1 OCEANOLOGICA ACTA- VOL 16- N°4

Sea levels Tidal variations of Marmara Sea straits Water level variations Oscillations

Niveau de la mer in the Variations dues aux marées Mer de Marmara Détroits maritimes Oscillations

Hüseyin YÜCE

Department of Navigation, Hydrography and Oceanography, Çubuklu, , 81647.

Received 7/04/93, in revised form 19/05/93, accepted 10/06/93.

ABSTRACT Characteristics of water level variations in the Sea of Marmara have been investi­ gated. Long-period oscillations of non-tidal origin have been identified in the analysis. These oscillations are due to regular fluctuations of water levet throu­ ghout the year caused by meteorological and oceanographie influences. Greater fluctuations are produced by local winds, river runoff and inflow from the . These fluctuations increase rapidly in spring and early August, the latter due to Black Sea fresh water inflow. The data show the presence of small amplitude diurnal tidal variations. Far from the entrances to the straits, the Sea of Marmara is not affected by short period tidal oscillations of the neighbouring . The · harmonie tidal constituents indicate the presence of mixed, mainly diurnal tides with a mean spring range of 1.50 cm at . Oceanologica Acta, 1993. 16, 4, 335-340.

RÉSUMÉ Variations du niveau de l'eau dans la Mer de Marmara L'observation du niveau de l'eau dans la Mer de Marmara montre des variations de faibles amplitudes dues aux marées diurnes. L'analyse des données révèle aussi des oscillations à long terme indépendantes de la marée. Loin de l'entrée des détroits, la Mer de Marmara n'est pas influencée par les oscillations à court terme de la marée dans les mers voisines. Les oscillations à long terme sont dues aux facteurs météorologiques et océanographiques. Les oscillations de plus gran­ de amplitude sont produites par des vents locaux, par le débit des rivières et par le flux en provenance de la Mer Noire. Ce dernier phénomène est plus prononcé au printemps et au début du mois d'aoOt. Les composantes harmoniques de le marée indiquent la présence de marées mixtes, essentiellement diurnes, avec une ampli­ tude moyenne de 1,5 cm à Erdek au printemps. Oceanologica Acta, 1993. 16, 4, 335-340.

INTRODUCTION extends over a distance of approximately 300 km, and consists of the Strait of Istanbul-SOI (Bosphorus: average The Sea of Marmara is a small of intricate confi­ depth 35.8 rn, length 31 km and width 1.6 km), the Sea of guration located between the Aegean Basin of the Eastern Marmara and the Strait of Çanakkale-SOC [: Mediterranean and the Black Sea (Fig. 1). It occupies a average depth 55 rn, length 60 km and width 4 km (Fig. 1)]. central part of the Turkish Straits System (TSS), which Water exchange through the TSS affects the oceanographie characteristics of the Black Sea and the . There

335 H. YÜCE

Figure 1 Location chart showing the tide gauge stations.

