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

. Temperature, salinity and oxygen data collected during April and June 2011 (M84/3 and P414 cruises respec-tively) 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 partic-ular, 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


Introduction
The circulation of the Eastern Mediterranean (EM) is deeply influenced by several driving forces, strong topographic constraints and internal dynamic processes (Robinson et al., 1991). The basin-scale thermohaline circulation of the EM is formed by two cells: the closed internal cell of the deep circulation encompassing the Adriatic-Ionian area and the Levantine basin, known as the EM conveyor belt, and the external thermohaline cell involving the exchange of water between the EM and the Western Mediterranean (Lascaratos et al., 1999). The first one, mainly driven by the formation of Adriatic Deep Water (AdDW), which eventually becomes the Eastern Mediterranean Deep Water (EMDW), is characterized by a renewal time of approximately 126 years . Indeed, the major source of the deep waters of the EM can be considered the Adriatic Sea.
Recent decades have shown dramatic changes in the circulation of the EM. During the late 1980s and early 1990s, the EM conveyor belt underwent a major climate shift known as the Eastern Mediterranean Transient (EMT) (Roether et al., , 2007Theocharis et al., 2002), during which dense waters of Aegean origin replaced the AdDW in the deep layers of the EM. The structure of the South Aegean Sea (namely, the Cretan Sea; CS) water column changed as exceptionally dense and salty water started to fill the deep Cretan basin and to overflow the sills of the Cretan Arc straits. Observations conducted in 1987, 1991 and 1995 indicated a reduced transport of Atlantic Water (AW) into the Levantine basin, where the salinity increased, altering the circulation pattern of the Levantine Intermediate Water (LIW) from its formation site towards the Ionian Sea Malanotte-Rizzoli et al., 1999). These drastic changes and the major contribution of dense water of Aegean origin continued for a period of 7-8 years. Due to its high density (σ θ > 29.35 kg m −3 ), the Cretan Deep Water (CDW) continued to fill the deepest parts of the Levantine and Ionian basins, uplifting the resident and older deep and bottom waters of Adriatic ori-54 V. Cardin: Thermohaline properties in the Eastern Mediterranean gin Roether et al., 1996;Malanotte-Rizzoli et al., 1997). CDW appeared to be the most important source of the new type of EMDW, the AdDW not being dense enough to spread over the bottom layer of the EM. The Aegean source apparently disappeared by about 1995 (Theocharis et al., 2002).
Hydrographic measurements obtained in 2003 indicate a new ventilation of the deep layers in the Adriatic and Ionian Seas (see Hainbucher et al., 2006;Manca et al., 2006;Cardin et al., 2011). The necessary preconditioning for deep convection in the Southern Adriatic (SA) and the formation of water dense enough to spread below the bottom water of EM origin were only re-established after 2005, following a salinity increase linked with the influence of EM water at intermediate levels. However, bottom waters encountered in the deep layers of the Ionian basin were warmer and saltier between (Rubino and Hainbucher, 2007. This implies the existence of a delayed link between characteristics of the intermediate layer of the SA and abyssal waters of the Ionian Sea (Bensi et al., 2013). Experimental observations show that the Northern Ionian Gyre (NIG) reverses on an almost decadal scale (Gačić et al., 2010). This reversal is driven by and responsible for internal mass redistribution within the EM. In particular, it seems mainly driven by variability in salinity (Gačić et al., 2011;Theocharis et al., 2014;Velaoras et al., 2014). Wind stress and other external forces play an important role as they interact with the abovementioned processes to shape the variability (strengthening or weakening) of the surface circulation regime and of the sub-basin ocean structures (Molcard et al., 2002;D'Ortenzio and Prieur, 2012;Pinardi et al., 2013). The result is a highly complex nonlinear system acting at very different spatial and temporal scales.
