The evolution and drivers of dense water formation (DWF) in the North–Central Aegean Sea (NCAeg) during winter 2021–2022 are studied using observations from two Argo floats and the output of an operational data-assimilating model. Dense water with ${\sigma }_{\theta }>29.1 {\mathrm{kg}\mathrm{m}}^{-3}$ was produced over most of the NCAeg, except for the northeastern part covered by Black Sea water (BSW), where the maximum surface density was $<29 {\mathrm{kg}\mathrm{m}}^{-3}$. The highest density waters were produced over the central and southern parts of the Lemnos Plateau and in the shallow coastal areas between Chios Island and the Edremit Gulf. Atmospherically driven transformation to the east of Lesvos Island resulted in the production of waters with anomalously high density and salinity, which flowed inside Skiros Basin, thus partly explaining its historically higher density and salinity compared to the rest of the NCAeg subbasins. The Skiros and Athos Basins were ventilated down to ${\sigma }_{\theta }$∼$29.35 {\mathrm{kg}\mathrm{m}}^{-3}$ horizons. The $29.1 {\mathrm{kg}\mathrm{m}}^{-3}$ isopycnal rose by ∼$200 \mathrm{m}$, and the $29.25 {\mathrm{kg}\mathrm{m}}^{-3}$ isopycnal overflowed above the ∼$400 \mathrm{m}$ sill depth filling the southern depressions of the NCAeg. Combining data from Argo floats, vessel casts, gliders, and a fixed-point observatory, the dense water produced in the NCAeg was observed spreading in the deep layer of the Central Cretan Sea for at least one and a half years after the formation. The cyclonic circulation of the newly formed water in the NCAeg has been observed directly for the first time using deep-drifting floats. The Eastern Mediterranean warming and salinification signal has propagated below the NCAeg sill depth. The winter average buoyancy loss was comparable to that of the peak of the Eastern Mediterranean transient (EMT) and other known years of DWF in the NCAeg; however, the high temperature of the upper layers due to long-term warming prevented the widespread formation of denser water.

The formation of dense water in the Aegean Sea is important as it affects the deep circulation and the hydrography of the Eastern Mediterranean Sea. In this study, the variability of dense water formation is investigated in relation to forcing mechanisms from 1947 to 2023 in the subbasins of the Aegean Sea, utilising in situ observations from various sources, which have been analysed in combination with satellite altimetry and reanalyses products. The analysis reveals that the Aegean Sea has been in a state of increased dense water formation since 2017 due to the combination of increased surface buoyancy loss and reduced Black Sea water inflow. Extremely high salinity has been recorded in the intermediate layers of the Aegean Sea since 2019. The anticyclonic circulation of the North Ionian gyre during 2017 and 2018 probably also contributed to the rapid transport of highly saline waters in the intermediate and, through dense water formation, the deep layers of the Aegean Sea in 2019. Until 2022, the dense waters formed during the peak of the Eastern Mediterranean Transient still occupied the bottom layers of some deep subbasins of the North and South Aegean; however, the $29.4 \mathrm{k}\mathrm{g} {\mathrm{m}}^{-3}$ isopycnal in the North Aegean and the $29.3 \mathrm{k}\mathrm{g} {\mathrm{m}}^{-3}$ isopycnal in the Southeastern Aegean have gradually deepened by 800 m, permitting the waters forming in the last ten years in the Aegean Sea to settle at ever greater depths. Temperature controls the density variability of the Cretan intermediate water up to the decadal time scale. Increased data availability since 2010 was sufficient to clarify that intrusions of dense water from the North–Central Aegean Sea contributed to the erosion of the Eastern Mediterranean transitional waters in the South Aegean Sea after 2017, as well as to raising the intermediate water masses of the South Aegean to shallower depths. The erosion of the transitional Mediterranean waters in the South Aegean Sea between 1947 and 1955 and 1973 and 1980 coincided with increased dense water formation in the North–Central Aegean Sea. During the peak of the Eastern Mediterranean Transient, the North Ionian circulation, the Black Sea water inflow, the Atlantic Multidecadal Oscillation, and the surface buoyancy fluxes favoured dense water formation in the Aegean Sea.

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.