is a two-layer current system. Brackish surface water of the Gunnerson and Ozturgut, 1974; Moller, 1928; Smith, 1946; Black Sea flows towards the Aegean Sea, and highly saline Ünlüata et al., 1991; Yüce, 1986). These studies were Mediterranean water flows toward the Black Sea at depth. focused mainly on the SOI, due to its decisive role in water The Sea of Marmara basinal area, which is about 11500 exchange between the Black Sea and the Aegean Sea. km2 (Ünlüata et al., 1991), is occupied mostly by the conti­ The sea level and its variability in the Sea of Marmara is nental shelf. The northern shelf is narrow, varying from 2 far from having been full y studied and the present paper to 13 km in width, and the southern shelf is much broader describes sea level variations in the Sea based upon recent [33 km wide (Bodur, 1991)]. The east-west length of the and historical data collected in the . basin is roughly 240 km and the north-south width is approximately 70 km (Ünlüata et al., 1991). Three sub­ basins with depths 1221, 1355 and 1276 mare oriented in an east-west direction (Fig. 1). DATA SOURCES AND ANALYSIS TECHNIQUES Tidal oscillations in the Aegean Sea are dominated by semidiumal constituents with supplementary contributions In order to investigate temporal and spatial variations of sea from the diurnal constituents. The most important consti­ level, sea-level data were collected from a temporary instal­ tuent is Sz, being typical of the Mediterranean (Defant, lation of mechanical OTT float-type tide gauges at Ortakoy, 1961). which is located in the northern part of the Sea of Marmara and at Bozcaada, in the northeastern part of the Aegean Sea The Black Sea is a subsided basin which bas a narrow shelf (Fig. 1). Hourly sampled data from the permanent stilling about 25 km wide in the area of the SOI entrance (Fig. 1). well R. Fuess tide gauge located at Erdek, and operated by Sealevel variations in the Black Sea region due to tide pro­ General Command of Mapping, were also analyzed. ducing forces are small. Tides are semidiumal in nature Although these stations records span larger time intervals, and their amplitudes are very small; in the centre of the only simultaneous short-term hourly sampled data at west and east coast, they are about 9 cm, in the centre of Bozcaada (two months, 1 March-30 April1989), Ortakoy the north coast () they are about 2-3 cm (Defant, and Erdek (four months, 1 March-30 July 1989) were utili­ 1961). In the eastern part of the southern coast, tidal oscil­ zed for power spectra and tidal constituent analysis. lations are very small in amplitude with a spring range of 0.74 and 1.9 cm in Samsun and Trabzon, respectively Water levels at hourly intervals were abstracted from the (Yüce, 1993 a). analogue records and data subjected to Fast Fourier Transform analysis techniques to calculate power spectra In the northern part of the SOI, tides are mixed, mainly in the records. These were calculated by utilizing the semidiurnal with a spring range of 4.4 cm. In the southern Seaspect Software developed by Lascaratos et al. (1990). part, they are mainly diurnal with a spring range of 2.50 In addition, data were analyzed for tidal constituents cm (Yüce, 1986). using a harmonie analysis program based on that of Characteristics of the water levet variations of the TSS Schureman (1940). Mean sea level (MSL) is derived by have only been partially studied in the past (Bogdanova, applying Doodson Xo filter (lOC-UNESCO, 1985) to the 1965; Çeçen et al., 1981; De Filippi et al., 1986; hourly values. ·

336 1 SEA LEVEL VARIATIONS, SEA OF MARMARA

1 cl

2 3 4 5 • 7 • 9 fO Il 12 13 14 15 t6 17 4 ' • 7 8 • 10 Il 12 15 .. 15 • 17 18 •• 2021 22 23 24 u 26 27 28 2t 30 MARCH 1989 MARCH 1989

Figure2 Figure 3

Water leve/ variations at Ortakëy: a) observed with MSL; b) astronomi­ Comparative water leve/ and MSL variations for the stations: a) cal tide; c) residual; d) barometric pressure (1-17 March 1989). Bozcaada, b) Erdek and c) Ortakiiy between 4-30 March 1989 (the ootum is arbitrary at each of the recording stations). Comparative meteorological data (wind speed and direc­ long-period oscillations are probably meteorologically tion, barometric pressure) were obtained for the sta­ induced. tion, located at the northern part of the Sea of Marmara Short-period oscillations are not particularly observed in (Fig. 1) from the State Meteorogical Service. the Erdek and Ortakoy data; this may be due to their effects being damped due to the locations of these installations. Short-period oscillations with a period of 3.1 hours obser­ RESULTS AND DISCUSSIONS ved at the station, located on the western part of Sea of Marmara, can be attributed to longitudinal seiche­ The records like motions (Yüce, 1993 b). . .. Comparative records of water level fluctuations between Water level records demonstrate that the area is one of 4-30 March 1989 are shown in Figure 3 for the stations low tidal amplitude. Figure 2 presents data from the of Bozcaada, Erdek and Ortakoy. This demonstrates that, Ortakoy tide gauge, located at the southern entrance of away from the SOC entrance, the Sea of Marmara is the SOI. Small amplitude tidal and non-tidal oscillations isolated from the tidal oscillations of the Aegean Sea. are superimposed upon higher amplitude long-period Far from the straits, the semidiurnal tides of the Aegean oscillations. Sea and Black Sea are not observed in the Sea of The long-period oscillations which occur in the records Marmara. Furthermore, spectral analysis of the houriy are due to both long-period tidal constituents and meteo­ sea level values from the Ortakoy station demonstrates rological influences, and can be exemplified by exami­ that diurnal variations are cleary dominant over semi­ ning the variations in mean sea level (MSL; Fig. 2). The diurnal variations. fluctuations have a period of severa! days. The mean On the other band, Figure 3 demonstrates co-oscillation of water level at Ortakoy is clearly inversely related to baro­ the western part of Sea of Marmara with the Aegean Sea metric pressure (Fig. 2), which provides evidence that the in the low frequency band. Another noteworthy feature

