Purpose. The purpose of the work is to study the spatiotemporal evolution of the cold intermediate layer in the deep part of the Black Sea in 2000–2021.

Methods and Results. The processes of formation of the cold intermediate layer in the Black Sea and its temporal evolution during 2000–2021 are studied based on the integrated application of satellite measurements of sea surface temperature, measurements from Argo floats, and two reanalyses providing regular three-dimensional temperature fields. Within the period under study, four stages of evolution were identified. During the first stage, from 2000 to 2005, the cold intermediate layer was close to the climatological norm, was actively renewed by winter convection, occupied the entire deep part of the sea and remained continuous almost until the end of the year. In 2006–2011 (the second stage), powerful ventilation in 2006 was followed by warm winters that induced the erosion of the cold intermediate layer off the southern and eastern coasts, its fragmentation and a reduction in its cold content. During the third stage (2012–2016), the cold intermediate layer was temporarily restored due to the cold winter of 2012, but during the subsequent warm years it became seasonal, disappearing completely by late summer. During the fourth stage (2017–2021), even after the cold winter of 2017, the cold intermediate layer moved closer to the surface, which accelerated its degradation, and by 2020 the layer with a core temperature below 8°C had completely disappeared.

Conclusions. During the period from 2000 to 2021, the classical cold intermediate layer of the Black Sea, with a core temperature below 8°C, underwent significant degradation, culminating in its complete disappearance by 2020. The main reason for its degradation is regional climate change, which is characterised by an increase in the frequency of warm winter periods and a reduction in the intensity and duration of winter convection. The acceleration of the cold intermediate layer degradation is also related to the rise of its core towards the surface, a decrease in its vertical extent, disruption of its continuity, and increased horizontal mixing by large-scale currents. The results obtained from reanalysis data are consistent with field observations and confirm the tendency for the cold intermediate layer to lose its classical defining characteristic, namely the presence of the 8°C isotherm. This fact indicates a significant restructuring of the thermal structure of the Black Sea.