The La Niña of and its impact on Australia

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1 The La Niña of and its impact on Australia Nicholas Klingaman NCAS-Climate and Walker Institute University of Reading

2 Introduction and Outline Review of the extreme weather in Australia in summer Brief introduction to Australian climate The Rockhampton and Brisbane floods Heavy rain falling on already-saturated soils The weather systems responsible for the flooding Is this climate change? The La Niña of (and now ) What are La Niña and El Niño? How does La Niña cause flooding in Australia? Severe Tropical Cyclone Yasi Summary and conclusions

3 Extreme weather events in December 2010: Central Queensland flooded, with particularly severe flooding in Rockhampton January 2011: Toowoomba and Brisbane flooded; also flooding in Sri Lanka, Philippines and Brazil February 2011: Severe Cyclone Yasi strikes Queensland between Cairns and Townsville. Source: telegraph.com.au Source: guardian.co.uk

4 Brief introduction to Australian climate Australia s coasts receive the vast majority of the rainfall. About 70% of Australia receives less than 500 mm of rainfall per year (about half of Reading s average rainfall). The northern third of Australia receives nearly all of its rainfall in summer (December-February). The southern third of Australia receives most of its rainfall in winter (June-August). Source: bigthink.com

5 The Rockhampton Floods The wettest December on record in Queensland; the second-wettest on record in Australia (since 1900). Much of Queensland received more than twice its average rainfall. The wet December followed the wettest spring on record in Australia.

6 The Rockhampton Floods Exceptionally heavy rain fell between Christmas and New Year s Eve: Rockhampton: 320 mm Carnarvon: 265 mm on 27 December alone Roughly five times the average rainfall for December Daily rainfall records fell along the Carnarvon Hills to the west of Rockhampton Thousands of square kilometres flooded.

7 The Rockhampton Floods The proximate cause of the heavy rainfall was the combination of Low pressure to the north, along the Queensland coast High pressure to the south, in the Tasman Sea The combined circulation around these systems directed warm, moist ocean air onshore, which subsequently rose over the Carnarvon Hills, producing heavy rainfall

8 The Brisbane Floods Southeastern Queensland received heavy rain 6-12 January 2011 Brisbane: 267 mm Toowoomba: 345 mm Flash flooding struck Toowoomba on 10 January Similar onshore wind pattern to Rockhampton floods

9 The Brisbane Floods Clouds and precipitation streaming in from the Pacific on 10 January. Large southward dip in the monsoon trough brought tropical moisture as far south as Melbourne, causing localized flooding. Tasmania experiences its wettest January on record.

10 The Brisbane Floods 2011 Brisbane City gauge reached a height of 4.46 metres, well below the record highest floods. The Wivenhoe Dam was constructed in the wake of the 1974 floods (5.45 m) to protect the city. Preliminary results: Without the Wivenhoe Dam, the river level at the City gauge would have been 6.5 metres (Neville Nichols, Monash University)

11 The cost of the Australian floods 15,000 homes destroyed 35 people killed

12 Climate change and extremes In the wake of the Queensland floods, many asked: Is it climate change? or: Would the Queensland floods have occurred if humans were not warming the climate through greenhouse-gas emissions? Climate change will affect the overall number and strength of extreme weather events. Two slightly easier and much more useful questions: Will floods in Queensland become more frequent? Will floods in Queensland become more intense?

13 Climate change and For the tropics as a whole, the frequency of extreme rainfall is expected to increase with warming temperatures. Clausius-Clapeyron: the atmosphere has a greater capacity for water vapour at higher temperatures. extreme rainfall This does not mean that global warming will be every rainfall event stronger, or that total rainfall will increase everywhere. Changes in frequency of extreme rainfall in the tropics with warming ocean temperatures Figure from Allan et al. (2010, Env. Res. Lett.)

14 Climate change and Queensland extreme rainfall For Queensland, doubling CO2 in a climate model increases the frequency of 100 mm/day rainfall by 50%: Present-day: 1 in 670 days Double CO2: 1 in 402 days Doubling CO2 also also increases the intensity of a 1-in-200 day event by 20%: Present-day: 54 mm Double CO2: 65 mm Frequency of different amounts of rainfall in Queensland during summer, for present-day and 2xCO2 climates

15 Brief introduction to El Niño and La Niña La Niña and El Niño are naturally occurring phenomena, in which ocean temperatures in the equatorial Pacific cool and warm, respectively. Events often develop in June-September, peak in December- February, then decay in March-May. La Niña and El Niño are episodic (irregular). El Niño does not necessary follow La Niña; neutral conditions can prevail for years. The strength and position of warm ocean temperatures shifts rainfall patterns around the world.

