Ocean storms

  • Image, Ocean storms.

    The ocean storm index estimates the number of days in a year when wind speeds exceed gale and storm force on the Beaufort Scale. In a gale, sea conditions are rough and waves can be over six metres high. In a storm, waves can be over 10 metres high. To put this into context, on land a near gale would make walking difficult, and a storm would cause some damage to roofs, chimneys, and trees. Climate change could lead to changes in the frequency and intensity of storms. More frequent and intense storms will likely be a stressor for habitats and species.

    We classified Ocean storms as a national indicator.

    Key findings

    Image, Trend not assessed.  Trend not assessed

    In New Zealand waters, the number of days each year when estimated wind speeds exceed storm thresholds is highly variable. From 1979 to 2015, the southern ocean region experienced more days where storm thresholds were exceeded (5.6–17.8 days) than the north-eastern region (0.2–1.8 days).

    From 1979–2015: 

    • the southern ocean region experienced an average of 10.9 days a year where estimated wind speeds exceeded storm force (Beaufort Scale 10 (BF10)), compared with an average of 0.9 such days a year in the northeast
    • the south experienced an average of 106.9 days a year where estimated wind speeds exceeded gale force (BF8) and the northeast experienced an average of 18.2 gale days a year
    • across our exclusive economic zone, there were on average 55.24 days a year where BF8 was exceeded and 4.34 days a year where BF10 was exceeded.

    In 2015:

    • 17.8 days exceeded storm force in the southern ocean region and 0.8 days in the northeast.

    Figure 1

    Average number of days wind speed exceeded storm force (Beaufort Scale 10), 1979–2015

    Note: Data are for six oceanic regions. The number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds are summed. The total hours are then divided into the number of days where the relevant wind speeds are reached, for each year.
    Source: University of Canterbury

    Figure 2

    Average number of days wind speed exceeded gale force (Beaufort Scale 8), 1979–2015

    Note: Data are for six oceanic regions. The number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds are summed. The total hours are then divided into the number of days where the relevant wind speeds are reached, for each year.
    Source: University of Canterbury

    Figure 3

    Number of days when wind speed exceeded storm force (Beaufort Scale 10), 2015

    Note: Data are for six oceanic regions. The number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds are summed. The total hours are then divided into the number of days where the relevant wind speeds are reached, for each year.
    Source: University of Canterbury

    Figure 4

    Number of days when wind speed exceeded gale force (Beaufort Scale 8), 2015

    Note: Data are for six oceanic regions. The number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds are summed. The total hours are then divided into the number of days where the relevant wind speeds are reached, for each year.
    Source: University of Canterbury

    Figure 5

    Note: Data are for six oceanic regions and our exclusive economic zone (EEZ). The number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds were summed. The total hours were then divided into the number of days where the relevant wind speeds were reached, for each year.

    Figure 6

    Note: Data are for six oceanic regions and our exclusive economic zone (EEZ). The number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds are summed. The total hours are then divided into the number of days where the relevant wind speeds were reached, for each year.

    Definition and methodology

    The ocean storm index estimates the number of days that wind speeds exceed gale and storm force on the Beaufort Scale. The Beaufort Scale is a widely used international classification that rates sea conditions from 0 (calm) to 12 (hurricane). We report on estimated wind speeds broken down to:

    • gales – measure 8 on the scale, have rough sea conditions with wind speeds of approximately 62–74 km per hour and wave heights of 5.5 metres 
    • storms – measure 10 on the scale, have wind speeds of approximately 89–102 km per hour and wave heights of 9–11.5 metres (McDonald & Parsons, 2016).

    Wind gust information is from the ERA-Interim reanalysis project (Dee et al, 2011), which uses observational data and model results. The wind data was for the years 1979 to 2015 and covered the New Zealand exclusive economic zone (EEZ), divided into six oceanic regions and scaled to the area covered.

    To calculate the ocean storm index, the number of three-hour time periods during which estimated wind speeds exceeded one of the thresholds are summed. The total hours are then divided into the number of days where the relevant wind speeds were reached, for each year.

    We were unable to assess the data for a trend because of the high variability between years. Data may have limits including high inter-annual variability and the occurrences of strong individual events that may influence results.

    For more information on the methodology covered please see Ocean storm index (McDonald & Parsons, 2016) and data quality information.

    Data quality

     Topic Classification   Relevance Accuracy 
     Sea level, temperature, and circulation  National indicator

    Image, Direct relevance.

    Direct

    accuracy-high
    High

    References

    Dee, DP, Uppala, SM, Simmons, AJ, Berrisford, P, Poli, P, Kobayashi, S, ... Vitart, F (2011). The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Quarterly Journal of the Royal Meteorological Society, 137, 553–597. http://dx.doi.org/ 10.1002/qj.828. Retrieved from http://onlinelibrary.wiley.com.

    McDonald, A & Parsons, S (2016). Ocean storm index. Prepared for the Ministry for the Environment. Christchurch: Department of Physics and Astronomy, University of Canterbury.

    Published 27 October 2016

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