Queensland Rainfall: Past, Present And Future

Queensland rainfall—past,present and future

Historically, Queensland has had avariable climate, and recent weatherhas reminded us of that fact. Afterexperiencing the longest drought inrecorded history, Queensland is nowemerging from one of the wettest yearson record.Heavy rains over the 2010–11 summerperiod resulted in almost the entirestate being activated under joint Stateand Commonwealth governmentnatural disaster relief and recoveryarrangements.Two of the strongest cyclones to havecrossed the Queensland coast—Yasi andLarry—have also hit in the pastfive years.Failure to adapt to climate change hasthe potential to severely impact on thelives of Queenslanders.Statistics on Queensland Queensland’s rainfall varies substantially from year to year and decadeto decade. El Niño and La Niña events explain about 25 per cent of thesevariations. Tropical cyclones and variations in coastal wind direction are alsoimportant drivers of rainfall. There is no clear long-term trend in annual or seasonal-mean rainfall inQueensland over the last 100 years—the period for which we have reliablerainfall records. The total annual rainfall in Queensland depends primarily on the number ofdays with heavy rain (more than 25 mm). The number of intense events variessignificantly from year to year. This dependence on heavy rainfall explains why Queensland experiencessuch intense floods and droughts: the state is highly sensitive to relativelyrare, variable and intense rainfall. As greenhouse gases continue to increase in the atmosphere, Queenslandcould experience a markedly shorter rainfall season and considerably moreintense rainfall events.2Queensland rainfall—past, present and future

Managing the risks from climate—a partnership approachQueensland’s economy relies heavily onsectors such as tourism, livestock andother agriculture, which are vulnerableto both natural variations in rainfall andshifts in rainfall due to climate change.To design adequate strategies formanaging climate impacts and reducingthe risk of harmful climate change, it isessential that we first understand thecauses of natural rainfall variations andthe potential effects of climate change.Scientists from the Departmentof Environment and ResourceManagement’s Queensland ClimateChange Centre of Excellence (QCCCE)have been working in partnership withthe Walker Institute for Climate SystemResearch at the University of Reading,United Kingdom (UK) to improveunderstanding of Queensland’s rainfall,how and why it has varied and howrainfall may in change in future decades.From 2009 to 2012, Dr NicholasKlingaman, a research scientist with theUK’s National Centre for AtmosphericScience-Climate and the WalkerInstitute, has worked with scientists fromQCCCE to investigate rainfall in the state.The Walker Institute is a world leadingclimate research centre established bythe University of Reading in the UK.These research findings will be usedto help inform state government andbusiness about the risks posed byclimate variability and climate change.Dr Nicholas KlingamanQueensland rainfall—past, present and future3

About Queensland’s climateThe majority of Queensland’s rainfalloccurs in summer (December-February).Far North Queensland receives 8001200 mm during this season, most ofwhich comes from the monsoon windsbringing tropical moisture from ocean toland. Summer rainfall totals are lower insouthern Queensland, as this region lieson the edge of the monsoon circulation,but still often exceed 500 mm.In spring and autumn, extra-tropicalcoastal low pressure systems fromthe south can bring rainfall to centraland southern Queensland. Winterand spring are very dry seasons in thenorth, where the monsoonal circulationreverses and warm, dry southerly windsblow from the interior of Australia.The Pacific south-easterly trade windsproduce rainfall along the eastern coastthroughout the year.10S20S30S40S120E1234130E56140E150E7 8 9 10 11 12 13 14 15 16Rainfall (mm x 100)Annual-total rainfall in mm, derived from the Specialised Informationfor Land Holders (SILO) gridded gauge dataset.4Queensland rainfall—past, present and future

How and why has Queensland rainfall variedin the past?There are no clear trends in annualtotal Queensland rainfall over the 20thcentury. Most striking in the rainfallrecord is the considerable naturalvariability from year to year and fromdecade to decade.The late 1990s and early 2000s weregenerally a dry period in Queensland,particularly in summer. Comparable dryperiods can be seen in the record in the1930s and 1960s. It is likely that none ofthese 20th and 21st century dry periodswere as severe as the Federation Droughtof 1896-1902.Percentage anomaly of May-April Queensland area-averaged 019001910192019301940195019601970Year at beginning of May-April year1980199020002010The red bars show the percentage deviation of each year’s rainfall from the longterm (1900-2010) average. The black line (11 year moving average) demonstratesdecade-to-decade changes in rainfall.Queensland rainfall—past, present and future5