In the present study, we have analysed temperature, salinity and oxygen data collected during April and June 2011 (M84/3 and P414 cruises, respectively) to examine the oceanographic characteristics of the EM basin. We focused our attention particularly on the properties and possible pathways of the AdDW, the EMDW and the LIW. Their evolutions are put in the context of the evolution of the EM large-scale oceanographic properties and of the circulation during the last three decades. Finally, we discuss the interannual variability of the preconditioning (salt redistribution) and air-sea interaction (heat fluxes) in the two main densewater source areas for the EM, i.e. the South Adriatic and Cretan seas.

M84/3 and P414 cruise data sets
The hydrographic data analysed in this study were collected in the course of two German oceanographic cruises: Meteor M84/3 (April 2011) and Poseidon P414 (June 2011).
The Meteor M84/3 cruise was set up to follow the demands and requirements of repeating hydrography as specified by the GO-SHIP group (http://www.go-ship.org/), i.e. to obtain a comprehensive set of physical and chemical parameters measured to the highest standards. The aim of the cruise was to cover the quasi-zonal section through the Mediterranean Sea, covering the main basins of the Mediterranean Sea, with dense sampling of physical and chemical parameters throughout the whole water column. The primary goal of the Poseidon P414 (June 2011) cruise was to identify the routes of the AdDW into the Ionian Sea, so as to study the spatial and temporal variability of the spreading and mixing of the EMDW (Fig. 1a).
The data set has two main components: the continuous profiles of conductivity, temperature and depth (CTD), and the discrete data from the water samples. It contains information from 61 hydrographic stations sampled during the April cruise (M84/3) (more information in Tanhua et al., 2013) and 33 full-depth standard hydrographic stations sampled in June 2011 during the P414 cruise. A SeaBird SBE911plus CTD-O 2 probe equipped with dual sensors of temperature, salinity and dissolved oxygen was employed, attached to a SeaBird carousel water sampler (24-bottle during the M84/3 and 12-bottle during the P414), together with an altimeter. Additionally, a fluorometer sensor was installed on the CTD during the M84/3 cruise and a digital reversing thermometer was attached to the probe for quality checking. At almost all stations, water samples for dissolved oxygen were taken at different depths throughout the whole water column. The oxygen samples were analysed onboard using a potentiometric Winkler titration method. From three depth levels, depending on the vertical profile of the stations, water samples were also taken for calibration of the salinity values, and they were analysed onboard using a Guildline Autosal Salinometer. Data were processed applying the Seabird software and a Matlab post-processing package. Spikes were removed from all data by applying the instrumental and climatological range criteria backed up by visual checks. Profiles were then averaged every 1 dbar. The overall accuracies are within 0.002 • C for temperature and 0.003 for salinity.

Long-term oceanographic variability
Vertical distributions of hydrographic data in the EM were examined to determine the long-term variability in the area since 1985. Three different approaches were applied: 1. Vertical distributions of temperature and salinity from the quasi-zonal section crossing the EM, from the Sicily Strait to the Levantine basin, were analysed by means of repeated surveys. We used data from all cruises throughout a trans-basin section: 1987 (Nellen et al., 1996), 1995 (Hemleben et al., 1996), 1999 (Pätzold et al., 2000), 2001 (Hemleben et al., 2003) and 2008 (Moutin et al., 2012). All these cruises took place as part 3. The temporal evolution of the water mass structure in the CS and SA was analysed on the basis of thermohaline parameters collected within the areas depicted in Fig. 1 between 1985 and 2011. Some of the data come from the MEDATLAS database (see Maillard et al., 2002;Manca et al., 2004), while some were collected within the framework of European and German National projects. The Hellenic Centre of Marine Research (HCMR) provided some of the profiles obtained between 2006 and 2011 for the CS sampled during the E1M3A observatory maintenance cruises. Further infor-56 V. Cardin: Thermohaline properties in the Eastern Mediterranean mation regarding the data set used for the SA can be found in Cardin et al. (2011).
For all data sets, derived parameters such as potential temperature, salinity and potential density were obtained from each original in situ temperature and conductivity profile. Hereinafter, temperature indicates potential temperature (θ ), salinity is reported according to the practical salinity scale, density indicates the potential density excess (σ θ ), and the oxygen corresponds to the dissolved oxygen.

