Lecture 16: El Nino and La Nina

• El Niño


  One of the phases within the El Niño Southern Oscillation (ENSO) phenomenon
• La Niña


  One of the phases within the El Niño Southern Oscillation (ENSO) phenomenon
• ENSO


  El Niño and La Niña are the two phases within this phenomenon, which occurs in the equatorial Pacific
• ENSO is an inter-annual phenomenon, with El Niño episodes occurring roughly every 2-10 years
• ENSO


  • Couples wind patterns in the atmosphere with sea surface temperature distributions and upper level wind patterns
  • The effects are not only local to the equatorial Pacific, but also global
• Normal conditions


  1. Warm sea surface temperatures in the western equatorial Pacific

  2. Cold sea surface temperatures in the eastern equatorial Pacific

  3. Water flowing from east to west due to convergence of trade winds

  4. Evaporation and convective precipitation in the western part of the basin

  5. Upwelling of cold, nutrient-rich water along the South American coast
• La Niña conditions


  An enhancement of normal conditions, with warmer than normal water in the western Pacific and colder than normal water in the eastern Pacific, leading to more precipitation in the west and stronger upwelling along the South American coast
• El Niño conditions


  1. Trade winds weaken, leading to a westward to eastward movement of water

  2. Warm sea surface temperatures migrate to the central and eastern Pacific

  3. Evaporation and convective precipitation spread eastward

  4. Downwelling of warmer water occurs, cutting off nutrient-rich upwelling and impacting fisheries
• El Niño and La Niña

  Modify atmospheric jet streams, which carry weather systems and control precipitation and temperature patterns globally
• During El Niño


  The Pacific jet stream is pushed southward, leading to cooler and wetter conditions in the southern US
• During La Niña


  The Pacific jet stream is pushed northward, leading to warmer and drier conditions in the southern US
• El Niño events can influence California through the development of the "pineapple express", an atmospheric river that brings high levels of water vapor and precipitation
• El Niño events can lead to more energized storms, known as nor'easters, that can bring heavy snowfall to the mid-Atlantic region, as seen in the "Snowmageddon" storm of 2010 and the 2016 East Coast blizzard
• During El Niño


  Increased vertical wind shear in the southeastern US, which is detrimental to hurricane development, leads to less vigorous Atlantic hurricane seasons
• The lack of an El Niño event in 2020 has allowed for reduced wind shear and a more active Atlantic hurricane season