Eastern Mediterranean Transient Research Articles

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

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.

A High-Resolution Numerical Model of the North Aegean Sea Aimed at Climatological Studies

A new, high-resolution model for the northern part of the Aegean Sea, aimed primarily at climatological research (relaxation and data assimilation-free climate simulations), is hereby presented, along with the results of a 28-year-long simulation covering the period from 1986 to 2013. The model applied is the Regional Ocean Modelling System (ROMS). A significant improvement over previous models of the Aegean introduced in this work is the replacement of parameterizations of the Dardanelles exchange by a fully three-dimensional simulation of the flow in the Strait. The incorporation of part of the Marmara Sea in the model domain enables the interaction with other regional climate simulations, thus allowing climatic variability of the exchange of the Mediterranean and Black Seas. An extensive validation is carried out comparing the model output with all the available observations from several different platforms, i.e., satellite sea surface temperature and height, T/S profiles from R/V ships, and HF radar surface currents velocity. We focus on the model’s ability to reproduce, to some extent, the distinct thermohaline features and circulation patterns that characterize this specific area of the Mediterranean Sea. Our findings, after comparing simulation results with all the available observations, revealed the model’s sufficiency to simulate very adequately the complex hydrology of the North Aegean Sea, and the model’s ability to reproduce incidents of deep-water formation that took place in the region in previous decades during the Eastern Mediterranean Transient (EMT).

Temporal evolution of oxygen and nutrients in the NE Ionian Sea from 1987 to 2008 under the influence of the Cretan and Adriatic Seas

The Eastern Mediterranean Transient (EMT) was a major climatic event, as it affected profoundly and for a long time period, the circulation of the deep and intermediate layers, with important consequences on the distribution of oxygen and nutrients in the Ionian Sea and more generally, in the whole Eastern Mediterranean basin.In this paper, nutrient and oxygen data collected by the Hellenic Centre for Marine Research, in the Northeastern Ionian Sea, the Western Cretan Sea and the western straits of the Cretan Arc, during the period 1987–2008, were used in order to reveal the detailed pattern of oxygen and nutrient distribution in the NE Ionian Sea, their temporal and spatial variability and their relevance to the two competing sources of deep water, namely the Adriatic and the Cretan Seas.The distributions of oxygen and nutrients before, during and after the EMT are presented and discussed in relation to the physical processes, the circulation and the dynamics of the area. In the post-EMT period there is evidence that deep waters of Adriatic origin are again present in the abyssal depths of the Ionian Sea. However, the oxygen and nutrients concentrations as well as the thermohaline properties in the study area are still far from those observed in the pre-EMT phase.

Recent Changes in Deep Ventilation of the Mediterranean Sea; Evidence From Long-Term Transient Tracer Observations

The Mediterranean Sea is a small region of the global ocean but with a very active overturning circulation that allows surface perturbations to be transported to the interior ocean. Understanding of ventilation is important for understanding and predicting climate change and its impact on ocean ecosystems. To quantify changes of deep ventilation, we investigated the spatiotemporal variability of transient tracers (i.e. CFC-12 and SF6) observations combined with temporal evolution of hydrographic and oxygen observations in the Mediterranean Sea from 13 cruises conducted during 1987-2018, with emphasize on the update from 2011 to 2018. Spatially, both the Eastern and Western Mediterranean Deep Water (EMDW and WMDW) show a general west-to-east gradient of increasing salinity and potential temperature but decreasing oxygen and transient tracer concentrations. Temporally, stagnant and weak ventilation is found in most areas of the EMDW during the last decade despite the prevailing ventilation in the Adriatic Deep Water between 2011 and 2016, which could be a result of the weakened Adriatic source intensity. The EMDW has been a mixture of the older Southern Aegean Sea dense waters formed during the Eastern Mediterranean Transient (EMT) event, and the more recent ventilated deep-water of the Adriatic origin. In the western Mediterranean basin, we found uplifting of old WMDW being replaced by the new deep-water from the Western Mediterranean Transition (WMT) event and uplifting of the new WMDW toward the Alboran Sea. The temporal variability revealed enhanced ventilation after the WMT event but slightly weakened ventilation after 2016, which could be a result of combined influences from the eastern (for the weakened Adriatic source intensity) and western (for the weakened influence from the WMT event) Mediterranean Sea. Additionally, the Mediterranean Sea is characterized by a Tracer Minimum Zone (TMZ) at mid-depth of the water column attributed to the rapid deep ventilation so that the TMZ is the slowest ventilated layer. This zone of weak ventilation stretches across the whole Mediterranean Sea from the Levantine basin into the western basin.

