Oceanic and coastal extreme waves

  • Image, Ocean and coastal extreme waves.

    See the latest oceanic and coastal extreme waves indicator.

    Extreme wave indexes estimate the occurrence of extreme wave events in coastal and oceanic waters. Our indexes use three significant wave height thresholds: four metres, six metres, and eight metres. Sea conditions are considered to be very rough when wave heights reach four metres. The Interislander ferry cancels all sailings when significant wave heights exceed six metres. In New Zealand, waves that would be considered extreme in the far north are not unusual in the far south. Extreme wave events can damage marine ecosystems and affect coastal infrastructure, ocean-based industries, and other human activities. At a global level we have a poor understanding of the likely impact of climate change on wave events.

    We classified Oceanic and coastal extreme waves as a national indicator.

    Key findings

    Image, Trend not assessed.  Trend not assessed

    Extreme waves occur more often in the south of New Zealand than in the north; and in coastal regions than in oceanic regions.

    • In 2015 in coastal regions, up to seven extreme wave events exceeded the eight-metre threshold in the South Island, but none exceeded this threshold for most of the North Island. In 2015 in oceanic regions, 18 such extreme wave events occurred in our southern-most region and one in the northern-most region. 
    • These patterns of extreme wave events (higher waves further south and in oceanic regions) occurred for all three wave-height thresholds. In 2015, 74 extreme wave events exceeded the four-metre height threshold in the southern oceanic region but only 30 such events occurred in the north ocean region. 
    • From 2008 to 2015, the only area that experienced no extreme wave events at any threshold of four metres or above was the Abel coastal area, in the north of the South Island. 

    Figure 1

    Number of waves exceeding 4 metres in oceanic regions, 2008–15 – interactive map

    Figure 2

    Number of waves exceeding 6 metres in oceanic regions, 2008–15 – interactive map

    Figure 3

    Number of waves exceeding 8 metres in oceanic regions, 2008–15 – interactive map

    Figure 4

    Number of waves exceeding 4 metres in coastal regions, 2008–15 – interactive map

    Figure 5

    Number of waves exceeding 6 metres in coastal regions, 2008–15 – interactive map

    Figure 6

    Number of waves exceeding 8 metres in coastal regions, 2008–15 – interactive map

    Figure 7

     Figure 8

    Definition and methodology

    Extreme wave indexes estimate the number of times a significant wave height exceeds one of three threshold values for at least 12 hours in 24 marine regions. The three wave-height thresholds are four metres, six metres, and eight metres.

    This indicator estimates the wave-height thresholds for each year from 2008 to 2015.

    Significant wave height is a measure of the ‘typical’ wave height in a place over a time period. It is four times the standard deviation of the water surface if, for example, you were to measure water moving up and down a jetty piling for an hour. The largest individual wave will typically have a height around twice the significant wave height.

    We use three wave-height thresholds because of the regional variation in extreme wave events. In general, the north experiences less exposure to consistently strong winds, and the waves generated by them, than the south. Four-metre tall waves are considered extreme in the northern-most parts of New Zealand but are more common in the south. For the southern-most parts of New Zealand, eight-metre waves better represent extreme wave events.

    We only include wave events where the relevant height threshold was exceeded for a minimum of 12 hours. This means that there was both a high tide (when overtopping and damage to coastal infrastructure, for example, is most likely) and a low tide during an event.

    We estimate extreme wave indexes for 24 regions around New Zealand, comprising 18 coastal and six oceanic regions. The 18 coastal regions cover the area from the shoreline to 100km from the coast and correspond to those used by the MetService for marine weather forecasts. The six oceanic regions cover New Zealand’s Exclusive Economic Zone.

    The indexes were generated using NIWA’s operational wave forecasting model (NZWAVE-12). This model has a 12km resolution and models wave heights using: 

    • wind from NIWA’s NZLAM-12 weather forecast model 
    • swell from NIWA’s global wave forecast model.

    The data series for extreme waves is too short to draw any conclusions about the frequency or intensity of wave events over time. There is also high variability in wave height between years.

    See Data quality information for more detail on the methodology used.

    Data quality

     Topic Classification   Relevance Accuracy 
     Climate and natural processes  National indicator

     Image, Direct relevance.
    Direct

     accuracy-high
    High

    References

    Gorman, R (2016). Extreme wave indices for New Zealand coastal and oceanic waters. NIWA Client Report HAM2016-014 prepared for the Ministry for the Environment.

     

    Published 27 October 2016

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