Wave hindcast for the New Zealand region: Deep‐water wave climate

Abstract

The wave evolution model WAM (WAve Model) has been implemented for the New Zealand region and used to simulate the generation and propagation of deep-water waves over a 20-year period . The model extends to include the relevant generation areas of the south-west Pacific and Southern Oceans. Input winds for the model were derived from the European Centre for MediumRange Weather Forecasts (ECMWF). The resulting synthetic wave climatology will provide a valuable tool for researchers and coastal planners, as it will help fill gaps in the available wave information for New Zealand waters. In this paper the hindcasts are described, and comparisons are made with wave height data from the altimeters flown on the TOPEX/ Poseidon and ERS1 and ERS2 satellite missions. Long-term mean significant wave heights from the hindcast were generally 0.3-0.5 m lower than values from "buoy-equivalent" altimeter data throughout the comparison region (150°E-170°W, 60°S-20°S). Hindcast distributions of significant wave height occurrence matched satellite data at the lowest wave heights and above the peaks of the distributions, but tended to overestimate occurrence below the peak and underestimate the occurrence of the largest wave events. The hindcast was then used to characterise the wave climate of the New Zealand region. Some prominent features noted were the large mean heights (3.6 m) in the high latitudes of the Southern Ocean, associated with strong prevailing westerlies. North of this band, waves largely propagate towards the north-east, with diminishing mean heights, further attenuated by the blocking effect of the New Zealand landmasses. This results in mean wave heights of c. 2 m in offshore waters north-east of New Zealand. Annual cycles of mean wave height with a range of c. 1 m were identified throughout the region. These were found to have minima in summer (December/January), and either unimodal maxima in winter (June/July) for tropical and temperate latitudes, or bimodal maxima (May and August) in southern waters. Longer-term variations were also noted in the form of correlations with the El Niño/Southern Oscillation. Positive correlations (R +0.2) were found off the north-east coast of the North Island, indicating a moderate tendency for increased wave heights there during La Niña conditions, whereas negative correlations were found south and south-west of New Zealand (R -0.2), and in the Fiji/Vanuatu region (R +0.4), reflecting wave height enhancements in the El Niño phase.

Fundamental and higher‐mode Rayleigh wave characteristics of ambient seismic noise in New Zealand

“…Microseisms from the Wairarapa are most intense when either eastward-tracking storms from the Tasman Sea impinge on the narrow (<15 km) Wairarapa shelf, or when Southern Ocean swell moves directly onto the shelf (observed from WAVEWATCH III hindcasts [Tolman, 2002]). ENE swell is prominent in the Bay of Plenty [Gorman et al, 2003] and can have maximum periods of about 16 s [Pickrill and Mitchell, 1979]. The signals from the Bay of Plenty (Figure 2d) occur shortly after trans-Tasman storms have passed across northern New Zealand (from WAVEWATCH III).…”

“…The mean of each trace is zeroed on the azimuth of the wave site location relative to the array center, with colors matching the azimuths of Figures 1 and 2e. The 50 m isobath was chosen as a reasonable depth at which high amplitude microseism excitation would be expected, though depths may actually be significantly greater than this since New Zealand has deeper waters adjacent to the coast and longer period waves than many other regions [Gorman et al, 2003]. Similar wave characteristics would still be expected at each azimuth if the excitation depth were greater.…”

“…[3] New Zealand's geographic isolation and $15,000 kmlong coastline expose it to a particularly energetic ocean, producing a high-amplitude seismic noise field [Pickrill and Mitchell, 1979;Gorman et al, 2003], and several New Zealand noise studies relating ambient noise spectra/ amplitudes to wave spectra/heights have been conducted previously [Kibblewhite and Ewans, 1985;Tindle and Murphy, 1999]. The energetic noise field makes the New Zealand region a suitable location for using ambient seismic noise for seismic tomography studies [Lin et al, 2007]; however, the long coastline and complex ocean regime that generate this noise field also result in more complex spatial and temporal noise distributions than may be observed in many other regions (e.g., continental USA).…”

Lithological control on the elevation of shore platforms in a microtidal setting

“…The wave climate for the Shag Point region is summarized from data output from the New Zealand regional hindcast of deep-water wave conditions over the period 1979-1998(Gorman et al, 2003a, 2003b. Wave spectra were interpolated from model output spectra at selected sites, and deep-water wave data were refracted toward the 10 and 30 m isobaths off the coast of Shag Point assuming shore parallel isobaths.…”

Variability and trends in the directional wave climate of the Southern Hemisphere

“…Description of the wave climate of the SH is limited to a few regional studies (e.g. Short and Trenaman, 1992;Laing, 2000;Scott et al, 2002;Gorman et al, 2003;Hemer et al, 2008), and global studies, where the attention paid to the SH is limited (e.g. Young, 1999;Cox and Swail, 2001;Sterl and Caires, 2005).…”