Formation of the coastal current in the Black Sea caused by spatially inhomogeneous wind forcing upon the upper quasi-homogeneous layer

A poorly known mechanism of formation of a warm coastal current by spatially nonstationary and nonuniform wind forcing in the northeastern part of the Black Sea is described. Owing to the blocking influence of the Caucasian Mountains, the northeasterly wind (nordost) west and east of Tuapse is characterized by a spatially nonuniform speed distribution over the Russian sector of the sea. In the first half of the year, during the nordost wind, the intense wind forcing upon the upper quasi-homogeneous layer (UQL) leads to its fast cooling due to the turbulent entrainment of cold waters from the shallow seasonal thermocline. Satellite data and measurements from ships obtained at the end of June-beginning of July 2006 showed that, during two days, the temperature of the UQL west of Tuapse dropped by 7–10°C, whereas, east of Tuapse where the wind was weak, it practically did not change. As a result, a narrow frontal zone between the warm (less dense) and cold (denser) upper layer waters oriented quasi-normally to the shore was formed. A hydrodynamic analysis showed that, in such situation, an intense jet of a near-shore density current is developed that transports warm waters to the northwest at a velocity of 40–60 cm/s. This estimate agrees well with satellite data and observations from ships. The satellite data allowed us to find that this jet reached the Kerch Strait region in a few days. During the time of its existence (approximately 2 weeks), the jet transports a large volume of water and can change the functioning regime of the coastal ecosystem. The mechanism of the near-shore current formation revealed may also function in other seas and oceanic regions.