Wave height analysis from 10 years of observations in the Norwegian Sea

摘要

Large waves pose risks to ships, offshore structures, coastal infrastructure and ecosystems. This paper analyses 10 years of in-situ measurements of significant wave height (H_s) and maximum wave height (H_(max)) from the ocean weather ship Polarfront in the Norwegian Sea. During the period 2000 to 2009, surface elevation was recorded every 0.59 s during sampling periods of 30 min. The H_(max) observations scale linearly with H_s on average. A widely-used empirical Weibull distribution is found to estimate average values of H_(max)/H_s and H_(max) better than a Rayleigh distribution, but tends to underestimate both for all but the smallest waves. In this paper we propose a modified Rayleigh distribution which compensates for the heterogeneity of the observed dataset: the distribution is fitted to the whole dataset and improves the estimate of the largest waves. Over the 10-year period, the Weibull distribution approximates the observed H_s and H_(max) well, and an exponential function can be used to predict the probability distribution function of the ratio H_(max)/H_s. However, the Weibull distribution tends to underestimate the occurrence of extremely large values of H_s and H_(max). The persistence of H_s and H_(max) in winter is also examined. Wave fields with H_s> 12 m and H_(max) > 16 m do not last longer than 3 h. Low-to-moderate wave heights that persist for more than 12 h dominate the relationship of the wave field with the winter NAO index over 2000-2009. In contrast, the inter-annual variability of wave fields with H_s > 5.5 m or H_(max) > 8.5 m and wave fields persisting over ~ 2.5 days is not associated with the winter NAO index.