The Black Sea Level Trends from Tide Gages and Satellite Altimetry

Transcription

1 ISSN , Russian Meteorology and Hydrology, 2013, Vol. 38, No. 5, pp Allerton Press, Inc., Original Russian Text A.A. Kubryakov, S.V. Stanichnyi, 2013, published in Meteorologiya i Gidrologiya, 2013, No. 5, pp The Black Sea Level Trends from Tide Gages and Satellite Altimetry A. A. Kubryakov and S. V. Stanichnyi Marine Hydrophysical Institute, National Academy of Sciences of Ukraine, ul. Kapitanskaya 2, Sevastopol, Ukraine Received October 30, 2012 Abstract Obtained are the estimates of the Black Sea level trends for the period of as derived from the tide-gage and satellite altimetry data. An estimated rate of the mean sea level rate calculated from the averaged altimetry data is 7.6 ± 0.3 mm/year that is by 2 3 times higher than the estimates for the previous periods. Such high values of the trend are evidently associated with the sea level variability features at the 10-year temporal scale. The Black Sea level trend is characterized by the high spatial variability: it amounts to mm/year in the coastal areas of the Black Sea basin that exceeds the trend in the deep-water part by times (4.5 6 mm/year). Such distribution is an effect of the cyclonic Rim Current intensification. Based on the difference in the sea level trends obtained from in-situ and altimetry measurements, the velocity of the vertical crustal motion is estimated for the Ukrainian coastal stations of the Black Sea. DOI: /S INTRODUCTION The sea level variations is an important climatic signal indicating the global environmental changes. The sea level rise influences directly the territories adjoining the water area, leads to coastline changes, and, hence, affects the economy of coastal countries. The majority of studies of the long-term variability of the Black Sea level are based on the measurements from tide gages installed at coastal stations [1 3]. These data embrace the long period of time and enable to obtain the estimates of trends for the period of more than a century [1 3]. However, the tide-gage measurements are carried out at the stations located at significant distance from each other that can result in the errors in the determination of the trend of common sea level associated with the local variability of the signal [20]. As demonstrated in [3, 8, 12], the rate of the sea level rise has a significant spatial variability associated with the ocean dynamics. The influence of these local factors can be rather considerable [12]. Besides, in case of tide-gage measurements, level recorders usually register the motion which is the result of lifting or sinking of the crust at the measurement point. Therefore, a signal measured by the tide gages consists of two parts: the signal associated directly with the sea level variability and the signal associated with the vertical crustal motion. The speed of the crustal movement reaches 5 mm/year at the typical values of the Black Sea level trend of 1 3 mm/year [2, 3], i.e., it makes up the major contribution to the total signal trend. In order to study the climatic variability of the sea level in the region from the tide-gage data, one need to separate the part of the signal associated with the sea level variations only. At the same time, the determination of the speed of the lifting (sinking) of the crust enables to investigate the features of coastal geodynamics. The operation of high-precision satellite altimeters which has started in 1992 since the Topex/Poseidon satellite launch, allowed to obtain the data on sea level variations not only in the coastal part but also in the open part of the Black Sea basin. The errors in the determination of the height of the level surface using the satellite altimetry are small and amount to ~2 cm [14]. The altimetry measurements give information on the absolute deviation of the sea level surface from the reference ellipsoid being a quadric surface roughly approximating the shape of the geoid [14]. Therefore, the estimates of the sea level trend obtained from the altimeter data are the real level variations independent of the lifting or sinking of the crust. Consequently, the difference in the sea level trends obtained from the satellite and tide-gage data indicates changing of the signal due to the vertical crustal motions at the coastal stations. It can be used for separating the respective part of the signal from the tide-gage data [10, 11, 19]. 329

