Beniamino B. Manca

Recent Changes in Eastern Mediterranean Deep Waters

Results from a recent hydrographic survey show that an influx of Aegean Sea water has replaced 20 percent of the deep and bottom waters of the eastern Mediterranean. Previously, the only source of such waters was the Adriatic Sea, and the waters of the eastern Mediterranean were in near-steady state. The flux changed the water characteristics and displaced older waters upward. Its cause was increasing Aegean Sea salinity, resulting from changes in either the circulation pattern or the large-scale freshwater balance. Current deepwater studies may be affected by the intrusion, but effects might be found also at shallower depths and over a larger region.

The large deep water transient in the Eastern Mediterranean

The recent changes in the thermohaline circulation of the Eastern Mediteranean caused by a transition from a system with a single source of deep water in the Adriatic to one with an additional source in the Aegean are described and assessed in detail. The name Cretan Sea Overflow Water (CSOW) is proposed for the new deep water mass. CSOW is warmer (θ>13.6°C) and more saline (S>38.80) than the previously dominating Eastern Mediterranean Deep Water (EMDW), causing temperatures and salinities to rise towards the bottom. All major water masses of the Eastern Mediterranean, including the Levantine Intermediate Water (LIW), have been strongly affected by the change. The stronger inflow into the bottom layer caused by the discharge of CSOW into the Ionian and Levantine Basins induced compensatory flows further up in the water column, affecting the circulation at intermediate depth. In the northeastern Ionian Sea the saline intermediate layer consisting of Levantine Intermediate Water and Cretan Intermediate Water (CIW) is found to be less pronounced. The layer thickness has been reduced by factor of about two, concurrently with a reduction of the maximum salinity, reducing advection of saline waters into the Adriatic. As a consequence, a salinity decrease is observed in the Adriatic Deep Water. Outside the Aegean the upwelling of mid-depth waters reaches depths shallow enough so that these waters are advected into the Aegean and form a mid-depth salinity-minimum layer. Notable changes have been found in the nutrient distributions. On the basin-scale the nutrient levels in the upper water column have been elevated by the uplifting of nutrient-rich deeper waters. Nutrient-rich water is now found closer to the euphotic zone than previously, which might induce enhanced biological activity. The observed salinity redistribution, i.e. decreasing values in the upper 500–1400 m and increasing values in the bottom layer, suggests that at least part of the transition is due to an internal redistribution of salt. An initiation of the event by a local enhancement of salinity in the Aegean through a strong change in the fresh water flux is conceivable and is supported by observations.

A synthesis of the Ionian Sea hydrography, circulation and water mass pathways during POEM-Phase I

Abstract In this paper we revisit, with a thorough in-depth analysis, the dataset collected in the hydrographic surveys of the international collaborative programme POEM (Physical Oceanography of the Eastern Mediterranean) in the period 1986–1987. The work has two major objectives. The first is to refine the dynamic picture of the Ionian upper thermocline sub-basin scale circulation, rather less definitive than the dynamic picture of the Levantine Sea circulation. The second is to identify the pathways of the major water masses of the basin not only in the near-surface, but also in the intermediate and deep layers. To our knowledge, this is the first work defining the pathways of the Levantine Intermediate Water (LIW) and of the Adriatic Deep Water (ADW)/Eastern Mediterranean Deep Water (EMDW) that characterize the intermediate and deep circulations. The major novel results can be summarized as follows. In the upper thermocline: (1) The Atlantic Ionian Stream (AIS) jet entering the Sicily Straits bifurcates into two main branches at 37°N, ∼ 17°E. It advects the Modified Atlantic Water (MAW) into the Ionian Sea interior. The first branch turning directly southward encloses an overall anticyclonic area comprising multiple centers around which the MAW is advected. (2) The second AIS branch extends further into the northeastern Ionian, where it too turns southward before crossing the entire Ionian Sea meridionally, advecting MAW on its left side and Ionian Surface Water (ISW) on its right. This branch of the jet is confined to the Ionian margin and does not pass around the Pelops gyre. (3) A new water mass, the LSW, is formed in the Levantine basin and enters the Cretan passage, then is first veered cyclonically south of Crete by the Cretan gyre and successively is entrained anticyclonically around the Pelops gyre, and then enters the Aegean Sea. (4) A permanent cyclone located in the northeastern Ionian determines the pathway of mixed Adriatic Surface Water/Ionian Surface Water (ASW/ISW). (5) A permanent cyclone is found in all the surveys near the tip of the Italian boot. This novel analysis of the LIW pathways shows that: (1) The major source of intermediate LIW during the period 1986–1987 was actually in the Levantine Sea. LIW formed there entered the Cretan passage, was veered cyclonically by the Cretan gyre south of Crete and then entered the southern Ionian Sea. The major LIW pathway was westward directly to the Sicilian Straits. (2) Secondary important LIW pathways were determined by the interior structures. The strong Pelops anticyclone was entraining LIW around its periphery and was determining the LIW northward pathway that closely followed the eastern Greek coastline. It was along this pathway that LIW entered the Otranto Strait. A further branch of LIW was entrained and recirculated around the multiple Ionian Anticyclones (IA) of the western Ionian Sea. (3) The Cretan cyclone is a feature confined to the upper thermocline-intermediate layer. It disappears at ∼ 400 dbar while the Pelops anticyclone is strongly barotropic below the upper 100 dbar and penetrates quite intense down to 800 dbar. (4) A further completely novel result concerns the new water mass found in the deep layer that spreads on the 29.15 kg/m3 isopycnal surface. This water mass, characterized by high salinity and high oxygen content, is formed inside the Aegean Sea and is observed to spread out all around the Cretan Arc Straits. The final fully novel result is the demonstration of a second pathway for the ADW exiting from the Otranto Strait that is transformed into EMDW and occupies the abyssal layers of the Ionian Sea interior. The traditional pathway for EMDW is along the isobaths along the western side of Italy but ADW was observed to be exiting from the Otranto Strait in the eastern Hellenic trench at 39.5°N, both during POEM-ON86 and POEM-AS87. This second pathway for EMDW follows isobath contours along the western side of Greece. The two EMDW routes converge and merge between 36°N and 35°N, so producing a deep layer of EMDW that occupies uniformly the abyssal plain of the interior of the Ionian Sea.

