PSU GEOSC 320 - ENSO and Climate Change (17 pages)

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ENSO and Climate Change



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ENSO and Climate Change

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Pages:
17
School:
Penn State University
Course:
Geosc 320 - Geology of Climate Change
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ENSO and Climate Change Tiffaney Stoy April 18 2003 Questions to keep in mind 1 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 Spanish El 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 dry An El Nino Study conducted by Matt Sittel of the Florida State University El Nino effects on the U S Warmer temperatures in the southeast cooler temperatures in the northwest Wetter in the Rockies midwest Ohio Valley extreme northwest and New England Drier in the extreme southeast and parts of California What 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 ridges El 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 system El 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 ENSO El Nino and Other Past Changes Analysis of Maiana coral SST records reveal high variability of ENSO since AD 1840 El 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 records El 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 much Timmerman Increased El Nino Frequency 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 allows for more inflow to take place More 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 forcing More 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 frequency What 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 reconstructing multi century records Maiana coral record shows 2 extended La Nina events 1855 1865 SW drought 1879 1887 no drought conditions Questions to think about 1 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


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