Introduction
In an experiment to examine wintertime water mass conversion in
the Aegean Sea, Charnock and Miller made pre-winter hydrographic
stations in October 1961 aboard R/V Chainand post-winter stations in
March 1962 aboard R/V Atlantis II. In 1961-62 the deep water in the
Aegean Sea had potential temperature of 14.15°C and salinity of
38.95‰ with a potential density anomaly of 29.25 kg m
-3
. For both the
October and March profiles, the deep water is separated from the sur-
face waters above 500 m depth by lower salinity (38.75‰) called
Transition Mediterranean Water by Theocharis et al.[1].
In September 1987 Meteorsurveyed the Eastern Mediterranean
including full-depth profiles in the Aegean Sea. In 1987 the deep water
properties in the Aegean were effectively the same as in 1961-62.
Notably, the 1987 profiles show a near absence of the low salinity
TMW as the minimum intermediate water salinity is about 38.9‰,
much higher than the TMW salinity of 38.75‰. Comparison of
September 1987 and October 1961 and March 1962 depth-averaged
salinities shows an increase in salinity of 0.03‰ to 0.05‰, which
would be equivalent to a local net evaporation of 1.5 m to 2.5 m or an
average extra net evaporation between 6.3 and 10.7 cm yr
–1
.
In January 1995, Roether et al. [2] made another Meteorcruise to
survey the modern deep water properties of the eastern Mediterranean.
Remarkably, newly formed deep water was found in the Aegean Sea
and it was spilling over the connecting sills and spreading out into the
deep eastern Mediterranean basins [2]. The new deep water is about
0.2°C colder, 0.12‰ saltier and 0.13 kg m
-3
denser than the deep
water of 1961-62 or 1987. The structure of the vertical stratification,
however, had returned to 1961-62 conditions with the reappearance of
low salinity intermediate waters (TMW).
Modelling
We develop a simple mixed layer model where the initial profiles
are taken to be the March 1962 observations of temperature and salin-
ity (and hence density) versus depth linearly interpolated to 20 m
intervals down to 2350 m. In time steps of one year, a net evaporation
of 10 cm of freshwater is imposed on the uppermost 20 m layer.We
run the model for 25 years, removing a net evaporation of 10 cm yr
–1
.
Then we continued to run the mixed layer model in monthly time steps
using monthly anomalies of e-p and air-sea heat exchange anomalies
from the SOC climatology [3] from March 1987 to December 1995.
In addition we continue to remove a net evaporation at a rate of 10 cm
yr
–1
(0.83 cm month
–1
).
Within the model, deep convection had reached 1950 m depth in
March 1987. By March 1988, the entire water column is mixed with a
bottom salinity of 38.99‰. After the winters of 1991-92 and 1992-
1993 when large wintertime heat and freshwater losses occur over the
Aegean, the deep water becomes colder as well as saltier.As a result
of mild conditions in 1993 and 1994, no new deep water is found in
March 1994 or in March 1995. By 1995, the deep water in the model
has a salinity of 39.01‰ and a potential temperature of 14.05°C. The
model deep water has cooled by 0.11°C and become saltier by 0.04‰
over the 8 years from 1987 to 1995.
Discussion and Conclusions
The model shows how persistent long-term changes in net evapora-
tion can slowly change the stratification in the Aegean basin until sud-
denly deep water formation occurs during a severe winter.We attribute
the observed increase in depth-averaged salinity from 1961-62 to 1987
to the increase in net evaporation following river diversion for irriga-
tion in Russia and Egypt [4].
The formation of new deep waters in the Aegean is therefore a result
of the two processes: a long, slow increase in the salinity due to
changes in the water budget and then a catastrophic deep water for-
mation event during a suitable cold, dry winter.The presence of low
salinity intermediate waters in 1995 while the newly formed deep
waters are still ?owing out of the Aegean over the sills into the deep
eastern Mediterranean [5] strongly suggests that deep water will not be
formed again for several years in the Aegean. On the basis of observa-
tions and understanding to date, it appears that salinity initially
increases near to where the water balance has changed; salty deep
water is then formed in a local deep basin which then spills out over
the sills spreading into the greater deep basin; within the local basin
the out?owing dense waters are replaced by lower salinity waters
which then shut off deep water formation. The cycle may repeat: we
eagerly await new observations of the next stage as the Mediterranean
becomes saltier and new model simulations brave enough to predict
what will happen next.
References
[1] Theocharis, A., E. Balopoulos, S. Kioroglou, H. Kontoyiannis, and A.
Iona. 1999. A synthesis of the circulation and hydrography of the South
Aegean Sea and the Straits of the Cretan Arc, Progress in Oceanography,
44, 469-509.
[2] Roether,W., B. B. Manca, B. Klein, D. Bregant, D. Georgopoulos, V.
Beitzel,V. Kovacevic and A. Luchetta. 1996. Recent changes in eastern
Mediterranean deep waters, Science,271, 333-335.
[3] Josey, S. A., E. C. Kent and P. K. Taylor. 1999. New insights into the
ocean heat budget closure problem from analysis of the SOC air-sea ?ux
climatology,Journal of Climate, 12, 2856-2880.
[4] Rohling, E. J., H. L. Bryden. 1992. Man-induced salinity and temper-
ature increases in western Mediterranean deep water,Journal of
Geophysical Research, 97, 11,191-11,198.
[5] Theocharis, A., K. Nittis, H. Kontoyiannis, E. Papageorgiou and E.
Balopoulos. 1999. Climatic changes in the Aegean Sea in?uence the
Eastern Mediterranean thermohaline circulation, Geophysical Research
Letters, 26, 1617-1620.
Rapp. Comm. int. Mer Médit., 36,2001
55
MECHANISM FOR NEW DEEP WATER FORMATION IN THE AEGEAN SEA
R. Boscolo
1
*, H. Bryden
2
1
Instituto de Investigacións Mariñas CSIC, Vigo, Spain - rbos@iim.csic.es
2
Southampton Oceanography Centre, Southampton, United Kingdom - h.bryden@soc.soton.ac.uk
Abstract
We analyse whether the observed newly formed deep water in the Aegean Sea could be formed locally by the combination of an increase
in net evaporation and wintertime water mass transformation. With a simple mixed layer model, we show that an effective increase in net
evaporation slowly increases the depth average salinity and decreases the stratification in the Aegean Sea until a severe winter leads to deep
convection and new bottom water formation. We argue that the increased amount in net evaporation over the Eastern Mediterranean basin
is due to river diversion projects in Russia and Egypt since the 1950´s.
Keywords : Aegen sea, water convection, air-sea interaction, depp-waters.