Is the Adriatic returning to dominate the production of Eastern Mediterranean Deep Water

Since the Aegean took over the deep water production of the Eastern Mediterranean at the end of the 1980s, the proficiency of the Adriatic as a formation site has been under question. The salt supply in the intermediate water enabling the Adriatic to produce dense water was diminished because of a salinity decrease by upwelling mid-depth waters. Tracer data indeed indicate that the deep layer in the Adriatic has not been ventilated for most of the 1990s. The data presented also show that the dilution of the intermediate water reached a peak in 1995, after which more ventilated and saline waters were added. The recent increase of salt supply to the Adriatic by an extremely saline intermediate water mass supplied from the Aegean, establishes the preconditioning required to resume dense water production in the Adriatic.

Property distributions and transient-tracer ages in Levantine Intermediate Water in the Eastern Mediterranean

We present distributions of chlorofluorocarbons (CFCs) and ages derived from them, of carbontetrachloride, and of hydrographic properties, in Levantine Intermediate Water (LIW) in the Eastern Mediterranean. The data originate from surveys of F/S METEOR in 1987 and 1995, which bracket the profound changes that have occurred in the Eastern Mediterranean deep waters, due to bottom water formation from Aegean Sea overflow and related enhanced upwelling (Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevich, V., Luchetta, A., 1996a. Recent changes in Eastern Mediterranean deep waters. Science, 271, pp. 333–335). As a framework for an interpretation, classical knowledge on LIW is summarized. A density horizon of σθ=29.05 is selected to characterize LIW, for which salinities and temperatures in 1995 were still similar to classical values. A principal result derived from the CFC-age distributions is that the enhanced upwelling of deep waters has been continuous up into the LIW layer. Newly formed LIW in both surveys is found to be distributed over an extended region which includes the Cretan Sea. The lowest CFC ages in LIW, amounting to several years, are found in this region. Smaller but significant apparent CFC ages are present in the mixed layer in a winter situation (1995). The CFC data are compatible with a formation of LIW by open-ocean convection. Outcropping of the isopycnal typical of young LIW was observed in the Aegean Sea in 1995, while to the east and southeast of the Rhodes Gyre no evidence of a major recent LIW formation was found. The CFC age distributions give an upper limit for the apparent travel time of LIW up to the Strait of Sicily of about 8 years. CCl4 is found to be chemically unstable in the Eastern Mediterranean (chemical lifetime in LIW <5 years), but this feature allows us to use this tracer as a low-life age marker. The present work can serve as a basis for future data evaluation by Mediterranean circulation models.

