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The astronomical tide and the residual level

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The astronomical tide

The astronomical tide is caused by the gravitational action of the Moon and the Sun on the mass of water, and depends on the position of the Earth with respect to the two stars. Since the relative motion of these celestial bodies is known with great precision, the evolution of the tide due to this interaction is also deterministically predictable, unlike, for example, meteorological forecasts.

On the coasts of the Northern Adriatic the tide occurs mainly with two high and two low daily (semi-day type). In coincidence with the new moons and full moons (syzygies) this phenomenon is very conspicuous and the maximum excursions are reached. In the vicinity of the first and last quarter of the lunar phases (quadrature) the tide presents itself with little pronounced excursions, showing only one high and one low during the day (diurnal type).

In this regard, it is important to note that the horizontal movement of the water mass is associated with the vertical motion of the sea level with a speed proportional to the amplitude of the tide (ebb and flow). This displacement affects the entire body of water and therefore, especially in coastal and shallow water areas such as the Gulf of Trieste, the exchange of water is continuously guaranteed. This exchange, being linked to the speed of the tidal currents, reaches its maximum in periods of syzygies and the minimum in periods of squaring.

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Example of astronomical tide prediction curves

The residual level

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The residual level (meteorological tide) is given by the difference between the observed level and the astronomical tide. Its variations depend on the meteorological forcing, mainly wind and atmospheric pressure. In this regard, the graph proposed below shows three curves of which the power plant represents the average sea level, for each day of the year, obtained starting from the daily values of the period 1939-2020 referred to the ZIT; the other two (upper and lower) derive from the central one respectively by adding and subtracting the corresponding standard deviation for each day. The deviations of the central curve from the average annual level depend on the action exerted locally by the wind and the atmospheric pressure. Without going into details, for which we refer to Ferraro and Crisciani (2001), a high atmospheric pressure determines the lowering of the level and a low atmospheric pressure determines its rise; as regards the winds, the continental ones, especially the Bora in the Gulf of Trieste, cause a lowering of the sea level and a delay in the incoming tide. The southern winds, Scirocco and Libeccio, cause water accumulation under the coast and, consequently, higher levels.

The three curves represent: i) the average sea level (central black) for each day of the year, obtained starting from the daily values of the period 1939-2020 referred to the ZIT; ii) the daily standard deviation (upper orange and lower blue curve)

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Again with reference to the graph proposed, the minimum level in the winter months is evident, linked to the prevalence of high pressure and continental wind, and a marked maximum in November, corresponding to the greater frequency of southern winds and low atmospheric pressure. The autumn peak is related to the phenomenon of high water which occurs mainly in October, November and December.

The following graph highlights the extreme sea level values ​​observed in the period 1939-2020 for each day of the year. The height is always referred to the ZIT. The black line in bold corresponds to the height of 277.8 cm and represents the plane of the Molo Sartorio, that is, with a good approximation, the height beyond which overflows occur. The red and blue curves are the absolute daily maximums and minimums respectively. Among the extreme values ​​observed we recall the maximum of 346 cm on 26 November 1969 (68 cm above the level of the Molo Sartorio) and the minimum of 30 cm on 14 February 1934 (248 cm below the level of the Sartorio Mole, not included in the graph).

extreme sea level values observed in the period 1939-2020 for each day of the year

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High water episodes with overflow. Overflows due to wave motion only, generally caused by the Libeccio wind, are excluded from this list. A = approximate height; S = estimated height; X = unknown height

Bibliography

Ferraro and Crisciani, 2001: Mareography in Trieste: scientific and technical aspects. CNR, Thalassographic Experimental Institute, Trieste, 104 pp.