Climate Change and Regional Ocean Water Mass Disappearance: Case of the Black Sea

Abstract

Data from profiling floats reveal that climate change in the Black Sea leads to the disappearance of specific water masses. The observed thermohaline change is possibly an amplified precursor of the changes to expect in the greater oceans. The warming trend in the cold intermediate layer (CIL) of ~0.05 °C/year was more than double the trend in previous decades, and its temperature approached that of the waters in the deeper layers (~9 °C), which signified its disappearance. This evolution was due to the warmer winters over the last 14 years. Intermittent major cold water formation events (only three during this period) could not sufficiently refill the CIL. A “density constriction” (minimum spread of density) was found at ~14.25 σt, and it separated two thermohaline regimes: temperature-dominated above and salinity-dominated beneath. Below this depth, the CIL is seen as a “corner” in the T-S diagram at ~18 salinity and ~9 °C, where the profiles make a curve. The variability in the T-S relationships at given σt levels in the CIL revealed trends dominated by diapycnic mixing with deeper layers (the high-salinity pool acts as a source of salt for the upper ocean). After 2010, salinity anomalies started to occur rhythmically with increasing amplitudes at the depths of the CIL. In the absence of a pronounced CIL in recent years, the relative role of salinity variability in the thermohaline state of the upper layers increased. Further trends and the plausibility of similar events in other similar environments are also addressed.