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Average annual sea level trend in Trieste since 1875 together with trends over various periods. Note the variability, and therefore the poor reliability, of the latter when the period analyzed is short. The period since 1992 is also covered by the satellite altimetry for the correction of the data.

The sea level in Trieste since 1875

The temporal variability scales of the sea level can be distinguished according to the factors that determine them. Variations on time scales up to the interannual are caused by meteorology, mainly wind and atmospheric pressure. Variations in the thermohaline characteristics of the ocean are also important from the seasonal scale upwards.

 

The prevailing winds on the Adriatic are the Bora, transversal to the basin, and the Scirocco, which blows along the basin. The Sirocco tends to accumulate water towards the northern end of the basin. In the North Adriatic the Bora favors the lowering of the level on the eastern coast and an increase on the western one. The Libeccio favors the increase of the level in the Gulf of Trieste.

 

Atmospheric pressure acts through the relationship known as the "reverse barometric effect". When the pressure is relatively low in the North Adriatic, water is sucked in from the South, raising the sea level, and vice versa. The combination of Scirocco and low local atmospheric pressure causes “high water”, ie the significant sea level rise observed on the northern coast.

 

Variations in temperature and salinity lead to changes in water density, which are also reflected in the sea level. This is the “steric effect”, characterized by an increase in the level when the water is heated and its decrease when the salinity increases. In the Gulf of Trieste the secular trend of the steric effect, about -4 mm / century, is negligible compared to that of the observed level, of the order of 100 mm / century.

 

On a global scale, after the glacial peak of about 21,000 years ago, the level has always increased on average, with variable speed. Between 2000 and 100 years ago, the increase was less than 2 cm per century. High-quality measurements at stations considered stable agree on trends between 1.5 and 2.0 mm / year in the 20th century , not considering vertical ground movements. During the last 30 years the trend is estimated at over 3 mm / year.


In the Mediterranean (excluding the Black Sea) there are only five stations with at least 80 years of data, namely Trieste (beginning in 1875), Genoa (1884), Marseille (1885), Venice (1871) and Marina di Ravenna (1873), the the last two affected by marked subsidence of anthropogenic origin.

 

The opening graph shows the trend of the average annual level in Trieste since 1875 together with the trends over various periods. Note the variability, and therefore the poor reliability, of the latter when the period analyzed is short. The period since 1992 is the one covered by the satellite altimetry.

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Series of overflow events in Trieste since 1875 (histogram in blue) and annual number of overflow events (histogram in red)

Compared to the average level of 1991-2009, according to the most recent IPCC projections for the 21st century, in 2050 the average level should be higher between 16 (in the most favorable scenario) and 63 cm (in the worst scenario), and in the 2100 between 30 and 250 cm. On the other hand, simulations of future scenarios would lead us to believe that the frequency and intensity of extreme high waters in Venice and Trieste are not destined to increase compared to the present, due to the simultaneous decrease in the frequency and intensity of meteorological events. that favor them.