Variability in the Cretan Sea (Eastern Mediterranean) from six years’ glider observations (2017–2023)

Abstract. The Cretan Sea is an intermediate and occasionally deep water formation area within the Eastern Mediterranean that accumulates and transforms water masses from the adjacent Aegean, Levantine and Ionian Seas. Six years of glider observations (2017–2023) in the Cretan Sea were analysed to study the properties, variability and dynamics of the water masses during the study period. The analysis revealed progressive warming and salinification of the intermediate and deep layers. The mean temperature increased by around 0.05–0.07 °C per year, and the salinity by approximately 0.02 per year. Furthermore, comparisons with climatological data from 2000 to 2015 show temperature departures of + 0.4 to 0.6 °C in the upper 400 m and salinity increases of up to + 0.3 at the surface. Both of these values decline with depth, highlighting the intensified warming and increased salinity near the surface and in the upper intermediate layers. Additionally, the analysis of salinity and temperature datasets revealed the formation of intermediate water annually, except in winter 2022 when an intense mixing event occurred in the Cretan Sea triggered by exceptionally cold atmospheric conditions. The mixed layer, as captured by the glider, extended below 600 m inside the Cretan basin. These newly formed waters almost reached the deep layers, significantly modifying the properties of the intermediate and deep waters, although full deep convection was not reached. The observed downward displacement of the TMW core below 1000 m is associated with the strong convective event as well as with the redistribution of the heat and salt in the intermediate and deep layers. These findings emphasize the importance of sustained, high-resolution observations in capturing both gradual trends and extreme events, and in improving our understanding of the evolving thermohaline circulation of the Eastern Mediterranean.