El Niño - Southern Oscillation (ENSO)Take away concepts and ideasTrade winds How do they affect the tropical oceans?Tropical surface and subsurface temperaturesTropical SSTs and rainfallTropical SSTs, Climate, & VegetationNormal State of the PacificEl Niño State of PacificSSTs and SST anomaliesLargest El Niños of 20th centuryPowerPoint PresentationSlide 12Slide 13Slide 14Slide 15Slide 16Slide 17Trade winds on the equator bring cold waters to surfaceNormal ConditionsSlide 20Transition to El NiñoFull El Niño conditionsSlide 23Slide 24Slide 25Slide 26Slide 27Walker CirculationSlide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Connecting El Niño and the SO (ENSO)General Description of ENSO ProcessesA very simplified view of ocean-atm couplingWhy does ENSO State tend to Oscillate?Slide 40Slide 41Prediction for Winter ‘09Slide 43Slide 44Slide 45Slide 46ENSO and Global WarmingEl Niño - Southern Oscillation (ENSO)Ocean-atmosphere interactionsTake away concepts and ideasWhat is El Niño, La Niña?Trade wind and Walker circulation.What is the Southern Oscillation?Tropical ocean-atmosphere coupling. ENSO changes in: SST gradients, surface winds, sea surface height, upwelling, productivity, thermocline depthWhy do tropical Pacific SSTs oscillate?Trade winds How do they affect the tropical oceans?TradewindsHadley CellsTropical surface and subsurface temperaturesTropical SSTs and rainfallTropical SSTs, Climate, & VegetationPapua New Guinea Coastal PeruNormal State of the PacificWarm poolSea surface temperature, SSTEl Niño State of PacificWarm pool shifts eastward“El Niño is a warming of the eastern tropical Pacific”SSTs and SST anomaliesAnnual averageSST (°C)SST Anomaly (°C)SST Anomaly = SSTobs - SSTavgLargest El Niños of 20th centurySST Anomaly (relative to the average state)19821997El Niño La NiñaOnce developed, El Niños are known to shift temperature and precipitation patterns in many different regions of the world. These shifts, although varying somewhat from one El Niño to the next, are fairly consistent in the regions shaded on the map below.Hadley circulationWinds cause a general westward motion of tropical surface waters, causing the warmest waters to “pile up” at the western Pacific (western Pacific warm pool)… and upwelling of cold waters in eastern basinsOcean Response to Atmospheric CirculationWarm poolTrade winds How do they affect the tropical oceans?TradewindsHadley CellsTropical surface and subsurface temperaturesWalker circulationTrade winds on the equator bring cold waters to surfaceEquatorial upwellingNormal ConditionsNote features:- East-west SST- upwelling- Thermocline depth- Sea surface height- Surface pressure- trade windsHadley and Walker winds vary in phase:when Hadley cell is strong, so is the walker circulationTransition to El Niño Note changes in- East-west SST- upwelling- Thermocline depth- Sea surface height- Surface pressure- trade windsFull El Niño conditionsWhat are changes in- East-west SST- upwelling- Thermocline depth- Sea surface height- Surface pressure- trade winds? ? ?SST anomalies for 1982 and 1997 El Niño EventsSometimes called the “warm phase”SST anomalies for 1968 and 1998 La Niña EventsSometimes called the “cool phase”Warm PhaseCold phaseCool SSTwarm SSTWalker CirculationBritish mathematician, director general of observations for India (formed after monsoon failure of 1877- 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 OscillationSir Gilbert WalkerSOI = Tahiti SLP - Darwin SLPAverage Climatology: 1933-1992TahitiDarwin+-**∆SLPDifference in Jan. SST between the two extreme ENSOepisodes of this centurynino3Blue = Atm pressure at DarwinRed = SST anomaly for the NINO3 regionPhysical coupling between the ocean and atmosphereConnecting El Niño and the SO (ENSO)The link between SO and El Niño was made convincingly by BjerknesMade extensive use of data gathered during 1957 (strong El Niño year)Realized that unusual events separated by half the circumference of Earth could be linked together as parts of a huge coupled phenomenon ENSO– involving both the ocean and the atmosphere.Prof. Jacob BjerknesGeneral Description of ENSO ProcessesBjerknes established the empirical connection between EN and SO. He also provided a hypothesis about the mechanism that underlies our present understanding.The key is to appreciate how odd the “normal” state is–The easterly trades drive westward currents, bringing the cold waters of the Peru Current from the South American coast–The coriolis force turns westward surface currents poleward, causing divergence and upwelling–The trade winds push the warm upper layer waters poleward as well as westward, pulling the thermocline to the surface in the east–All are due to the easterly winds, but the easterly winds are also due to temperature contrasts along the equator. Thus there are positive feedbacks to reinforce both El Niño and La Niña phasesA very simplified view of ocean-atm couplingWhy does ENSO State tend to Oscillate?Equatorial ocean dynamicsKey observations by Klaus Wyrtki in 1970’sEl Nino is preceded by a transfer of warm water from west to eastThis transfer is thought to trigger a warm eventWhat triggers the movement of water?–In cold phase, waters cold (and low) in east, warm (and high) in west–Warm water from west sloshes back and overshoots equilibrium- positive feedbacks mentioned before set of El Niño conditionsWhy does ENSO State tend to Oscillate?Shortest Answer: Equatorial ocean dynamics Short answer: Differing timescales of Kelvin and Rossby Wave propagationLonger answer: The oscillation is made possible due to the asymmetry between eastward and westward oceanic motions (see 2-D Animation in notes). Along the equator there is a relatively fast eastward motion called an equatorial Kelvin wave. Peaking somewhat off the equator are westward motions called Rossby waves. Time scale: <1 yr for the Kelvin waves to shift the warm pool eastward [once trades relax]; 2 yrs for the Rossby waves to return the warm pool to the west, to await another wind relax. KelvinRossbyHere is a 3-D animation the tropical Pacific as it cycles through an El Niño then La Niña event. The surface shown is sea-level (in cm) and the surface is colored according to the SST anomalies