On the Applicability of CMEMS Reanalysis Data for Investigation of the Cold Intermediate Layer in the South-Eastern Part of the Baltic Sea

Vertical thermohaline structure in the southeast part of the Baltic Sea is complicated: The salinity field has two layers due to the general estuarine circulation, while the water temperature stratification changes seasonally from two-layered to three-layered. Simulation of the re-stratification processes in such a basin is still a challenging problem, even for the most advanced models. In this study, using a comparison with the independent field data, we have examined how well the structure of the Baltic Sea Cold Intermediate Layer (CIL) is reproduced by the Copernicus Marine Environment Monitoring Service (CMEMS) reanalysis data. It was found that the water salinity within the CIL and the halocline is overestimated, and the upper CIL boundary (the thermocline) in the reanalysis data is considerably shallower than that in the field data. These conclusions are similar to other studies comparing model and field data in the Baltic Sea for various applications. The reanalysis data demonstrate certain deviations in the water temperature, salinity, and depth values at the CIL upper and lower boundaries. However, the general structure of the CIL is reproduced well, including its salinity-homogeneous and salinity-gradient sublayers. Moreover, the T-S diagrams indicate that the salinity-gradient sublayer in the south-eastern and central basins in summer contains waters similar to those observed within the upper mixed layer in spring in the south-western sea basins. This confirms the applicability of the CMEMS reanalysis (with certain caution regarding the temperature/salinity/depth values) for the investigation of such a complicated process as the formation of the Baltic CIL in spring when the seasonal vertical mixing ceases and denser waters from the eastern part of the sea are expected to renew the lower part of the CIL in the Baltic Proper.