Recent warming and stratification changes in the southern Tyrrhenian Sea

Abstract

Warming of the upper ocean is expected to strengthen stratification and reduce the efficiency of wind-driven mixing, affecting water-column stability, nutrient supply, biological productivity, and oxygen ventilation. Despite its importance in the climate system and its rapid change, long-term observations of water column stratification remain limited in the Mediterranean Sea. Additionally, current knowledge on increasing stratification in the southern Tyrrhenian Sea does not clarify whether observed changes reflect local dynamics or a broader regional response. Here we analyze a decade (2015–2025) of bimonthly CTD profiles collected at a fixed offshore site along the Cilento coast, complemented by Mediterranean Sea Physics Reanalysis fields spanning 1987–2025, to quantify changes in thermohaline properties and vertical stratification. The results reveal a pronounced seasonal cycle and marked interannual variability. Trend estimates indicate significant upper-ocean warming, increasing heat content, decreasing density, and a strengthening of thermally driven stratification. Mixed layer depth shows a tendency toward shoaling, particularly during winter, although its magnitude depends on the dataset and sampling strategy. Multi-decadal ocean reanalysis indicate that these changes are not isolated but part of a longer-term evolution of the upper ocean, with the last decade representing its most pronounced phase. The agreement between in situ observations, multi-decadal ocean reanalysis, and previous regional studies supports the interpretation that the changes observed in the study area are consistent with the ongoing transition of the Mediterranean upper ocean toward warmer and more stratified conditions, with potential implications for marine and coastal ecosystems.