The movement of water masses in the Adriatic Sea

I would like to begin with an introductory remark: as you probably know, a public debate is currently taking place on the consequences of potential hydrocarbon extraction in Croatian waters. Research into the environmental impact of the oil fields (with special regard to Natura 2000  areas) commissioned by the Croatian Ministry of the Economy clearly shows that a cross-border impact is expected, mainly during the preliminary seismic surveys in the northern Adriatic (especially in zone 1, which extends up to the Gulf of Trieste), whereas during regular activity it would only occur in the case of accidents. However, accident management is unfortunately only touched upon in the research, which is concerning.

If an oil spill occurred in zones 1 or 2, which is at the latitude of the Istrian Peninsula, the south and west winds would carry the oil slick, which in a few hours or days could reach the Slovene sea and coast. The response of competent authorities to these accidents can be far more effective (and faster) if it can be predicted where and how the oil spill will spread on the sea surface. These predictions can be made using a reliable hydrodynamic model of currents in the Adriatic Sea and a model of pollutant spill, which on the basis of currents simulates the circulation and dispersal of pollutants in the sea and identifies the locations where the spill could reach the coast.

Rapid response is very important, as it is best if these spills are already contained on the surface of the sea – once this pollution reaches the coast its consequences are usually very difficult (and very expensive) to remove or to reduce.

This is the reason why Croatia promises to carry out for each intervention in these areas further detailed research on cross-border impact. However, the cross-border impact of Croatian hydrocarbon extraction on the Slovene sea and coast will not be covered any further in this article. This impact is actually more complex than the simple physics of the Adriatic Sea, as it primarily influences the ecology, biology, chemistry, geology and geography of the northern Adriatic region. This article briefly describes the general circulation in the Adriatic Sea and the Gulf of Trieste, and presents some numerical simulations of movements of water masses in the northern Adriatic.

The Adriatic basin has an elongated shape and runs in a southeast–northwest direction (see Figure 1). In the north, the sea is quite shallow, only up to 40 metres deep. Southeast from the line connecting Zadar and Ancona, the sea deepens to over 200 metres and in the southernmost part it inflows into the South Adriatic Pit, which lies on the line between Ulcinj and Bari and is over 1200 metres deep. The eastern Adriatic coast is steep, rugged and has many rocky islands, whereas the Italian side has gentle inclines and predominantly sandy beaches. Numerous rivers flow into the Adriatic Sea, significantly affecting its density, salinity and circulation (Artegiani et al., 1997).

Figure 1: Adriatic Sea Bathymetry (seabed topography).

jadran topografija

The circulation of the Adriatic can be divided roughly into two components. The first is the climatic component, which is determined by the balance of the heat flux between the sea and the atmosphere, by freshwater river inflows and by seawater density distribution in the Adriatic basin. This component influences the flow of currents on a seasonal and annual scale and confirms the folk wisdom which says that currents in the Adriatic Sea flow along the east coast towards the north and along the west coast towards the south. In addition to the above mentioned Adriatic currents, there are also seasonal gyres measuring over ten kilometres in diameter that have a cyclone rotation (anticlockwise) as shown in Figure 1 (adapted from Artegiani et al., 1997).

The second component that influences the flow of currents on a daily and hourly time scale is the tide, which enters the Adriatic Sea through the Strait of Otranto in the south from the Mediterranean, and wind circulation (currents which occur due to local winds). One could claim that the climatic component represents a kind of current background, which is daily and locally modified by the local tide, temporarily increased river inflows and currents caused by local winds. A chart showing the present flow of currents can thus differ considerably from seasonal circulation.

Figure 2: Seasonal upper-layer circulation in the Adriatic Sea (adapted from Artegiani et al., 1997).

jadran

The tide is the change in sea level caused by gravitational forces between the Sun, the Earth and the Moon. In addition to the tide, the change in sea level reflects other impacts, for example meteorological (change in air pressure, winds etc.). The difference in sea level rise and fall depends greatly on the local sea depth. In the northern Adriatic region, where the sea is the shallowest, this difference is the most prominent. The changes in elevation due to tidal bores, which enter the Adriatic from the Mediterranean, are shown in Figure 3. However, the tide does not greatly affect water mass circulation, as it is manifested as waves, which are primarily a movement of disturbance in the elevation of sea level and in the speed of sea waves. To put it simply, these waves present tidal velocity in the Adriatic Sea between high tide and low tide in one direction and between low tide and high tide in the opposite direction, causing this “swaying” in a few days to neutralise. In other words, if the water in the Adriatic Sea was only moved by the tide there would be no significant currents.

