IL NUOVO CIMENTO VOL. 18 C, N. 1 Gennaio-Febbraio 1995 Do Tidal Harmonic Constants Depend on Mean Sea Level? An Investigation for the Gulf of Trieste. F. CRISCIANI (1), S. FERRARO(1) and B. PATTI(2) (1) Istituto Sperimentale Talassografico del CNR - Trieste, Italy (2) Istituto di Tecnologia della Pesca e del Pescato del CNR Mazara del VaUo, Trapani, Italy (ricevuto il 13 Dicembre 1993; revisionato il 4 Luglio 1994; approvato 1'11 Luglio 1994) Summary. -- The correlation between the mean sea level and the main seven tidal harmonic constants is investigated on the basis of 648 monthly observations, from 1939 to 1992, taken in the Gulf of Trieste. Some statistically significant correlations are singled out when the considered time interval is divided into classes of 54 corresponding months. These results allow us to conjecture that the observed variability of the harmonic constants is partially originated by the monthly mean-sea-level fluctuations, modulated by the meteorological regime. PACS 92.10.Hm - Surface waves, tides, and sea level. 1. - Introduction. One of the traditional research activities of the Istituto Talassografico of Trieste, since its origin, is the sea level monitoring in the Gulf of Trieste and the subsequent data processing, both for standard and specific investigations. The sea level elevation is measured by means of an analog tide-gauge (made by Seibt-Fuess and recently Busum-Ott) which is periodically calibrated by a hydrometer. The sea level heights are referred to the zero of the tide-gauge, which is located 233.315 cm below the IGM (Geographic Institute of the Italian Armed Forces) horizontal datum point n. 39. The station location was not subject to position changes, is far from areas of strong tectonic movements and is not exposed to fresh-water invasion. A detailed descrip- tion is found in [1]. Monthly and yearly mean-sea-level heights are published by the Permanent Service for Mean Sea Level at Birkenhead (UK). A basic procedure that connects hourly data with the astronomic forecast is the evaluation of the harmonic constants, by means of which the forecast tide is expressed as linear combination of sinusoidal components. By following the Doodson method [2] the harmonic constants are deduced on the basis of the hourly data of 29 consecutive days of observations and consist of a given set of amplitudes and phases that are the coefficients of the expansion of the sinusoidal components of the astronomical tide. Every coastal situs has a typical set of harmonic constants. In the case of the Gulf of Trieste, there are seven main components which can be easily computed by means of a suitable 15