__ ERDEK

---- ORTAI<ÔY

E "' .,"'> .J .,... ;;::0

Figure4 -,.,

MSLs and 400 hour/y mean sea levels from Ortakëy and Erdek between 1 March-30 July 1989 (the datum is arbitrary at each of the recording stations). MARCH APRil. MAY JUNE JUI.Y 1969

337 H.YÜCE from Figure 3 is the inverse water level variations obser­ Table 1 ved between northern and southern parts of the Sea of Distribution of energy percentages in the records (one year hour/y data at Marmara. This is clearly evident and observed throughout each of the stations). the records in MSL variations. Long-period sea level variations and predominant tidal constituents are analysed Frequency (cpd) Bozcaada Erdek Ortakoy in more detail below, interaction of the Aegean Sea with the Sea of Marmara in the low frequency (sub-tidal) band <0.8 45.4 92.5 91.9 merits further study. 0.8-1.2 5.3 3.3 5.0 1.2-1.8 1.4 0.5 0.5 Long-period variations 1.8-2.2 46.6 1.0 1.5 >2.2 1.3 2.7 1.1

Long-period oscillations can be investigated by examining the variations in MSL. MSLs derived by Xo filtering the Long-period oscillations produced by meteorological and Erdek and Ortakoy data are shown in Figure 4, superimpos­ oceanographical phenomena may be revealed in filtered ed on 400-hour low-pass filtered mean water level. Figure 4 sea-level data. MSL data and 400-hour low-pass filtered demonstrates once again the inverse water level variations values presented in Figure 6 display characteristics of long between northern and southern parts of the basin at daily period sea level variations. and long time scales. lt also indicates the transient and Variations in MSL depend on variability of wind patterns dynamic nature of the MSL variations. This inverse oscilla­ over the Sea of Marmara, the influence of which is more tion observed between north and south may be attributed to strongly observed at the northem and southem parts. On an winds. Northerly winds cause wind set-up and set-down in annual basis, northerly winds are dominant with southerlies the southern and northern parts respectively. Transient, occuring 20 % of the time (De Filippi et al. 1986). Short­ abrupt high water levels are observed in the northem part, at term effects of winds are clearly evident in transient Ortakoy, during southerly wind periods. Correlation func­ increases in sea level. Southerly winds which are dominant tions between Erdek and Ortakoy show peak(- 0.75) value in winter increase water level at the northem part. Sea-level at zero time lag, decreasing slowly and reaching (- 0.18) at variations caused by the changes in the outflow from the sixty-day time lag, which shows the similar character of this Black Sea depend upon freshwater input cycle of the Black relation, across a range of frequencies. Sea which is mainly governed by the Danube and the other great rivers. During the period of 1928 to 1959 maximum The results from the spectral analysis of Ortakoy hourly sea monthly runoff was found in May (Gunnerson et al., 1974). level and Florya barometric pressure data are plotted as Highest monthly sea level observed in early August at normalized power against frequency in cycles per day (cpd; Erdek suggests about a two-three month travel time from Fig. 5). These show that the water leve! fluctuations are the Black Sea, which is consistent with the surface salinities dominated by the energy input from long period fluctuations observed in the SOI. Local runoff may be the reason for (0 to 0.25 cpd) with minor contributions at diurnal frequen­ higher water elevations observed in spring (Fig. 6). cies. Long-period pressure variations are also evident in the barometric pressure spectrum, especially for < 0.3 cpd (Fig. Numerical analysis of the energy distribution is presented 5); these variations could be directly related to long-period in Table 1. The energy contributions, in different frequen­ fluctuations in water level. Long-period sea level variations cy bands, are expressed as percentages of the total energy observed at Ortakoy and Erdek reflect cyclonic systems per­ in the records. iod in the region, which is between three to ten days and These percentages confirm, quantitatively, the dominance often result in winds of 8 to 10 mis (hourly speed) sustained of the long-period energy inputs (approximately 92 %) on over one to two days (Büyükay, 1989). water level in the Sea of Marmara.