Another problem in the northern Adriatic region is the annual flooding along the lower lying shores. Some of these floods have been recorded in Venice and Piran, and material damage in such cases can amount to millions of Euros. These situations are caused by two physical phenomena – tide and storm surges. When in the Adriatic region the strong south-eastern wind (the sirocco or jugo) blows for a few consecutive days, it pushes large quantities of water towards the northern Adriatic Sea. The sea level thus rises by up to half a metre and causes a storm surge in the sea. When the wind abates or it cannot maintain the locally increased sea level, the water in the entire Adriatic longitudinally oscillates up and down, like in a bathtub. The basic frequency proper to such oscillation lasts around twenty-two hours, while the oscillation itself decreases fairly slowly and can last for over ten days. In addition, if this oscillation coincides with high tide, it may cause significant flooding along northern Adriatic shores. In the middle Adriatic region, the sea is so much deeper that such extreme events occur less frequently than on the Slovene coast. The sea promenade in Split is flooded less often than Tartini Square in Piran. Such events cannot be prevented, but timely prediction on the basis of tidal and oceanic models can significantly reduce material damage and possibly save people’s lives.

Figure 3: Tidal elevation changes of sea level in the Adriatic region

adritide_el_20130916

With regard to the flow of currents in the Gulf of Trieste, due to variable river inflow, especially of the Isonzo River, and to sporadic strong wind (the burja), the circulation is quite complicated and difficult to generalise on a single pattern (Malačič and Petelin, 2009). The strong burja, which blows over the northern Adriatic region and the Kvarner Gulf from the Nanos Plateau and the Dinaric Alps, with the influx of cold continental air masses, quickly stirs the water in the gulf, typically causing noticeable cooling (and consequently condensing) of the water and intensifying the outflow of fresh water along the Italian coast towards Venice. The outflow of fresh water is a dominant feature of the flow of currents in the winter, when the burja blows and causes the surface outflow of the water along the northern coast of the Gulf of Trieste, more specifically from the shallows past Monfalcone and Grado towards Venice (Malačič and Petelin, 2009; Cosoli et al., 2013). This outflow is proven by measurements of surface currents (Cosoli et al., 2013) and waves, as well as by mathematical models, which calculate the flow of currents in the Adriatic Sea under realistic conditions (winds, rivers, tide). As the burja at the same time pushes the water towards the Venetian Lagoon and Chioggia, the sea level is higher near Venice than in the Gulf of Trieste, which is why the pressure gradient force in these situations causes deep compensation currents towards the Gulf of Trieste, in the middle and in the southern parts of the mouth of the gulf. A rough description of the winter circulation in the Gulf of Trieste is as follows: the water inflows at the lower middle and southern part of the gulf, goes past Savudrija and Piran and outflows in the northern part of the gulf through shallows near Monfalcone and Grado. When the flow rate of the Isonzo/Soča River is high, it can, in a period without strong winds (for example in the summer), cause anticyclonic surface circulation in the gulf, which is compensated by cyclonic circulation in the depths (Malačič and Petelin, 2009).

Figure 4: Average flow of currents in the Gulf of Trieste between March 2011 and October 2012. On the left: Flow of currents, measured with surface wave radars. On the right: Flow of currents, calculated with the mathematical and physical circulation model NAPOM (Adapted from Cosoli et al., 2013).

tokovanje

General impressions of the circulation in the Adriatic Sea and in the Gulf of Trieste unfortunately do not suffice to foretell the direction of movement during a certain time interval of a particular final volume of water mass, which is at the starting time at a determined known location. Its further movement will be determined by the river inflows, the tide, relatively unsteady local winds and the general sea circulation.