The Mediterranean Sea Overturning Circulation

AbstractThe time-mean zonal and meridional overturning circulations of the entire Mediterranean Sea are studied in both the Eulerian and residual frameworks. The overturning is characterized by cells in the vertical and either zonal or meridional planes with clockwise circulations in the upper water column and counterclockwise circulations in the deep and abyssal regions. The zonal overturning is composed of an upper clockwise cell in the top 600 m of the water column related to the classical Wüst cell and two additional deep clockwise cells, one corresponding to the outflow of the dense Aegean water during the Eastern Mediterranean Transient (EMT) and the other associated with dense water formation in the Rhodes Gyre. The variability of the zonal overturning before, during, and after the EMT is discussed. The meridional basinwide overturning is composed of clockwise, multicentered cells connected with the four northern deep ocean formation areas, located in the Eastern and Western Mediterranean basins. The connection between the Wüst cell and the meridional overturning is visualized through the horizontal velocities vertically integrated across two layers above 600 m. The component of the horizontal velocity associated with the overturning is isolated by computing the divergent components of the vertically integrated velocities forced by the inflow/outflow at the Strait of Gibraltar.

Archaeological, geomorphological and cartographical evidence of the sea level rise in the southern Levantine Basin in the 19th and 20th centuries

Sea level variability affected by sea water mass is strongly associated with global, regional and local climate. In this context, the eastern Mediterranean Sea has been intensively investigated in recent decades because of its sensitivity to climatic and environmental variables, due to the influence of the Eastern Mediterranean Transient (EMT). The sea level in Israel during the Crusader period (12th-13th centuries CE) was found to be −0.5 ± 0.20 m relative to the present mean sea level (MSL). The difference between the Crusader sea level and the present-day MSL raises some questions which bring us to the aim of this study: estimating the timeline of the changes in sea level elevation in the eastern Mediterranean over the last two centuries. Archaeological evidence from areas of low tidal range, such as the Mediterranean Sea, can provide significant information on sea level changes for times when instrumental measurements where not yet available (e.g., before 1955 in Israel). The method employed in this study integrates two dimensions: The vertical—estimating the changes in sea levels relative to the present MSL, based on archaeological evidence; and the horizontal—determining the coastline changes, based on coastal architectural and geomorphological structures appearing in historical maps. Both the structural and the cartographic evidence for sea level changes date to the 19th Century, and indicate a rise of 0.36 m over the last two centuries.Findings attesting to horizontal changes, indicate a gradual migration of the coastline landward, to the east since 1863 and a rapid change in the coastal geomorphology at the beginning of the 20th century. The sea level increase from the 19th century might be, in part, a consequence of regional trends and, in part, a result of a gap between method accuracy (archaeology and modern measurements). Nevertheless, the drastic change in the geomorphology of the coastline may indicate an extreme meteorological event, such as a storm at sea, accompanied by a local rise of sea level, but further research is required to verify this.