2 330 KUBRYAKOV, STANICHNYI Fig. 1. Black Sea level trends at different points of altimetry tracks for the period of The joint analysis of tide-gage and satellite measurements of the sea level variations enables to study the peculiarities of the long-term variability of the sea level in the whole area of the Black Sea, to compare the estimates of its trends computed by two methods from the data of sea level measurements, and to determine the speed of vertical crustal motions. DATA The authors used the sea level measurement data received from the coastal stations Odessa, Sevastopol, Yalta, and Feodosiya in the period of The measurements were carried out using the pole and the recorder with the temporal resolution of 12 hours for Feodosiya and 6 hours for other stations. The satellite altimetry data are obtained in the form of the sea level anomalies along the tracks of Topex/Poseidon and Jason-1 satellites. The altimetry measurements are analyzed for eight tracks of Topex/Poseidon satellite for the period from April 1993 to 2002 and of Jason-1 satellite for (see Fig. 1). The spatial resolution of the data is ~7 km and the temporal resolution is ~10 days. The data were obtained from the Division for Space Oceanography of Collecte Localisation Satellites (CLS). The regional dataset developed specially for the Black Sea was used. Its main difference from the global dataset is the possibility to obtain the altimetry measurements closer to the shore and the high spatial resolution of the data ( When forming the datasets, traditional correction for such measurements was introduced to the initial altimetry data [14, 18]. However, the standard dynamic atmospheric correction to the high-frequency barotropic response of the sea level to the atmospheric impact was not included because first of all this correction is used for the separation of the level gradients associated with the geostrophic circulation [9]. The full signal of the altimeter taking account of all processes that affect the sea level variations should be used for the joint analysis of the altimetry level with the tide-gage measurement data. BLACK SEA LEVEL TRENDS Using the data of tide gages, the values of the sea level trends were computed for each station by the least-squares method for the period from April 1993 to December 2005: Station Trend, mm/year Odessa Sevastopol Yalta Feodosiya The analysis demonstrated that at the stations located on the coast of Crimea the sea level increases with the rate varying from 1.5 mm/year in Feodosiya to 9.5 mm/year in Yalta. According to the tide-gage data, the mean sea level in Odessa drops with the rate of 4.2 mm/year. The trend averaged for all stations under consideration is mm/year. The Black Sea level trends from the satellite altimetry data were determined using the least-squares method for the spatially averaged level and for each point of altimetry tracks. The mean Black Sea level increases with the rate of mm/year for the period of corresponding to the period of tide-gage measurements (Fig. 2). This value exceeds by two times the estimates obtained from the averaged data of tide-gage measurements. Such noncoincidence of the results is associated with the local variability

3 THE BLACK SEA LEVEL TRENDS FROM TIDE 331 Fig. 2. Integral variations of the Black Sea level and the trend line from the altimetry data (black line; the trend is mm/year) and tide-gage data at Sevastopol station (grey dotted line; the trend is mm/year). of the sea level at coastal stations and, in particular, with the significant contribution of vertical tectonic motions to the total signal of the sea level variations. As noted in the monograph [3], the sea level variations obtained from the measurement data from Sevastopol station correspond mostly to the total change in the Black Sea level. In Fig. 2, the interannual variability of the sea level is presented according to the altimetry data and the measurements at Sevastopol station. The sea level trend for this station was 8.3 mm/year that roughly corresponds to the value determined from the altimetry data. The obtained values exceed significantly the estimates obtained from the tide-gage measurement data in [1 3] for the previous period of , where the computed Black Sea level trend was mm/year. The similar increase in the rate of the sea level rise for the World Ocean is also noted in some papers [12, 13]. In particular, in the review paper [12] it is noted that the sea level trends in the World Ocean in increased from 1.7 mm/year (the value typical of the 20th century) to 3.3 mm/year. Nevertheless, these values are smaller by two times than those obtained for the Black Sea in the present paper. This large difference between different periods can be associated with the peculiarities of the 10-year variability of the sea level that influences greatly the value of the sea level trend (as demonstrated in [16]). It should be noted that the estimates of the trend computed by the authors are smaller by 2 3 times than the values of mm/year obtained in [17] from the satellite altimetry data for the roughly corresponding measurement period of The series of the sea level variability presented in [17] do not correspond to the real the altimetry measurements. In particular, the amplitudes of the altimetry sea level given in [17] are higher by times than those observed in the Black Sea (see [4, 21] for the comparison). Thus, the estimates obtained in [17] are overestimated and do not correspond to the reality. The values of the trend at each point of altimetry measurements, where the full measurement series for the period of existed, were obtained for the spatial analysis of the interannual variability of the sea level (Fig. 1). The computed values of the trend vary from 4.5 mm/year in the deep-water part to 9 mm/year in the periphery of the basin. Such distribution of values can be the effect of the cyclonic Rim Current intensification during that period. Due to the cyclonic vorticity of the Rim Current, the dynamic level increases in the periphery as the velocity of the current increases, and decreases in the deep part. In Fig. 3, the sea level trend distribution is demonstrated along the track No. 83 of Topex/Poseidon satellite passing from south to north in the eastern part of the sea (Fig. 1). It is clear that the considerable increase in the sea level is observed in the area of the cyclonic Rim Current with the rate of mm/year that is mm/year higher than the trend values in the deep-water part. ESTIMATES OF THE SPEED OF VERTICAL CRUSTAL MOTIONS As mentioned above, the sea level trend obtained from the tide-gage measurement data TG is the sum of two trends: of vertical crustal motions W and the sea level variations SL: TG W SL.

