Kostas Nittis

Recent changes in deep water formation and spreading in the eastern Mediterranean Sea : a review

Abstract Observations of the last decade testify that the characteristics of the deep thermohaline circulation in the Eastern Mediterranean Sea have changed thoroughly. The source of the most dense waters of the basin has moved from the Adriatic to the Aegean Sea. This new source has proved to be very efficient since the estimated formation rate for the period 1989–95 was more than 1 Sv, about three times more than the dense water formation rate of the Adriatic Sea. These new waters with hydrological characteristics, that are warmer and more saline, have replaced almost 20% of the older deep waters of the basin, and have uplifted the deep isopycnals by about 500 m. This major event can be attributed to important meteorological anomalies in the Eastern Mediterranean and to changes of circulation patterns. The extended dry period of 1988–93 and the exceptionally cold winters of 1987 and 1992–93 created favorable conditions for increased formation of dense water in the Aegean. Furthermore, changes in the circulation patterns in the intermediate water range (Levantine Intermediate Water LIW and Cretan Intermediate Water), themselves possibly linked to meteorological anomalies, appear to have played an important role in the redistribution of salt. As a result of an interruption to exchanges between the Ionian and Levantine Basin, the salinities in the latter started to rise, high salinity waters were diverted into the Aegean [ Malanotte-Rizzoli, P., Manca, B. B., Ribera dAcala, M., & Theocharis, A. (1998). The Eastern Mediterranean in the 80s and in the 90s: the big transition emerged from the POEM-BC observational evidence. Rapport du Commission International de la Mer Medittanee, 35, 174–175] and the westward transport of LIW was reduced. An additional effect of the deep water discharge from the Aegean and the resulting uplifting of mid-depth waters was to lower salinities in the LIW layer. This effect is most strongly felt in the Ionian Sea. A 3-D primitive equation numerical model for the Eastern Mediterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them. Under appropriate atmospheric forcing the model successfully reproduces the main characteristics of the transient. These results indicate that the observed changes can be, at least partially, explained as a response of the Eastern Mediterranean, and more specifically of the Aegean, to atmospheric forcing variability.

Water masses and circulation in the central region of the Eastern Mediterranean: Eastern Ionian, South Aegean and Northwest Levantine, 1986–1987

In the framework of the major multinational coordinated POEM-II-86 (March–April 1986) and POEM-V-87 (September–October 1987) cruises in the Eastern Mediterranean, high resolution hydrographic (CTD) data were collected by R.V. Aegaio in the eastern Ionian Sea, south Aegean Sea and northwest Levantine Basin. The intercalibrated data sets were analyzed objectively for an optimal estimation of the basins circulation in two different seasons. The analysis reveals a rather complex general circulation pattern consisting of basin, sub-basin and mesoscale features during both study periods. The major results concern the advection of the Modified Atlantic Water (MAW) along its eastward route, the spreading of the Levantine Intermediate Water (LIW) and its trapping by the eddy fields; the interaction between the south Aegean Sea and the eastern Mediterranean; and the reverse circulation in the Cretan Sea during winter and summer. From the hydrography and the dynamic height maps, synthetic schemes show the main circulation features in the two seasons.

Evolution and status of the Eastern Mediterranean Transient (1997–1999)