Experiment in eastern Mediterranean probes origin of deep water masses

During the last decade the oceanography community has focused much attention on the Mediterranean Sea. One reason for the growing interest is that the Mediterraneans impact on the Northern Atlantic Ocean is more significant than previously realized. The warm, salty Mediterranean water tongue exits the Gibraltar Straits and spreads throughout the North Atlantic at all depths between 1000 and 2500 m. The second reason for the surge in interest is the well-recognized role of the Mediterranean Sea as a laboratory for studying ocean processes that are important in global climate dynamics [Malanotte-Rizzoli and Robinson, 1991; Malanotte-Rizzoli and Robinson, 1994].

Adriatic Deep Water and Interaction with the Eastern Mediterranean Sea

Being a dilution basin, the Adriatic Sea exports a relatively fresh water to the adjacent Ionian Sea. This water obviously must pass through the Strait of Otranto in the surface layer. In addition, the Adriatic loses heat every year, and the resulting buoyancy loss generates a dense water that has to be exported to the Ionian in the bottom layer of the Strait of Otranto. To balance these losses, a compensating flow brings more saline and warmer waters from the Eastern Mediterranean through the surface and intermediate layers. In sum, water exchange through the Strait of Otranto consists in three main components: the outflowing surface Adriatic water, the outflowing Adriatic Deep Water (ADW), and the inflowing more saline and warmer waters of Mediterranean origin.

Flow Patterns of the Main Water Masses Across Transversal Areas in the Southern Adriatic Sea: Seasonal Variability

The results here presented regard four seasonal multidisciplinary surveys conducted in the Southern Adriatic basin, in the framework of the collaborative research program PRISMA (Programma di Ricerca e Sperimentazione del Mare Adriatico), with the aim of investigating the longitudinal fluxes across a series of significant transverse sections. The thermohaline properties of the main water masses and their seasonal and spatial variability over an entire annual cycle were analyzed. Five water masses take part in these processes; namely the Adriatic Surface Water (ASW), the Ionian Surface Water (ISW), the Levantine Intermediate Water (LIW), the Adriatic Deep Water (ADW), which overflows the Sill of Otranto to form the main component of the Eastern Mediterranean Deep Water, and finally the Northern Adriatic Dense Water (NADW). In order to follow the spatial extension of these water masses and their seasonal variability, the distribution of thermohaline properties along the transverse sections were considered. The major results concern the advection of the NADW, which has been observed to flow southward over the Pelagosa Sill and subsequently into the southernmost western shelf area, reaching the Otranto Strait. The horizontal thermohaline fields, objectively analyzed, show the main pathways of the above-mentioned water masses. Important signals in the seasonal variability of the baroclinic basin-wide circulation have been noted. Finally, a quantitative measure of the cross-sectional areas occupied by the intermediate and deep waters along the transverse sections located at the Gargano passage and Otranto Strait have been calculated and are discussed.

The Eastern Mediterranean in the 80’s and in the 90’s: The Big Transition Emerged from the Poem-Bc Observational Evidence

The thermohaline circulation of the Eastern Mediterranean underwent a dramatic change between 1987 and 1995. In 1987 the “engine” of the Eastern Mediterranean “conveyor belt” was the convective cell of the Southern Adriatic, while in 1995 the active convection region moved to the Aegean Sea. This change actually started as early as 1991. The phenomenological evidence of the POEM programme shows that in 1987 the source of Levantine Intermediate Water (LIW) mass was the Levantine basin and the bottom water mass was formed in the Southern Adriatic. In 1991 all the intermediate/deep water masses on the horizons σθ = 29.00 to 29.18 kg/m3 were formed inside the Aegean sea, from which they spread out into the entire Eastern Mediterranean through the Cretan Arc Straits.

Long-term sustained observing system for climatic variability studies in the Mediterranean

Abstract During the CIESM workshop (Monaco, 22–24 April 2002) entitled “Monitoring Hydro- logical Trends in the Mediterranean”, the review of existing data sets and analyses has revealed important variability in the dynamics and hydrological characteristics, in the past century, ranging from interannual to decadal time scales. These variations are related mainly to local forcing and larger scale atmospheric parameters (NAO, ENSO, Indian monsoon). Moreover, abrupt events contribute to modification of the Mediter- ranean conveyor belt, also influencing the biogeochemical environment and the Mediter- ranean outflow in the Atlantic Ocean. The participating scientists have agreed on the need of a sustained multi-component long-term monitoring system based on both oceanographic and atmospheric observations and modelling. The group also proposed an expansion and strengthening of the ongoing relevant large operational programs by a “climatic approach” component.