El Niño is a major reorganization of the equatorial climate system that affects regions far from its point of origin in the Western Equatorial Pacific

Occurs roughly every 6 years

Onset recognized by climatic effects

• warm surface waters
• collapse of fisheries
• heavy rains in Peru/Ecuador/central Pacific
• droughts in Indonesia
• change in typhoon tracks

• Atmospheric pressure differential between Tahiti and Darwin
• Normally low pressure in Darwin, high in Tahiti
• El Niño: high pressure in Darwin, low in Tahiti
• Change in pressure differential results in weakening of easterly equatorial winds

• Strong easterly winds:
• Pile up warm water in the western Pacific-- thermocline deep in western Pacific, shallow in eastern Pacific
• Drive equatorial upwelling-- as thermocline is shallow upwelling brings nutrients to surface waters along the equator
• Strong coastal winds:
• Cause upwelling of nutrient-rich water along West coast of South America
• Upwelled nutrients in coastal and equatorial regions support phytoplankton growth fisheries

• Pressure at Darwin rises, pressure at Tahiti drops
• Easterly winds weaken
• Warm surface water surges back across central Pacific
• Thermocline drops in the east, rises in the west
• Equatorial upwelling decreases because of lower wind strength
• Coastal upwelling continues but deeper thermocline so only warm low nutrient water upwells
• Phytoplankton population collapses, fisheries collapse

• Normal conditions:
• Strong winds blowing over cold upwelled water too dense to rise
• Rain occurs in western Pacific over warm ocean water
• El Niño conditions:
• Warm water moves east across Pacific, warm moist air rises
• Heavy rains in central and eastern Pacific

• Atmosphere driving the ocean, ocean driving the atmosphere positive feedback loop, no end
• Atmosphere responds very quickly to the ocean, the ocean responds more slowly, "remembers" the past winds
• Themocline moves across Pacific to the east as a sub-surface wave, another wave moves to the west
• Waves reflect off Asia and return to central Pacific raising thermocline and bringing cold water near the surface again
• Breaks the feedback loop
• El Niño and La Niña alternate every few years, frequency may be changing

• Fishery collapse, seabird population collapse
• Lakes in desert region of S America lake fishery
• Coastal estuaries produce shrimp
• Coastal flooding malaria epidemics
• Crop failures:
• wet regions have droughts
• dry regions have floods
• Hurricane/typhoon tracks change Tahiti, Hawaii
• Cost 1982/83 El Niño ~ $8 billion

• Build up of warm pool in W Pacific and wind bursts from Asia initiates
• Currently can predict about 12 months in advance
• Allows farmers to plant appropriate crops
• Frequency of onset and severity may be increasing. Global climate change?