Numerical Investigation of the Upper Biologically Acting Turbulent Layer of the Black Sea

Abstract

As it is known, the Black Sea ecosystem a dramatically changed during the last few decades. Upper mixed layer of the Black Sea, which is biologically active is polluted by many highly toxic substances. They are getting in the Black Sea by inflow of rivers and ground waters. These substances heavily loaded with nutrients containing nitrogen and phosphorus and contaminated with industrial and mining wastes. Anthropogenic eutrophication developed in the Sea a dramatically destroys the marine food chain and living biological organisms. Besides, the oil floods became intensive in last years as sea transportations have significantly increased. It is clear, that the process of transport and evolution of any polluting substance is closely connected to dynamic processes ( circulation, wind-driven turbulence , etc.) Thus the modeling of spreading of the substances with underlying some biochemical transformations are a complex problem, which in turn includes the modeling of marine dynamic processes. In this paper some features of the Black Sea upper mixed layer (UML) are numerically investigated by using 3-D basin-scale model of the Black Sea dynamics (BSM-IG, Tbilisi, Georgia). For this goal this model is functioned with variable coefficients of vertical turbulent eddy viscosity and diffusion (VTV-VTD during) one modelling year. These coefficients as last version of the Pacanovsky -Philande parameterization (PP-P) developed by Bennis are embedded in the model. The Black Sea UML depth variability is estimated by using a well known criteria of temperature Dt-0.2 . Using criteria of temperature the results analysis shows: : On the one hand the mixed layer has physically different configuration and variable depths which is undergoes a significant variability in the inner annual time scale ; on the other hand it is combined to the following results: In wintertime the intense wind-driven turbulence promotes strong mixing. Mixed layer has almost same homogenous structure when above Black sea the strong wind was replaced by weak wind. Besides, it is characterized by the clear tendency to deepen mixed layer depth approach to 26m especially in Georgian water area . In the same time gradient of Richardson numbers at the Sea upper levels varied in the ranges 0 <R <<1 and R >> 1 are in a good agreement according to these strong and weak wind respectively. This result demonstrates that: In wintertime similar alternation of the winds affect and change circulation of the Sea. It in turn in small time does not have the ability to transform the structure of the mixed layer . This result is in direct connection of the thermohaline impact, which plays the additional role to grows UML depth in the Black Sea.