Abstract The Eastern Mediterranean Transient (EMT) was the major climatic event in the circulation and water mass properties of the Mediterranean in the last century. In this paper, we describe the present status of the EMT and its evolution since 1995 using hydrological and tracer data from 1997 to 1999. Few but important changes have been observed in the circulation pattern. The intrusion of the Asia Minor Current (AMC) that carries the saline Surface Levantine Waters into the Aegean has been reduced compared to the picture of late 1980s. This means that one of the mechanisms that increased the salt content of the Aegean during the peak of the EMT is no longer present. The Modified Atlantic Water (MAW) signal that has been weakened in the Levantine Basin during the early stages of the EMT has also been re-enhanced. The Aegean still functions as a source of deep (Cretan Deep Water, CDW) and intermediate waters (Cretan Intermediate Water, CIW) for the Eastern Mediterranean, although with modified characteristics. The most important changes in the thermohaline structure of the Cretan Sea (southern Aegean Sea) are the weakening of the signal of the old Mediterranean mid-depth waters and the modification of the properties of the CDW both leading to a reduced stratification. The outflowing CDW is no longer dense enough to reach the bottom of the adjacent basins, but ventilates layers between 1500 and 2500 m. Only the deep eastern Straits of the Cretan Arc are still active in the discharge of CDW, while at the western Strait (Antikithira), the density of the outflowing water was reduced significantly. The intermediate water CIW formed in the Aegean is characterized as a shallow CFC-12, temperature and salinity maximum layer, and differs much from the “old” CIW formed before the EMT, which was found in the layer below the Levantine Intermediate Water (LIW). The new CIW extends into the Ionian Basin through Antikithira Strait. It has lately been observed to enter the Adriatic, where its high salinity is expected to re-establish deep-water formation in this basin. The spreading of the CDW that had been deposited in the Cretan Passage in the first phase of the EMT has progressed further. The entire bottom layer of the Levantine Basin is now covered by the CDW. In the Ionian, the CDW has reached the Straits of Sicily and Otranto. Similar pathways in the Ionian are followed by the new shallower outflow of the CDW.

Dense water formation in the Aegean Sea: Numerical simulations during the Eastern Mediterranean Transient

Dense water formation processes in the Aegean Sea (eastern Mediterranean) are studied using a three-dimensional numerical ocean model. The simulations cover the period 1979-1994 during which major changes that affected the thermohaline circulation of the whole Mediterranean Sea were recorded. Sensitivity studies that focus on the role of freshwater budget are presented, and the results are evaluated against available hydrological data of the same period. The very cold winters of 1987, 1992, and 1993 and the extended dry period 1989-1993 that affected the whole eastern Mediterranean Sea are the main driving mechanisms, corresponding to 50% and 32%, respectively, of the excessive deepwater volume formed in the Aegean after 1987. The reduced Black Sea Water outflow during the same dry period was another important forcing mechanism, contributing 18% to the total formation, while the increased inflow of saline waters from the Levantine Sea after 1992 was an additional preconditioning factor. The locations and mechanisms of water formation processes are identified with combined analysis of data from the March 1987 oceanographic cruise in the Aegean Sea and the respective model results for that period. Deep water is found to be formed mainly through open ocean convection in the central and north Aegean Sea, while the contribution of shelf areas is limited. Intermediate water is also formed through open ocean convection in the southern Aegean Sea during cold winters as well as in the central and northern Aegean during mild winters. The total volume of dense water formed during 1979-1994 corresponds to an annual formation rate of 0.24 Sv for deep water and 0.34 Sv for intermediate water.

Model intercomparison in the Mediterranean: MEDMEX simulations of the seasonal cycle

The simulation of the seasonal cycle in the Mediterranean by several primitive equation models is presented. All models were forced with the same atmospheric data, which consists in either a monthly averaged wind-stress with sea surface relaxation towards monthly mean sea surface temperature and salinity fields, or by daily variable European Centre for Medium Range Weather Forecast (ECMWF) reanalysed wind-stress and heat fluxes. In both situations models used the same grid resolution. Results of the modelling show that the model behaviour is similar when the most sensitive parameter, vertical diffusion, is calibrated properly. It is shown that an unrealistic climatic drift must be expected when using monthly averaged forcing functions. When using daily forcings, drifts are modified and more variability observed, but when performing an EOF analysis of the sea surface temperature, it is shown that the basic cycle, represented similarly by the models, consists of the seasonal cycle which accounts for more than 90% of its variability.