Atmospheric forcing
Air-sea heat fluxes at the air-sea interface were obtained from meteorological data such as mean sea level pressure, total cloud cover, wind speed at 10 m above the mean sea level, air temperature, dew point temperature at 2 m above the mean sea level, and skin temperature. These data were derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim data set with approximately 0.7 • spatial resolution, and downloaded on a Gaussian grid with 0.25 • × 0.25 • resolution in latitude and longitude (interpolated by the ECMWF system from the original data). Total heat fluxes (Q net ) were obtained as the sum of the heat flux components: the solar radiation (Qs) was calculated using the modified Reed formula (Schiano, 1996), the net long-wave radiation (Q B ) was obtained by applying the Bignami et al. (1995) formula, while the sensible (Q H ) and latent (Q L ) heat fluxes were obtained from the bulk aerodynamic formulas (Kondo, 1975). More information on the formulas applied, procedures and coefficients can be found in Cardin and Gačić (2003). Total winter heat fluxes were calculated as a time-integral for the period spanning 1 December to 31 March for the years 1985-2011. Normalized heat fluxes for each area were calculated applying where Q norm is normalized value; X is individual Q net to normalize, µ is arithmetic long-term (1985-2011) winter average calculated from winter daily mean values and σ is longterm (1985-2011) winter standard deviation.

2011 Thermohaline conditions
The monthly average of the absolute dynamic topography (ADT) in the EM, referred to April and June 2011, are shown in Fig. 1b and c. Data are obtained from the Live Access Server of the AVISO web portal (http://las.aviso.oceanobs. com/las/getUI.do). The ADT is the sum of sea level anomaly and Synthetic Mean Dynamic Topography computed combining in situ drifter velocities and altimetry measurements. The error of the Synthetic Mean Dynamic Topography in the study area is in the range 1-3 cm (Rio et al., 2007). In this study, the ADT maps are used only to show the major circulation features in the upper thermocline, as some of the stations sampled during the M84/3 and P414 cruises were positioned at the edges or across dynamic structures that influence the water mass pathways. In the EM, several well-developed sub-basin-scale cyclonic or anticyclonic gyres, permanent or quasi-permanent, and interconnected by jets and meandering currents (Malanotte-Rizzoli et al., 1997), were observed in both April and June 2011. Temperature and salinity along the quasi-zonal section crossing the EM, from the Sicily Strait to the Levantine basin, is shown in Fig. 2. In April 2011, the easternmost part of the transect was characterized by the presence of the Levantine Surface Water (LSW), which occupied the first 50 dbar and showed high temperature (θ > 18 • C) and salinity (S > 39.15). Beneath this layer, a core of LIW with a salinity maximum of 39.24 on stations 293 (and 292, not shown) protruded westward ( Fig. 2b), occupying the 150-350 dbar layer. Water characterized by S > 39.20 and θ > 17.50 • C flowed out from the CS (St. 297) through the eastern Cretan Arc spreading into the Levantine Sea, and breaking the main LSW/LIW tongue with outcrops of the isohalines (see also Fig. 6). The westernmost part of the surface layer was mainly occupied by the AW within the upper 130 dbar and confined by the 38.80 isohaline, contained within the Ionian circulation. AW propagated eastward along the pathway observed from the dynamic heights (ADT, Fig. 1b, c). Hence, from the analysis of Fig. 1b, c and Fig. 2 we can deduce that the AW veered due to the action of an intense anticyclonic mesoscale structure centred over the deepest area of the Ionian (at ∼ 19 • E, 36 • N), the edge of which we sampled at stations 305, 303 and 302. AW intrusion into the Levantine basin is very variable and may depend on the circulation present in the Ionian (Gačić et al., 2010). In 2011 the NIG circulation was just reversing to cyclonic, as reported in Gačić et al. (2014). Previously, the main effect of ∼ 5 years of anticyclonic NIG (2006)(2007)(2008)(2009)(2010), when the AW meandered in the northernmost part of the Ionian Sea, was a general increase in salinity in the Levantine basin (see the long-term analysis and discussion in Sect. 4) and a general decrease in salinity in the SA.
