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9.2: El Niño and La Niña Phases

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    El Niño

    However, every few years, key ingredients of the Walker Circulation begin to shift, leading to the formation of an El Niño. Figure \(\PageIndex{1}\) shows the Walker Circulation during the El Niño phase.

    Cross-section of the Pacific Ocean during El Niño. Details in caption.
    Figure \(\PageIndex{1}\): The Walker Circulation in an El Niño phase. (Public Domain; Schematic Diagrams via NOAA/PMEL) Alternative description of image. 
    1. The trade winds, which usually carry warm water to the western Pacific, either slow down or reverse completely, allowing for:
      1. Warm water to collect in the eastern Pacific
      2. More cold water to collect in the eastern Pacific 
    1. In turn, this allows for the collection of warmer, rising air, and thus a  ______ pressure system to build over the eastern Pacific.
      1. low
      2. high
    1. On the other hand, because the trade winds weaken, there is ______ warm water present in the western Pacific.
      1. more
      2. less 
    1. Therefore, there is less warm air over the western Pacific, resulting in _______ pressure and _______ weather.
      1. lower; wetter
      2. higher; wetter
      3. higher; drier
      4. lower; drier
    1. Thus, we can conclude that the Walker Circulation ______ during an El Niño.
      1. strengthens
      2. reverses
      3. does not change

    La Niña 

    La Niña is the opposite of El Niño. During this phase of the Walker Circulation, rather than weakening or reversing from the Neutral Phase, the trade winds intensify. Figure \(\PageIndex{2}\) shows Walker Circulation during the La Niña Phase

    Cross-section of the Pacific Ocean during La Niña. Details in caption.
    Figure \(\PageIndex{2}\): The Walker Circulation during La Niña. (Public Domain; Schematic Diagrams via NOAA/PMEL) Alternative description of image.
    1. This intensification of the east-to-west trade winds results in ______ warm water collecting over the western Pacific, resulting in ______ warm, rising air.
      1. more; less
      2. more; more
      3. less; more
      4. less; less
    1. On the other hand, the eastern Pacific experiences ______ sea surface temperatures and _______ air.
      1. even cooler; cooler, sinking
      2. warmer; warmer, rising

    Let's check if we can identify these changing sea surface temperature patterns and wind directions using the TAO/Triton buoy array in the equatorial Pacific. Figure \(\PageIndex{3}\) shows the TAO/Triton measured mean sea surface temperature, winds, and anomalies for December 2015. 

    TAO/TRITON data from December 2015. Details in caption.
    Figure \(\PageIndex{3}\): Sea Surface Temperatures and Anomalies for December 2015. (Public Domain; Monthly mean SST and Winds via NOAA/PMEL) Alternative description of image.
    1. According to the lower panel in Figure \(\PageIndex{3}\), during December 2015, the most significant temperature anomalies were:
      1. Very cold (colder than -2°C below normal)
      2. Closer to average
      3. Very hot (hotter than +2°C above normal)
    1. The largest anomalies were located:
      1. In the western Pacific
      2. In the central and eastern Pacific
    1. Trade wind anomalies, especially in the middle and western half of the map, were:
      1. Mainly west-to-east and towards the Equator
      2. Mainly east-to-west and away from the Equator
      3. Light and varying in direction
    1. These conditions indicate that the Walker Circulation was in a(n) ______ phase.
      1. La Niña
      2. El Niño
      3. Neutral

    Figure \(\PageIndex{4}\) shows data from the TAO/Triton array during December 2010.

    TAO/TRITON data from December 2010. Details in caption.
    Figure \(\PageIndex{4}\): TAO/Triton Sea Surface Observations and Anomalies for December 2010. (Public Domain; Monthly mean SST and Winds via NOAA/PMEL) Alternative description of image.
    1. Considering mean temperatures, trade winds and anomalies in Figure \(\PageIndex{4}\), we can confidently say that during December 2010, the Walker Circulation was in a(n) ______ phase.
      1. Neutral
      2. El Niño
      3. La Niña

    Additional Reading

    In this investigation, we didn't cover the impacts of El Niño and La Niña because the focus was on identifying the signatures and detecting their presence. If you want to learn more about the effects of El Niño and La Niña on our weather, check out the following links.

    This page titled 9.2: El Niño and La Niña Phases is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Neel Desai & Alicia Mullens .