El Ni ñ o-Southern Oscillation: Past, Present, and Future

1. El Niño-Southern Oscillation: Past, Present, and Future Oceanography lecture, Kim Cobb

2. Why study the El Niño-Southern Oscillation? 1. It is the largest source of year-to-year global climate variability. 2. It carries negative societal consequences, both economic and humanitarian. 3. Improved forecasts minimize negative societal consequences. 4. It’s a fascinating, mysterious, and complex natural phenomenon.

3. What is the El Niño-Southern Oscillation? A natural climate cycle in the tropical Pacific that alternates between warm (El Niño) and cold (La Niña) phases every 2-7 years. a birds-eye view of 2 of the largest El Niño events of the century: and the 2002/2003 El Niño event:

4. What is the El Niño-Southern Oscillation? A natural climate cycle in the tropical Pacific that alternates between warm (El Niño) and cold (La Niña) phases every 2-7 years. a birds-eye view of 2 of the largest El Niño events of the century: and the 2002/2003 El Niño event:

5. This system is tightly coupled strong trade winds cool in East warm in West Average tropical Pacific conditions - trade winds blow from East to West - warm water piles up in West Pacific, driving deep atmospheric convection - cold, nutrient-rich water is brought to the surface in the East Pacific (upwelling)

6. The Bjerknes Feedback: trade winds weaken upwelling slows Eastern Pacific warms Tropical Pacific conditions during El Niño Peruvian fisherman (1800’s): named mysterious warm, nutrient-poor waters “El Niño” Sir Gilbert Walker (1924): named East-West seesaw in sea-level pressure the “Southern-Oscillation” Jacob Bjerknes (1969): explains feedbacks that link the “El Niño” and “Southern Oscillation” phenomena http://www.pmel.noaa.gov/tao/elnino

7. trade winds strengthen upwelling intensifies Eastern Pacific cools …and the reverse for La Niña

8. In Oregon: ENSO influences - coho salmon spawning - agriculture - water resource management - ski conditions El Niño Impacts - the impacts are not confined to the tropical Pacific - these departures from “normal” climate carry serious economic and social costs - La Nina’s effects are roughly the opposite of El Nino’s effects - improved ENSO forecasts minimize the costs

9. ENSO affects temperatures and rainfall in Georgia

10. Predicting ENSO extremes 1982 El Niño 1997 El Niño El Niño La Niña Devastation caused by the 1982 El Niño made ENSO prediction a top priority for U.S. and other countries.

11. I. Short-term Predictions 6-month prediction: - requires careful monitoring of tropical Pacific ocean + atmosphere - first ENSO model unveiled in 1987 - models predicted a weak El Nino event in 1997 The tropical Pacific observing system http://www.pmel.noaa.gov/tao/

12. II. Long-term Predictions 1982 El Niño 1997 El Niño El Niño La Niña Are severe El Niño events becoming more frequent? How will ENSO change as the Earth warms?

13. ENSO and Global Warming in the 21st century “This winter's El Niño gives us a taste of the extreme erratic weather that our children and grandchildren can expect more of unless we take action to reverse the trend of global warming.” then Vice President Al Gore, 1998 “Warm episodes of the El Niño-Southern Oscillation (ENSO) Phenomenon . . . have been more frequent, persistent and intense since the mid-1970s, compared with the previous 100 years.” Intergovernmental Panel on Climate Change, 2001 “Confidence in projections of changes in future frequency, amplitude, and spatial pattern of El Niño events in the tropical Pacific is tempered by some shortcomings in how well El Niño is simulated in complex models.” IPCC, 2001

14. The instrumental record of ENSO is too short to answer some key questions: 1. Are late 20th century El Niño events more frequent and more severe than those of the recent past? 2. Is there a correlation between average global temperature and El Niño activity? 3. How much and how fast has ENSO changed in the past?

15. ENSO and coral d18O in the Central Tropical Pacific (CTP) SST and rainfall anomalies during the 1982 El Nino During El Niño events, positive SST and precipitation anomalies both contribute to negative coral d18O anomalies in the CTP Interpretation of coral d18O on lower frequencies relies on assumption that warm SST drives higher precipitation in the CTP, and vice versa

16. Corals: The geologic record of ENSO CORALS from the tropical Pacific record ENSO in the geochemistry of their skeletons This live coral is ~50yrs old This fossil coral grew from ~1320-1390A.D. CORALS from 125 thousand years ago contain the signature of ENSO

17. Fossil corals collected during fieldtrips in 1998, 2000, and 2005 Beached fossil corals range from gravel-sized to 2m-diameter (~100-150yrs). rare longer cores: ENSO & decadal variability common short cores: mean climate

18. Palmyra 40 cores U/Th dated 28 cores undated Christmas 18 cores U/Th dated 63 cores undated Fanning 33 cores undated The Line Islands Fossil Coral Collection • Strategy • - rare long cores (40-100y) for variability • common short cores (10-20y) for mean • climate

19. 17th Century Spliced Coral d18O Records splice - absolute d18O values agree (give consistent picture of mean climate) - large, well-reproduced El Niño events in mid-17th century - low-amplitude, less well-reproduced decadal variability

20. 1997 El Niño El Niño La Niña 2. ENSO only Coral reconstructions of tropical Pacific climate 1. Raw records Canals freeze in Europe “Little Ice Age” Greenland green “Medieval Warm Period” warmer 1°C colder

21. Probing the coral record of ENSO: 1. Are late 20th century El Niño events more frequent and more severe than those of the recent past? Not necessarily. 2. Is there a clear relationship between average global temperature and El Niño activity? No. 3. How much and how fast can ENSO change? ENSO can double in strength in less than 5 years.

22. Chasing Late Holocene trends in ENSO variance Equally important are reconstructions of tropical Pacific mean climate through the late Holocene…. Are corals up to the challenge? Reduce error bar by using many small corals? Are we up to the challenge? Clement et al., 1999 Moy et al., 2002 Tudhope et al., 2001 Woodruffe et al., 2003 McGregor et al., 2004 Correge et al., 2000 Cobb et al., 2003, in progress

23. Conclusions 1. The future of ENSO remains uncertain. 2. We have much to learn about ENSO from the geologic record. Food for Thought Coral reefs are disappearing at alarming rates worldwide, due to the combined influence of rising ocean temperatures and human disturbances (pollution, dynamite fishing, etc). Even if ENSO does not change in a “greenhouse world”, Man has perturbed the environment to the point that an El Niño event could be the “straw that breaks the camel’s back”.

24. Borneo caves Dai and Wigley, 2000 Annually-resolved precipitation records from Warm Pool stalagmites • Goal: generate well-dated records • of interannual to millennial climate • in an ENSO center of action • Emphasis: • calibration • reproducibility • Potential Resolution: • sub-annual • Time coverage: • 1 million years ago • to present photos of three Borneo speleothems that formed during the last 70,000 years, including two that contain records of 20th century climate