Southern Annular Mode

  • Image, Southern Annual Mode.

    A consistent band of westerly wind flows across the Southern Hemisphere and circles the South Pole. The Southern Annular Mode (SAM) describes how this band moves, either north towards the equator (negative phase) or south towards Antarctica (positive phase). A negative phase typically causes increased westerlies, unsettled weather, and storms in New Zealand. A phase can last several weeks, but changes can be rapid and unpredictable.

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

    We classified Southern Annular Mode as supporting information.

    Key findings

    The Southern Annular Mode (SAM) has been increasing (becoming more positive) since 1970. However, the period 1887–2005 exhibited long-term oscillations, so we expect to observe increasing and decreasing phases. Over this period, SAM had an indeterminate trend at the 95 percent confidence level.

    • SAM values can vary widely over time periods of weeks or months.
    Figure 1

    Graph, Annual average southern annular mode, 1887–2016.

    Note: The index is annual averages from two datasets: 1) Lamont-Doherty Earth Observatory (LDEO), 1887–2005; 2) Climate Prediction Center (CPC) 1979–2016.
    Figure 2

    Graph, Monthly average southern annular mode index, 1979–2016.

    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 (SAM) can last several weeks, but changes phases quickly and unpredictably.

    The SAM is also known as the Antarctic Oscillation or the Southern Hemisphere Annular Mode. SAM is based on actual readings or modelled anomalies of high or low pressure over a period.

    The SAM index used here is the difference between the zonal mean sea-level pressure (the average sea-level pressure around a latitude circle) between the latitudes 40°S and 65°S. Phase changes in the SAM cannot be predicted more than a few days in advance. They can happen quickly and tend to continue for several weeks (NIWA, 2006).

    During a negative SAM phase, the Southern Hemisphere low-pressure belt moves north towards the equator. In New Zealand, this can cause more frequent westerly winds, unsettled weather, and storm activity over most of the country. Rainfall can increase and temperature can decrease along the west coast of New Zealand during a negative phase (NIWA, 2006). Over the southern oceans, there are relatively less frequent westerly winds and less storm activity. This pattern reverses during a positive phase, when the low-pressure belt moves south towards Antarctica.

    More positive SAM phases have occurred in recent decades. This is linked to the Antarctic ozone hole (NIWA, 2006). Ozone depletion strengthens winds around the polar region and increases pressures over New Zealand, which may result in weaker westerly winds over New Zealand in early spring when the ozone hole is prominent (NIWA, 2007).

    Data quality

    We classified Southern Annular Mode 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

    NIWA (2006). The Southern Annular Mode and New Zealand climate. Retrieved from www.niwa.co.nz.

    NIWA (2007). Stratosphere holds potential for predicting NZ climate. New Zealand Climate Update, 80(4). Retrieved from www.niwa.co.nz.

     

    Archived pages

    See Southern Annular Mode (archived October 2017).

     

    Updated 19 October 2017

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