The deep layer below 2500 dbar (Fig. 2) is topographically divided into two by the EM Ridge (see its location in Fig. 1a, highlighted by the black dotted line). West of it, a large part of the Ionian basin was dominated by EMDW of Adriatic origin, still the coldest (θ < 13.48 • C), freshest (S < 38.73) and densest (σ θ ≈ 29.20 kg m −3 ) water mass of the area, with the highest oxygen value (204 µmol kg −1 ). The signature of the AdDW flowing out through the Otranto Strait was also found in profiles measured in the northernmost part of the Ionian, as will be shown later. East of it, the deepest part (below 2000 dbar) of the Levantine basin was filled by EMDW of Aegean origin, with similar σ θ but slightly warmer and saltier (θ > 13.6 • C, S > 38.78) than the EMDW found in the Ionian, and with lower oxygen levels (between 185 and 190 µmol kg −1 ) (see Fig. 2c).
The Cretan Sea water that flowed from the Kasos Strait showed salinity higher than ∼ 39.15 occupying the first 250 dbar and protruding like a tongue into the Levantine (Fig. 3, transect B). However, its area of influence was mainly confined to the north part of the EM Ridge; south of this area it encountered the AW at the surface, while it mixed with the LIW in the intermediate layer. Very dense water occupying the deep layer of the CS remained "isolated" by virtue of the presence of the sill. Notably, the densest water was found in the CS at station 288 (θ ≈ 13.95 • C, S ≈ 39.06 and σ 0 > 29.30 kg m −3 ; Hainbucher et al., 2014). This water remained blocked in the CS (see St. 289), filling the layer below 1000 dbar. The T -S diagram (Fig. 3d) shows the characteristics of four stations along section bb . Two of them (St. 290, green and St. 296, orange) are located in the Levantine basin and they revealed the typical long-lasting pronounced T -S inversions already described in Hainbucher et al. (2014) for this cruise, and explained by Roether et al. (2014) in detail. This inversion was caused by a noticeable increase in temperature and salinity throughout the EM deep waters due to an anomalously abundant and dense outflow of water of Aegean origin produced during the EMT.
Station 289 (magenta) is located directly above the sill of the Kasos Strait where an inversion of higher temperature and salinity values at around 500 dbar is found. This might indicate that two kinds of Cretan deep water mass, an older and a newer, are present in this area. Station 288 (red) is located in the Aegean Sea and reaches its minimum salinity values at around 1150 m, indicating the presence of CDW at that level.
From its formation area westwards, the LIW pathway was directly influenced by the NIG and by the anticyclonic vortex present around 35.5 • N/19.0 • E that trapped low-salinity waters in its centre, as seen in Figs. 1b, c and 3. There, the LIW bifurcated: one branch flowed westward directly to the Sicily Channel while the other one flowed northwards along the eastern shelf-break of the Ionian. To describe the LIW pathway into the northern Ionian and into the Adriatic, temperature, salinity and density meridional sections along transect cc are presented in Fig. 4. The LIW seems to be influenced by the Pelops anticyclone (PA in Fig. 1b, at ∼ 36.5 • N), as revealed by the down-sloping of the isopycnals, and by the water exiting the CS through the Antikythera Strait (right part of the panels). The southernmost stations of the transect showed the presence of the AW in the first 100 dbar while the rest of the stations showed the LIW spreading northward into the Ionian basin as a warm (θ ≈ 15.0 • C) and  as to a reduction in the relatively high dissolved oxygen content associated with the AdDW. The western meridional (N-S) section dd (Fig. 5) links the SA and Ionian Seas along the Italian coast, ending at the western edge of the quasi-zonal transect crossing the EM. This field distribution complements the information inferred from Fig. 4, indicating that the LIW did not enter the Adriatic, while the cyclonic circulation observed is coherent with the ADT distribution shown in Fig. 1b and c. High-salinity waters characterized the intermediate layer spreading from east to west in a cyclonic circulation with the core of the LIW located between 200 and 300 dbar, whereas low-salinity waters were confined to the surface layer along almost the whole section shown from the northern part of transects (b) and (c), depicted in Figs. 4 and 5. The latter also shows two cores of high salinity water, the first located at St. M309 and the second at St. M305, both catching a less diluted signal of the LIW spreading westward from the Levantine. Beneath the LIW, low-salinity waters (38.72) with relatively high oxygen content (198 µmol kg −1 ) filled the deepest part of the central Ionian. They can be attributed to relatively fresh and recently ventilated water of Adriatic origin. The oxygen minimum was located in the layer 1000-1300 dbar, with values around 180 µmol kg −1 , showing that this layer was scarcely renewed.