Water masses and hydrography during April and June 2016 in the Cretan Sea and Cretan Passage (Eastern Mediterranean Sea)

Two successive cruises in the Cretan Sea and Cretan Passage (West Levantine basin) during April and June 2016 surveyed the physical characteristics of the water masses in these large Eastern Mediterranean basins. Data confirm that the hydrographical status is far different from that of the Eastern Mediterranean Transient (EMT) period; however, the basins have not yet returned to the pre-EMT status. In the Cretan Sea, intermediate water masses of both Cretan and Levantine origins are detected. The stagnating bottom waters of this basin still hold high salinity, density, and dissolved oxygen values, remnant of the EMT deep water formation episodes. Characterised by low salinity and oxygen values, transitional waters of Mediterranean origin are present between intermediate and bottom layers throughout the Cretan Sea. Intermittent weak outflow of warm and saline masses of Cretan origin towards the Eastern Mediterranean is observed at the bottom of both east and west Cretan Straits. In the Cretan Passage, there is no sign of the Ierapetra anticyclonic gyre, possibly related to the seasonality of the gyre or linked to larger scale Eastern Mediterranean circulation variability. The observed surface circulation in this area is comprised of a series of smaller gyres between the Cretan Cyclone and the Rhodes Gyre. The bottom waters of the Cretan Passage present a west-to-east gradient of increasing salinity and decreasing oxygen related to the propagation of new Adriatic Deep Water from the Ionian Sea towards the Levantine basin.

Molecular composition of dissolved organic matter in the Mediterranean Sea

AbstractThe molecular composition of marine dissolved organic matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid‐phase extraction (SPE‐DOM) and molecularly characterized using Fourier‐transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE‐DOM transported by the shallow overturning circulation of the Mediterranean Sea by following the increase in molecular weight (+20 Da), oxygenation (+5%), degradation index (Ideg +22%), and the proportional decrease of unsaturated aliphatic compounds (+34%) along the Levantine Intermediate Water. This reworked SPE‐DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg −25%). In the deep eastern and western overturning cells, the molecular composition of the deep waters varied according to their area and/or time of formation. SPE‐DOM of the waters formed in the Aegean Sea during the Eastern Mediterranean Transient (EMT) was more processed than the DOM in pre‐EMT waters formed in the Adriatic Sea (molecular weight and the proportion of unsaturated aliphatic compounds were increased by 5 Da and 9%, respectively). Furthermore, pre‐EMT waters contain more reworked SPE‐DOM (Ideg +7%) than post‐EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a laboratory basin where degradation processes and diagenetic transformations of DOM can be observed on close spatial and temporal scales.

High-resolution regional modelling of natural and anthropogenic radiocarbon in the Mediterranean Sea

Abstract. A high-resolution dynamical model (Nucleus for European Modelling of the Ocean, Mediterranean configuration – NEMO-MED12) was used to give the first simulation of the distribution of radiocarbon (14C) across the whole Mediterranean Sea. The simulation provides a descriptive overview of both the natural pre-bomb 14C and the entire anthropogenic radiocarbon transient generated by the atmospheric bomb tests performed in the 1950s and early 1960s. The simulation was run until 2011 to give the post-bomb distribution. The results are compared to available in situ measurements and proxy-based reconstructions. The radiocarbon simulation allows an additional and independent test of the dynamical model, NEMO-MED12, and its performance to produce the thermohaline circulation and deep-water ventilation. The model produces a generally realistic distribution of radiocarbon when compared with available in situ data. The results demonstrate the major influence of the flux of Atlantic water through the Strait of Gibraltar on the inter-basin natural radiocarbon distribution and characterize the ventilation of intermediate and deep water especially through the propagation of the anthropogenic radiocarbon signal. We explored the impact of the interannual variability on the radiocarbon distribution during the Eastern Mediterranean Transient (EMT) event. It reveals a significant increase in 14C concentration (by more than 60 ‰) in the Aegean deep water and at an intermediate level (value up to 10 ‰) in the western basin. The model shows that the EMT makes a major contribution to the accumulation of radiocarbon in the eastern Mediterranean deep waters.

