MetOcean Solutions scientist publishes long-awaited study on the Taranaki wave climate

MetOcean Solutions’ wave modeller, Dr Henrique Rapizo, is the lead author of the recently published study that fills a two-decade-long knowledge gap of the New Zealand (NZ) Taranaki regional wave climate. “Wave climate refers to long-term statistics of sea state parameters that provide critical knowledge for engineering applications, coastal management and safe operations at sea,” explains Henrique. “We carried out a high-resolution 40-year wave hindcast for the Taranaki waters and examined several aspects of the climate in the region.”  

Taranaki is a region of economic significance due to its offshore oil and gas production, which in turn accounts for the largest share of exports in Port Taranaki, making it the country’s third-largest export port. The region is also well-known by both domestic and international surfers for its many surf breaks. In fact, the Regional Policy Statement for Taranaki (2010) identified 80 surf breaks as being of “high-quality or high-value” for the region. 

Southern Royal Albatross cruising along, unaffected by the rough conditions in the South Taranaki Bight (Photo by Stine Sorensen). 

The idea for the study came to life during NZ’s 2020 lockdown response to Covid-19. Henrique was working on a MetOcean Solutions consultancy project, comparing the 2019 winter waves in Taranaki waters to the long-term regional climate. Being a wave expert and enthusiast, Henrique felt inspired to keep digging beyond the scope of the original project. He involved another wave expert and friend, Dr Victor Godoi from the Marine Science Institute (ICMar) at the Federal University of Maranhão in Brazil. “I went to him (Victor) with the idea because he had recently finished his PhD studying this region, and we started brainstorming. There was no official collaboration, grant or project, so it was really a hobby study - a lockdown inspiration,” recalls Henrique.  

The Simulating Waves Nearshore (SWAN) model was used for the 40-year multi-nest wave hindcast. A 4-level dynamic nesting approach was applied to downscale wave spectra from the global implementation of the WaveWatchIII model to the shallow nearshore areas. The hindcast data was then validated against observational data (from buoy and satellites) that indicated that the data was suitable for wave climate analysis. “Although previous studies have investigated wave conditions around the Taranaki region, they have not used high-resolution data covering the last four decades, as we did,” explains Victor. He adds; “the data we employed allows for a detailed spatial and temporal wave climate analysis, which is essential for engineering applications.” 

Bathymetry used in the model domain implemented in the hindcast simulation: (left panel) only part of the global domain is displayed so that the bathymetry can be visualised properly; (top right panel) larger SWAN domain “NZ north” and (bottom right panel) smaller SWAN domain “Taranaki.” The model output locations used for the analysis (Source: Rapizo et al. 2021) 

Henrique says that the model showed that “the region is periodically battered by large swell generated by persistent strong winds blowing over long fetches in the Southern Ocean.” The study also revealed that the average and extreme significant wave heights have increased over the past 40-years, with extreme waves showing the most pronounced trends. “Our results suggest that beaches in Taranaki have become more susceptible to erosion processes and multi-hazard effects by the combination of higher waves with storm surges and high tides.” says Victor. 

 Trends in annually averaged storm wave parameters over the period 1979-2018: (a) independent Hs peaks above the 90th percentile; and (b) number of events. Hatched areas show statistical significance at the 90% confidence level. Upper panels show examples of two sites where statistically significant trends are relatively high (Source: Rapizo et al. 2021). 

The study found that Taranaki experiences the largest storm peaks in autumn, but that storm events are most frequent in winter. Furthermore, the region is also affected by climate patterns, and waves typically increase during El Niño and negative Antarctic Oscillation phases. Despite the increasing size of storm peaks, it appears that the number of storms has decreased in recent decades. Victor explains that Taranaki is an interesting area because “waves from a wide range of directions approach the coast, and so you can clearly see the differences in wave conditions among the different coastline sectors.” 

Monthly climatology of storm significant wave height Hs (left panel) and number of storm events (above right figure), defined as those independent events whose Hs peaks are higher than the 90th percentile, for the period 1979-2018 (Source: Rapizo et al. 2021). 

The full abstract is provided below. 

Analysis of the New Zealand’s Taranaki regional wave climate using high-resolution modelling 

H.Rapizo (a), V.A.Godoi (b), J.Perez(a), T.Durrant(c), R.Guedes(c)

a) MetOcean Solutions, 5 Wainui Road, Raglan 3225, New Zealand 

b) Marine Science Institute (ICMar), Federal University of Maranhão, Av. dos Portugueses, 1966, São Luís, 65080-805, Brazil 

c) Oceanum Ltd, Raglan, New Zealand 

https://doi.org/10.1016/j.rsma.2021.101806 

Understanding the regional wave climate is essential for engineering applications. The last two decades have not been included in assessments of the wave climate of New Zealand’s Taranaki region, where the country exploits offshore oil and gas. To make up for this lack of assessment, we carried out a high-resolution long-term (1979–2018) wave hindcast and examined several aspects of the climate in the region. The hindcast data validation against buoy and satellite data showed good agreement and suitability for wave climate analysis. Seasonal wave distributions reveal a bi-modal pattern, composed of (1) more energetic and long period (13–15 s) westerly–southwesterly swells; and (2) shorter-period (∼8 s) southwesterly wind-seas. These signatures result from the region’s high exposure to swells generated by persistent strong winds blowing over long fetches in the Southern Ocean, and to local storm-associated wind-seas, respectively. The largest waves are found offshore, with mean Hs value reaching 2.83 m, 90th percentile 4.3 m, 99th percentile 6.1 m, and maximum 10.8 m. Storm wave monthly climatologies show that storm peaks are largest in the austral autumn, especially in May, while the number of events is the largest in July. Trends in Hs statistics reveal an increase over the past 40 years, with higher rates at higher percentiles. Storm peaks have also increased, by up to 8 cm/decade, whereas the number of storm events has decreased. In agreement with previous work, we found relationships between Hs and climate patterns. Waves get larger in Taranaki waters during El Nino events, as a result of stronger southwesterly winds, and during negative Antarctic Oscillation phases, as westerly winds displace northward.