ENSO and Climate ChangeTiffaney StoyApril 18, 2003Questions to keep in mind1. What is ENSO? What causes it?2. What proxies help determine El Nino/La Nina occurrences?3. What general climatic effects does ENSO have around the globe?4. What was ENSO like in the past?5. What theories exist about the future of ENSO? Will anthropogenic effects change the variability of ENSO?What is (ENSO) El Nino -Southern Oscillation?• Warming of eastern Pacific which occurs when prevailing tradewinds die down, allowing warm water to migrate eastward, warm waters warm the atmosphere causing strange weather patterns• Sea level falls in the west and rises in the east by as much as 25 cm• Reduced upwelling in eastern Pacific, strengthening temperature anomaly• Occurs roughly every 3 to 7 years• La Nina is the opposite, occurs with onset of opposite effects, cooler conditions, occurs about a year or so after El Nino• Can be determined by the SOI - Southern Oscillation Index• Means “little boy” in SpanishEl Nino Effects• Intense rainfall in areas• Severe drought in other areas, such as over Australia and Indonesia• Weakened Summer Monsoon season in South Asia• Water-borne diseases (hepatitis, cholera) as well as Vector-borne diseases (malaria, yellow fever) have cycles associated with El Nino• Increase in Pacific tropical cyclones• Broad influence of global economy• In America, NW and Great Plains wet, Northeast dryAn El Nino Study conducted by Matt Sittel of the Florida State UniversityEl Nino effects on the U.S.Warmer temperatures in the southeast, cooler temperatures in the northwestWetter in the Rockies, midwest/Ohio Valley, extreme northwest and New England, Drier in the extreme southeast and parts of CaliforniaWhat Proxies help to understand past El Nino’s occurrences?• **Corals (150 year record)- SST conditions, upwelling, salinity, precipitation, river runoff, all linked to large scale variability– Radiocarbon concentrations - changes in upwelling -> thermocline• Low upwelling (El Nino) - High 14C and vice versa– El Nino - Negative delta 18O anomalies in coral skeleton– La Nina - Positive delta 18O anomalies– Sr/Ca ratios• Tree rings/Ice Cores - (Radiocarbon dated) 3-D atmospheric response world wide as well as ocean-atmosphere variations, Ex. evidence of prolonged drought in N. America from last millenium• Sediments/Schlerosponges (millions of years old) - Thermocline histories, Precipitation• Other proxies - Archaeological remains, geomorphic events, historical documents, flood deposits, beach ridgesEl Nino during the Holocene• Holocene (5000-10000 years BP) colder warm season, not colder cold season• Weaker variability during this time, 4 years but weak events (like mid-20th century)• Sediments analyzed show the weakness• But weakness due to weak orbitally forced changes (seasonal changes) in tropical Pacific, caused by instability that occurs • Note: Because the ENSO system is non-linear, ENSO changes do not correlate exactly with orbital variations, but small forcings are capable of causing large alterations in this systemEl Nino during the Eocene• Eocene (55 - 35 million years ago) was a warmer era, poles more different today than tropics were• They thought if ever a time for a continuous El Nino state, it would be during Eocene, but theories suggest Pacific didn’t get as warm• Lake sediment records have shown evidence that El Nino frequency was not high• Therefore, likely that a continuous El Nino state didn’t occur even though Eocene was considerably warmer• During this time, no drastic thermocline difference, atmospheric-ocean dynamic changes or ENSOEl Nino and Other Past Changes• Analysis of Maiana coral SST records reveal high variability of ENSO since AD 1840El Nino and Other Past Changes• Before 1880 - Decadal variability 12.5 years• 1880-1920 - ~3 year variability• Since beginning of 20th century, coral records show slight warming• Around 1955- 4 year variability which continues through record• 1976- abrupt change to warmer and wetter conditions in tropical Pacific indicative of coral recordsEl Nino and Future Climate• Theory 1- More El Nino events in the future caused by greenhouse gases/global warming• Global warming could have 10C temperature rise• Will this warming combined with an increase in greenhouse gases increase ENSO occurrences? There is still debate over this theory• This is because ENSO and climate variability dependent on many background conditions• In the past, there was a greater time span between events when cooler and drier and when anthropogenic forcing was absent• Modeling results suggest more frequent and stronger ENSO events are possible in the future, but not sure how muchTimmerman (“Increased El NinoFrequency in a Climate Model Forced by Future Greenhouse Warming”, 1999)– states that increasing greenhouse gases allow for more El Nino like conditions– SST anomalies would be greater, explains why variabilty has intensified over past several decades– also states that the equatorial thermocline (most important factor) becomes stronger as a result of an increase in greenhouse gases,temperatures in deep ocean don’t change as much, gradient allowsfor more inflow to take placeMore on the Future...• Theory 2 - If oscillation between warm and cool sessions due to global warming stops, Earth could suffer from continuous El Nino state that would warm the Earth very quickly (Run Away Greenhouse• Theory 3 - Change will occur towards more events but not due to anthropogenic reasons• Changes have been seen before anthropogenic forcingMore on the Future...• Theory 4 - Decreased El Nino events for the future• Moy, C.M. et al (“Variability of ENSO Activity at Millennial Timescales during the Holocene Epoch”, 2002)• AD 1250, Little Ice Age, 27 events/100 years• 3000 yrs. BP, millennial scale cold period, ENSO frequency elevated• 2000 yrs. BP, Roman Warm Period, ENSO frequency at low level• Start of Modern Warm Period, 1940, 4-5 events/100 years• Therefore, they believe that future warming will lead to a decline in El Nino frequencyWhat else will be done?• Re-examination of proxy data, need new and better techniques to study the samples, new approaches to analyses of samples• Samples from single sites may not consistently represent ENSO conditions• More sampling needed, especially in sub-tropics• Existing samples insufficient in