Analyses of hourly measurements of ocean wave heights along the U.S. East Coast, collected since the 1970s by three buoys of the National Data Buoy Center, document a progressive increase during the summer months when hurricanes are most important to wave generation. In contrast, the waves measured during the winter, generated by extratropical storms, have not experienced a statistically significant change. Summer waves with significant wave heights greater than 3 m, which can be directly attributed to specific hurricanes, have increased at a rate of 0.059 m/y (1.8 m in 30 years) according to records from buoy 41002 offshore from Charleston, South Carolina, with a lower rate of 0.024 m/y (0.7 m in 30 years) recorded by the Cape Hatteras buoy (41001); both trends are statistically significant at the 90% level. A still lower rate is found for the Cape May buoy (44004), 0.017 m/y, suggesting that there is a systematic latitude variation. Histograms of the ranges of significant wave heights measured during the hurricane season show that the most extreme occurrences during the 1996–2005 decade are both higher and more common than occurred 30 years ago, at the beginning of buoy measurements, having increased from about 7 m to higher than 10 m. The waves recorded by the buoys depend on the annual numbers of hurricanes that followed tracks northward into the central Atlantic, how close their tracks approached the buoys, and the intensities (categories) of those hurricanes. Examinations of the storms that have occurred since 1980 indicate that the primary explanation for the progressive increase in wave heights has been an intensification of the hurricanes, with increased numbers of storms a contributing factor.
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Vol. 2008 • No. 242