LONG TERM TREND OF THE SEA LEVEL AT THE ROMANIAN LITTORAL
The analysis of the longest sea level record at Constanta, reveals a significant rising trend. Also, the interdecadal changes, as related to the variations of the river discharge, are investigated.
Key words: Sea level, Black Sea, Danube
Sea level recording in Romania dates back to 1859, when European Commission of the Danube, in order to improve the navigation conditions at the Danube mouths, initiated the observation of the sea level by setting up a visual tide staff (VTS) where three readings were daily made. These values were afterwards used for the entire Danube leveling.
Beginning in 1933 a float operating gauge was put in function in Constantza and nowadays is still operational in the same place.
Different institutions were in charge with the exploitation of this tide gauge such as Maritime Harbour Service or Military Hydrographic Service.
Romanian Marine Research Institute as being subordinated to the Ministry of Waters, Forests and Environment Protection, was assigned to carry on this activity since 1970, the year of its foundation, and this work is ongoing. After 1974, three other tide gauges were put into function for a better coverage of the entire Romanian littoral.
1. TIDE GAUGE NETWORK AT THE ROMANIAN LITTORAL
RMRI tide gauges, which are operational are located in the following points: Constantza, Tomis, Sulina and Mangalia (Fig. 1).
Description of the tide gauges:
Constanta: (Fig. 1) and (Fig. 2) and (Picture 1); and (Fig 3a) and (Fig 3b) ;
Geographical position: 44°10'21.0" N, 28°39'34.0" E.
Float operating tide gauge, OTT type, functional in Constantza Port since 1933, recordings on paper chart, changed once a week.
Two VTS are used, one of them for control, and three readings are made daily.
Tomis: (Fig. 1) and (Picture 2) and (Fig. 3b), situated in a small marina, about 1.5 km north from Constantza Port. Float operating and VTS. Three VTS readings are made daily.
Sulina: (Fig. 1) and (Picture 3), situated at the northern limit of the Romanian littoral.
Geographical position: 45°09'45.6" N, 29°43'37.2" E.
Float operating tide gauge, type SUM, functional since 1977, daily charts. It is placed near the meteorological station 6 km downstream Sulina locality, on the southern side of the jetty. An horizontal pipe is connecting the stilling well with the open sea. Violent storms damaged the VTS and a new one is to be fixed this year.
Mangalia: (Fig. 1) and (Picture 4), situated in the southern area of the Romanian littoral, is placed in Mangalia harbour.
Geographical coordinates: 43°48'30"N, 28°35'30"E.
Float operating, SUM type, daily chart, VTS readings three times a day. Due to the sand accumulation which obstructed the stilling well it was relocated in Dec. 1998 in a new position, on the opposite quay of the same port.
2. SOME CHARACTERISTICS OF THE HYDROLOGICAL AND METEOROLOGICAL REGIME 0F THE AREA
Situated at mid latitudes, the northwestern part of the Black Sea presents the general conditions imposed by a temperate continental climate, with cold winters and warm summers. Siberian anticyclone influence alternates with the southern Mediterranean circulation. Consequently the rivers which debauch in this area will present an annual high in spring season when the precipitation in their hydrographic basins are in excess and a minimum in warm seasons when evaporation processes become excedentary.
Considering the main components of water balance in the area of our littoral (Altman, 1990; Bondar et al. 1991), that is continental runoff represented by Danube discharge, which is 60% of the total fresh water discharged into the sea, next to precipitation and evaporation, the high sea levels in the first part of the year occur in the condition of a considerable increase of the river volume and important precipitation quantities (53% in May - August) and a reduced evaporation (37% in the first six months of the year. (Selariu, 1971). Analysis of our data regarding Danube’s annual regime indicates a maximum mean discharge in May, 23.74 km3. Maximum discharge of 38.89 km3 occurred in May 1970. The lowest mean discharge 11.82 km3 , was recorded in October, with a minimum of 5.89 km3 in October 1946.
The wind regime at the Romanian littoral is somehow inconstant but in certain seasons prevalent frequencies may occur (Climate of Romania). Long term average of the wind (1941 - 1997) indicate high frequencies from northern and western directions, 14.9% north and 15% west, where the highest mean speed was recorded as well (north 6,0 m/s). South and southeast winds present a notable frequency, 11.9% and 9.3% respectively. Due to the coastline orientation, onshore winds produce a sea level rise (northerly and easterly), while offshore wind have an opposite effect (Fig. 4) . A significant change of the sea level is induced by strong winds blowing from the same direction for more than 48 hours. Also, short term changes in the sea level are due to the sub-basin seiches (Blatov et al., 1984).