High-resolution neodymium characterization along the Mediterranean margins and modelling of <i>ε</i><sub>Nd</sub> distribution in the Mediterranean basins

Abstract. An extensive compilation of published neodymium (Nd) concentrations and isotopic compositions (Nd IC) was realized in order to establish a new database and a map (using a high-resolution geological map of the area) of the distribution of these parameters for all the Mediterranean margins. Data were extracted from different kinds of samples: river solid discharge deposited on the shelf, sedimentary material collected on the margin or geological material outcropping above or close to a margin. Additional analyses of surface sediments were done in order to improve this data set in key areas (e.g. Sicilian strait). The Mediterranean margin Nd isotopic signatures vary from non-radiogenic values around the Gulf of Lion, (εNd values ∼ −11) to radiogenic values around the Aegean and the Levantine sub-basins up to +6. Using a high-resolution regional oceanic model (1/12° of horizontal-resolution), εNd distribution was simulated for the first time in the Mediterranean Sea. The high resolution of the model provides a unique opportunity to represent a realistic thermohaline circulation in the basin and thus apprehend the processes governing the Nd isotope distribution in the marine environment. Results are consistent with the preceding conclusions on boundary exchange (BE) as an important process in the Nd oceanic cycle. Nevertheless this approach simulates a too-radiogenic value in the Mediterranean Sea; this bias will likely be corrected once the dust and river inputs will be included in the model. This work highlights that a significant interannual variability of εNd distribution in seawater could occur. In particular, important hydrological events such as the Eastern Mediterranean Transient (EMT), associated with deep water formed in the Aegean sub-basin, could induce a shift in εNd at deep/intermediate depths that could be noticeable in the eastern part of the basin. This underlines that the temporal and geographical variations of εNd could represent an interesting insight of Nd as tracer of the Mediterranean Sea circulation, in particular in the context of palaeo-oceanographic applications.

Mediterranean circulation perturbations over the last five centuries: Relevance to past Eastern Mediterranean Transient-type events.

The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910 ± 12, 1812 ± 18, 1725 ± 25 and 1580 ± 30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Multiple equilibria and overturning variability of the Aegean-Adriatic Seas

The Eastern Mediterranean Transient (EMT) – a transition and amplification of the Eastern Mediterranean Sea deep water source from the Adriatic Sea to the Aegean Sea – was observed in the mid-90′ and stimulated intense research. Here we demonstrate, using an oceanic general circulation model, that the meridional overturning circulation of the Eastern Mediterranean has multiple equilibria states under present-day-like conditions, and that the water exchange between the Aegean and the Adriatic Seas can drastically affect these states. More specifically, we found two stable states and a hysteresis behaviour of deep water formation in the Adriatic Sea when changing the atmospheric (restoring) temperature over the Aegean Sea. In addition, the overturning circulation in both seas exhibits large decadal variability of the deep water formation. The Aegean-Adriatic relationship can be summarized as follows: warm and saline water of the Aegean can either flow in the sub-surface to the Adriatic, switching “on” deep water formation in the Adriatic by increasing its salinity, or the Aegean water can feed the deeper layer of the Ionian and Levantine basins, turning “off” the deep water formation in the Adriatic. The “off” steady state resembles some aspects of the EMT in which the Adriatic source of deep water was weakened when the Aegean source became active.

Changes in the non-crustacean zooplankton community in the middle Adriatic Sea during the Eastern Mediterranean Transient

Background and Purpose: Here we presented changes in the non-crustacean zooplankton community in the years characterized by the large scale changes in the thermohaline circulation in the East Mediterranean known as the Eastern Mediterranean transient (EMT) and stronger inflow of colder and less saline Modified Atlantic Water (MAW) into the Adriatic Sea. Material and Method: Monthly samplings from February 1995 to February 1996, were performed at fixed station Stoncica near the Island of Vis in the open oligotrophic Middle Adriatic waters. Zooplankton samples, taken vertically from 100 m to the surface using a Nansen net with 125-µm mesh size were analyzed in detail for the following zooplankton taxa: Hydromedusae, Calycophorae, Ctenophora, Pteropoda, Heteropoda, Polychaeta and Chaetognatha. Results and Conclusions: Among the investigated non-crustacean zooplankton three species were registered for the first time, while one species reappeared after years of absence. Compared with earlier data there was a dramatic change in dominant species of calycophoran medusae, pteropods and polychaetes. Data presented herein provide baseline information that is essential for the evaluation of impact of hydroclimatic changes on the zooplankton community, which started in the East Mediterranean in the 1990s and are still on going. Keywords : zooplankton, hydroclimatic changes, Middle Adriatic