Dense intermediate water outflow from the Cretan Sea: A salinity driven, recurrent phenomenon, connected to thermohaline circulation changes

Data collected from different platforms in the Cretan Sea during the 2000s decade present evidence of gradually increasing salinity in the intermediate and deep intermediate layers after the middle of the decade. The observed gradual salt transport toward the deeper layers indicates contributions of dense water masses formed in various Aegean Sea subbasins. The accumulation of these saline and dense water masses in the Cretan Sea finally led to outflow from both Cretan Straits, with density greater than typical Levantine/Cretan Intermediate water but not dense enough to penetrate into the deep layers of the Eastern Mediterranean. We name this outflowing water mass as dense Cretan Intermediate Water (dCIW). A retrospective analysis of in situ data and literature references during the last four decades shows that similar events have occurred in the past in two occasions: (a) in the 1970s and (b) during the Eastern Mediterranean Transient (EMT) onset (1987–1991). We argue that these salinity-driven Aegean outflows are mostly attributed to recurrent changes of the Eastern Mediterranean upper thermohaline circulation that create favorable dense water formation conditions in the Aegean Sea through salinity preconditioning. We identify these phenomena as “EMT-like” events and argue that in these cases internal thermohaline mechanisms dominate over atmospheric forcing in dense water production. However, intense atmospheric forcing over an already salinity preconditioned basin is indispensable for creating massive deep water outflow from the Cretan Sea, such as the EMT event.

Characteristics of the summer 1987 flow field in the Ionian Sea

We present an extensive analysis of the first complete data set in the northern Ionian Sea collected during the Physical Oceanography of the Eastern Mediterranean (POEM) general circulation survey of September 1987. A four water mass structure of the basin is found to be represented by a first internal baroclinic Rossby radius of deformation of 11.8 km. The horizontal correlation scales decrease with depth, and the subsurface flow is dominated by anticyclonic gyres. Large-scale circulation trends of the temperature and salinity covariance matrices are compensated below 200 m, and only the gyre scales (∼100 km) persist at intermediate and deep levels. The empirical orthogonal functions of the data set show that an horizontal scale separation exists between the first and higher vertical modes of the dynamic height field.

Diagnostic and prognostic numerical studies of LIW formation

Abstract We use a hierarchy of diagnostic and prognostic numerical methods to investigate the formation mechanisms of Levantine Intermediate Water (LIW), determine the formation sites and estimate its annual formation rates. Although the three methods are different and use different hydrological/atmospheric forcing data, all of them reach the same qualitative and quantitative results: under mean climatological conditions the LIW is formed during winter in the Rhodes cyclonic gyre with an annual formation rate of 1.0 Sv. The preferred formation site is mainly controlled by the preconditioning of the hydrology, i.e., the presence of the Rhodes gyre, rather than the spatial distribution of surface forcing, which plays a significant role only during extreme meteorological episodes. The interannual variability experiments show that during severe winters, the formation rate is not increased significantly but instead, deep waters are formed in the center of the gyre, and the LIW formation area is extended towards the northeast Levantine. During mild winters, the formation rate is reduced considerably (

Structures and Characteristics of Newly Formed Water Masses in the NW Levantine During 1986, 1992, 1995

In early spring of 1986, 1992 and 1995, newly formed deep water masses were observed in the cyclonic Rhodes Gyre of the Eastern Mediterranean at depths reaching ∼1000 m in 1995 and exceeding 1000 m in 1986 and 2000 m in 1992. Levantine Intermediate Water (LIW) formation was observed at localized source areas south of the East Cretan Straits at the periphery or near the center of the Ierapetra Anticyclone. Adittional massive LIW formation was observed in 1992 in anticyclonic circulation structures north of Cyprus. The lateral scales of the newly formed water masses in cyclonic structures appear roughly proportional to the penetration depth of the convection, so that a large lateral scale indicating a massive production would be associated with a deep rather than an intermediate formation.