1.0

0.8

----- PRESSURE o.~ a: -- WATER LEVEL w 3': 0 0.4 0..

o. 2 Figure 5 Spectral analysis of: a) hour/y water leve/ at OrtakOy; and b) barometric pressure at 0 1 2 Floryafor the period of 1 March-30 July 0 0.24 0.48 0.72 0.96 1989. FREOUENCY 1 Cycles/day)

338 1 SEA LEVEL VARIATIONS, SEA OF MARMARA

Figure6 -400 hourly 220- meon level& Annual progress of MSL and 400 hour/y mean sea levels at Erdek station. 200 e .!! IBO ..J ILl > ILl ..J 160 . Ir ILl 1- ~ 140 3:

120 JAN FEB MAR APR MAY JUL JUN AUG SEP OCT NOV OEC

Diurnal tidal energy inputs dominate over semidiurnal tidal Table2 energy inputs, but contribute only sorne 3-5 % of the total tidal + non-tidal energy. Numerical analysis of the energy Tidal harmonie constituents (amplitude values in centimetres). distribution at Bozcaada once again demonstrates the dominance of tidal energy input (52 %) on water levet fluc­ Mz Sz Kt 01 Mean Mean tuations in the Aegean Sea. Comparing with the Erdek Spring Neap For STATIONS Range Range Number results, this indicates dissipation of Aegean Sea tides through the SOC, and presence of small amplitude diurnal BOZCAADA 7.0t 3.11 1.50 1.64 20.24 7.80 0.31 tides away from the straits. 0ELIBOLU 1.14 1.62 0.41 0.36 5.52 -0.96 0.28

E!IDEK 0.42 0.3t 1.69 0.81 1.46 0.22 3.42 Tidal constituents 0RrAK0Y 0.69 0.49 1.66 0.86 2.16 0.40 2.33

Hourly sampled data from observations have been analys­ ed for their tidal harmonie constituents. The amplitudes of from the Aegean Sea. Lunar fortnightly (Mf) and monthly the four main constituents are presented in Table 2; these (Mm) tides are 1.23; 1.64 cm and 1.91; 1.35 cm for Erdek - ··~ are the semi-diurnallunar (Ml) and solar (S2) constituents, and Ortakôy respectively. _,. and the soli-lunar diurnal (Kt) and main lunar diurnal (Ot) constituents. Mean neap 2(M2- S2) and spring 2(M2 + S2) tidal ranges and form number F =(Kt + Ot)/(M2 + S2), are CONCLUSIONS included in the Table. The tidal amplitudes demonstrate the relative importance Sea-level variations in the Sea of Marmara are mainly due of the various constituents. At Erdek and Ortakôy, the most to prevailing meteorological and oceanographie conditions important constituents is Kt. Large amplitude diurnal in the region. Water-level fluctuations are dominated by constituents cause the predominantly diurnal tides in the energy inputs from long period oscillations of non-tidal ori­ Sea of Marmara; the small amplitude semidiurnal consti­ gin. Ranges of greater magnitude, produced by wind, river tuents ·cause the minor diurnal inequalites. runoff and water flow from the Black Sea, are observed. The form numbers demonstrate that the tides at Erdek are Co-oscillation of the Sea of Marmara with the Aegean Sea diurnal, but at Ortakôy tides are mixed, predominantly in long-period range is also observed. diurnal. In the Aegean Sea, at Bozcaada, tides are mixed, The Sea of Marmara is not large enough to generate its mainly semidiurnal in nature. ln the western part of the own tides. Co-oscillation with the neighbouring larger Sea of Marmara, at Gelibolu, mixed, mainly semidiurnal basins is governed by water exchange along the connec­ tides are observed. A semidiurnal tidal pattern of the ting shallow, narrow, long straits. Tides are very small in Aegean Sea is observed in the vicinity of the Sea of amplitude. Due to the depth, width and morphology of the Marmara entrance and the Black Sea. Semidiurnal tides SOC, the Sea of Marmara is more direct!y affected from mostly dissipate in the SOI and consequently only diurnal the Aegean Sea. Away from the straits, diurnal tides are tides are observed at Ortakôy. These fmdings indicate that observed which are superimposed and generally masked the Sea of Marmara is much more strongly influenced by the long-period oscillations.

Acknoweldgements

The author wishes to express his sincere thanks to his col­ to this study and Mrs. Arzu Kaya and Mrs. Gülay Ôztoprak leagues, especially to Eng. Bedri Alpar, Eng. Berkay for their patient help in typing and producing the figures of and Dr. Nazmi Postacio~u for their valuable contributions this manuscript.

339 J H.YÜCE

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