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Seasonal and interannual variability of M_2 tide is investigated by the harmonic analysis of the sea level observed at 63 tide stations along the coast of Japan for about 40 years. The amplitude of M_...2 tide turned out to have a distinct seasonal modulation that differs from one area to another. The approximate magnitude of the seasonal modulation relative to the long-term average of the amplitude of M_2 tide is (a) 4 % along the coast of the Tsushima Strait, (b) 10 % along the coast facing the Japan Sea, (c) 2-3 % along the northeast coast of Honshu facing the Pacific, (d) 2 % in Ariake Bay, and (e) 6% in Osaka Bay. This means that, in a crudest sense, the seasonal variation of the amplitude of M_2 tide is proportional to its long-term average. The amplitude attains its maximum in (a) March along the coast of the Tsushima Strait, (b) February along the coast facing the Japan Sea, (c) August along the northeast coast of Honshu facing the Pacific, (d) June in Ariake Bay, and (e) December in Osaka Bay. Seasonal variability of M_2 tide is not clear around the Ryukyu Islands and along the coast from southeast of Honshu to south Kyusyu facing the Pacific. As regards interannual variability, the amplitude of M_2 tide mostly showed a long-term decrease at a rate of 0.5 % a year or less in the past 20 years. This trend of decrease has been reported in recent articles on the tide of Ariake Bay, Tokyo Bay, and so on. Moreover that trend was often overlaid by a conspicuous though slight modulation of 4 % or less with periods of 10 to 20 years. Similar decadal oscillations are observed for the mean sea level as well, which has been inferred to be associated with a decadal change of climate on a global scale. The decadal modulation of the amplitude of M_2 tide was found in antiphase with that of the mean sea level at more than 70 % of the tide stations analyzed here. As a whole, various characteristics of the amplitude of M_2 tide, either mean or variability, have one extreme along the coast facing the Japan Sea and the other along the coast facing the Pacific.続きを見る
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