Adriatic Sea Level

Full title: WEB PRESENTATION OF TIDES AND SEA LEVELS ALONG THE CROATIAN COAST OF THE ADRIATIC SEA AND CONSTRUCTION OF THE CORRESPONDING DATABASE

Sponsor: Ministry of Science and Technology of the Republic of Croatia

Project goals: To supply any potential user with the information about tidal predictions for any location along the Croatian coast of the Adriatic Sea. To provide the real-time information about the measured sea levels at one tide gauge station (Split), and the scientific analyses of measured data. To construct a database of the sea level data.

Potential users: The sea level data are necessary within a wide range of activities related to the sea. Major application fields, which also define the users of the project, are as follows:

Safety of navigation in the Adriatic Sea (passenger and cargo ships traffic, especially when manoeuvring in harbours, nautical tourism, fisheries and mariculture, small craft navigation);
Hydrographic survey and other sea and undersea researches (bathymetric survey and charting, especially when laying various submarine installations - fibre optic and power cables, pipelines and sewage outfalls, offshore platforms);
Impact on the coastal infrastructure (the analyses of extreme sea levels, flooding estimates, vulnerability analyses coupled with other oceanographic parameters, particularly surface waves);
Scientific and expert analyses of the sea level changes on the time scales ranging from several minutes and hours (free oscillations - seiches), 12 and 24 h (tides) to several days and weeks (storm surges excited by synoptic disturbances and planetary waves);
Analyses of the sea level on seasonal and interannual time-scales and calculation of trends with particular emphasis on the current problem of global sea level rise;
Legislative application, especially when defining the maritime law terminology e.g. the coastline, maritime demesne and territorial sea.

ABOUT TIDES

Phenomenon of the tides has fascinated people since ancient times, particularly those whose activities are related to the sea. The studies of the Sun's and Moon's actions on the water movements have been enhanced during the last centuries, particularly in recent times when entire human race is faced with the problem of global climate change. Long-term sea level oscillations (periods exceeding 1 min) are basically agitated by tidal and atmospheric forces, primarily by air pressure and wind.

Tidal force is caused by gravitational attraction forces of the Sun and Moon on the Earth's water masses, as well as by the centrifugal force generated by the Earth's revolution. The force is periodical, having strongly pronounced diurnal and semidiurnal components. Tidal force generates regular sea level oscillations, which are called tides. It can change the sea level by more than 10 m in the world oceans, whereas in the Adriatic Sea which is a semi-enclosed basin, such changes range from 30 cm in the South up to 1 m in the North Adriatic.

Meteorological influence on the sea level changes is twofold: forced oscillations - storm surges - which are more important in the domain of synoptic and planetary formations, and free oscillations - seiches. Storm surges are basically driven by the air pressure, but also with winds which can be of great importance in the coastal areas. Free oscillations represent a response of the sea to the rapid changes of meteorological parameters, with the amplitude influenced by the basin topography, too. Both processes may cause the flooding of the coastal lowland areas, especially if occurring together during the spring tides.

Sea level fluctuations occurring on larger periods can have periodic and aperiodic behaviour. The strongest periodic oscillations have seasonal characteristics, related to the changes in density but also to the seasonal changes of meteorological parameters. Aperiodic sea level changes are driven by aperiodic forcing of the meteorological processes on interannual and climatic scales, dynamic processes in the sea and geological-tectonic changes of the sea bottom. Recently, special attention has been paid to the anthropogenic influence on the sea level changes. Industrial and agricultural activities have increased the concentration of the so-called greenhouse gases, such as carbon dioxide, methane, CFC gases and others. Greenhouse gases absorb the long-wave Earth's radiation, consequently increasing the Earth's atmosphere temperature. This effect, called the greenhouse effect, induces the rise of the sea level which is primarily the result of thermal expansion of the upper ocean, but also of the melting of ice-caps and glaciers.

TIDES IN THE ADRIATIC SEA

Tidal dynamics in the Adriatic Sea was described by a number of authors in the last century. Adriatic tides are of mixed type, which means that in some periods semidiurnal tides are predominating, exchanging the sea level rise and fall twice a day, while in some periods diurnal tides prevail, causing the sea level rise and fall once a day. In 1960 Italian geophysicist Polli described the acting of tidal force on the Adriatic sea level by using seven basic tidal constituents, neglecting the influence of other constituents as they do not significantly change the sea level. The amplitudes and phases of constituents were estimated by analysing the data collected on 30 tide gauges, namely of the semidiurnal constituents M2 (main Moon), S2 (main Sun), K2 (Moon-Sun) and N2 (elliptical Moon), and of the diurnal constituents K1 (Moon-Sun), O1 (main Moon) and P1 (main Sun).

