Extreme waves in New Zealand waters

A detailed climatology of extreme wave events for New Zealand waters is presented, in addition to estimates of significant wave height (Hs) for up to a 100-year return period. Extreme events were explored using 44 years (1958-2001) of wave hindcast data. Comparisons to buoy data at three locations around New Zealand showed negative biases in the model, which nevertheless provided a suitable basis for trends, spatial distribution, and frequency analyses. Results indicate some similarities to patterns previously shown in the mean wave climate, with the largest waves found in southern New Zealand, and the smallest ones observed in areas sheltered from southwesterly swells. The number of extreme events varies substantially throughout the year, while the differences in intensity are more consistent. Events occur more/less frequently in winter/summer months. The greatest mean annual variability of extreme Hs is found on the north coasts of both the North and South Islands, where more locally-generated storms drive the extremes. The interannual variability is largest along the north coast of the country and on the east coast of the South Island, suggesting relationships with La Niña-like effects and the Southern Annular Mode, respectively, which past work showed to be important drivers in these regions. Moreover, the known trend for a more positive Southern Annular Mode may explain the increasing number of extreme events shown in our study.