GEOPHYSICAL RESEARCH LETTERS, VOL. 9, NO. 12, PAGES 1305-1308, DECEMBER 1982 EVIDENCE OF COASTAL SEICHE EXCITATION BY TIDE-GENERATED INTERNAL SOLITARY WAVES G. S. Giese, 1 R. B. Hollander, 1 J. E. Fanchef, 2 & B. S. Giese 3 1provincetown Center for Coastal Studies, Provincetown, MA 02657 2National Ocean Survey, NOAA, Rockville, MD 20852 3Department of Oceanography, University of Washington, Seattle, WA 98195 Abstract. Coastal seiches at Magueyes Island, Puerto Rico, occur in distinct fortnightly groups with maximum seiche activity following new and full moon by 7 days. The size of the fort- nightly groups depends upon the amplitude of the preceding spring tide and upon the Caribbean sur- face water temperature. Temperature records from the upper thermocline on the slope seaward of the shelf edge off Magueyes Island indicate the ar- rival of semidiurnal packets of negative internal solitary waves. We suggest that these internal waves are generated by tides in the southeastern Caribbean Sea and that they are responsible for exciting the coastal seiches. This hypothesis is supported by the characteristics of coastal seiching at Puerto Princesa on Palawan Island in the Philippines which indicate excitation by the Sulu Sea internal waves reported by Apel,Holbrook and Tsai (1980). Introduction Coastal seiches are long, standing waves which occur when the water of harbors, bays or open shelves is bodily disturbed. They are commonly recorded at tide stations as sea level oscilla- tions with periods ranging from minutes to hours. Coastal seiches may result from direct disturbances by meteorological or seismic forces at the surTace or bed of the coastal basin (Wil- son, 1966). More frequently, however, they are generated by long waves reaching the coast from the open sea (Miles, 1974). Such seiche-produc- ing long waves can be generated by distant mete- orological disturbances at the sea surface (Munk, 1962) and by seismic disturbances at the sea bed (Matuzawa, Kanbara and Minakami, 1933). We report here evidence that coastal seiches can be initiated by an additional open sea wave mechanism: excitation by internal solitary waves generated by tidal flow of stratified water over bottom discontinuities. First we summarize the results of our studies of seiches on the south coast of Puerto Rico, which lead to the hypothe- sis that seiches can be excited by internal waves. Following that, we describe a test of the hypothesis through a study of tide records from Puerto Princesa, the Philippines, a coast known to be impinged upon by tide-generated solitary waves (Apel, Holbrook and Tsai, 1980). The results presented here are drawn from a many-faceted study which has spanned 14 years. A more complete and extensive presentation of the results is in preparation. Copyright 1982 by the American Geophysical Union. Paper number 2L1552. 0094-8276 / 82 / 002L-155253. O0 Seiche Observations in Puerto Rico: Developing the Hypothesis Seiches off southwestern Puerto Rico were first described by Harris (1907) whofound os- cillations with a mean period of 45 minutes at Guanica Harbor (Fig. la). Wefound oscillations with a slightly longer period (50 minutes) at nearby Magueyes Island (Fig. la, b and c). Using a variation of Merian's formula (Defant, 1961) we calculated the period of the fundamental free oscillation of the shelf at Magueyes Island to be 51 minutes. While the usual height of the Magueyes seiches is in the range of 3-5 cm, os- cillations have been recorded with heights up to 31 cm which may be compared to the mean (diurnal) tidal range of 21 cm (U.S. Department of Com- merce, 1982). We examined the Magueyes Island tide records from 1955 through 1971 and found usable records for 4,780 days, or the equivalent of 13.1 years. In tabulating the daily number of oscillations (with heights greater than 3 cm) we noted a fortnightly pattern. Therefore, we plotted the number of oscillations per day against the "age" of the moon, expressed in days after new and full moon. The results (Fig. ld) indicate that the seiche activity is highly correlated with lunar phase and that the maximum activity occurs approximately 7 days after new and full moon: that is, 7 days after the time at which the semidiurnal tide-producing force reaches a maximum. This result surprised us because of the long phase lag and because the tides at this station are predominantly diurnal. (The diurnal tide varies principally with lunar declination. ) At Magueyes Island the major semidiurnal tidal constituents, M2 and S 2, have a combined height (2 x sum of amplitudes) of only 2.9 cm (Zetler and Cummings, 1972), which is less than the minimum size of the seiches included in our data. We studied further the relationship between seiche activity and the semidiurnal tide by ex- amining the number of oscillations which oc- curred during each fortnightly period following new and full moon. An average of 25 oscilla- tions occurred per fortnight. This is illus- trated by the area beneath the curve in Fig. ld. However, on any given fortnight, the total num- ber of oscillations varied from 0 to as many as 149. Figure 2a illustrates that the mean number of oscillations per fortnight varies inversely with the absolute difference in time (days) be- tween the preceding new and full moon and the perigee nearest to it. The greatest number of oscillations occurs during fortnights following new and full moons near perigee, which enhances the spring tides (Mariner, 1926). This result 1305