Spatiotemporal Dynamics of Vertical Mixing Hotspots in the Caspian Sea: Physical Drivers and Ecological Implications

Vertical mixing is fundamental to thermohaline circulation, deep-water ventilation, and biogeochemical cycling in enclosed seas, yet its spatiotemporal variability remains poorly quantified. Using a validated ۹-year (۲۰۱۰–۲۰۱۸) ۳D ocean circulation model, we identify three distinct vertical mixing hotspots with unique physical drivers: (۱) Deep Basin winter convection (December–March, Kh ≈ ۰.۰۱ m²/s), driven by surface cooling; (۲) Eastern Slope upwelling/frontal hotspot (June–September, Kh ≈ ۰.۰۱ m²/s), where shear instability and internal wave breaking overcome strong stratification (N² ≈ ۰.۰۰۰۵ s⁻²); and (۳) Volga Shelf river plume hotspot (April–August, Kh ≈ ۰.۰۱–۰.۰۰۱ m²/s), driven by plume instabilities and bottom friction. From ۲۰۱۵ to ۲۰۱۸, winter mixed-layer depth decreased by ۳۰–۵۰ m and surface Kh declined by a factor of ۲–۳, consistent with recent interannual variability and strengthened stratification in the region. In contrast, deep Kh (۱۰۰–۲۰۰ m) showed a slight increase (typically ۰.۰۰۱ to ۰.۰۱ m²/s), indicating vertical decoupling. Nutrient flux estimates show the Eastern Slope sustains summer supply on the order of ۱۰ to ۱۰۰ μmol N m⁻² day⁻¹ (one to two orders of magnitude higher than the stratified interior ≈ ۰.۱ μmol N m⁻² day⁻¹), explaining persistent coastal productivity. Deep-water ventilation timescales exceed ۲۰۰۰ years below ۳۰۰ m, highlighting extreme vulnerability to hypoxia in the isolated deep layers. As basin-scale winter convection weakens under warming, lateral-vertical exchange via slope mixing hotspots becomes increasingly critical. These findings provide a mechanistic framework for physical-biological coupling in enclosed basins and inform fisheries management and climate adaptation in the Caspian Sea and similar systems worldwide.

کلیدواژه ها:

Vertical mixing ، Eddy diffusivity ، Mixing hotspots ، Caspian Sea ، Nutrient transport