Changes in the non-crustacean zooplankton community in the middle Adriatic Sea during the East Mediterranean Transient

Background and Purpose: Here we presented changes in the non-crustacean zooplankton community in the years characterized by the large scale changes in the thermohaline circulation in the East Mediterranean known as the Eastern Mediterranean transient (EMT) and stronger inflow of colder and less saline Modified Atlantic Water (MAW) into the Adriatic Sea. Material and Method: Monthly samplings from February 1995 to February 1996, were performed at fixed station Stončica near the Island of Vis in the open oligotrophic Middle Adriatic waters. Zooplankton samples, taken vertically from 100 m to the surface using a Nansen net with 125-µm mesh size were analyzed in detail for the following zooplankton taxa: Hydromedusae, Calycophorae, Ctenophora, Pteropoda, Heteropoda, Polychaeta and Chaetognatha. Results and Conclusions: Among the investigated non-crustacean zooplankton three species were registered for the first time, while one species reappeared after years of absence. Compared with earlier data there was a dramatic change in dominant species of calycophoran medusae, pteropods and polychaetes. Data presented herein provide baseline information that is essential for the evaluation of impact of hydroclimatic changes on the zooplankton community, which started in the East Mediterranean in the 1990s and are still on going. Keywords: zooplankton, hydroclimatic changes, Middle Adriatic

Water column distribution of stable isotopes and carbonate properties in the South-eastern Levantine basin (Eastern Mediterranean): Vertical and temporal change

Water column distributions of the oxygen isotopic composition of sea-water (δ18OSW) and the stable carbon isotope ratio of dissolved inorganic carbon (δ13CDIC), total alkalinity (AT) and the pH (total scale) at 25°C (25°CpHTotal) were investigated along the Southeast Mediterranean (SE-Med) shelf and open water, during 2009–2010. While, the vertical profiles of δ18OSW lacked a clear depth signature, those of δ13CDIC were characterized by a structure that reflects the major water masses in the Levantine basin, with noticeable vertical gradients.The δ13CDIC Suess effect of the Levantine water column was estimated from the difference between the average profiles of 1988 and 2009–2010 (Δδ13CDIC). We observed δ13CDIC temporal change, which indicates propagation of anthropogenic CO2 (Cant) to depth of about 700m. The Modified Atlantic Water (MAW; 0–200m) and the Levantine Intermediate Water (LIW; 200–400m) exhibited a depletion rate of −0.13±0.03 and −0.11±0.03‰ decade−1, respectively, representing ~50% of the atmospheric change, while the deep water of the Adriatic source (700–1300m) did not change during this period. A Δδ13CDIC depletion trend was also recognized below 1350m, corresponding to the Aegean source deep water (EMDWAeg) and therefore associated to the Eastern Mediterranean Transient (EMT) event.Anthropogenic CO2 accumulation rate of 0.38±0.12molCm−2yr−1 for the upper 700m of the SE-Med, over the last 22yr, was estimated on the basis of mean depth-integrated δ13CDIC Suess effect profile. Our results confirm lower accumulation rate than that of the subtropical North Atlantic, resulting due to the super-saturation with respect to CO2 of the well-stratified Levantine surface water. High pCO2 saturation during summer (+150μatm), in oppose to a small degree of under-saturation in winter (−30μatm) was calculated from surface water AT and 25°CpHTotal data. However, the δ13CDIC depletion trend of the LIW and the EMDWAeg supports isotopically light Cant penetrating into the Levantine interior during convection events, such as the EMT.

