El Niño

2. El Niño

3. What is El Niño? • Warming of sea surface waters in the central and eastern tropical Pacific Ocean. • El Niño: the ocean part: Warm phase of ENSO: El Niño - Southern Oscillation • Southern Oscillation: the atmospheric part; a global wave pattern • La Niña: is the cold phase of ENSO: Cool sea temperatures in tropical Pacific • EN events occur about every 3-7 years

4. Peak El Niño occurs about Christmas season ... • “Christ Child” • “The Boy” • “The Little One” • Cf: La Niña • “The Girl”

5. ENSO A natural mode of the coupled ocean-atmosphere system ENSO: EN and SO together: Refers to whole cycle of warming and cooling. ENSO events have been going on for centuries (records in corals, and in ice layers in glaciers in South America) ENSO arises from air-sea interactions in the tropical Pacific

6. Measuring El NiñoBuoys

11. Excerpt: RIME OF THE ANCIENT MARINER Samuel Taylor Coleridge The fair breeze blew, the white foam flew, The furrow followed free:(The trades) We were the first that ever burst Into that silent sea Down dropt the breeze, the sails dropped down, Twas sad as sad could be; And we did speak only to break (The doldrums) The silence of the sea! All in a hot and copper sky The bloody Sun, at noon,(Vicinity of the ITCZ) Right up above the mast did stand, No bigger than the moon. Day after day, day after day, We stuck, nor breath nor motion: As idle as a painted ship Upon a painted ocean.

12. SSTsandSSTanomalies • There is a close link between tropical SSTs and precipitation • Not simple: involves total SST and its gradients • Winds blow down pressure gradient toward warmest waters • Convergence over/near warmer waters • Subsidence in cooler regions • For the most part: • High SSTs determine where the action is! • Low SSTs determine where it isn't!

13. ENSO Impacts Occur around the world Droughts (e.g. Australia, Africa, Brazil, Indonesia) Floods (e.g. Peru, southern USA)

14. Darwin and Tahiti

16. Southern Oscillation pattern: annual mean sea level pressure

17. SO: sea level pressure patterns: Seasons

18. SO: surface temperature and annual precipitation

19. 0 200 400 0 200 400 Jan July April Oct Kessler subsurface ocean temperature climatology used for TAO array

20. Subsurface ocean temperatures along equator from TAO array Total (top) and anomaly (bottom) Development of 1997-98 El Niño

21. lag -8 -4 0 4 8 Evolution of ENSO Ocean heat content 1980-1998 Trenberth et al. 2002

22. So ENSO is predictable • We can track the subsurface temperatures and their evolution • We can model these and make predictions for up to about a year ahead. • Statistical methods work quite well • But can not deal with the different flavors of El Niño

23. Processes involved in ENSO Different flavors of El Niño Links with global warming

24. A major source of climate variability from year to year is from the El Niño phenomenon

25. El Niño theories • El Niño: Start of, lifetime, and time between events • Previous El Niño sows the seeds for the next • Leads to quasi-cyclic behavior • El Niños occur every 3 to 7 years or so. • Bit like a pendulum • El Niños occur 31% of time, La Niñas 23% • The ocean is “primed” for an event to occur • The trigger may be random “weather” • “Delayed Oscillator” • Ocean waves are a key: • Rossby (off equatorial, westwards traveling) • Kelvin (equatorial, eastwards traveling) Waves reflect off boundaries and return altered

26. Rainfall in thunderstorms and organized convection gives rise to latent heat release in the atmosphere, which determines atmospheric heating patterns. The heating patterns set up low level convergence and upper level divergence that drives Rossby waves in the atmosphere. These propagate to other regions (teleconnections) and determine the atmospheric circulation locally.

27. Increased SSTs Enhanced convection Upper troposphere: outflow, divergence Anticyclonic pair Wave-train Storm tracks: southward, enhanced northward, diminished Trenberth et al. (1998)

28. The drought of 1988: cost about $40 billion

29. Star Tribune Aug 2 1993 Minneapolis, MN FLOODS Upper Mississippi Basin Source: Trenberth

30. Different flavors of El Niño Events come in different “flavors” El Niño refers to a warming of the tropical Pacific but this does not take account of surrounding areas and character of event. Tropical winds and rainfall respond to total SSTs and so depend on time of year and details. Surface wind convergence is in vicinity of warmest water, not anomaly. Small changes in SST can change region of warmest water by 1000 km or more!

35. Sources of variability in ENSO • There is evidence of changes in ENSO about 5000 years ago from paleo evidence: that ENSO ceased • Model studies also show this • Implies that small forcings are able to cause large alterations in the behavior of ENSO (nonlinear) • Changes can be abrupt • Also the 1976 shift • Timmermann et al. (1999) suggest that greenhouse warming will induce greater ENSO activity marked by more large amplitude cold (La Nina) events and the possibility of abrupt regime shifts. • The forced response is a shift in the probabilities of states preferred. • Character of change is model dependent

36. Quelccaya Ice Cap Peru Annual layers of ice reveal El Niños Courtesy Lonnie Thompson

37. Records of ENSO in the mid-Holocene • Modern drought-adapted pollen taxa not present before 5000 years ago; ENSO variability much weakened or absent (McGlone et al. 1992; Shulmeister and Lees 1995). • GBR coral indicates interannual hydrologic variability nearly absent compared to modern, background conditions warm; ENSO variability absent (Gagan et al. 1997). • New Guinea coral indicates interannual variability significantly damped; ENSO variability weak (Tudhope et al. 2001). • Flood deposits in lake occur less frequently before about 5000 years ago; strong El Niño events more rare (Rodbell et al. 1999). • Presence of warm-loving fauna suggests permanent warm state of eastern Pacific SST. Persistent El Niño; cool La Niña’s absent (Sandweiss et al. 1996). Summary from Cole 2001

38. El Niño and Global warming 1998 warmest year on record, 2001 second warmest El Niño contributes to global warming There is a pattern of more and bigger El Niños in past 20 years El Niño years La Niña years Is global warming contributing to changes in El Niño? Likely, yes, to some extent. Which part is natural variability?

39. Global warming  Heating   Temperature  & Evaporation   water holding capacity   atmospheric moisture    greenhouse effect  & rain intensity   Floods & Droughts

41. There is a mini global warming with El Niño: 0.24°C peak in 1998, 0.17°C for year Trenberth et al. 2002

42. lag -8 -4 0 4 8 mo Evolution of ENSO: surface temperatures 1950-78 left 1979-1998 right Shows how and where warming is manifested. Trenberth et al. 2002

43. How will El Niño events change with global warming? • El Niño involves a build up and depletion of heat as well as major redistribution of heat in the ocean and the atmosphere during the course of events. • Because GHGs trap heat, they interfere. • Possibly expand the Pacific Warm Pool. • Enhance rate of recharge of heat losses. • More warming at surface: enhanced thermocline  enhanced swings • More frequent El Niños? • Some models more El Niño-like with increased GHGs. • But models do not simulate El Niño well • Nor do they agree • The hydrological cycle is expected to speed up with increased GHGs. Increased evaporation enhances the moisture content of the atmosphere which makes more moisture available for rainfall. ENSO-related droughts are apt to be more severe and last longer, while floods are likely to be enhanced.