A. Lascaratos

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.

General Circulation of the Eastern Mediterranean

Abstract A novel description of the phenomenology of the Eastern Mediterranean is presented based upon a comprehensive pooled hydrographic data base collected during 1985–1987 and analyzed by cooperating scientists from several institutions and nations (the POEM project). Related dynamical process and modeling studies are also overviewed. The circulation and its variabilities consist of three predominant and interacting scales: basin scale, subbasin scale, and mesoscale. Highly resolved and unbiased maps of the basin wide circulation in the thermocline layer are presented which provide a new depiction of the main thermocline general circulation, composed of subbasin scale gyres interconnected by intense jets and meandering currents. Semipermanent features exist but important subbasin scale variabilities also occur on many time scales. Mesoscale variabilities modulate the subbasin scale and small mesoscale eddies populate the open sea, especially the south-eastern Levantine basin. Clear evidence indicates Levantine Intermediate Water (LIW) to be present over most of the Levantine Basin, implying that formation of LIW is not localized but rather is ubiquitous. The Ionian and Levantine basins are confirmed to form one deep thermohaline cell with deep water of Adriatic origin and to have a turnover time of one and a quarter centuries. Prognostic, inverse, box and data assimilative modeling results are presented based on both climatological and POEM data. The subbasin scale elements of the general circulation are stable and robust to the dynamical adjustment process. These findings bear importantly on a broad range of problems in ocean science and marine technology that depend upon knowledge of the general circulation and water mass structure, including biogeochemical fluxes, regional climate, coastal interactions, pollution and environmental management. Of global ocean scientific significance are the fundamental processes of water mass formations, transformations and dispersion which occur in the basin.

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.

Mediterranean Sea Circulation

Allan R. Robinson, Wayne G. Leslie, Division of Engineering and Applied Sciences, Department of Earth and Planetary Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA Alexander Theocharis, National Centre for Marine Research (NCMR), Aghios Kosmas, Hellinikon 16604, Athens, Greece Alex Lascaratos, Department of Applied Physics, Oceanography Group, University of Athens, University Campus, Building PHYS-V, Athens 15784, Greece

Numerical simulation of the interannual variability of the Mediterranean Sea upper ocean circulation

Numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the Mediterranean. From January 1980 to November 1988, the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents. The model prediction of the interannual fluctuations of the Sicily Strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region. The current anomalies persist for many months after a Winter atmospheric anomalous disturbance has occurred over the basin. The Eastern Mediterranean basin is the area where the interannual ocean response is most pronounced.

The Ocean Response to Low-Frequency Interannual Atmospheric Variability in the Mediterranean Sea. Part I: Sensitivity Experiments and Energy Analysis

Abstract In this study a general circulation model is used in order to investigate the interannual response of the Mediterranean Basin to low-frequency interannual variability in atmospheric forcing for the period 1980–88. The model incorporates a realistic scheme for the air–sea interaction physics, has 31 levels in the vertical, and a quarter of a degree horizontal resolution. The simulations show the strong seasonal and interannual signal of the upper thermocline Mediterranean general circulation. Interannual variability of the basin has an eventlike character (anomalous winter wind curl for 1981 and 1986, heat flux winter anomalies in 1981 and 1987) and it is mainly forced by wintertime anomalies;for example, it is locked to the seasonal cycle. The Ionian and the eastern Levantine areas are found to be more prone to interannual changes. The Gibraltar mass transport undergoes small seasonal changes around an average value of 0.95 Sverdrup (Sv) while the Sicily Strait transport is characterized by much ...

A mixed-layer study of the formation of Levantine intermediate water

A mixed-layer model is used to investigate the formation of Levantine Intermediate Water (LIW) over the Eastern Mediterranean. The one-dimensional model is initialized with climatological hydrography and integrated over the Levantine basin with forcing by climatological surface fluxes. Realistic and repeated seasonal mixed-layer cycles are obtained if the annual surface heat input and water loss are offset by a parameterized horizontal advection. The model integrations show that LIW is formed during winter in the mixed layer of the Northwestern Levantine. The preferred formation region for LIW is found through idealized experiments to be controlled by the preconditioning of the hydrography, especially that of the cold, cyclonic Rhodes gyre, rather than by the pattern of the climatological fluxes. The annual-mean formation rate of LIW is estimated to be 1.0 Sv using the climatological surface fluxes. The magnitude of the annual surface fluxes alters the formation rate and modifies the formation region. An additional annual heat flux reduces the formation rate of LIW, whereas an extra cooling enhances it, as well as forming waters denser than LIW in the center of the Rhodes gyre.

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.

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.