Changes in wave climate off Hiratsuka, Japan, as affected by storm activity over the western North Pacific

Abstract

Changes in wave climate off Hiratsuka during 1980-2003 are investigated on the basis of hourly observed wave and atmospheric data obtained at a tower 1 km off Hiratsuka in Sagami Bay, Japan. First, the interannual variability of significant wave heights (Hs) for the summer mean (June-August, SHs) is analyzed. A large difference between wave spectrum (E(f, t)) in high SHs years and that in low SHs years is found at the frequency of 0.09 Hz. It is revealed that the difference is related to the changes in enhanced tropical cyclone (with central air pressure below 980 hPa) activity in the western North Pacific. Second, the interannual variability of Hs for the winter mean (December-February, WHs) is investigated. It is found that there is a large difference between E(f, t) in high WHs years and that in low WHs years at the frequency of 0.11 Hz. It is revealed that the difference is related to the changes in southerly wind intensity (integral of southerly wind speed by hours) associated with extratropical cyclones. Third, the interdecadal variability of WHs is described. The time series of anomalies of WHs, smoothed by means of a 7-year moving average, is divided into three periods: 1980-1985, 1986-1996, and 1997-2003. The first empirical orthogonal function of 10-m meridional sea surface winds captures the interdecadal variability over the storm track. The smoothed WHs exhibits similar variation to that of the leading principal component of the winds. Copyright 2005 by the American Geophysical Union.