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• Describe the general atmospheric and oceanic circulation patterns that characterize La Nina and El Nino
• Describe the effects of La Nina and El Nino• Explain some of the related ecological effects of El Nino• Explain coral bleaching and how it relates to El Nino and La
Nina• Explain how hurricanes are impacted by El Nino/La Nina• Explain how we know that ENSO is natural• Describe the relationship between ENSO and global warming• Describe the current ENSO conditions
The Southern Oscillation
• Naturally occurring cyclical change in Pacific ocean and wind circulation patterns
• Paleorecord indicates it has been occurring for 50 million years ago
• Has global teleconnections• Occurs every 2-7 years, a temporal scale larger than
seasons• Global warming does not cause El Nino, but there
may be interactions between the two.• Two endpoints of oscillation, La Nina and El Nino• Also referred to as ENSO
The Southern Oscillation
Walker circulation: normal or average conditions
• West Pacific – Warm ocean water– Low pressure– Unstable atmospheric conditions
• East Pacific– Cold water upwelling– High pressure– Stable atmospheric conditions
El Nino
• Tradewinds slow and reverse direction• Eastern Pacific
– Upwelling ceases– Warmer surface water– STHP weakens– Wet and unstable conditions
• Western Pacific– Cooler surface water – Low pressure weakens– Drier and stable conditions
El Nino
Drawing/animation
El Nino: Flooding and mudslides in California and South American coast
El Nino: Drought and fire in western Pacific
El Nino: Loss of cold water upwelling and diminished phyoplankton blooms in E. Pacific
Animation
Phytoplankton: primary producers
• Serve as food for zooplankton and higher trophic levels
Diatoms
CyanobacteriaDinoflagellates
Coccolithophorids
El Nino: Collapse of fisheries along South American coasts due to loss of cold water
upwelling
Increased rainfall and vegetation cover in arid regions of western
US
El Nino: Hanta virus outbreaks in Western US
http://www.cdc.gov/ncidod/diseases/hanta/hps/noframes/elnino.htm
La Nina
• Amplification of normal Walker circulation– Easterly tradewinds grow stronger (E to W)– More warm water piles up in western Pacific– Lower pressure, more wet and unstable in western
Pacific– STHP in eastern Pacific grows stronger, cooler
and drier conditions– Greater upwelling
Drawing
La Nina
Drawing
Hurricanes and the Southern Oscillation
Hurricane tracks for La Nina (left) and El Nino years (right) 1950-2001
Hurricanes and the Southern Oscillation
• Fewer hurricanes in Atlantic/Caribbean with El Nino– Reversal of tradewinds and strong more southerly
subtropical jet weaken hurricanes through increased wind shear
• More hurricanes in Atlantic/Caribbean with La Nina– Tradewinds blowing from east to west favor
hurricane development.
– Subtropical jet stream further north
bleaching
Coral bleaching and the Southern Oscillation
• Caused by prolonged high sea surface temperatures. At high temps:– Zooxanthellae (photosynthetic algae) in coral
decrease production of photosynthate for coral animal
• These changes result in the expulsion of zooxanthellae from coral polyps
• Corals deprived of color and food, decline and death
• El Nino brings coral bleaching to central-eastern Pacific and Caribbean.
• La Nina brings bleaching events to Australia and the western Pacific
ENSO and global warming models
• ENSO not caused by global warming
• ENSO is natural cycle but influenced by global warming
• More El Nino conditions under global warming?
• Greatest variability in ENSO over past century than the preceding 7000 years
Normal ENSO fluctuations during Eocene (50 mya)
Normal ENSO fluctuations during Eocene warm period (50 mya)
Current Southern Oscillation conditions
• Synopsis:• The chance of El Niño is at 60-65% during the
Northern Hemisphere fall and winter.
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf
Other ENSO states and oscillations
• Modokai El Nino• North Atlantic Oscillation• North Atlantic Oscillation• Arctic Oscillation• Pacific Decadal Oscillation
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