Thermohaline Intrusions as Seen by Argo Floats: The Case of the Black Sea

Abstract Understanding the dynamics of thermohaline intrusions is crucial for predicting changes in water masses and their impact on marine ecosystems, especially in highly stratified semi‐enclosed seas and other coastal environments. We use high‐resolution (up to 1–2 m) Argo profiling float data collected over 15 years in the Black Sea, an excellent test area for studying thermohaline intrusions. Our analysis challenges the conventional view of stagnant intermediate and deep waters, revealing active mixing processes that reshape the thermohaline structure. We identified two main mechanisms driving these intrusions, related to dense water inflows from the Marmara Sea and boundary mixing enhanced by frontal instabilities. Argo data also allowed us to identify areas with favorable conditions for double‐diffusive processes. The variability of intrusions is due to changes in the thermohaline state of the upper ocean as well as to quasi‐periodic changes in the inflow caused by local conditions. Trends in the intensity and frequency of intrusions indicate shifts in water mass properties that are likely to be associated with climate variability and extreme weather events. Such trends can affect nutrient cycling, oxygen distribution and the overall stability of the water column, thereby affecting biogeochemical cycles and the resilience of marine ecosystems. Similar ventilation mechanisms may operate in other highly stratified marine systems, such as the Baltic Sea and the Arctic Ocean, so our findings may have wider implications for understanding climate‐induced changes in water masses at regional and global scales.