El Niño Southern Oscillation

  • Image, Climate oscillations.

    The El Niño Southern Oscillation (ENSO) is the movement of warm equatorial water across the Pacific Ocean and the atmospheric response. It occurs every 2–7 years, typically lasting 6–18 months. ENSO has three phases: neutral, El Niño, and La Niña. In New Zealand an El Niño phase in summer can bring increased westerly winds, more rain in the west, and drought in the east; in winter it can lead to cooler southerly winds. During a La Niña phase we may experience more north-easterly winds, wetter conditions in the north and east, and higher sea levels.

    The ENSO is one of three climate oscillations that affect our weather. These changes in air pressure, sea temperature, and wind direction can last for weeks to decades, depending on the oscillation.

    We classified El Niño Southern Oscillation as supporting information.

    Key findings

    There was an El Niño phase in late 2015 to early 2016, when average southern oscillation index values were below -1 for July–October 2015 and in January, February, and April 2016.

    Figure 1

    Graph, Monthly average southern oscillation index, 1986–2016.

    Note: The southern oscillation index (SOI) is calculated from the difference between the standardised surface air pressures at Tahiti and Darwin, Australia. A negative or El Niño phase corresponds with monthly averages below -1.0, a positive or La Niña phase corresponds with monthly averages above 1.0.
    Figure 2

    Graph, Annual average southern oscillation index and de-trended NZ temperature, 1909/10–2015/16.

    Note: The de-trended annual average temperature anomaly is calculated using the national temperature time series. De-trending removes the effects of the long-term trend from the data to help see variability and emphasise short-term changes.
    We did not calculate annual average southern oscillation index values for the years where there were missing data for some months.
    The annual average southern oscillation index is for the year beginning 1 June and ending 31 May the following year because this a captures typical ENSO cycle. The de-trended temperature is for the calendar year, beginning 1 January and ending 31 December the same year. For example, the period 1909/10 in the graph represents 1 May 1909–31 May 1910 for ENSO and the calendar year 1909 for de-trended temperature.
    The unusual drop in temperature over 1992/93 is a result of the volcanic eruption at Mount Pinatubo, the Philippines.

    Definition and methodology

    Three climate oscillations affect New Zealand:

    • Interdecadal Pacific Oscillation (IPO) lasts 20–30 years
    • El Niño-Southern Oscillation (ENSO) occurs every 2–7 years and lasts around a year
    • Southern Annular Mode can last for several weeks, but changes phases quickly and unpredictably.

    The southern oscillation index (SOI), which measures ENSO, is calculated from the difference between the standardised surface air pressures at Tahiti and Darwin, Australia, and represents the strength of the tropical trade winds. This difference is represented by a positive or negative SOI. For example, a negative SOI represents below normal air pressure at Tahiti and/or above normal air pressure at Darwin, and weaker than normal trade winds.

    • A neutral climate phase corresponds with prolonged SOI monthly averages between −1.0 and 1.0.
    • A negative or El Niño phase of the SOI corresponds with prolonged monthly averages below −1.0.
    • A positive or La Niña phase corresponds with prolonged monthly averages above 1.0.

    Generally, the prevailing winds across the tropical Pacific run east to west, moving warm sea-surface water from the central tropical Pacific westward. This causes warm water to pool near Indonesia, where sea levels can be about 50cm higher than those in Ecuador (NIWA, nd). In the eastern Pacific, colder, nutrient-rich water wells up to the surface to replace the lost warm water. For ENSO this is a neutral phase.

    During an El Niño phase the prevailing east to west winds weaken. This leads to a reduced area of nutrient-rich surface cold water in the eastern Pacific tropics, resulting in less primary production from phytoplankton (very small photosynthetic organisms forming the main basis of marine food chains). In New Zealand, an El Niño phase in summer can be associated with increased winds from the west, leading to more rain in the west and drought in the east (NIWA, nd). An El Niño phase during winter can be associated with more winds from the south, cooling both the land and the sea (NIWA, nd).

    During a La Niña phase, the prevailing east to west winds strengthen, resulting in colder than normal sea-surface temperature in the eastern Pacific tropics. In New Zealand, a La Niña phase may mean warmer temperatures across the country (NIWA, nd). There may also be more north-easterly winds, more rain in north-eastern parts of the North Island and less rain to the south and southwest of the South Island (NIWA, nd). Mean sea level in the South Island can increase during a La Niña phase. However, this relationship is highly variable (Goring & Bell, 1999). ENSO accounts for less than 25 percent of variance in seasonal rainfall and temperature patterns at most sites in New Zealand (NIWA, nd).

    Data quality

    We classified El Niño Southern Oscillation as supporting information.

    Relevance

       This supporting information is a partial measure of the ‘Natural pressures’ topic.

    Accuracy

       The accuracy of the data source is of high quality. 

    See Data quality information for more detail.

     

    References

    Goring, DG & Bell, RG (1999). El Niño and decadal effects on sea-level variability in northern New Zealand: A wavelet analysis. New Zealand Journal of Marine and Freshwater Research, 33(4), 587–598. Retrieved from www.tandfonline.com.

    NIWA (nd). El Niño and climate forecasting. Retrieved 5 June 2015 from www.niwa.co.nz.

     

    Archived pages

    See El Niño Southern Oscillation (archived October 2017).

     

    Updated 19 October 2017

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