Investigating dynamic topography variations of the Caspian Sea and their impact on Iranian Shoreline changes using satellite altimetry data

The Caspian Seaclassed as the world’s largest lake, lies between Europe and South Western Asia (between 45.43°to 54.20°longitude east and 36.33°to 47.07°latitude north). The Caspian Sea level has changed widely over time. These changes have occurred gradually and incrementally leading to landward and seaward migration of the coastline. Therefore, it is very important to study and predict futurechanges of the Caspian Seacoastline. Today, experts in atmospheric and marine physics from all around the world consider the Caspian Sea as a natural dynamic model of oscillatory processes in watersurface.High annual rate of water level changeshas made oscillatory processes of this lake different from those of oceans. With the advent of satellite altimetry in 1973, highly accuratemonitoring of sea level has been made possible. The present study seeks to investigate the trend of dynamic topography changes in the Caspian Sea and determine the effects of changes in thesea level on the southern coastline.

Various sets of satellite data have been used in the present study. Long-term average ofglobal sea level data was obtained from MSS_CNES.CLS15. Covering a period of 20 years (1993 to 2012),these datasets are produced based on information received from different satellitealtimeters. Mean sea level is calculated foreach point of the network created atthe Caspian Sea (with a distance of 0.25°). The correlation between altimetry data and sea level changes is calculated using gravity changes. Investigating these changes leads us to equipotentialgeomagnetic surfaces called geoid. Geoid is an equilibrium surface of the Earth’s gravitational field showingapproximately the average leveloffree water. Mean sea level does not coincide with geoid and theirdifference at any given point is called absolute dynamic topography. In this study, GOCE model was used to calculate geoid value at every point of the network created at 1′distance from the Caspian Sea. Aviso Altimetry dataset was used to obtain sea level anomaly data. Mean sea level was obtained by adding dynamic topography mean to geoid height.In order to obtain average dynamic sea topography,MDT values were calculated for all the points created in the Caspian Sea. Afterwards, sea level anomaly was added to the mean dynamic sea topography to obtain absolute dynamic topography. Daily SLA data of the Caspian Sea were extracted with a resolution of 0.25° from AVISO and CNES.CLS15 SLA ultrasound satellites and interpolated at the specific location created on the Caspian Seanetwork with a resolutionof 1′.Aabsolute dynamic topography were calculated on a daily basis. These calculations were repeated for a 20 year period (7305 days) from 1993 to 2012 using MATLAB and in this way, a complete database including the Caspian Sea surface topographic datawas obtained for this period.

Following the calculation of the mean ADT data obtained fromall over the Caspian Sea, time series of daily Sea Level Fluctuations were extracted. These time series indicated that despite the positive trend of the Caspian Sea water level changes in both 1993-1995 and 2000-2005 periods, the overall trend of water level changes over the 20-year period is negative. Moreover, examining sea level changes over this 20-year period shows thatthe highest altitude (-25.914m) has occurred on June 1st, 1995, while the lowest altitude (-27.20) has occurred on November 26th, 2012. In addition, March 20th, 2002 and June 29th, 2005 have experienced two abrupt changes  ​​of -26.843m and -26.26m in the time series. In this time series, an upward trend is observed until June 1st, 1995, while a decreasing trend of 93 cmis observed from March 20th, 2002 over a period of approximately 7 years. Between March 20th, 2002 to June 29th, 2005 (a period of approximately 3 years), we observe a decreasing trend of 61 cm. Over a 7-year period (until late 2012), we also observe a 97cm decreasing trend. Altimetry data received from three stations located in the Caspian Sea are used to verify the results obtained from the above mentioned method. Examination of these values and comparing them with the values obtained from the method used in the study confirms the resulting trend. In orderto investigate the shoreline changes caused by changesin the Caspian Sea water level,the southern shoreline of the Sea is mapped based on the obtained trend.Days with the highest and lowest sea level over the 20-year study period were extracted from satellite images. Mapping and overlayingthe coastlines based on the information related to these two time series, changes have been observedthroughthe Caspian coastlines. However, these changes are more significant in the South Eastern Gorgan Bay (Miankale) due to the smaller slope of the South Eastern Caspian Sea compared to other areas of the Sea.

Investigating changes of the Caspian Sea level shows anegativetrend of changes, with a -1.287 m difference between thehighest and lowest altitudes. Of course, the trend has not always been negative over these years. For an instance, a positive trend was observed from 1993 to1995 and from 2000 to 2005. Results indicate that the Caspian Sea dynamics of water level fluctuations changes rapidly and long-term prediction of the Caspian Sea water level cannot be very accurate. However, it can be concluded that the Caspian water level changes will continue its decreasing trend in the future. This negative trend of sea level changes has resulted in the seaward migration of the Caspian coastline, which has began in 1995 and still is present today. This has resulted in drying up of more than 12850 hectares of the GorganGulf.