To get an impression of how unpredictable can be the movement of particles which float in the sea, look at the simulation in Figure 5. It shows the journey of an initial number of fictitious particles which were scattered at a certain location in the sea and were then carried by sea currents. The movement of these particles is thus similar to the movement of local sea currents. In addition to solely moving with the sea currents, their movement is enhanced by diffusion, whose movement along the streamlines causes additional spreading of the observed particles group. The colour of the trajectories changes solely with time: at the beginning of the simulation, the trajectories are dark blue, at the end they are red, and in between they are in other colours. The changing colour of the particle trajectories is only for greater clarity, so that the particles can be more easily observed on their journey.

Note that these simulations do not show the spreading of potential oil spills on the sea; in a best-case scenario, they only show it for the first few hours. Later, the spreading of the oil spill can differ greatly from the simulations shown in this article. Simulations of an oil spill require accurate information on the type of the fuel, as different fuel types behave differently when they are spilled (some fuels, for example, evaporate quickly while others emulsify, fuse or sink). The simulations in this article show the tracking of sea currents, more specifically, how a passive tracer (for example, a colorant) moves and spreads if it is at the beginning accumulated in a water mass which is subjected to the real flow of currents (calculated with mathematical and physical models of the Adriatic Sea) and turbulent diffusion processes in the northern Adriatic.

Figure 5: Simulation of the tracking of particles released in the northern Adriatic Sea on 1st July 2014. The colour of the particles trajectories changes with time for greater clarity.

tracer_20140701_01

The sea currents that determine particle movement were calculated using a mathematical and physical sea model based on the initial condition, tide, river inflows and atmospheric parameters (for example, the wind over the sea) and calculates the hydrodynamics (currents and sea level elevations) and the thermodynamics (heat flux between the sea and the atmosphere, changes in temperature, salinity and density of the sea water) in the northern Adriatic region. As can be observed in the simulation, the movement of the particles is quite lively and, at least at first sight, quite hectic, although on a longer time scale the paths of the particles in the observed time period reflect the movement pattern by which the water flows along the east coast towards the north and along the west coast towards the south. However, these particle paths apply only to this particular release location and this particular simulation time period. Release at another location or at another time could lead to quite different particle journeys in the northern Adriatic.

To sum up, this article briefly describes some features of the circulation in the Adriatic. Knowing the (physical, chemical, biological, ecological and other) state of the Adriatic is important in many regards as it enables the right measures to be taken and the damage from maritime accidents to be minimised, as well as the appropriate management of the maritime environment. For shallow coastal seas like the northern Adriatic, which due to an increased coastal population density and two large ports in the cities of Koper and Trieste is already polluted, not to mention the announced oil extraction, this is particularly important.

 

Author: Matjaž Ličer. A physicist and researcher at the Marine Biology Station of the National Institute of Biology. He works on mathematical simulations of the physics of the sea and is currently most interested in the interactions between the ocean and the atmosphere and the impact of these processes on flood prediction in continental Slovenia. Besides focusing on ocean physics, he is also too slowly preparing his PhD thesis in continental philosophy and is trying to watch as many films as possible in the meantime. In the autumn of 2014, the ZRC Publishing House published his translation of Einstein’s Relativity: The Special and General Theory. Follow him on Twitter at @MatjazLicer.

 

Sources:

  1. Artegiani, D. Bregant, E. Paschini, N. Pinardi, F. Raicich, A. Russo, “The Adriatic Sea General Circulation. Part II: Baroclinic Circulation Structure”, Journal of Physical Oceanography, American Meteorological Society 1997.
  2. Malačič, B. Petelin, “Climatic Circulation in the Gulf of Trieste (northern Adriatic)”, Journal of Geophysical Research, American Geophysical Union 2009.
  3. Cosoli, M. Ličer, M. Vodopivec, V. Malačič, “Surface circulation in the Gulf of Trieste (northern Adriatic Sea) from radar, model, and ADCP comparisons”, Journal of Geophysical Research: Oceans, American Geophysical Union 2013.
  4. Ličer, “Nevednost, ki stane: o smiselnosti raziskovanja morja pri nas”, Dnevnikov Objektiv, Dnevnik, 8th June, 2013.

 

Title photo: via Wikipedia.

 

Translated by: Valentina Rebec.

 

Več od Metina lista

Organi za notranje zadeve (9.)

Z organi za notranje zadeve sem se srečeval na takih krajih, kot...
Beri dalje