The system od currents found at a control station in the middle Adriatic
Abstract
Direct measurement of currents at three different depths were carried out at Stončica control station in four seasons during the period extending from 1959 through 1964.
In the surface layer the current direction showed the clockwise swinging such that it described a full circle in a year’s time. Schematicaly, the changing of the directions of current may also be presented this way: NW in winter, N in spring. SE in summer, and SW in automn. Such an arrangement of directions stands in close connexion with the distribution of water density in various seasons. The highest Adriatic density values are found in winter in the Northern Adriatic, and the lowest ones in summer in the same region. This fact is associated with the phenomenon consisting in the occurrence, in that area, of the Adriatic minimum of dynamic depths in winter and of their maximum in summer. The changes in the sea level, occurring in the Northern Adriatic, may be considered, in comparison with the Middle Adriatic, as a periodic wave phenomenon whose period extends over a year’s time, since the summer sea level (caused by the distribution of density) is higher in the northern than in the middle Adriatic, while the winter level is higher in the middle than in the southern Adriatic. These changes in sea level are in connexion with the described system od currents in the Middle Adriatic. There is a flow from the middle to the northern Adriatic (NW-direction) in winter, when the sea level in the latter is lower than in the former; exactly the opposite occurs in summer. In the periods of transition, besides, occurres the clockwise swinging of direction due to Coriolis’s force. The whole phenomenon is considered to be analogous to that observed in the tide wave where also, in the course of a tidal cycle, the direction of flow in the open sea swings entirely around the compass due to Coriolis’s force.
The NE-direction is not represented in the surface layer but it occurs at a depth of 50 metres, while the SW-direction, on the contrary, is not found at that depth but it is strongly marked in the surface layer. Freshwater discharge from the River Neretva is supposed to be responsible for that since the discharge, while favouring the SW flow, does not allow the NW flow to develop. This is confirmed by the observed fact that the SW flow is the strongest in the surface layer during the period when the Neretva reaches its maximum level.
At a depth of 50 metres, where the NW direction generally prevails, the annual cycle of changes in the direction of flow is not pronounced. The prevalence of the NW-direction, which represents the direction of flow into the Adriatic, is connected with the earlier established fact that the Mediterranean advection water is dominant in the layer concerned. This is why that layer is characterized by water of higher salinity.
Essential changes in the direction of current, occuring from year to year, were observed at a depth of 100 meters. While the inward flow (NW) prevailed in some of the surveyed years, the outward flow (SE) was prevalent in one of them. This may be in connexion with the earlier observed phenomenon of the varying extent of Mediterranean advection in the Adriatic illustrated by the fluctuation of salinity values of Adriatic waters. It was thus observed that an inward flow occurred in the years with a higher average salinity in the Middle Adriatic, while an outward flow followed in the years with a lower average salinity. The interdependence of these two phenomena is of a great importance, and further investigations should be undertaken in this direction.