Maps of salinity distributions (Fig. 6a) at 100 dbar and at the isopycnal surface of σ θ = 29.05 kg m −3 (Fig. 6b), i.e. the typical horizon of the LIW (Malanotte-Rizzoli et al., 1997), provide evidence of the major water masses occupying the basin from the Sicily Channel to the Cretan Passage and their circulation patterns. The AW entered the Ionian Sea from the Sicily Channel and protruded towards east and north, occupying an extended portion of the west half of the Ionian basin. A front running from 36 to 40 • N separated lowsalinity waters of Atlantic origin (< 38.45) from Ionian Surface Waters (38.60). This pattern confirms that the anticyclone positioned at 19 • E (Ionian gyre in Fig. 1b and c) acts as a sorting point from which the AW divides into two branches, one flowing to the east and the other proceeding northward. Bifurcated pathways of LIW were also recognizable from the salinity distribution at 29.05 kg m −3 (Fig. 6b): (i) the tongue defined by S ∼ 39.0-39.05 spread directly from the Cretan Passage towards the Sicily Channel; (ii) the other branch of LIW was embedded in the NIG, and extended north of the Peloponnesus peninsula along the eastern border of the Ionian basin. On its way towards west and northwest the LIW mixed with the surrounding waters, decreasing its salinity.

Long-term variability in the Eastern Mediterranean
The long-term variability along the quasi-zonal section, from the Sicily Strait to the Levantine basin, is discussed here through the analysis of the vertical distributions of temperature and salinity sampled during six repeated surveys carried out in different oceanographic stages of the EMT: 1987, which represents the pre-EMT phase; 1995 when the EMT was in full swing -i.e. a large amount of Aegean-derived deep water was still deposited in the near-bottom waters around the Cretan Arcs and the Adriatic salinity maximum decreased; 1999 and 2001 showing the evolution of the EMT; and, finally, data from cruises carried out in 2008 and 2011 which reflect the more recent oceanographic situation. Figures 7 and 8 show the vertical distributions of temperature and salinity respectively. For a detailed discussion on the major findings from the cruises carried out in 1987, 1995 and 1999, the reader is referred to the works by Klein et al. (1999Klein et al. ( , 2000, Malanotte-Rizzoli et al. (1999), Manca et al. (2003) and Roether et al. (2007). A similar approach was used by Touratier and Goyet (2011) to determine the distributions of properties (θ , S, oxygen) along a west-east transect in the EM, but considering all stations (from the Medar/Medatlas II database) within a band of 200 km and for years 2000-2001. Data from the M5/6 cruise in 1987 show the hydrographic conditions prior to the EMT in the Ionian and Levantine basins, when the AdDW was clearly the main contributor to the EMDW (Figs. 7a and 8a). This distribution is considered a reference for the climatological state of the EM . Indeed, the deep western and central Ionian showed a temperature of 13.30 • C and salinities lower than 38.68. On the other hand, observations conducted during the EMT phase, in 1995 and1999 (Figs. 7b, c and 8b, c respectively), revealed the presence of Aegean waters outside the Cretan Passage, with large salinity and tempera- ture values, lying at the deepest layer of the EM. Enhanced salinity and temperature relative to 1987 (values higher than 38.84 and 13.60 • C) were already ubiquitous in early 1995 (Roether et al., 2007), with the exception of the western Ionian deep layer which was still occupied by low-salinity and low-temperature waters (S < 38.68, θ < 13.30 • C). In 1995, waters of Aegean origin at the Cretan Passage were clearly recognizable by a dome of the ventilated bottom waters associated with θ > 14.0 • C and S > 38.78. An analogous