El Niño- Southern Oscillation‘ENSO’

El Niño- Southern Oscillation

1. ENSO events2. Normal conditions3. ENSO conditions4. Cause of ENSO events

Unit 3 Lecture 9

• Ocean/atmosphere interaction ~ 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

ENSO Events

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

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

Cause of ENSO Events - Normal Conditions

• Cool, nutrient-rich coastal surface waters ~ upwelled waters come from below

the pycnocline

Normal Conditions - Peru and Ecuador

• Major fishery i.e. in the 1970’s 22% of all fish production came from coastal waters off of Peru and Ecuador

OCEAN

NOAA

• High pressure system over coastal Peru and Ecuador ~ dry in winter

Normal Conditions - Peru and Ecuador (cont’d)

ATMOSPHERE

NOAA

• Warmer than normal, nutrient deplete coastal surface waters ~ upwelled water comes from above

pycnocline

ENSO Conditions - Peru and Ecuador

OCEAN

• 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

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

Cause of ENSO Events - ENSO Conditions

• Low pressure system over coastal Peru and Ecuador ~ wet in winter~ heavy rains cause flooding and landslides on usually dry land

ENSO Conditions - Peru and Ecuador (cont’d)

ATMOSPHERE

NOAA

• 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

ENSO Conditions - Global

• More rain• Cooler• Increase in number and strength of

tornadoes ~ only two F4 tornadoes in FL occurred

during strong El Niño years• Less hurricanes

ENSO Conditions - Florida