Presentation on theme: "2. Natural Climate Variability"— Presentation transcript:
1 2. Natural Climate Variability 2.1 Introduction2.2 Interannual Variability2.3 Decadal Variability2.4 Climate Prediction2.5 Variability of High Impact Weather
2 2.1 Introduction: What is Interannual and Decadal Variability? Time series of SSTs in the East Pacific (region Nino3.4)
3 2.1 Introduction: What is Interannual and Decadal Variability? Time series of SSTs in the North Atlantic highlighting the Atlantic Multidecadal Oscillation (AMO) – source Knight et al (2005).
4 2.1 Introduction: What is Interannual and Decadal Variability? The NAO index is defined as the anomalous difference between the polar low and the subtropical high during the winter season (December through March).The positive phase is associated with more storminess in the Atlantic storm track and the negative phase with reduced storminess.
5 2.1 Introduction: Why is Climate Variability important? Interannual-to-Decadal Variability impacts Societies:Food resourcesWater securityHealthDemographicsEnergyTime series ( ) of average normalized April-October rainfall departure for 20 stations in the West African Soudano-Sahel zone (11-18N and West of 10E); following methodology of Lamb and Peppler, 1992).
6 2.1 Introduction: Can we predict SI Variability?
7 Section 2.2 Interannual Variability El Nino Southern Oscillation (ENSO)(i) Observations(ii) Theory for ENSO(iii) ImpactsInterannual variability in Atlantic SSTsThe North Atlantic Oscillation (NAO)
8 2.2.1 ENSO: ObservationsPhilander, S.G.H, 1990: El Nino, La Nina and the Southern OscillationUseful El Nino pages:
9 Mean State of the tropical Pacific coupled Ocean-Atmosphere System Warmest SSTs in west, “cold tongue” in eastPrecipitation associated with warmest SSTsEasterly trade winds advect equatorial surface waters westward
11 The Walker Circulation Mean ascent, and low surface pressure, over warmest SST associated with deep convectionSubsidence, and high surface pressure, in non-convecting regionsEquatorial trades blow from high to low pressure (balanced by friction since Coriolis force =>0)Low slpHigh slp
12 El Nino During El Nino trade winds slacken E-W tilt of thermocline & upwelling of cold water are reduced.SST rises in central/eastern equatorial PacificChanges Walker Circulation
23 What about La Nina?In La Nina conditions SST in the central and eastern equatorial Pacific is unusually cold & easterly trade winds are unusually strongDec 1982Nov 1988La Nina conditions sometimes occur in the year following an El Nino event (e.g followed 1987 El Nino)
24 Walker CirculationBritish mathematician, director general of observations for India (formed after monsoon failure of worst famine in Indian history)Arrived in 1904, shortly after huge famine caused by droughtGoal to predict Indian MonsoonFound that many global climate variations, including Monsoon rains in India, were correlated with the Southern OscillationFig. 9.
25 The Southern Oscillation Index DarwinTahitiMean pressure is lower at Darwin than TahitiThe term “Southern Oscillation” was also coined by Gilbert WalkerThe SOI measures the strength of the Pacific Walker circulation
27 ENSOBjerknes recognised that the “El Nino” warming of the ocean was related to variations in the SOI.During El Nino:SOI is lowTrades are weakPrecipitation is enhanced over central equatorial Pacific (indicated by low OLR)
28 The “Nino3” SST indexNote that El Nino events do not occur regularly
29 Why is El Nino important? Major climate and economic impacts on countries around the tropical Pacific, and further afield.Droughts in some regions, floods in othersCollapse of coastal fishery in Peru (largest average annual catch of marine fish in the world)
30 Impacts on global climate, ecosystems and society
41 3-D animation of the tropical Pacific as it cycles through El Nino then La Nina. The surface shown is sea level (cm) and the surface is colored according to the SST anomalies associated with each event.
42 Chronology of Events in the History of Understanding El Niño and La Niña late 1800s Fishermen coin the name El Niño to refer to the periodic warm waters that appear off the coasts of Peru and Ecuador around Christmas.1928 Gilbert Walker describes the Southern Oscillation, the seesaw pattern of atmospheric pressure readings on the eastern and western sides of the Pacific Ocean.
43 1957 Large El Niño occurs and is tracked by scientists participating in the International Geophysical Year. Results reveal that El Niño affects not just the coasts of Peru and Ecuador but the entire Pacific Ocean.1969 Jacob Bjerknes, of the University of California, Los Angeles, publishes a seminal paper that links the Southern Oscillation to El Niño.
44 1975 Klaus Wyrtki, of the University of Hawaii, tracks sea levels across the Pacific and establishes that an eastward flow of warm surface waters from the western Pacific causes sea surface temperatures to rise in the eastern Pacific.1976 Researchers use an idealized computer model of the ocean to demonstrate that winds over the far western equatorial Pacific can cause sea surface temperature changes off Peru.
45 1982 A severe El Niño develops in an unexpected manner, but its evolution is recorded in detail with newly developed ocean buoys.1985 Several nations launch the Tropical Ocean-Global Atmosphere (TOGA) program, a 10-year study of tropical oceans and the global atmosphere.1986 Researchers design the first coupled model of ocean and atmosphere that accurately predicts an El Niño event in 1986.
46 1988 Researchers explain how the "memory" of the ocean--the lag between a change in the winds and the response of the ocean--influences terminations of El Niño and the onset of La Niña.The array of instruments monitoring the Pacific, plus coupled ocean-atmosphere models, enable scientists to warn the public of an impending El Niño event.