Dense Water Formation in the North–Central Aegean Sea during Winter 2021–2022

Abstract

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 σθ>29.1 kgm−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 kgm−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 σθ∼29.35 kgm−3 horizons. The 29.1 kgm−3 isopycnal rose by ∼200 m, and the 29.25 kgm−3 isopycnal overflowed above the ∼400 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.

Evidence for Reduced Black Sea Water Outflow to the North Aegean

“…Long records of Sea Surface Salinity (SSS) from the Marmara Sea (Beşiktepe et al, 1994) suggest that the reduction in buoyancy input could also be attributed to more salty BSW being exported through the Dardanelles in winter 1987. Observational evidence (Beşiktepe et al, 1994;Potiris et al, 2024;Sayin et al, 2011), as well as numerical simulations (Potiris et al, 2024;Vervatis et al, 2013), clearly suggest that the areas of the Central and North Aegean most prone to deep-water production are those areas lacking the surface BSW layer at the surface, that is, areas of high SSS. The surface BSW not only hinders dense water formation by isolating the intermediate and deeper layers, but also reduces heat-losses to the atmosphere (as a cooler surface water mass) and thus controls the magnitude and sign of buoyancy fluxes, as recently clarified by Tragou et al (2022).…”

Design and Optimization of the Teardrop Buoy Driven by Ocean Thermal Energy

“…The introduction of the Argo program, coupled with rapid advancements in underwater exploration technology, has created an enabling environment for the advancement of exploration buoys [1]. Currently, there are over 3000 profiling buoys actively operating in the ocean, making significant contributions to marine environmental observation, weather forecasting, and monitoring atmospheric changes, among other critical areas [2,3]. Conventional buoys rely on batteries for power, which cease functioning once their power is depleted.…”