Abstract
AbstractMore than 6,000 profiles from profiling floats in the Black Sea over the 2005–2020 period were used to study the ventilation of this basin and the mixing pathways along isopycnals. The layer of the minimum potential vorticity (PV), the Black Sea pycnostad, approximately follows the core of the cold intermediate layer, similar to the case of oceanic mode waters. However, unlike in the ocean, the horizontal patterns of PV are shaped by cyclonic gyre circulation. There is a principle difference in the probability distribution of the thermohaline properties presented in geopotential coordinates from those presented in density coordinates. In the latter case, several mixing pathways, which are not known from previous studies, dominate the ocean states. These formed after three intermittent events of cold water formation. The density ratio decreased three times during the last 15 years, revealing the decreasing role of temperature in the vertical layering of the Black Sea halocline. The basin‐wide distribution of PV above σθ = 16, which is where the maximum vertical density gradient appears, is opposite to the distribution below this depth. This finding suggests a complex change in the mesoscale dynamics in different layers. Comparisons of observations with data from the Copernicus Black Sea operational model demonstrate that the mixing parameterizations of models need further improvements.
Highlights
The cold intermediate layer (CIL) in the Black Sea is a layer of minimum temperature sandwiched between the seasonal thermocline and the permanent halocline
The observations collected from profiling floats contributed substantially to advancing the oceanography of the Black Sea
It was shown that the cold intermediate water (CIW) has much in common with the mode waters in oceans
Summary
The cold intermediate layer (CIL) in the Black Sea is a layer of minimum temperature sandwiched between the seasonal thermocline and the permanent halocline. The CIL is formed in the winter as a result of strong cooling. During CIL formation, surface waters are subducted, and the subsurface layers are ventilated. The formation of cold intermediate water (CIW) is spread over large areas with regional enhancements due to specific dynamics, such as slope convection, fronts, open ocean eddies and coastal anticyclones V. Stanev & Staneva, 2001; E. V. Stanev et al, 2003). The strong stratification in the seasonal thermocline tends to reduce vertical mixing, shielding the CIL from the warm surface water in summer
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