm tracks and cold fronts, leading to a reduction in rainfall. 19
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