4 332 KUBRYAKOV, STANICHNYI Fig. 3. Black Sea level trends along the track No. 83 of Topex/Poseidon satellite for the period of At the same time, the sea level variations determined with the altimeter ALT are counted from the reference ellipsoid and do not depend on the crustal motions. Then, the sea level trends obtained from the altimetry observations depend on the sea level variations only: ALT SL. It follows from the previous formulae that the difference between the values of the rise in the sea level computed from the altimetry data and those registered at the coastal station defines the speed of the vertical crustal motion at the corresponding station [7, 8], i.e., W TG SL TG ALT. To determine W, the tide-gage measurements of the sea level were interpolated for the time of remote measurements at the nearest point of altimetry track. Afterwards, the series were obtained of the difference between the sea level at the station and the sea level at the nearest point of the track. Using the least-squares method, the values of trends for the obtained series of differences were computed. The values of the vertical speed of crustal motion at the stations are the following: Station W, mm/year Odessa Sevastopol Yalta Feodosiya It was obtained that the immersion of the shores with the rate of 5.1 and 8.8 mm/year takes place in the area of Odessa and Eastern Crimea, respectively. At Sevastopol and Yalta stations, the speed of vertical motions is insignificant (0.1 and 0.7 mm/year, respectively). The speed of the crustal motion computed in the present paper using the altimetry data corresponds in part to the estimates obtained in the previous papers. They may vary significantly depending on the methods used (see the detailed review in [3]). In particular, the sinking of the crust in Odessa is corroborated almost in all papers investigating the speed of vertical geodynamic motions [5 7]. The works [5] and [7] with the help of different methods of tide-gage data processing, registered the crust lifting in the area of the mountainous Crimea that corresponds with the results of the present research. The vertical speed of the crustal motion for Sevastopol station computed in [15] using the altimetry data is also close to zero. At the same time, in [6] it is obtained from the analysis of tide-gage data on the sea level that all Ukrainian Black Sea stations are subjected to the immersion of the shores (at Sevastopol station the sinking occurs with the speed of 3 mm/year). CONCLUSIONS In the period of the Black Sea level has risen. The distribution of values of its trend is spatially inhomogeneous and varies significantly for different points of tide-gage and altimetry measurements. The estimates of the sea level trend obtained from the spatially averaged altimetry data amount to mm/year that is more by several times than the estimates for the previous periods. Such high values of the rise of the sea level are evidently associated with the features of the sea level variations at the 10-year temporal scales. The study of the spatial variability of the trend in the Black Sea demonstrated that the rate of the sea level rise rate in the coastal areas is mm/year and exceeds by times the values of the sea level trend in the deep-water part. These differences can be caused by the Rim Current intensification that has the

5 THE BLACK SEA LEVEL TRENDS FROM TIDE 333 cyclonic vorticity and results in the sea level rise in the coastal part and in the sea level drop in the deep-water part of the basin. The estimates of vertical crustal motions at the Ukrainian coastal stations are computed using the altimetry and tide-gage measurements. It was obtained that the sinking of the shores takes place in the area of Odessa and the Eastern Crimea with the rate of 5.1 and 8.8 mm/year, respectively. At Sevastopol and Yalta stations, the lifting takes place with the insignificant vertical speed (0.1 and 0.7 mm/year, respectively). The values of the trend computed from the data at different coastal stations and at different points of altimetry track differ considerably from each other. This stresses that in order to obtain spatially averaged dataset for the whole area of the basin the accurate values of the Black Sea level variations one need to use. 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