An assessment of the ocean wave climate of New Zealand as represented in Kidson's synoptic types

Abstract

ABSTRACTRecent studies have shown large increases in global significant ocean wave height since the genesis of satellite‐borne altimeter wave observations in the mid‐1980s. The island nation of New Zealand (NZ) has many coastal communities and ecosystems which are sensitive to changes in ocean wave climate. In this article we document regional ocean wave height in the seas around NZ, employing significant wave height output from the ECMWF (European Centre for Medium‐Range Weather Forecasts) Re‐analysis (ERA)‐Interim reanalysis. Initially, we perform a comparison against independent satellite altimeter observations in deep waters around NZ. Results indicate that the ERA‐Interim well represents wave heights in this region, although there is some underestimation of extremes. We apply the Kidson types, a synoptic climatology of NZ lower atmospheric circulation to the wave fields, finding that mean swell and wind‐sea heights greatly depend on local weather conditions. High waves in the south of the country coincide with local strong south‐westerly winds, while those in the north are dominated by distantly generated swells, punctuated by strong local events.Linear trend analysis of ocean wave height shows increases that are statistically significant in many areas. Trends vary by location and season, are larger in winter than summer, and appear to change according to different seasonal forcings. We find that summer wave trends are congruent with mean sea‐level pressure gradient trends in large sectors of the Southern Ocean, suggesting that distant forcing is important. In summer, the height of swells has increased over the period examined, while those of locally generated wind‐sea have decreased. This is consistent with the movement south and intensification of the near‐surface westerly jet.In winter, trends are forced via intensification and shifts in the dominant NZ wind patterns. Pressure gradient congruent trends are large only in the vicinity of NZ, and both the heights of swell and wind‐sea have increased over the period.