The 1987 Aegean dense water formation: A streamtube investigation by comparing theoretical model results, satellite, field, and numerical data with contourite distribution

We here discuss a detailed investigation of the dense water formation, evolution and spreading in the Aegean Sea during the year 1987, immediately prior to the onset of the Eastern Mediterranean Transient (EMT). We use hydrologic data collected during the LIA cruise; satellite images for SST (Sea Surface Temperature), and PROTHEUS data (a coupled ocean–atmosphere numeric model) along with theoretical streamtube models. These hydrological analyses are related to late Quaternary sedimentary drifts in the Cyclades Plateau and in the Myrtoon Basin. Our analysis shows that streamtube dynamics provide a novel model of dense water evolution and spreading in the Aegean Basin. Applying this model to dense water masses observed in winter and spring 1987 near Samothrace and over the Limnos-Lesbos Plateau, results in a geostrophic flow of this dense, cold water towards the Limnos-Sporades Channel, in the North Aegean Sea. There it mixes with dense water from the Limnos-Lesbos Plateau and finally both move geostrophically towards the Cyclades Plateau. These results indicate that most of the dense water observed near the Cyclades, formed initially about 3months earlier at Samothrace and Limnos shelves. During its long pathway it partially mixed with adjacent water masses. Although our analysis concerns only one year of dense water analyses, these results are thought to reflect a more general and recurrent phenomenon in the Aegean basin. Indeed, high-resolution (Airgun 10in.) seismic-reflection data from the Cyclades Plateau reveal the presence of late Quaternary sediment drifts. These observations are concordant with results from our theoretical model. This suggests a direct link between such a dense-water cascading and contourite dynamics. The continuation of sediment drifts into the deep basin floor (≈900m deep) of the Myrtoon Basin, moreover, indicates a cascading character of such bottom currents at the flanks of the basin, a feature that set further investigations.

Late quaternary bottom-current activity in the south Aegean Sea reflecting climate-driven dense-water production

The Eastern Mediterranean Transient (EMT) represents a recent (early 1990s to today) water circulation phenomenon, during which more arid climatic conditions in the Mediterranean lead to the production of very dense, surface water masses in the north Aegean Sea. The produced north Aegean dense water (NADW) sinks to seafloor, flows to the south Aegean Sea and, then, spills into the Eastern Mediterranean Sea through the Cretan Straits, contributing to the rejuvenation of the bottom water. A large dataset of vintage high-resolution seismic-reflection profiles indicates that the floors of the basins in the southwest Aegean Sea are characterised by late Quaternary erosional gullies, abraded surfaces, and channel-related sediment drifts. Such structures provide evidence for the presence of strong bottom currents, which originate from the southward propagation of NADW masses. The basins have depths of 300m to 1000m and are connected to each other by narrow (a few km) straits and sills. The dominance of abraded surfaces and erosional gullies on the northern flank of the basins and in the narrow straits/sills between them provides evidence that the southward propagation of the NADW occurs by the filling and flooding of each basin by dense water, which then spills and cascades into the next basin southward. Seismic stratigraphy suggests that the formation of the drifts started at marine isotope stage (MIS) 11 (420ka BP). The coincidence of at least three erosional surfaces within the sediment drifts with interglacial MIS 11, 5, and 1, respectively, suggests that bottom currents were mostly active during high sea-level stands and sluggish during glacial low sea-level stands. A 90–100m drop of the sea level below the current level would have resulted in the emergence of the eastern Cyclades Plateau and, thus, in the disconnection of the southwest Aegean Sea from the north Aegean Sea. Several (0.05–0.5m thick) beds of gravelly sand with algal and mollusc debris, laminated to mottled sand, and mottled mud are observed in the sediment cores analysed in this paper; these beds can be confidently correlated from core to core, and are interpreted as bottom-current deposits. The separation of these beds by hemipelagic sediments indicates that bottom-current activity in the southwest Aegean is not permanently established but episodic. Six episodes of intense bottom-current activity are recorded in the MIS 2–4 (10–80ka) sediments and they appear to correlate well with the cold and dry Younger Dryas and Heinrich 3, 4, 5, 6, and 7 events. In contrast, at least three episodes of intense bottom-current activity are recorded in the cores during the Holocene. The higher frequency of strong bottom-current episodes in the southwest Aegean Sea during the Holocene is attributed to the higher sea-level stand that makes the circulation of water masses in the Aegean Sea more sensitive to climatic changes.

