Mesoscale Water Dynamics and Chlorophyll-a/TSM/SST Distributions off the Eastern Coast of the Middle Caspian

The study of seasonal changes in water dynamics and the corresponding distributions of chlorophyll-a (Chl-a) and Total Suspended Matter (TSM) in the Kazakhstan sector of the eastern coast of the Middle Caspian Sea (42–45°N, 49.5°N – coastline on the East) was carried out based on a joint analysis of synchronous satellite-derived maps of sea surface temperature (SST), concentration of Chl-a and Water Leaving Radiance (0.551 µm) in different months of the year (July–December 2023 and January–March 2022). It has been established that in summer, with pronounced density (thermal) stratification of waters, the main source of waters rich in chlorophyll-a and suspended matter entering the surface layer of the study area are periodically breaking through from the Northern Caspian streams of water with a width of approximately 10 to 40 km, spreading in the meridional direction from the southern coast of the Tyub-Karagan Peninsula to the south for a distance up to approximately 250 km. The dynamics of such jets have been identified as follows: they propagate mainly along the front of low-productivity (in the surface layer) coastal upwelling with its inherent elements of instability; in the region of approximately 43°30’N–44°N, a division of the jet into two streams is sometimes observed, one of which continues to move south, while the other is directed northwest along an approximately 100-meter isobath. In the cold period of the year (October–March), with intensive vertical mixing of waters, increased concentrations of Chl-a and TSM correspond to coastal areas of reduced SST due to seasonal cooling, mainly within the 50-meter isobath. An additional source of increased concentration of Chl-a seaward of 20–50 m are waters from the Southern Caspian involved in movement along the periphery of the Derbent cyclonic gyre.