Direct measurements of sea currents on the Mangistau shelf of the Caspian Sea 

<p>The Mangistau shelf refers to the area off the coast of the Mangyshlak (Mangistau) Peninsula in the Kazakh sector of the Caspian Sea. This area is important in many socio-economic respects, including oil and gas production, fisheries, maritime transport and tourism. Obviously, information about sea currents is essential for ensuring these activities. However, for the Mangistau shelf, the most recent in situ data on currents, as well as on other oceanographic characteristics, date back to 1988, and even then such measurements in this area were only episodic. Over the past decades, the climate in the Caspian Sea region and the level of the Sea itself has changed significantly, and it can be expected that the regime of shelf currents has also undergone changes.</p><p>As part of a joint Russian-Kazakh project, moored autonomous buoy stations equipped with current meters, as well as water temperature gauges were deployed at two shelf locations for the time period of 69 days. The stations were installed on June 2 and recovered on August 9, 2021. The measurements during the entire observation period were conducted at 20-min sampling rate. We also used 20-min data on wind speed and direction, as well as on air temperature, obtained at a nearby meteorological station. In addition, satellite images of the infrared and visible bands from  MODIS Aqua radiometer (total of 39 images for the period of the field measurements) were analyzed. At the beginning of the moorings operation period, a CTD survey of the thermohaline structure from the surface to the bottom was performed at each of the stations.</p><p>The interpretation of the newly obtained data points on the following conclusions about the pattern of the coastal circulation on the Mangistau shelf during the study period. We hypothesize that this circulation is formed under the influence of two competing mechanisms. Firstly, the direct local action of the northwestern winds prevailing in this region contributes to the development of currents in southeasterly direction. Secondly and oppositely, the larger-scale factors such as vorticity of the wind stress and basin-scale variability of the thermohaline fields, as is known, tend to lead to the formation of a cyclonic circulation in the Central Caspian and, consequently, the development of currents of a northwestern direction along the eastern coast of the sea. In periods when the northwest wind stress is sufficiently strong, the first mechanism prevails, and an alongshore current to the southeast is formed, accompanied by coastal upwelling, clearly expressed in enhanced thermal stratification and presence of very cold waters at shallow depths. However, during periods of weakening of the local wind, the balance of forces controlling the circulation changes in favor of the second mechanism, and a cyclonic current in the northwest direction develops, while upwelling stops or weakens, which leads to an increase in temperature in the bottom layer and a gradual relaxation of stratification. This conceptual scheme, however, is still only hypothetical, and its confirmation requires further, longer-term studies.</p>