CIESM Congress 2001, Monaco, article 0073

Rapp. Comm. int. Mer Médit., 36,2001

Introduction

In the late 80's and early 90's an abrupt climatic change (the EMT)

occurred in the Eastern Mediterranean Sea, by which the deep water

formation site moved momentarily from the Adriatic Sea to the

Southern Aegean Sea (Cretan Sea). [1], [2], [3], [4], [5]. The origins

and causes of the EMT have been the object of some controversy.

Some [2], [3], stress the combined role of the salinity increase in the

Cretan Sea during the late 80's and the very cold winters of 92-93

which triggered the very large amount of high density waters produced

in the Cretan Sea, others [4], claim that the EMT was solely caused by

extra cooling, and the observed salinity increase is a consequence and

not a forerunner of the event, while finally [5] based on data analysis

of water masses at various years, claim that the EMT was triggered in

the Northern Aegean where dense waters were formed due to low tem-

peratures and possibly reduced in?ow of the low salinity Black Sea

waters in to the Aegean which then in?uenced the Southern Aegean

Sea.We further investigate the question of the EMT with a series of

numerical experiments focusing mostly on the eventual role of the

Black Sea waters and the Northern Aegean Sea.

The numerical model

We use an implementation of the POM model with a horizontal res-

olution of 1/20 of a degree (~ 5.5 km) in both horizontal directions

with 30 sigma levels in the vertical. The model is initialized with the

MED4-MODB data set and is forced with monthly values of a 'per-

petual year' forcing atmospheric data set derived from the 1979-1993

6-hour ECMWF reanalysis data. The model includes a parameteriza-

tion of the Dardanelles out?ow with a salinity of 28.3 psu and a mean

volume of 10000 m

/sec. The results of the 3

year of the climato-

logical run are kept as out reference experiment to which all other sub-

sequent runs are compared.

Numerical experiments

We conducted a series of 5 numerical experiments (including the

reference run). With these experiments out intention is a) to investigate

the possible effects of the Black Sea waters and b) to examine the role

of extra cold winters in the area. See table 1.

The switching-off of the Dardanelles out?ow is of course an

extreme condition which we nevertheless impose to validate their

impact in winter dense water formation in the Northern and Southern

Aegean Sea. The extra cooling is applied by decreasing the air tem-

perature by 2° Celsius from January up to March in each consecutive

year which results in an SST decrease of 0.5° C approximately.We

believe this to be more realistic than the 2° C drop in SST used in [4]

which in fact implies a much more important decrease of atmospheric

temperatures (of the order of 8° C). All above experiments were run

for three years following the climatological run and the results of the

last year are compared to climatology.

Results and discussion

In experiment 2 we observe an increase in deep waters densities in

the Northern A

egean (by 0.05 sigma-t units), which is further

enhanced in experiment 3 (0.1 sigma-t units). A very small increase

(0.01) of density is observed in the Western Cretan sea. No EMT pro-

duced. In experiment 4, we obtain slightly lighter waters in the

Northern Aegean compared to experiment 3 and a relatively important

increase of densities in the Western Cretan Sea by .05 (29.28). Still no

EMP produced although a small volume of dense waters starts to

appear in the Western Cretan Sea. Finally the experiment 5 results are

an almost perfect combination of experiment 3 and 4 with no syner-

gies observed.

From all the above we conclude that without the salt precondition-

ing observed in the Cretan Sea (not simulated in any of our experi-

ments) the EMT is not realistically produced. In our various experi-

ments we have tried various forcings that might be responsible for the

EMT with the exception of one which is the very important salinity

increase observed in the data of the Cretan Sea prior to the occurrence

of the EMT.We ended up not producing the EMT. In contrast to [3]

where this effect was simulated and the EMT successfully produced.

We conclude that the salt preconditioning (either due to increase of

evaporation and/or increase of salty water in?ow from the Levantine

basin due to circulation pattern changes as mentioned in [3], play a

major crucial role in producing the EMT.

References

1-RoetherW., Manca B., Klein B., Bregant D., Georgopoulos D., Beitzel

V., KovacevicV. and Luchetta A., 1996. Recent changes in the Eastern

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

2-Theocharis A., Nittis K., Kontoyannis H., Papageorgiou E., 1999.

Climatic changes in the deep waters of the Aegean sea and their in?uence

in the deep thermohaline circulation of the Eastern Mediterranean (1986-

1997).Geophysical Research Letters, 20, 1617-1620.

3-Lascaratos A., Roether W., Nittis K. and Klein B., 1999. Recent

changes in deep water formation and spreading in the Eastern

Mediterranean Sea. Progress in Oceanography, 44, 1-3, 5-36.

4-Wu P., Haines K. and Pinardi N., 2000. Towards an understanding of

deep water renewal in the eastern Mediterranean. J of Phys. Ocean., 30,

443-458

5-ZervakisV., Georgopoulos D. and Dracopoulos P. 2000. The role of the

North Aegean in triggering the recent Eastern Mediterranean climatic

changes.Jour. Of Geoph. Res., 105,26,103-26,116

A NUMERICAL STUDY ON THE CAUSES OF THE EASTERN MEDITERRANEAN TRANSIENT :

THE ROLE OF THE NORTHERN AEGEAN AND THE BLACK SEA WATERS

Alex Lascaratos* and Eleni Hatziapostolou

Dept. of Applied Physics, Physical Oceanography Group, University of Athens, Greece -alasc@oc.phys.uoa.ge, ehatz@oc.phys.uoa.gr

Abstract

We investigate the role that the Northern Aegean Sea and the Black Sea waters might have played in triggering the Eastern Mediterranean

Transient. Our results indicate that neither a decrease (even a total absence) of the low salinity Black Sea waters and/or a simple cooling

of the Northern and Southern Aegean can be considered responsible for the production of the EMT in the Southern Aegean Sea.

Keywords : Aegean Sea, Circulation models, Deep waters, Eastern Mediterranean

Table 1: Description of numerical experiments conducted and

corresponding forcing applied

ExperimentsDardanellesExtra cooling in degrees

Celsius

N. AegeanS. Aegean

Yes

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