distribution was found in 1999 (see Theocharis et al., 2002;Manca et al., 2003) at the Cretan Arc and at the easternmost part of the Levantine basin (Figs. 7d and 8d), blocked by the bathymetry constraint of the EM Ridge. In the same years, signs of new EMDW of Adriatic origin were already evident in the western continental slope of the Ionian Sea, i.e. indicating that a major change occurred between 1999 and 2003 in the abyssal part of the Ionian basin, where AdDW returned gradually to become the principal contributor to the EMDW (Hainbucher et al., 2006;Manca et al., 2006;Rubino and Hainbucher, 2007;Bensi et al., 2013). In 2008 (Figs. 7e and 8e) only the Levantine basin was still under the influence of the warm and saline CDW that overflowed during the EMT, whilst the EMDW that resided in the Ionian abyssal layer was influenced by more recent AdDW with modified properties, being warmer and saltier (θ = 13.40 • C, S = 38.72) than in the past. These results are consistent with the tendency toward the warming of and salinification of the AdDW pointed out by Rubino and Hainbucher (2007) and seen also in 2011 (Figs. 7f and 8f) when, in general, temperature and salinity in the abyssal layer of the Ionian Sea (θ ≈ 13.40 • C, S ≈ 38.75) indicated almost everywhere a continuous relaxation of the EMT since the early 1990s.
The analysis of the upper layer (0-500 dbar) in Figs. 7 and 8 showed that the decadal variability of the Ionian and Levantine thermohaline properties is strongly related to the variability of the AW pathway, which in turn is influenced by the circulation regime in the Ionian Sea. In the last decades, different regimes alternated, leading to an increase (when AW was transported toward the Ionian basin by anticyclonic circulation) and to a decrease (when AW was transported toward the Levantine basin by cyclonic circulation) in the salinity in the Levantine basin. This variability is clearly evident in Fig. 8, where the salinity of the upper thermocline, especially in the Levantine basin, appears to oscillate on multiannual or decadal scales. A relative minimum in salinity was evident in 1987 (Fig. 8a) and was followed by an increase in 1995. At the end of 1980s the AW flow in the Ionian increased and an anticyclonic circulation up to 39 • N arose (Malanotte-Rizzoli et al., 1997). This anticyclonic period lasted until 1996. The two salinity distributions in 1987 and 1995 ( Fig. 8a and b, respectively) represent the limits of a period during which the salinity progressively increased. Different mechanisms acting separately or even in combination could potentially explain part of such variability. Gačić et al. (2010Gačić et al. ( , 2011 and Theocharis et al. (2014) showed that internal mechanisms seem to prevail over the atmospheric forcing in engaging the reversal of the circulation in the Ionian, which in turn modifies the position of the Atlantic-Ionian Stream responsible for the salinity distribution in the EM. In contrast, Pinardi et al. (2013) suggested that the reversal was wind driven.
The massive production and outflow of CDW was the principal manifestation of the EMT. To observe the evolution of the thermohaline properties of the water column in the CS (Fig. 9, upper panel) and in the SA (Fig. 9, lower panel), Hovmöller time diagrams of the thermohaline properties during the period 1986-2011, based on all CTD profiles sampled in the areas indicated in Fig. 1a, were obtained. Special attention was given to the density = 29.20 kg m −3 ; this is the maximum density recorded in 1987 at the reference level of 1300 dbar in the proximity of the Kasos Strait, and it is considered the threshold density for filling the deep layers of the EM. The 900 and 800 dbar levels highlighted represent the outflow levels above the Kasos and Otranto sills, respectively.