Spreading of tides in the Adriatic is usually described by the largest M2 and K1 constituents, while other semidiurnal and diurnal constituents behave similarly to the respective M2 and K1. Common characteristic of the semidiurnal tides is rotating around the so-called amphydromic point that is placed between Ancona and Šibenik, having small amplitudes there. The amplitudes are increasing rapidly from that point to the northwest, with the maximum values inside the Gulf of Trieste (M2 tide amplitude equals 26 cm). Semidiurnal amplitudes are also increasing from the amphydromic point to the southeast, reaching the maximum between Pelješac peninsula and Italian coast, but their values are less than half of the maximum ones occurring in the North Adriatic.

Diurnal tides are propagating from the Croatian to the Italian coast, whereas the amplitudes are rising from the South to the North Adriatic. K1 amplitude varies from 5 cm at Dubrovnik to 18 cm in the Gulf of Trieste. Mean daily tidal range along the eastern Adriatic coast, based on the long-term sea level measurements, is estimated to be 22 cm at Dubrovnik, 23 cm at Split, 25 cm at Zadar, 30 cm at Bakar and 47 cm at Rovinj.

Forced oscillations of the sea level are characterized by no significant disruption of the hydrostatic balance in the sea. Such behaviour is aperiodic and mainly caused by the forcings of long-lasting and strong winds as well as by pronounced air pressure excesses. The lower is air pressure the higher is sea level and vice versa. The acting of wind on the sea surface is complex, depending on the topography of the basin and on the wind speed, direction and duration. In the Adriatic Sea the southeast winds (Sirocco) raise the sea level, especially in the North Adriatic, where a long-lasting Sirocco and low air pressure can raise the water level up to 1 m. Wind influence is less important in the South Adriatic, where the air pressure dominantly influence the sea level changes, up to 30 cm.

STORM SURGES AND SEICHES IN THE ADRIATIC SEA

Free oscillations - seiches - occur as a sea response on the sharp changes of meteorological parameters over the basin, especially the wind. Along-basin winds can pill up the water on the closed end of a basin, creating the sea level difference between the open and closed ends of the basin. If the wind suddenly changes its speed and/or direction, hydrostatic balance is disrupted and it tends to be restored by periodical oscillations of the sea level. Seiche periods are determined by the dimensions and topography of the basin, whereas the amplitudes depend on the changes of the wind speed and direction. The most pronounced seiche in the Adriatic Sea is uninodal seiche having a period of 21.2 h, but the second mode with a period of 10.8 h is significant as well. The main Adriatic seiche represents a uninodal oscillation with nodal line placed in the Otranto Strait, while the second mode has nodal lines in the Otranto Strait and on the section Pelješac - Monte Gargano. In addition, the seiches of the Middle and North Adriatic treated as single basin can appear notably as well, having periods of 8.2 and 6 h. Seiches also occur in smaller semi-enclosed regions, such as Middle Adriatic (4 h), Rijeka Bay (2 h), Kaštela Bay (1 h), but also in small bays and harbours (Vela Luka Bay - 15 min, Split harbour - 7 min, Zadar harbour - 4 min).

SEASONAL AND INTERANNUAL CHANGES OF THE ADRIATIC SEA LEVEL

Seasonal oscillations of the Adriatic sea level are dominantly driven by the annual course of the total heat budget at the sea surface, having as a consequence heating or cooling of the upper layers. Vertical stratification usually starts to develop in the spring season (April-May), when thermocline (pycnocline) is formed, enhancing and deepening in summer. In autumn, thermocline (pycnocline) weakens as a result of enhanced synoptic activity over the Adriatic, and disappears during the November-December period, followed by a homogeneous vertical structure that remains during the winter season (January-March). Generation, development and destruction of thermocline (pycnocline) cause changes of the sea density. Consequently, the sea level seasonally oscillates, and this process is called the steric effect. The highest sea levels occur during the pycnocline destruction in autumn (October-December), whereas the lowest ones are found in the spring and summer seasons.

Interannual sea level changes are driven by climatic fluctuations of the meteorological parameters, e.g. of the air pressure, surface heat flux, precipitations etc. Such changes are estimated at several centimetres in the Adriatic Sea, which makes it impossible to calculate precisely the sea level trends. However, the sea level trends reveal a decrease in the sea level rise rate in the Adriatic. Moreover, vertical crustal movements cannot be neglected when calculating the trends, as they can enlarge the sea level trends at some places and cause a high rate of the sea level fall at others (e.g. Scandinavia). According to the recent researches, the decreasing of the sea level rise rate in the Adriatic and Mediterranean Seas is a result of a negative trend in precipitation and the fresh water inflow into the sea, and of changes in the bottom circulation and water masses in the Mediterranean Sea. However, a strong positive trend in the sea level observed during the last ten years underlines the importance of interannual and interdecadal fluctuations.

But what if the sea level rises for 50 cm in the Adriatic, as it is projected for the global scale? Then the vulnerability of coastal areas and infrastructure will be multiplied as the destructive storm surges are going to occur almost every year, especially in the North Adriatic. But, the consequences of sea level rise will also stroke other areas, particularly lowland areas and deltas such as agricultural Neretva River delta, which will be salted and therefore uncultivable. Such areas can be protected by constructing the defending walls, but the price is rather high.

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