THE ANALYSIS Why Venice was flooded by water (and why it is important to know)

On the exceptional phenomenon that brought “very high” water to Venice (and other lagoon and northern Adriatic cities) a few weeks ago, all kinds of things were heard. Some have called global warming and even Greta Thunberg into question, while others have downplayed it, and some have given explanations that are completely out of the air.

We asked the team at Meteomed, one of the most reliable weather forecasting systems for the Mediterranean, to tell us in detail what happened and why. The following article is by Fabio Da Lio, one of the experts who follows the Adriatic for Meteomed (the company has several professionals working “in the zone,” to use football parlance).

An interesting read that we recommend in order to dispel popular myths and move with greater critical awareness in the web landscape often made up of improvised meteorologists.

SYNOPTIC ANALYSIS OF THE EVENT OF NOVEMBER 12, 2019

Between November 11 and 17, 2019, the synoptic-scale situation over Italy saw the continuation of a highly dangerous hydrogeological configuration. A large depression circulation centered over Western Europe, fed by continuous influxes of cold North Atlantic air, carried a series of disturbances headed toward our country, all of them particularly intense. The presence over the Balkans of a high-pressure field actually blocked the perturbative flow right over Italy, causing repeated bad weather conditions over many regions with sometimes remarkable meteomarine effects.

In particular, violent sea storms swept over the Adriatic coasts, from Friuli Venezia Giulia to Romagna but also Apulia, due to persistent Sirocco winds that reached Storm/Fortunale force (above 90km/h); in this regard an exceptional high tide value was recorded in Venice on the evening of November 12 (187cm on the middle sea), with flooding of the entire city and the lagoon islands (Chioggia, Pellestrina, Lido di Venezia). Below are the ground pressure and winds at 9 p.m. on November 12, 2019:

When there are sudden drops in atmospheric pressure, on the order of a dozen hPa, the baric gradient wind (related to the pressure change) is joined by an additional contribution, the isallobaric wind. The isallobaric wind is generated when over areas with a range of 500-1000km, sudden drops in pressure are followed by sudden rises of the same magnitude in a short time. Under such conditions, air masses are pushed more from areas where pressure is rising sharply to adjacent areas where it is still falling. This is what happened specifically on the Adriatic Sea on November 12.

THE ADRIATIC AND THE EXCEPTIONAL WATER HIGH INVENICE
The Adriatic Sea is a semi-enclosed basin, running transversely from the southeast (Otranto Channel) to the northwest (Gulf of Venice). The Sirocco wind, coming from the southeast, exerts extensive action on this sea, as it acts right along its major axis. Since they have the entire basin at their disposal, the sirocco currents can push considerable amounts of water particularly toward the Gulf of Venice and its Lagoon, but also toward the Gulf of Trieste and the Grado Lagoon (Fig.1).

It should be remembered that the Adriatic is a shallow sea, a factor that results in greater fluctuations in its state if it is sufficiently stressed. This in fact happened in the early morning of Nov. 12, when stormy Sirocco winds were reported over the Lower Adriatic, with gusts up to 90-120km/h along the Salento coast and waves up to 5 meters high.

In this regard, it is worth mentioning the
sessa phenomenon
, which is a periodic motion that is originated by an almost stationary wave in a closed or partially closed body of water. Sessa waves consist of oscillatory motions that originate as a result of sudden drops in atmospheric pressure. Over the Adriatic Sea, the fundamental sessa oscillation has a characteristic period of about 22 hours, while other secondaries have a period of 11 hours.

The phenomenon of high water in Venice
The typical situation that results in the occurrence of high water over the Venice Lagoon is characterized by the presence of a depression over the Tyrrhenian Sea, strong sciroccal winds along the Middle-Lower Adriatic, and Bora winds over the Upper Adriatic; in fact, there is the collision of wave trains caused by the two wind systems. It should also be remembered that the tide is mainly affected by two contributions:

1. Astronomical, resulting from the interaction between the Earth-Moon-Sun
2. meteorological, consisting of the effect of wind and atmospheric pressure on water

Thus, the meteorological contribution can become of extreme importance to the tide, so much so that it can range from a few tens of centimeters to more than a meter of water, which is in addition to the astronomical contribution. In the specific case of last Nov. 12, the astronomical tide chart marked a maximum peak of about +55cm on the mid-sea at 10:55 p.m. (Fig.2).

Wind, with its surface action can push water masses, while local low pressures are also of considerable importance: where the atmosphere presses less, the sea will tend to rise(inverse barometric effect).

In the evening hours of that Tuesday, November 12, the sea buoyancy caused by the seiche coming from the Lower Adriatic Sea was compounded by the ‘storm surge’ (storm wave) resulting from a meso-minimum low pressure, which abruptly deepened as it moved up from the upper Marches toward the Po Delta, heading right toward the Venetian Lagoon.

In this regard, atmospheric pressure over Venice dropped from 1000hPa in the morning to 987hPa recorded around 9:30 pm; a baric drop of 13hPa, of which as much as 6hPa was lost in less than 3 hours. The entrance of the low pressure over the Lagoon during the evening, in conjunction with the peak tide expected right at 11 p.m., elicited a rise of more than 130cm of water (meteorological contribution) resulting in the exceptional value of +187cm (Tab.1).

Counterclockwise gale-force winds were blowing around the low pressure area, reaching speeds of 70-90km/h but gusting at times to 120km/h (value recorded by the ISMAR-CNR platform), agitating the waters of the Lagoon and causing waves close to 5 meters high in the open sea, with considerable damage to the coastlines between Veneto and Friuli Venezia Giulia. Finally, it should be noted that the high water phenomenon was also recorded over cities such as Trieste and Piran, as well as in the Grado Lagoon.

CONCLUSIONS
Themeteomarine event that affected Venice and its Lagoon on the evening of November 12, 2019 was characterized by an unfortunate concomitance of factors. Adding to the period of maximum tide predicted that evening was the contribution of the seiche, which came from the Lower Adriatic, as well as the storm surge derived from the passage of a meso-minimo that moved into the Lagoon from Romagna, further strengthening the local winds, which were already in themselves stormy from the Scirocco offshore.

The meteomarine event that affected Venice and its Lagoon on the evening of November 12, 2019, was characterized by an unfortunate coincidence of factors. Adding to the period of maximum tide predicted that evening was the contribution of the seiche, which came from the Lower Adriatic, as well as the storm surge derived from the passage of a meso-minimo that moved into the Lagoon from Romagna, further strengthening the local winds, which were already in themselves stormy from the Scirocco offshore.

HIGH WATER IN VENICE, LAGOON AND TRIESTE: PICTURES

WHO ARE OUR EXPERTS
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