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

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.

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

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

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) strong trade winds cool in East warm in West This system is tightly coupled

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 Tropical Pacific conditions during El Niño trade winds weaken upwelling slows Eastern Pacific warms The Bjerknes Feedback:

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

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

ENSO affects temperatures and rainfall in Georgia

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

6-month prediction: - requires careful monitoring of tropical Pacific ocean + atmosphere - first ENSO model unveiled in models predicted a weak El Nino event in 1997 I. Short-term Predictions The tropical Pacific observing system

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

“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 ENSO and Global Warming in the 21st century

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?

ENSO and coral  18 O in the Central Tropical Pacific (CTP) During El Niño events, positive SST and precipitation anomalies both contribute to negative coral  18 O anomalies in the CTP Interpretation of coral  18 O on lower frequencies relies on assumption that warm SST drives higher precipitation in the CTP, and vice versa SST and rainfall anomalies during the 1982 El Nino

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 ~ A.D. CORALS from 125 thousand years ago contain the signature of ENSO

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

Palmyra 40 cores U/Th dated 28 cores undated Christmas 18 cores U/Th dated 63 cores undated Fanning33 cores undated The Line Islands Fossil Coral Collection Strategy - rare long cores (40-100y) for variability - common short cores (10-20y) for mean climate

17 th Century Spliced Coral  18 O Records splice - absolute  18 O 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

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

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.

Tudhope et al., 2001 Woodruffe et al., 2003 McGregor et al., 2004 Correge et al., 2000 Cobb et al., 2003, in progress 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? Moy et al., 2002 Clement et al., 1999

1. The future of ENSO remains uncertain. 2. We have much to learn about ENSO from the geologic record. Conclusions 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”.

Dai and Wigley, 2000 Borneo caves 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 20 th century climate Annually-resolved precipitation records from Warm Pool stalagmites