1. El Niño- Southern Oscillation
‘ENSO’

2. El Niño- Southern Oscillation
Unit 3 Lecture 9
El Niño- Southern Oscillation
ENSO events
Normal conditions
ENSO conditions
Cause of ENSO events

3. ENSO Events Ocean/atmosphere interaction Occurs on a 3-4 year cycle
surface water temperature/ atmospheric pressure
Occurs on a 3-4 year cycle
Earliest written records – 1500’s Spanish explorers
Phenomenon usually observed around Christmas
season in waters off Peru and Ecuador

5. Warm surface temp → Low-pressure tropical cyclones (hurricanes or typhoons)

8. Cause of ENSO Events - Normal Conditions
Tradewinds
Tradewinds ‘push’ surface waters at western edge of basin
Warm water piles up in west “Pacific warm pool” , sea surface 1m higher
Pycnocline found at 100m depth at western edge of basin and only at
30m depth at eastern edge of basin
Coastal winds ‘push’ waters offshore (i.e. via Ekman transport)
cold nutrient-rich waters subsurface waters upwelled

9. Normal Conditions - Peru and Ecuador
OCEAN
Cool, nutrient-rich coastal surface
waters
upwelled waters come from below
the pycnocline
Major fishery
i.e. in the 1970’s 22% of all fish
production came from coastal waters
off of Peru and Ecuador
NOAA

10. Normal Conditions - Peru and Ecuador (cont’d)
ATMOSPHERE
High pressure system over coastal Peru and Ecuador
dry in winter
NOAA

11. ENSO Conditions - Peru and Ecuador
OCEAN
Warmer than normal, nutrient
deplete coastal surface waters
upwelled water comes from above
pycnocline
Fishery crashes (lower catch, higher
prices at fish market)
causes death of organisms adapted to
cooler, nutrient rich surface waters
upsets and shifts food web dynamics
NOAA

12. Cause of ENSO Events - ENSO Conditions
Tradewinds Relax or Cease
Tradewinds no longer ‘push’ surface waters at western edge of basin
Pacific warm pool begins to flow back towards eastern edge of basin
via Equatorial Counter Current
starts to move in Sept., makes it to S. America in Dec. or Jan.
pycnocline flattens out
now found at a greater depth in the east
Coastal winds still ‘pushing’ waters offshore (i.e. via Ekman transport)
warm nutrient-deplete surface waters upwelled

13. ENSO Conditions - Peru and Ecuador (cont’d)
ATMOSPHERE
Low pressure system over coastal Peru and Ecuador
wet in winter
heavy rains cause flooding and landslides on usually dry land
NOAA

14. ENSO Conditions - Global
Flooding in North and South America (low pressure system)
landslides, erosion
Droughts in Australia (high pressure system)
fires
Switching position of pressure zones effects jet stream flow
increase in tropical storms in the Pacific
decrease in tropical storms in the Atlantic

16. ENSO Conditions - Florida
More rain
Cooler
Increase in number and strength of
tornadoes
only two F4 tornadoes in FL occurred
during strong El Niño years
Less hurricanes