Interdecadal Variability of the Black Sea Cold Intermediate Layer and Its Causes

The long-term variability of the Black Sea intermediate layer temperature is analyzed using an updated database for the period from 1951 to 2017. The existence of interdecadal temperature variations in the vicinity of the cold intermediate layer (CIL) with a period of similar to 50 years and the amplitude to 0.7 degrees C is confirmed. It is shown that the CIL cooling/warming is accompanied by its upward/downward displacement. The analysis of temperature fields in the sur face and subsurface layers constructed from data averaged over different decades revealed that the cold and warm climate periods are characterized by changes in the ratio between the intensity of the two main CIL sources: winter convection in the open sea and subduction of cold water from the northwestern shelf. A dramatic sea surface temperature rise during the recent 20 years led to the fact that currently observed temperature minima in the upper 100-m layer considerably exceeded the minima registered during the period of the previous quasiperiodic temperature rise in the mid-20th century.