Oceanic and coastal extreme waves
What is measured
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. Each is measured for a period of at least 12 hours.
Why it is important
Extreme wave events can damage marine ecosystems and affect coastal infrastructure, ocean-based industries, and other human activities.
Changing wave characteristics can have impacts on natural systems, as most coastal and near-shore biological communities can be damaged or destroyed by extreme wave action (Ummenhofer & Mehl, 2017).
In another example, extreme waves can disrupt ferries, such as those crossing the Cook Strait. Sailings are often cancelled when significant wave heights exceed six metres.
It is important to report on extreme waves to gain greater insight into their frequency, particularly as sea level and storm surges are projected to increase and can compound wave effects.
Key findings
- In 2017 in coastal regions, 17 extreme wave events exceeded the eight-metre threshold; 15 of these were in the South Island, one in the North Island (Castlepoint), and one in the Chatham Islands.
- In 2017 in oceanic regions, 16 extreme wave events exceeded the eight-metre threshold, whereas 37 occurred in 2016.
- Coastal extreme waves exceeding the four-metre threshold were recorded for the first time in the Abel (Tasman) coastal region in 2016, and again in 2017.
- Below the regional scale (at approximately 8000 square kilometre resolution), trends in the number of extreme wave events per year were observed between 2008 and 2017. These show:
- increasing occurrences of extreme wave events at the six- and eight-metre thresholds to the east and south of New Zealand
- decreasing occurrences at the six-metre threshold on the North Island’s west coast and to the north of Bay of Plenty.
Where this data comes from
NIWA
View data tables
Ocean acidity and climate change data
Related indicators
Related content
Technical report
About the data
An extreme wave event is defined as a continuous 12-hour period during which the significant wave height equals or exceeds one of three height thresholds: four, six, or eight metres (Gorman, 2018).
These thresholds were set based on the regional variation in extreme wave events. In general, the north of New Zealand is less exposed to consistently strong winds (and the waves generated by them) than the south. This means four-metre-tall waves are considered extreme in the northern-most parts but are more common in the south. For the southern-most parts of New Zealand, eight-metre waves better represent extreme wave events. A threshold of four standard deviations is used because wave height variability is common.
We use a duration threshold of 12 hours as it allows semi-diurnal tides to cover their full low water to high water range. This means that there was both a high tide (when, for example, overtopping and damage to coastal infrastructure 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 60 nautical miles from the coast and correspond with those used by MetService for marine weather forecasts. The six oceanic regions are intended to cover the area within New Zealand’s Exclusive Economic Zone (EEZ). For further information on these regions, see Gorman (2018).
From 2008 to 2015, the indexes were generated using NIWA’s operational wave forecasting model (NZWAVE-12) (Gorman, 2016). This model has a 12-kilometre resolution and models wave heights. The indices for 2015 to 2017 have been generated using NIWA’s NZWAVE-8 model. This model has an 8-kilometre resolution. Both models are calculated using:
- wind from NIWA’s NZLAM-12 weather forecast model
- swell from NIWA’s global wave forecast model.
2015 extreme wave data is provided in both NZWAVE-12 and NZWAVE-8 format to allow comparison between the model resolutions. Analysis found that the different models are generally comparable.
For the purpose of displaying data on the map, data have been aggregated into approximately 8000 square kilometre hexagons using the mean number of annual exceedances from the original pixels. The long-term mean was then calculated for all years within each hexagon.
In addition, we tested for trends in the annual number of exceedances over time within each hexagon using the Mann Kendall test. This test is non-parametric and assumes that there is a monotonic trend over the nine years. But with the short time period of available data, it is too early to say whether this trend is environmentally robust as longer-term climate cycles, such as the Interdecadal Pacific Oscillation (IPO), which may have influenced the trend results. For completeness, all trends are displayed, including those considered non-significant.
Due to model constraints, the ocean regions do not cover the full extent of our EEZ. Some additional exceedances could be reasonably expected with additional coverage, particularly for the southern region which experiences the highest-energy wave climate.
The data is based on modelling used to quantify the number of storm events exceeding the measured thresholds for wave height and duration. A 12-hour duration threshold has been used for this model. This means that the data takes into account ongoing extreme wave events over this 12-hour period but will not necessarily measure one-off rogue or short-term wave/storm events which can still cause significant damage and disruption to our marine environment and activities.
Data quality
The accuracy of the data source is of high quality.
Oceanic and coastal extreme waves are a direct measure of the ‘Climate and natural processes’ topic.
Stats NZ and the Ministry for the Environment must report on topics related to the five environmental domains: air, atmosphere and climate, fresh water, land, and marine. These topics identify key issues within each domain.
Topics for environmental reporting describes the topics for each domain.
Data quality information has more information about the criteria we use to assess data quality.
References
Gorman, R (2016). Extreme wave indices for New Zealand coastal and oceanic waters. Prepared for the Ministry for the Environment. NIWA Client Report No: HAM2016-014.
Gorman, R (2018). Extreme wave indices for New Zealand coastal and oceanic waters updated to 2017. Prepared for the Ministry for the Environment. NIWA Client Report No: 2018325HN.
Ummenhofer, CC, & Meehl, GA (2017). Extreme weather and climate events with ecological relevance: A review. Philosophical Transactions of the Royal Society B: Biological Sciences, 372(1723). https://doi.org/10.1098/rstb.2016.0135.
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