Causation of Large-Amplitude Coastal Seiches on the Caribbean Coast of Puerto Rico

Abstract

Sea-level oscillations at supertidal frequency with amplitudes of the order of the mean tidal range have been reported from the Caribbean coast of Puerto Rico. Analysis of a 10-year time series of digital tide data from Magueyes Island, Puerto Rico, demonstrates that sea-level variance at the fundamental normal mode (sciche) frequency of the shelf has a pronounced fortnightly distribution with a maximum occurring 6?7 days after new and full moon. The sieche variance also shows a bimodal seasonal distribution with an inverse relationship to easterly wind stress. It is argued that the sciches are excited by internal waves generated by strong tides in the southeastern Caribbean. Support is provided by airborne radar imagery showing sea-surface patterns suggesting the presence of internal waves near the southern Aves Ridge, and by the results of two field experiments, carried out during times when large-amplitude sciches were expected, to search for evidence of internal wave forcing near the shelf break. During the first experiment, large negative-amplitude, pulse-like internal waves were recorded 6 km seaward of the shelf break during a period of strong sciche activity. Such pulses were not observed during the second experiment. However, high-frequency temperature variance 2.3 km seaward of the shelf break, possibly resulting from internal surf, increased with depth and reached a maximum 6?7 days following new moon, again suggesting the presence of internal waves. The 10-year time series analysis shows that large tides are necessary, but not sufficient, to generate high sciche activity. This is supported by the two field experiments; during the first, large-amplitude sciches occurred as expected, while during the second experiment they did not. We suggest that this behavior is related to variations in stratification which in turn alter the energy transfer from tides to seiches.