Historically, wave data coverage of New Zealand's coast has been poor, particularly for directional records. With very few data sets available of more than 1 year's duration, it has been difficult to establish accurate wave climatologies. To help fill in the gaps in our wave records, the wave generation model WAM (WAve Model) has been implemented over a domain covering the south-west Pacific and Southern Oceans. The model has been used to hindcast the generation and propagation of deep-water waves incident on the New Zealand coast over a 20-year period (1979-98), using winds from the European Centre for Medium-Range Weather Forecasts (ECMWF). The resulting synthetic climatology is expected to provide a valuable tool for researchers and coastal planners. The hindcasts were compared with data from wave buoy deployments at eight representative sites around the New Zealand coast. With appropriate interpolation and correction for the effects of limited fetch and sheltering by land, the hindcast was found to provide a satisfactory simulation of wave conditions at sites on exposed coasts. Regression between measured and hindcast significant heights at the four deep-water sites (100-120 m) achieved scatter indices (ratio of root mean square error to mean) averaging 0.28. At the four shallower sites (30-45 m), the corresponding scatter index averaged 0.49, indicating that for regions of complex coastal topography, deep-water spectra do not represent inshore conditions well. Wave spectra can be considerably modified by the processes of refraction and shoaling. To address these effects, nearshore wave transformations in the outer Hauraki Gulf were investigated using the shallow water model SWAN (Simulating WAves Nearshore), which was used to derive wave statistics at nearshore locations from deep-water wave spectra obtained from the hindcast. The simulations were validated using data from an inshore site in 30 m water depth at Mangawhai on the north-east coast of the North Island. Use of the nested model improved the agreement between model and measured significant wave height, decreasing the scatter index from 0.50 to 0.26. The suite of tools provided by the hindcast and localised, shallow water models can provide accurate new wave information for most of New Zealand's coastline.

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.

Wave data derived from radar altimeters carried on four satellite missions are combined into a wave climatology for New Zealand waters. These data provide extensive observations of wave conditions around New Zealand, where the paucity of measurements has previously hindered definition of the wave climate. The data span the period 1985 to the present with the exception of a 2-year gap in 1989-91. The spatial distribution of the long-term mean of significant wave heights (SWH) indicates a strong latitudinal variation in the south-west Pacific, with values of over 4 m at latitudes of 50-60°S and under 2.5 m towards the tropics. The shadowing of New Zealand is quite marked; a result of the dominant contribution of south-westerly wave events. The annual range of the mean SWH also varies over the region; within 0.6 m in the north and 1.3 m in the south. A principal component analysis of the monthly anomalies in mean SWH identifies spatial patterns of variation. Some components vary with the local wind more than others suggesting that some anomalies are associated with wind sea and some with swell. Some patterns also appear to vary with the Southern Oscillation Index and can be related to the wind anomalies associated with El Nino events. Frequency distributions of SWH are also determined, and it is noted that in the north of the region the spatial pattern of the high waves differs considerably from the means. M99071

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A procedure has been developed for estimating the wave climate in New Zealand waters. The key elements are the specification of surface wind fields and the diagnosis of wave conditions using a numerical wave model. Wave data have been generated for a period in 1989 and the deduced significant wave heights have been analysed. The distributions at a number of selected sites indicate wave features consistent with existing knowledge of the longer-term wave climate-despite differences between wind characteristics for this season and the longer-term norm as derived from ship observations. The wave-fields were partitioned into wind-sea and swell components and the probability distributions for each feature were also found to be consistent with longer-term studies.