Record-high salinity and interannual dense water formation variability in the Aegean Sea coincide with reduced inflow of Black Sea Water

The observed warming and salinification of the eastern Mediterranean Sea (EMed) during the last decades could impact the dense water formation (DWF) and consequently the thermohaline circulation of the basin. The main drivers of interannual DWF variability in the EMed are the atmospheric heat fluxes, the internal redistribution of salt, and the exchange of heat and salt at its straits. Extremely high salinity has been observed recently in the Levantine and Adriatic Seas due to the superposition of the basin’s decadal variability and long-term trend.

In the Aegean Sea, a major DWF site of the EMed, record-high salinity is recorded in the upper and intermediate layers since 2018. Argo floats observations also show that the Aegean Sea is in the most prolonged state of increased DWF after the Eastern Mediterranean Transient (EMT) period.

The causes of increased salinity and DWF in the Aegean Sea were investigated using: 1) in situ hydrographic observations, 2) satellite observations of sea surface height (SSH), 3) in situ- and model-based gridded hydrography reanalysis products, 4) atmospheric forcing from model reanalysis, and 5) information of Black Sea Water (BSW) inflow in the Aegean Sea from a box model, literature, and an SSH-based index coined in this study.

The long-term increase of salinity in the Aegean Sea follows that of the EMed, and its interannual/decadal variability is dictated by the reversals of the North Ionian circulation and the inflow of BSW. The BSW inflow has a negative trend which results in the decreased dilution of the Aegean Sea and the reduced salinity difference between the Levantine and Aegean Seas, especially after 2012. Surface buoyancy loss also presents significant decadal variability, with peaks in 1993/2003/2012/2022 and anomalously high winter-mean heat loss from 2017 onwards, which coincides with the known post-EMT DWF events in the Aegean Sea. The record-high salinity observed in the Aegean Sea is attributed to the decreased BSW inflow and the anticyclonic reversal of the North Ionian circulation. The decreased BSW inflow and increased surface heat loss can explain the persistence of high salinity and increased DWF in the Aegean Sea from 2020 onwards, despite the cyclonic circulation of North Ionian Sea and the drop of salinity in the Levantine Sea since 2019. In particular, analysis from Argo observations and output of a data assimilating hydrodynamic model developed in the context of the project “Coastal Environment Observatory and Risk Management in Island Regions AEGIS+” revealed widespread formation of water with σ₀ > 29.35 kg/m^-3.

Post-EMT periods of minor DWF in the Aegean Sea coincide either with the anticyclonic circulation of the North Ionian Sea or with the increased surface buoyancy loss over the Aegean Sea, as a synchronous DWF-favoring phase of both drivers has not occurred yet. However, the common denominator of all major post-EMT DWF events in the Aegean -also true for the EMT period- is the reduced inflow of BSW, which seems to be controlled by the freshwater budget of the Black Sea rather than the temperature-driven increase of SSH in the Mediterranean Sea.