Background: 1899: A disastrous famine strikes India when monsoons fail.

• British Director-General of Observatories in India, Sir Gilbert Walker, sets out to understand phenomenon
  • goal to predict monsoons so that famine can be averted
    
    
  • Walker collected weather records from all over the globe to establish the correlation of all these records
    
    
  • Identified connections between weather/climate patterns in remote parts of the world
    
    
  • Walker never succeeded in predicting the monsoons (we still can't do this)
    
    

• Walkers discovery of global climate correlations led to identification of a key part of the El Niño phenomenon
  
  
• During 1920's G.T. Walker noted that:
  • When atmospheric pressure unusually high in western equatorial Pacific, it tended to be unusually low in southeastern tropical Pacific, and vice-versa
    
    
• This opposite relationship, or seesaw, between pressure in eastern and western tropical Pacific became known as the Southern Oscillation (SO)
  
  
• How It Works: Normal Conditions
  • During normal years strong trade winds blow westward from high pressure over eastern Pacific to low pressure over Indonesia; drag cool water with them, which warms as it crosses Pacific upwelling in eastern Pacific (off Peru)
    
    
  • nutrients brought to the surface by the cold Peru undercurrent support a rich concentration of fish
    
    
    • Fisherman in coastal waters of Peru recognized that an annual influx of warm water (Peru Counter Current) would temporarily result in reduced catches
      
      
    • Fishermen traditionally take some time off to repair their nets, etc
      
      
    • Usually occurs around Christmas. Thus the name El Niño (the Child)
      
      
  • Effects
    • Sea surface temperatures normally low in E, high in W
      
      
    • Sea level higher in W; where there is a thick layer of warm water
      
      
    • This warm water warms the air, causing convective air uplift over western Pacific (Australia)
      
      
    • Results in High rainfall in Western Pacific
      
      
    • This situation maintained by large pressure difference across southern tropical Pacific
      
      
      
      
• How It Works: El Niño Conditions
  • Occasionally, the low pressure cell normally located in the western Pacific migrates eastward

• High rainfall further East
  • Heavy rains and storms affect the central Pacific and droughts begin in Australia and Indonesia.
    
    
• May actually get east-blowing trades
  
  
• The warm waters reach the Pacific coast in December. Ocean adjusts by varying slope of thermocline (line separating warm, surface waters from cold, deeper waters)
  • Slope of thermocline maintained by trade winds
    
    
  • "Normally" expose cold water offshore Peru
    
    
  • When trade winds collapse, slope of thermocline reduces, capping cold waters with warm
    
    
• Effects: This causes the El Niño current to remain in place for months, or even throughout the year.
  • Fisheries suffer during these times since cold nutrient-rich waters are replaced with warm, nutrient-poor waters
    
    
  • Every few years it's especially intense, warmer than usual, penetrates further south, accompanied by heavy rains
    • Severe "El Niño Events" cause mass mortalities, reduced oxygen content of water
      
      
    • 1972-1973, reduced Peruvian anchovy catch from 10.3 Million tons to 4.6 million tons; this used for animal feed; raised poultry prices around the world (US more than 40%).
      
      
    • One of worst events on record in 1982
      
      
  • Other parts of Peru receive unusual amounts of rain, even in normally arid regions
    
    
  • Those years became known as "Años de Abundancia" -- (years of abundance).
    • Crops can be grown and animals grazed in regions usually inhospitable to agriculture.
      
      
    • Down-side to these years is that floods often accompany the unusual rainfall.
      
      
  • Torrential rains along west coast of North & South America (California)
    
    
  • Drought and fires in Australia
    
    
• These years became known as El Niño years - years in which the warm El Niño current persisted
  
  
• The term El Niño was thus originally used to refer to the seasonal warm current that came to coastal Peru every year.
  
  

• Today we reserve this term for more unusual warming of eastern tropical Pacific waters that persist through much or all of the year.
  
  

• Phenomena has an average oscillation period of 3 years (2 - 10 year range)
  
  
• El Niño usually ends 12 to 18 months after it starts.
  
  
• La Niña seems to be associated with weather phenomenon opposite to that of El Niño
  • For example Indian Ocean monsoons are dryer than usual in El Niño years and wetter than usual in a Niña years.
    
    
• Recent El Niño episodes
  • 1977-1978, 1982-83, 1986-87, 1991-92, 1993-94 Mississippi floods), 1994-1996 (some consider 1990-1996 the longest single El Niño)
    
    
  • 1982-1983: the worst El Niño of the century
    • Encephalitis along the east coast (mosquitoes)
      
      
    • Bubonic plague in New Mexico (fleas)
      
      
    • Shark attacks off Oregon (warm water)
      
      
    • Drought in Spain & N. Africa
      
      
• Changing frequency of El Niños
  • During the Little Ice Age
    • Every 7-10 years
      
      
  • This century
    • Every 3-5 years
      
      
  • Past 20 years
    • Six episodes
      
      
• Why?
  • Global warming????