The time evolution of salinity in the CS shows, firstly, a remarkable increase in a large part of the water column (from 400 dbar to the bottom) in the period 1985-1992. As demonstrated by Theocharis et al. (1999), in the subsequent period (1992)(1993)(1994) the strong decrease in temperature due to the very cold winters in 1992 and 1993 caused the accumulation of a huge amount of dense water (σ θ > 29.20 kg m −3 ), as shown by the rapid rise of the 29.20 kg m −3 isopycnal (Fig. 9c, upper panel). At the same time, in the SA (Fig. 9c, lower panel) the salinity was < 38.70 from 200 dbar to the outflow depth, slightly increasing until 1990 (S > 38.70), and then decreasing until about 1997. The 29.20 kg m −3 isopycnal depth rose from 600 dbar (in 1985) to about 250 dbar (in 1992). The maximum density observed at the overflow depth was 29.25 kg m −3 .
In the period 1992-1995, the CS density at the outflow level of the Kasos Strait remained higher than 29.25 kg m −3 and corresponds to the period of the most intense and continuous outflow of the new CDW into the Levantine basin . In the SA, in contrast, density rapidly decreased to values ≈ 29.20 kg m −3 only in the deepest portion (650-800 dbar) of the water column involved in the exchange with the Ionian Sea. In this period, the EMDW of Adriatic origin present in the Ionian Sea and in the Levantine was progressively displaced by the new denser CDW.
Our analysis confirms what was hypothesized by Roether et al. (2007) regarding the evolution of the Cretan water thermohaline properties post-EMT (from 1995) at the outflow level -i.e. that due to the slow deepening of the 29.20 kg m −3 isopycnal, the density levels exceeded pre-EMT values for several years. The strong CDW outflow was compensated for by an increased inflow of surface Levantine waters and by the Transitional Mediterranean Water, which in the CS was traceable by a salinity minimum (Theocharis et al., ) between 200 and 400 dbar in 1993(Theocharis et al., -1995 deepening and losing its characteristics around the second half of the 2000s. The presence of Transitional Mediterranean Water presumably inhibited further significant vertical mixing and dense water production after 1993. As shown by Klein et al. (2000) and Cardin et al. (2011) to increase again until 2005. It is evident from Fig. 9 that sufficient preconditioning was established to resume the AdDW production during the first half of the 2000s. This was the result of a massive import of LIW and Cretan Intermediate Water (CIW) from the Ionian. However, it was only after 2005 that the AdDW at the overflow depth at the Otranto sill regained a density > 29.20 kg m −3 .
During the last part of the period studied, namely from 2005 to 2011, salinity at the intermediate level in the CS continuously increased, reaching absolute maximum values of > 39.10 in 2011. A concomitant temperature increase that prevented effective convection able to produce as much dense water to fill the deep Cretan basin was observed. In the same period in the SA the salinity of the whole water column decreased, as did the density after a relative maximum (29.20 kg m −3 at 600 dbar) in 2006.
The role of the preconditioning, linked to the basin circulation, in promoting the alternation of dense water formation in the CS with that in the SA is important, but the relevance of the air-sea interaction cannot be neglected. Therefore, interannual variability in the normalized heat flux (Q norm ) was analysed in the two sites for the period December to March (Fig. 10). The general behaviour in the two sites was similar. Particularly noteworthy is the heat loss which occurred in the winters 1991-1992 and 1992-1993 in the CS and which triggered the active phase of the EMT. Another period of intense winter heat loss was from 2002 to 2006, for both sites. In this period, in the SA both preconditioning and heat loss were favourable to the AdDW production, whilst in the CS the presence of the above-mentioned Transitional Mediterranean Water counteracted the noticeable heat loss reported in Fig. 10. From 2007 to 2011 neither basin was subject to significant heat loss. The increased temperature in the CS and the decreased salinity in the SA made the situation in the two basins very similar in terms of the density at their own outflow depths. Also, thermohaline properties in the CS were still very different from those recorded in the pre-EMT phase.