The influence of the El Niño Southern OscillationAbout 25 per cent of the year-to-yearand decade-to-decade variability inQueensland rainfall is driven by theEl Niño Southern Oscillation (ENSO).El Niño and La Niña describevariations in equatorial Pacific Oceantemperatures. During El Niño thetropical Pacific is abnormally warm,during La Niña it is abnormally cold.El Niño events in Queensland tend toexperience drier conditions, conversely,La Niña events are associated withwetter than normal conditions.Queensland's rainfall is sensitive, notonly to the magnitude of El Niño and LaNiña events, but also to their position inthe Pacific.Events that are focused in the centralPacific have a much stronger impacton Queensland than events that arefocused in the eastern Pacific, near thewest coast of South America.During the 1920-1950 IPO warm phase,there was hardly any connectionbetween ENSO and rainfall inQueensland. Cool phases of theIPO have been associated with anabnormally strong link between ENSOand Queensland rainfall.Warm phases of the IPO are alsoassociated with weak ENSO variations;relatively stable, normal levels of rainfallin Queensland; fewer tropical cyclonesin the Coral Sea; and a northward andeastward migration of the South PacificConvergence Zone (SPCZ), the band ofheavy rainfall that parallels the eastcoast of Queensland. The SPCZ normallylies off-shore, but during La Niña eventsit moves closer to Queensland’s coast,bringing heavy rainfall to the state.The effect that El Niño and La Niñaevents have on summer rainfall inQueensland has varied from one decadeto the next. As well as El Niño/La Niñaevents, the Pacific Ocean experiencesslower temperature variations overone or more decades, which is calledthe Inter-decadal Pacific Oscillation(IPO). The IPO has a period of about20-30 years. Its positive (warm) phaseresembles El Niño, while its negative(cool) phase resembles La Niña. El Niñoand La Niña events can occur ‘on top’ ofeither phase of the IPO.6Queensland rainfall—past, present and future

030SDarwinCooktownCairnsMJO causesAlice Springsmonsoon “bursts”Brisbaneand “breaks”PerthAdelaide SydneyMelbournePositive SAMshifts westerly beltpoleward60S70E100E130EMJO spawnstropical cyclonesHL160ENiño-3 (EastPacific El Niñoand La Niña)El Niño shiftstropical cyclonegenesis region eastPositiveSAM enhancesSE tradesCool SST anomaliesin positive Inter-decadalPacific OscillationaliesST anomWarm S tive IPOin posiCool SST anomaliesin positive IndianOcean DipoleNiño-4 (Central PacificEl Niño and La Niña)Warm SSTs inpositive IPOMadden-JulianOscillationsalieoman IPOTSS itivearm osW in pCool SST anomalies in positiveInter-decadal Pacific Oscillation30NPositive phase ofSouthern Annular Mode170W140W110W80WKey climate drivers of rainfall in Queensland.SST - Sea Surface TemperatureSAM - Southern Annular ModeSE - South EastMJO - Madden Julian OscillationQueensland rainfall—past, present and future7

Other influences on Queensland rainfallAutumn rainfall in Queensland dependson the behaviour of the end of themonsoon season. Ocean temperaturestend to be warm around easternAustralia when autumns are wet, andcool when autumns are dry. It may bepossible to use ocean temperatures topredict autumn rainfall.In South East Queensland, rainfall isdriven by the strength and moisturecontent of onshore easterly winds, whichare often connected to blocking anticyclones (high pressure systems) in theTasman Sea.Coastal rainfall in South EastQueensland has declined since in the1950s, associated with a reduction in thenumber of coastal cyclones and weakeronshore easterly winds. This decline isparticularly evident in summer rainfall.Cape York summer rainfall is tied to thefrequency and intensity of land-fallingtropical cyclones, most of which formin the Coral Sea. El Niño is associatedwith fewer tropical cyclones strikingQueensland, while La Niña is associatedwith more tropical cyclones. Changesin the frequency, intensity or tracks ofthese cyclones under climate changecould have substantial consequences fortotal rainfall in this region, as well as forrisks to human life and infrastructure.Rainfall over South East Queenslandshows substantial decade-to-decadevariations that, unlike state-widerainfall, are not associated with largescale climate drivers such as PacificOcean temperatures. The source of thisvariability and whether the variations arepredictable remain open questions.5SPacific OceanTropicalcyclonesCoral SeaDarwin15SCooktownCairnsIndian OceanAlice SpringsSE ey35SAdelaideCut-offlowsTasman SeaMelbourneLPrevailingwesterliesL45SHSouthern Ocean55S110EBlocking highsin Tasman SeaSub-tropicalanti-cyclone(summer)Blocking highsin Southern Ocean130E150EThe main weather systems that bring rain to d rainfall—past, present and future170E

Daily rainfall and extreme eventsOn average, roughly half of Queensland’srainfall comes from days with relativelyintense precipitation (accumulationsgreater than 25 mm). This dependenceis highest in the heavily-populatedcoastal regions, with 40-60 per cent ofthe total annual rainfall falling on theseheavy rain days. Coastal Queenslandexperiences an average of around20 heavy rain days per year; in the drysouth-west, there are on average only1 or 2 days heavy rain days each year.The number of heavy rain days variesconsiderably from year to year; thisexplains much of the year to yearvariability in rainfall. La Niña is alsoassociated with more heavy rainfallevents, while El Niño yearsexperience fewer.This dependence on heavy rain dayslikely explains why Queenslandexperiences such intense floods anddroughts: the state is highly sensitiveto relatively rare, variable andintense rainfall.Previous studies have identified thatover Queensland as a whole, thecontribution of extreme events to theannual rainfall total has grown over the20th century. If this trend continues inthe 21st century, it will have profoundhydrological implications for the regionin terms of surface runoff, river flowsand flooding frequency. Similar upwardtrends in the contribution of dailyextreme events to total rainfall havebeen found in other regions of the worldduring the second half of the20th century.Queensland rainfall—past, present and future9