El Niño- Southern Oscillation ‘ENSO’

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

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

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

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

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

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

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

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

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

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

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