The alternating capability of the Cretan and Adriatic seas in producing dense water by means of the internal salt redistribution between the Ionian Sea and Levantine basin has already been pointed out by Gačić et al. (2011),  and Velaoras et al. (2014). Our comparative analysis also suggests that the evolution of thermohaline proper-  ties of the two potential dense water source areas is subject to the complex interactions among preconditioning, air-sea interactions, and local dynamics and circulation.
The spatial and temporal evolution of the water mass structure was also analysed through the comparison of vertical profiles of temperature and salinity within two areas: the central Ionian and the central Levantine (see Fig. 11a and b for vertical profiles and Fig. 1 for the location of the areas). This analysis includes the data from cruises already discussed in the previous paragraph. The inset graphs enhance the oceanographic characteristics for the deep layer below 1500 dbar. Salinity and temperature profiles for both areas ( Fig. 11a and b, zoomed areas) are mostly indicative of changes in the water mass structure related to the EMT. In 1987, the central Levantine below 2500 dbar showed a vertical homogeneity in temperature and salinity in the ranges ≈ 13.34 • C and ≈ 38.66, respectively. In 1991, a slight increase in salinity from ∼ 500 dbar down to the bottom was seen. In 1995, the salinity increase was remarkable, especially below 2300 dbar where it jumped from 38.71 to ∼ 38.84 because of the new dense CDW outflow . Its signature was tracked until 2007, when the salinity started to decrease. Meanwhile, in the central Ionian the salinity continued to increase, the abyssal layer being filled by newly formed AdDW, saltier and warmer; this was still evident in 2011. The most recent data (2008)(2009)(2010)(2011) in the deep layer in the central Levantine show a salinity and temperature decrease, probably a result of the basin internal mixing (Roether et al., 2014).

Conclusions
This paper discusses data collected in 2011 during the Meteor M84/3 (April 2011) and the Poseidon P414 (June 2011) cruises in the context of the spatial and temporal evolution of the large-scale oceanographic properties of the EM during the last three decades. Additionally, we discussed the interannual variability of the preconditioning and air-sea interaction (heat fluxes) in the two main dense water source areas for the EM, i.e South Adriatic and Cretan Seas.
Our results, which are the most recent analysis for the whole EM, reveal that the thermohaline properties in the study area were still far from those observed in the pre-EMT phase (1987). The 2011 oceanographic conditions in the deep layers of the central Ionian, however, lie between the thermohaline characteristics of the EMT and those of the pre-EMT phase, indicating a possible slow return towards the latter.
The abyssal layer of the central Ionian experienced the effect of a post-EMT phase, which started in about 2003, characterized by the arrival of newly formed AdDW waters, completely different from the recent past in that they are saltier and warmer. In the following years, this new EMDW of Adri-atic origin seemed to have propagated towards the abyssal Levantine basin.
This study highlights the relationship between the hydrological property distribution of the upper layer in the Levantine basin and the alternate circulation regimes in the Ionian, which modulates the salinity distribution in the EM.
The comparative analysis of the evolution of the thermohaline properties in the two potential dense-water source areas, SA and CS, demonstrates that, as far as dense-water accumulation is concerned, the alternation of the two sites is strongly influenced by the preconditioning driven by the reversal of the NIG. Nevertheless, the role of the air-sea interaction and the local dynamics should be taken into account in assessing the decadal variability of the EM thermohaline cells.
supported by a grant from the Deutsch Forschungsgemeinshaft -Senatskommision für Ozeanographie (DFG), TA 311/3-1. Funding from the University of Hamburg supported the Poseidon P414 cruise. V. Cardin and G. Civitarese thank the CIESM for supporting their participation in the Meteor Cruise. The Italian Ministry of Education, University and Research (MIUR) supported this research under the RITMARE (Ricerca ITaliana per il MARE) and MedGES (Studio integrato dell'influenza dei processi fisici e dei fattori antropici sullo stato degli ecosistemi costieri e profondi del Mediterraneo Centrale) national projects. The Topic Editor is grateful to the two anonymous referees for their contributions which have helped to greatly improve the quality of this paper.