CIESM Congress 2001, Monaco, article 0079

Rapp. Comm. int. Mer Médit., 36,2001

Introduction

The Mediterranean is a semi-enclosed sea characterized by an anti-

estuarine thermohaline circulation resulting from the excess of evapo-

ration over precipitation plus river runoff. North Atlantic surface water

that enters the Mediterranean moves eastward and spreads from Sicily

Straits through the southeastern Mediterranean (SEM) undergoing

r e s

ive modifications due to air-sea interaction. The wa

becomes warmer and saltier and reaches the EM as Modified Atlantic

Water (MAW) (1-3). During the summer, a shallow ephemeral water-

layer, defined as the Levantine Surface Water (LSW) is formed (4,5).

The surface water layer displays a high annual-interannual variability

related mainly to the position of atmospheric pressure cells (6). Recent

studies (2,6-8), have shown that the EM thermohaline circulation has

changed considerably since’87. Until `87 the Levantine Intermediate

Water (LIW) was formed in the Rhodes Gyre and the Deep Water

(DW) originated in the south Adriatic Sea. After that, and due to

changes in large scale freshwater balance, the Cretan/South Aegean

Sea became an additional source of DW.

Because of the sensitivity of the EM to large amplitude interannual

variability (6) we initiated this study that aims to determine the

seasonal and annual variability of SEM water-masses as well as their

temperature and salinity characteristics.

Sampling and Methodology

Water samples were taken during twelve consecutive bi-monthly

cruises (June`96 May`98) on board of the R/V Shikmona.The upper

700 m were sampled at a permanent station (32°28.23’N; 34°36.61’E),

~30 km off the Israeli coast. During each cruise, continuous vertical

profiles of T, S and O

concentration were measured by a Sea-Bird

SBE 9 + electronic CTD. Water sample were taken from 1.7-liter

Niskin bottles mounted on a General Oceanics Rosette and were

stored in darkness at ~4°C for the

O analyses. The

O measure-

ments were carried out using a VG SIRA-II mass spectrometer.The

values are reported in ‰VSMOW.The analytical precision is 0.01‰,

and the reproducibility is better than 0.1‰.

Results and Discussion

O variability in surface water

The results show that the

O values of the LSW varied repeated-

ly between 1.3 and 1.7‰VSMOW, without a distinct seasonal pattern.

The stable isotope signal of the surface layer is very sensitive to local

and short-scale climatic events of days and weeks. During summer, the

O variability is in?uenced mainly by the evaporation rate. During

winter, the system is more complex and the variability is the result of

the combined effect of evaporation rate, rainfall

O value and rain-

fall amount, as well as the winter overturn rate (thickness of the mixed

layer). During rain/snow storms, the

O values of the LSW dropped

to ~1.4‰. In other winter events, higher

O values of ~1.6‰ result-

ed from the overturn and mixing of the surface layer with the denser

underlying LIW.

The seasonal and interannual d18O variations with depth

Unlike the frequent variability of the surface-water

O values, the

LIW and DW had nearly constant values of ~1.5 and ~1.3‰, respec-

tively, that generally correspond to the salinity and temperature pro-

files (Fig. 1). Between October and December `96, exceptionally high

OLIW values of up to ~1.8‰ occurred between ~200-400 m water

depth, probably re?ecting an extreme formation event of intermediate

water at the northern part of the Levantine basin. This

O increase

indicates a change in the freshwater forcing or a redistribution of salin-

ity in the northern EM similar to the dry climatic event that occurred

between `87 and `95 (3,7,8).

Conclusions

The eastern Mediterranean water-masses (off the Israeli coast) have

a distinct oxygen isotopic signature controlled simultaneously by sev-

eral physical processes related to the atmospheric forcing. The

variations of the surface water results from local and short-scale cli-

matic events of days and weeks. Changes in evaporation rates, rainfall

amount and its

O value, and the winter overturn rate may play an

important role in shaping the

O signal during the winter. Because

the LIW and DW masses originate in the northern EM, abrupt evapo-

ration/precipitation changes in the surface waters at the source area

could be detected by the

O values of these water masses.

References

1. Manzella G.M.R., Gasparini G.R. and Astraldi M., 1988. Water

exchange between the eastern and western Mediterranean through the

Strait of Sicily.Deep Sea Res., 35: 1021-1035.

2. Malanotte-Rizzoli P., Robinson A.R., Roether W., Manca B.,

Bergamasco A., Brenner, S. Civitarese G., Georgopoulos D., Haley P.J.,

Kioroglou S., Kontoyannis H., Kress N., Latif M.A., Leslie W.G., Ozsoy

E., Ribera d’Alcala M., Salihoglu I., Sansone E. and Theocharis A.,

1996. Experiment in Eastern Mediterranean probes origin of deep water.

AGU - Earth in Space, 9: 8-10 (Source: EOS 77, 305).

3. Malanotte-Rizzoli P., Manca B.B., d’Alcala M.R., Theocharis A.,

Brenner S., Budillon G. and Ozsoy E., 1999. The Eastern Mediterranean

in the 80s and in the 90s: the big transition in the intermediate and deep

circulations.Dyn. Atmos. Oceans, 29: 365-395.

4. Malanotte-Rizzoli P. and Hecht A., 1988. The general circulation of

the Eastern Mediterranean Sea: a review of its phenomenology and mod-

eling.Oceanol. Acta, 11: 323-335.

5. Hecht A., 1992. Abrupt changes in the characteristics of Atlantic and

Levantine intermediate waters in the Southeastern Levantine basin.

Oceanol. Acta, 15: 25-42.

6. Pinardi N. and Masetti E., 2000. Variability of the large scale general

circulation of the Mediterranean Sea from observations and modelling: a

review.Palaeogeogr., Palaeoclimatol., Palaeoecol., 158: 153-173.

7. Klein B., Roether W., Manca B.B., Bregant D., Beitzel V., Kovacevic

V. and Luchetta, A., 1999. The large deep water transient in the Eastern

Mediterranean.Deep-Sea Res., 46: 371-414.

8. Roether W., Manca B.B., Klein B., Bregant D., Georgopoulos D.,

BeitzelV., KovacevicV. and Luchetta, A., 1996. Recent changes in

Eastern Mediterranean deep waters. Science,271: 333-335.

THE D18O VARIABILITY OF SOUTHEAST MEDITERRANEAN WATER MASSES (OFF THE ISRAELI COA

)

Bettina Schilman

*, AhuvaAlmogi-Labin

, Barak Herut

Geological Survey of Israel, Jerusalem , Israel. bettina.shilman@mail.gsi

Israel Oceanographic & Limnological Research, National Institute of Oceanography, Haifa, Israel

Abstract

The annual and seasonal variability of salinity, temperature and oxygen isotopic composition (

O) of the southeastern Mediterranean

water-masses (off the coast of Israel) was studied during two consecutive years (June’96 – May’98). The

O values of the Levantine

SurfaceWater varied between 1.3 and 1.7‰VSMOW, without a distinct seasonal pattern. The variability in

O values during summer

is due to changes in evaporation rates. While during winter, the combined effect of winter overturn and evaporation rates, as well as rain-

fall and its

O signal, modify the surface-water

O values. The intermediate and deeper waters have nearly constant values of ~1.5‰

and ~1.3‰ respectively throughout the year.

Key-words: oxygen isotope composition, water masses, southeastern Mediterranean

Fig. 1. d18O, temperature and salinity variations during the stu-