ENSO

• the main cause of year to year variability in global atmospheric weather patterns affecting climate over many regions (and global mean temperatures slightly)
• ENSO has El Nino and La Nina phases
• during El Nino the tropical eastern Pacific becomes unusually warm in winter/spring and this continues for about a year
• The pressure gradient between the east and west Pacific weakens hence the trade winds weaken - Southern Oscillation
• La Nina - colder than average SSTs in the tropical eastern Pacific and stronger trade winds
• La Nina is an enhancement of normal conditions
• El Nino conditions:

  • warm surface water propagates eastward -> warmer than average SSTs over the East and Central Pacific
  • surface pressure rises in the west and falls in the east
  • PGF lessens hence surface trade winds weaken
  • region of heaviest rainfall moves eastward as well
• La Nina conditions:

  • enhancement of normal conditions
  • higher than normal pressure in the East Pacific, lower than normal pressure in the West Pacific -> PGF -> surface trade winds strengthen
  • rising air and heavier-than-normal rainfall in the West Pacific
  • sinking air and drier weather over the East Pacific
• El Nino can cause flooding in tropical South America and dry conditions in Indonesia/Australia and the reverse for La Nina
• the changes to the atmospheric circulation affect neighboring regions hence ENSO impacts extend outside of the tropics