Wave hindcast for the New Zealand region: deep-water wave climate

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 (1979-98). 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 Medium-Range 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 ate 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 (150degreesE-170degreesW, 60degreesS-20degreesS). 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 Nino/Southern Oscillation. Positive correlations (R approximate to +0.2) were found off the north-east coast of the North Island, indicating a moderate tendency for increased wave heights there during La Nina conditions, whereas negative correlations were found south and south-west of New Zealand (R = 0.2), and in the Fiji/Vanuatu region (R approximate to +0.4), reflecting wave height enhancements in the El Nino phase.