3. SEA LEVEL ANNUAL CHARACTERISTICS
As previously mentioned, sea level evolution presents high values in the first part of the year and low ones in the second (Fig. 5). Mean monthly values, which are positive all over the year, range between 6.84cm in October an 21.24cm in May. The maximum mean value, 46.40cm occurred in March 1970, even if the Danube discharge will reach its maximum mean monthly value in May of the same year, 38.89 km3. The low, negative values specific for autumn, -13.20cm in October and -14.20cm in November are exceeded in February 1949, when in the condition of a low volume of the Danube (7.16 km3), severe winter conditions have been recorded.
Highest amplitudes, 54.90cm and 54.40cm occurred in February and respectively March, while lowest amplitude, 35.20cm, occurred in September and corresponded with Danube’s volume lowest amplitude 16.27km3 in September as well.
4. SEA LEVEL LONG TERM EVOLUTION
A general, preliminary approach of the sea level long term evolution indicates that a slight rise of the values is evident. Soviet scientist Altman (1990), considering the entire Black Sea basin, indicated a rising tendency between 1875 and 1985, especially in the last 50 years. His estimate is 1.5-2.0mm/year. Romanian scientists (Banu, 1961; Bondar & Filip, 1963; Selariu, 1971) analyzed the sea level evolution in different periods and they all identified a rising tendency. Using data between 1933 and 1956, Banu (1961) indicated a mean value 12.7cm and a tendency of 0.425 cm/year. Selariu (1971), for the period 1933 - 1969, found 13.02cm as mean value and a tendency of 0.256 cm/year. Later on, studies of the Romanian Marine Research Institute (RMRI) found 13.5cm for 1933 - 1978 and 14.3cm for the period 1933 - 1983
Analysis of the data set between 1933 and 1998, emphasize the following: aspects :
- annual means are positive, with only two exceptions: -2.44cm in 1943, the lowest annual mean, and -1.22cm in 1983;
- highest annual mean (29.70cm) occurred in 1970, when exceptional runoff was recorded;
- negative values in the entire period represent only 9.7% and they occurred between September and February;
- the mean sea level value, 14.2cm is positive relative to initial reference zero;
- for the entire period, the linear rising trend (Fig. 6) has a value of 0.128 cm/year;
-Danube's mean water flow at the outlet into the Black Sea during 1858-1988, with an average value of about 191km3/year also has an increase trend in time (Bondar);
- the interannual and interdecadal variations have large amplitudes, and a long period oscillation could be detected;
- the averages computed for the 1946-1955 decade is 10.8cm, while that of the 1966-1975 reaches 18.6cm;
- spectral analysis reveals strong oscillations with periods of 2.5 years and 4 years, similar to those of the river discharge (Fig. 7);
- the correlation of the variables in the time domain (monthly averages) is significant (Fig. 8);
-wind regime has only a temporary influence and does not affect sea level long term evolution;
The analysis of the data measured at the Constantza sea level gauge during 1933-1998 period revealed the existence of a rising trend, in good agreement with the estimates made by other authors for different locations (Gorjachkhin, 1995) or for the entire Black Sea basin (Altman et al., 1990). This is also consistent with the contemporary eustatic trend.
Also, significant interannual and interdecadal variations have been observed and estimated through spectral analysis. They are correlated with the changes in the Danube discharge, the main contributor to the river input into the sea. However, the time series are not long enough to allow for the secular oscillations to be correctely assessed.
ALTMAN E. N., BEZBORODOV A. A., BOGATOVA Yu. I. - 1990 - Practical Ecology of Marine Regions. Black Sea., Naukova Dumka, Kiev, 252p (in Russian).
BANU A. C. - 1961 - Observations and measurements on the recent and secular oscillations of the Black Sea waters at the Romanian shore. Hidrobiologia, vol II., p. 127-160 (in Romanian).
BLATOV A. S., BULGAKOV N. P., IVANOV V. A., KOSAREV A. N., TUZHILIN V. S. - 1984 - Variability of the hydrophysical fields ot the Black Sea, Gidrometeoizdat, Leningrad, 240 p. (in Russian).
BONDAR C., STATE I., CERNEA D., HARABAGIU E., - 1991 - Water Flow and Sediment Transport of the Danube at its Outlet Into the Black Sea. Meteorology and Hydrology, vol 21.1, p. 21-25.
BONDAR C., FILIP I. - 1963 - Contribution to the study of the Black Sea level. Studii de hidrologie vol IV, (in Romanian).
GORJACHKIN I. N., IVANOV V. A. - 1995 - Interannual changes of the sea level in the Northwestern part of the Black Sea. In: Investigations of the Azov - Black Sea basin / Collected papers, Eremeev et al. (ed.), Sevastopol, p. 18-21 (in Russian).
SELARIU O. - 1971 - On the Black Sea level oscillations at Constantza. Hidrotehnica, vol. 3, p. 169-176 (in Romanian).
STOENESCU, St. M., TASTEA D. - 1962,1966 - Climate of Romania, 2 vol, 164+277p. (in Romanian)
RMRI: Study on the Black Sea level, 1973-1985 (unpublished manuscripts).