Why does a cold layer remain beneath the Black Sea surface during summer?

The cold layer that persists beneath the surface of the Black Sea during the summer is a remnant of the previous winter's cooling.

During the winter, the sea undergoes significant cooling, which creates an upper mixed layer that extends to depths of approximately 160 to 330 feet (50 to 100 meters). As summer arrives, solar heating warms the surface waters to temperatures between 73 and 79 °F (23 and 26 °C). However, because the vertical intermixing of water in the Black Sea is limited due to its stratified nature, this warming does not penetrate deeply. Consequently, the water at depths of about 160 to 240 feet (50 to 75 meters) remains at the cooler temperatures established during the winter, typically around 45 °F (7 °C). This phenomenon is an example of a seasonal thermocline, where a distinct layer of temperature transition forms due to seasonal solar heating.12