long-term trend of the sea level at the romanian littoral viorel malciu and vasile diaconu romanian marine research institute bd. mama

Long-term trend of the sea level at the Romanian littoral
Viorel Malciu and Vasile Diaconu
Romanian Marine Research Institute
Bd. Mamaia, Constantza 8700, Romania
Introduction
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 made daily. These values were afterwards used for the entire
Danube leveling.
In 1933 a float-type gauge was placed at Constantza and it 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.
Tide gauge network at the Romanian littoral
RMRI tide gauges that are operational are located in the following
points: Constantza, Tomis, Sulina and Mangalia (Fig. 1).
Description of the tide gauges:
Constantza: (Fig. 1, Fig. 2, Picture 1, Fig 3a, 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, Picture 2), 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, 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. A horizontal pipe
connects the stilling well with the open sea. Violent storms damaged
the VTS and a new one is to be installed this year.
Mangalia: (Fig. 1, Picture 4) Situated in the southern area of the
Romanian littoral in Mangalia harbour. Geographical coordinates are
43°48’30”N, ­­28°35’30”E. Float operating, SUM type, daily chart, VTS
readings three times a day. Due to sand accumulation that obstructed
the stilling well, it was relocated in Dec. 1998 to a new position on
the opposite quay of the same port.
Characteristics of the hydrological and meteorological regime of the
area
Situated at mid latitudes, the northwestern part of the Black Sea
experiences 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 have 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 dominant.
The main components of the water balance in the area of our littoral
(Altman, 1990; Bondar et al. 1991) is continental runoff via the
Danube discharge, which is 60% of the total fresh water discharged
into the sea, and 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 of
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 variable, but in certain
seasons prevalent frequencies may occur (Climate of Romania).
Long-term averages 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).
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
6.84 cm in October and 21.24 cm in May. The maximum mean value, 46.40
cm occurred in March 1970, and the Danube discharge reached its
maximum mean monthly value in May of the same year, 38.89 km3. The low
values in the autumn, -13.20 cm in October and -14.20 cm in November,
are exceeded in February 1949, when the volume of the Danube is low
(7.16 km3), and severe winter conditions were recorded.
The highest amplitudes, 54.90 cm and 54.40 cm, occurred in February
and March, respectively, while lowest amplitude, 35.20 cm, occurred in
September and corresponded with the Danube volume’s lowest amplitude
of 16.27 km3 in September as well.
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 1.5-2.0 mm/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 12.7 cm and a tendency of 0.425 cm/year. Selariu (1971),
for the period 1933 - 1969, found 13.02 cm as mean value and a
tendency of 0.256 cm/year. Later on, studies of the Romanian Marine
Research Institute (RMRI) found 13.5 cm for 1933 - 1978 and 14.3 cm
for the period 1933 – 1983.
Analysis of the data set between 1933 and 1998 reveals the following
aspects :
- annual means are positive, with only two exceptions: -2.44 cm in
1943, the lowest annual mean, and -1.22 cm in 1983;
- the highest annual mean (29.70cm) occurred in 1970, when exceptional
runoff was recorded;
- negative values in the entire period represent only 9.7% of the data
and these occurred between September and February;
- the mean sea level value, 14.2 cm, is positive relative to the
initial reference zero;
- for the entire period, the linear rising trend (Fig. 6) has a value
of 0.128 cm/year;
- the Danube’s mean water flow at the outlet into the Black Sea during
1858-1988, with an average value of about 191 km3/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 10.8 cm, while
that of the 1966-1975 reaches 18.6 cm;
- 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);
- the wind regime has only a temporary influence and does not affect
the long-term evolution of the sea level.
Conclusions
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 accurately assessed.
References
----------
Altman, E. N., A.A. Bezborodov and Y.I. Bogatova, 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., N.P. Bulgakov, V.A. Ivanov, A.N. Kosarev and V.S.
Tuzhilin, 1984. Variability of the hydrophysical fields at the Black
Sea, Gidrometeoizdat, Leningrad, 240 p. (in Russian).
Bondar C., I. State, D. Cernea and E. Harabagiu, 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., and I. Filip, 1963. Contribution to the study of the Black
Sea level. Studii de hidrologie vol IV, (in Romanian).
Gorjachkin I. N., V.A. Ivanov, 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, S.M., and D. Tastea, 1962,1966. Climate of Romania, 2 vol,
164+277p. (in Romanian).
RMRI: Study on the Black Sea level, 1973-1985 (unpublished
manuscripts).

Figure 1. Map of the Constantza area.

Fig. 2 Correlation between sea level and hydro-meteorological
parameters

Fig. 3 Annual evolution of the sea level at Constantza and Danube
Discharge (1933 - 1998)

Fig.4 Evolution of the monthly and annual means of the sea level at
Constantza and their trend

Fig. 5 Spectral density of the sea level at Constantza and Danube
discharge

Fig. 6 Correlation between sea level monthly means and Danube
discharge

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