16 Development of the La Niña event

17 Brief introduction to El Niño and La Niña El Niño La Niña Departures of ocean temperatures in the central equatorial Pacific from average conditions (Source: KNMI Climate Explorer from UK Met Office data) There are insufficient reliable data to determine whether El Niño and La Niña are changing with global warming.

18 Queensland annual (May-Apr) rain (mm) Historical relationships between La Niña and Australian rainfall On average, El Niño and La Niña are responsible for approximately 25% of the year-to-year variations in Queensland rainfall. Strong La Niña 1974/ /11 Weak La Niña Neutral Weak El Niño Strong El Niño Strong La Niña events result in large floods in Queensland, but stronger El Niño events do not result in stronger droughts. Nino-4 index (degrees Celsius)

19 Historical relationships between La Niña and heavy rainfall La Niña increases by 20-70% the risk of heavy rainfall in Queensland El Niño reduces by 10-40% the risk of heavy rainfall in Queensland La Niña minus neutral years Difference in the number of 1-in-30 day rainfall events per season El Niño minus neutral years

20 Climate change or natural variability? Scientists expect warming temperatures to increase the frequency and intensity of extreme rainfall, particularly in the tropics. There is a strong historical link between La Niña and strong monsoon seasons in Australia, including with the frequency of extreme rainfall. There is no robust evidence that global warming has influenced the frequency or intensity of La Niña; the event was strong, but not outside the range of past events. Climate change Strong La Niña More heavy rainfall Heavier rainfall

21 Severe Cyclone Yasi Severe Tropical Cyclone Yasi was the strongest cyclone to strike Queensland in at least 100 years. The cyclone struck between Cairns and Townsville with winds of 180 mph (290 km/h) and a seven-metre-high storm surge. One death and AU$3.5 billion ( 2.27 billion) in damages

22 Severe Cyclone Yasi Yasi was Australia s Katrina The track of Severe Cyclone Yasi as it passed across northern Queensland

23 Historical relationships between La Niña and tropical cyclones Observed tropical-cyclone tracks during the seven El Nino and La Nina events from Compared to El Niño, La Niña doubles the risk of a tropical cyclone striking eastern Australia (Callaghan and Power, 2010) All years in which more than one tropical cyclone has struck eastern Australia have been La Niña years (Callaghan and Power, 2010).

24 Historical relationships between La Niña and tropical cyclones Changes in tropical-cyclone track and genesis locations for El Niño years minus La Niña years

25 Summary and conclusions Rainfall records fell across Queensland in December 2010 and January A persistent pattern of onshore easterly winds produced heavy rainfall in coastal regions. La Niña is associated with stronger summer monsoons in Australia and more frequent tropical cyclones. Warm ocean temperatures around Australia, enhanced by La Niña, provided moisture and energy to amplify cyclones and monsoon depressions. It is difficult to separate the effects of natural variability and climate change on a single extreme event.

26 In a warmer world, we expect to see fewer tropical cyclones. Climate change and tropical cyclones The decrease may not be evenly spread across the globe; some regions may have more cyclones. The intensity of tropical cyclones, particularly the strongest ones, is expected to increase. Temperature changes over Estimates from three models of the change in the number of days with a tropical cyclone: minus From Thorne From Lavender et al. (2010) et al. (2011).

27 Climate change or natural variability? Climate change is expected to reduce the number, but increase the intensity of tropical cyclones in the Southwest Pacific. La Nina increases the number of landfalling tropical cyclones in eastern Australia, but has no effect on their overall intensity. Was Yasi enhanced by global warming? Would Yasi have even formed in a warmer world? In an El Nino year, would Yasi have simply have moved out to sea? Climate change Strong La Niña More tropical cyclones Stronger tropical cyclones

28 Should we blame ourselves? Humans are definitely warming the climate through emissions of greenhouse gases. A warming climate is affecting and will affect weather patterns, especially the number and strength of extreme weather events. The effects of climate change are being seen and will be seen in the overall behaviour of extreme events. Natural variations, like La Nina, still have an important role in determining when and where extreme weather will occur. For individual events, trying to separate natural variations from climate change is not easy and (in my opinion) not particularly useful for adapting to climate change.