Jet Currents in the Eastern Middle Caspian Sea: Satellite Observations, In-situ Measurements, and Numerical Modeling

This paper compares jet currents of surface waters in the eastern Middle Caspian (warm jet streams from the Southern and Northern Caspian to the Middle Caspian and currents in the coastal upwelling zone), identified through the analysis of satellite images of sea surface temperature (SST) and biooptical parameters (chlorophyll-a, suspended matter) from different years, with known results of in-situ measurements and numerical modeling using eddy-resolving hydrodynamic models. Good agreement is noted between the spatiotemporal characteristics of two types of jets of the Southern Caspian origin (propagating along the periphery of the cyclonic gyre over the Derbent Depression in October–June and along the eastern coast within approximately the 50-meter isobath in July–September), identified during satellite monitoring, and the results of numerical modeling. Using daily SST maps from the GHRSST (The Group for High Resolution Sea Surface Temperature) project, the movement of the South Caspian water jet was traced in August 2014 to approximately the latitude of Cape Peschany and then, in the form of a thinner jet with lower SST, to the southern coast of the Tyub-Karagan Peninsula. The estimated propagation velocity of the South Caspian jet based on SST maps (approximately 12–22 cm/s at different stages of its northward progress) also agrees well with the model-estimated average monthly velocity (approximately 8–12 cm/s). The unusual northward propagation of the transverse upwelling jet, observed in successive satellite images in September 2003, can be explained by taking into account known instrumental measurements of the current structure during winds unfavorable for upwelling. The main similarities and differences between the results of satellite monitoring and numerical modeling of jet streams from the Northern Caspian, propagating near the eastern coast of the Middle Caspian from the Tyub-Karagan Peninsula to the south, are discussed.