Modelling of the anthropogenic tritium transient and its decay product helium-3 in the Mediterranean Sea using a high-resolution regional model

Abstract. This numerical study provides the first simulation of the anthropogenic tritium invasion and its decay product helium-3 (3He) in the Mediterranean Sea. The simulation covers the entire tritium (3H) transient generated by the atmospheric nuclear weapons tests performed in the 1950s and early 1960s and is run till 2011. Tritium, helium-3 and their derived age estimates are particularly suitable for studying intermediate and deep-water ventilation and spreading of water masses at intermediate/deep levels. The simulation is made using a high-resolution regional model NEMO (Nucleus for European Modelling of the Ocean), in a regional configuration for the Mediterranean Sea called MED12, forced at the surface with prescribed tritium evolution derived from observations. The simulation is compared to measurements of tritium and helium-3 performed along large-scale transects in the Mediterranean Sea during the last few decades on cruises of R/V Meteor: M5/6, M31/1, M44/4, M51/2, M84/3, and R/V Poseidon: 234. The results show that the input function used for the tritium generates a realistic distribution of the main hydrographic features of the Mediterranean Sea circulation. In the eastern basin, the results highlight the weak formation of Adriatic Deep Water in the model, which explains its weak contribution to the Eastern Mediterranean Deep Water (EMDW) in the Ionian sub-basin. It produces a realistic representation of the Eastern Mediterranean Transient (EMT) signal, simulating a deep-water formation in the Aegean sub-basin at the beginning of 1993, with a realistic timing of deep-water renewal in the eastern basin.

Thermohaline properties in the Eastern Mediterranean in the last three decades: is the basin returning to the pre-EMT situation?

Abstract. Temperature, salinity and oxygen data collected during April and June 2011 (M84/3 and P414 cruises respectively) are analysed to derive the oceanographic characteristics of the Eastern Mediterranean (EM) basin. These observed characteristics are compared with those from previous cruises over the period 1987–2011. As a result, the interannual and decadal variability of the EM thermohaline properties are discussed in the context of the evolution of the Eastern Mediterranean Transient (EMT) and of the general circulation of the basin. We found that the state of the EM is still far from the pre-EMT conditions, though the 2011 results possibly indicate a slow return to this status. In particular, a comparison between thermohaline property evolution deriving from interannual variability of the preconditioning and air–sea interaction (heat fluxes) in the South Adriatic and the Cretan Seas reveals aspects of the alternation of the two dense water sources (Adriatic and Aegean) during the last three decades, which have strong implications for the hydrographic characteristics of the intermediate and deep layers of the Ionian and Levantine basins.

INTER-annual/decadal variability of the north Aegean Sea hydrodynamics over 1960-2000

Results from a high-resolution hindcast model experiment, supported by available observations, reveal an increasing salinity trend in the north Aegean during the Eastern Mediterranean Transient (EMT), largely controlled by increases in the flow rate and salinity of water masses of Levantine origin entering the domain through the Myconos-Ikaria strait as a response to an acceleration of the Aegean thermohaline cell. Changes in the Dardanelles inflow (increasing salinity) and in the surface freshwater flux (increasing Evaporation-Precipitation), although both contribute to a higher salt content of the basin during the EMT, play a minor role in the inter-annual/decadal variability of the freshwater budget. A long-term decreasing temperature trend is observed from the 1960s to the early 1990s. It is superimposed on the salinity-preconditioning phase over the 1980s and early 1990s. Both signals are, concomitantly, favouring conditions for intense Dense Water Formation (DWF) in the north Aegean Sea. In addition, the northward displacement of the Black Sea Water front over the EMT, leads to the expansion of convective cells towards the north and to higher formation rates associated with both colder and saltier surface waters.