Investigating changes in the Atlantic Waters characteristics along the
Egyptian Mediterranean Coast.
M.A. Said+, M.A. Gerges, I.A. maiyza, M.A. Hussien and A. A. Radwan
National Institute of Oceanography and Fisheries, Alexandria, Egypt
+
mamsaid2@hotmail.com,
Abstract:
The paper investigates the changes in characteristics of the Atlantic
Waters (AW) as they move eastwards along the Egyptian coast in the Southeastern Mediterranean.
The study analyzed a long series of temperature, salinity and σt data,
collected by several expeditions that were carried out by research vessels of
different nationalities, including Egypt, during the period 1959-2008, averaged
for the winter and summer seasons.
The paper also examined the long-term (50 years) changes that occurred
in the characteristics of the water masses off the Egyptian coast as a result of
damming the Nile River in 1965 and the subsequent cessation of its discharge
into the Mediterranean. These changes were considered in terms of their
possible contribution to the observed changes in the characteristics of the AW
along the Egyptian coast.
The results show that the sea surface temperature of the southeastern
Mediterranean waters off the Egyptian coast varied between 16.6-18.5oC in
winter, and between 22-28oC in summer. Furthermore, the salinity of the coastal
waters off the Egyptian coast has, on average, increased from 26.675 in 1964
before the erection of Aswan Dam, to around 38 in the 1970s and reached more
than 39 in 2008.
Vertically, only one water mass could be observed in winter in the upper
200 m layer, whereas in summer, three distinct water masses could be observed.
The subsurface water mass, which is of Atlantic origin, occupying the 50-150 m
layer and characterized by low salinities ranging from < 38.60 to 38.80, runs
throughout the study area from west to east and spreads over the range of
density between 27.5-28.5 σt.
Temperature and salinity anomalies indicated increasing trends for both
temperature and salinity that reached 0.62oC/dec and 0.067/dec, respectively for
the Mediterranean surface waters. For the Atlantic water, the trends were
0.56oC/dec for temperature and 0.035/dec for salinity. These results confirm that
the increase of temperature and salinity of AW with time are attributed to both
anthropogenic modifications, especially the Nile damming, and the local
climatic changes, which need further investigation.
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Introduction:
Surface salinity trends of the southeastern Atlantic waters (AW) during
1980's–1990's reached 0.04/decade, with relatively low values (~0.01/dec) just
west of the Strait of Gibraltar (Reverdin et al., 2007). This AW, occupying the
upper 200 m layer is likely to flow into the Mediterranean Sea, through the
Strait of Gibraltar, with its general characteristics of S≈36.0-36.5, θ≈13.5-20°C
and potential density t≈26.5-27 kgm-3 (Millot, 2007).
The surface Atlantic Waters, flowing into the Mediterranean are subject to
evaporation and mixing with the underlying waters, causing a progressive
increase of the salinity from 36.25 in the Gibraltar area to 37.25 in the Strait of
Sicily and to values higher than 38.50 in the Levantine Sea. Its west to east path
across the Mediterranean can be tracked by a subsurface salinity minimum
(Lacombe and Tchernia, 1960), representing the signature of their Atlantic
origin.
The present work aims to give better understanding of the long-term
changes in the Atlantic waters passing along the Egyptian Mediterranean coast,
and to show the seasonal variability of the salinity of the inflowing AW due to
the mixing processes and the interannual variability.
Materials and methods:
The hydrographic data used in the present study were taken from several
expeditions carried out by Egypt and different countries from within and outside
the Mediterranean region, for the last 50 years (1959-2008). The temperature
and salinity were averaged and mapped on a ½°×½° grid for winter and summer
seasons.
Results and Discussion
As a result of the erection of Aswan High Dam in 1965, the yearly fresh
water discharge of the Nile River into the southeastern Mediterranean
remarkably decreased. The annual cycle of the discharge has also changed. At
present, the discharge is only through Rosetta, and the maximum discharge is
recorded in winter months. Such a change in both the total amount and pattern
of fresh water discharge had obviously affected the characteristics of the coastal
waters off Nile Delta. The most pronounced and direct effect of the damming of
the Nile River is evidently reflected on the salinity distribution in the coastal
water off the Egyptian coast.
In winter, the surface water temperature varied between 16.6 and 18.5°C,
with slightly colder or warmer spots. The surface salinity changes between
38.60 and 39.30, with a general trend of increasing eastwards. The most
prominent feature of the salinity distribution at the surface is the presence of a
nucleus of salinity >39.00 that lies between longitudes 27-29°E. This nucleus is
characterized by low temperature (16.6°C) and high density 28.7t. The above
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feature coincides with the location of the well- recognized gyre known as Mersa
Matruh gyre. In summer, the surface water temperature varied between 22 and
28°C, except in an area with slightly cold water. This is the area of the abovementioned Mersa Matruh gyre which lies between longitudes 27° and 29°E.
In order to study the vertical space variability of the hydrographic
parameters, the average winter and summer values of each of the water
temperature, salinity and density t were presented on a vertical section taken
parallel to the Egyptian Coast along latitude 32°30'N and between 25 o 30' and
34oE longitudes.
Only one surface water mass could be observed during winter in the upper
200 m layer. This surface water mass is characterized by temperature values
ranging from 15° to17°C, salinity maximum in the range of 38.90 - >39.10 and
corresponding density values of 28.5-28.9 t. This water mass was previously
observed and discussed in detail by Said et al (2007).
Three water masses could be observed in the upper 250 m layer in
summer, as follows: The surface water mass, occupying the upper layer from 30
to 50 m depth, with temperature values of 22° to 28°C and salinity 38.8 to 39.20.
The subsurface water mass with temperature values of 16 to 22°C and minimum
salinity (<38.60-38.80). This water mass is of Atlantic origin, characterized by
oxygen maximum of values >5.2 ml/l (Said & Eid, 1994) and occupies the 50150 m layer. Below this layer, the Levantine intermediate water mass (LIW) of
temperature <16°C and maximum of salinity (38.90-39.10) is clearly identified.
This water mass is formed in some regions of the Eastern Mediterranean, from
where it spreads
Time series summary plot for long-term comparison of water temperature
and salinity for the Mediterranean surface waters and the Atlantic waters along
the Egyptian Coast are shown in Figs 1 & 2. The seasonal cycle of the local
temperature differs markedly from that of the salinity. For the Mediterranean
surface waters, the temperature and salinity anomalies fluctuated between
negative and positive values with a general trend of increasing temperature and
decreasing salinity throughout the study period. During the last 20 years (19882008), the temperature and salinity decadal trends reached 0.62 oC/dec and
0.067/dec respectively.
For Atlantic waters, the temperature anomaly was negative from 1988 to
2000 and then turned to positive till 2008, giving temperature trend of
0.56oC/dec for the last 20 years. Meantime, the salinity anomaly of AW was
positive indicating a salinity trend of 0.035/dec (Fig.1). This increase of
temperature and salinity of AW with time is therefore confirmed to be attributed
to anthropogenic modifications, especially due to the damming of the Nile River
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in addition to local climatic changes as suggested earlier by Rohling and Bryden
(1992) and Bethoux et al. (1990)
Conclusions
The observed increase with time of temperature and salinity of the
Atlantic Water in the Eastern Mediterranean off the Egyptian coast is hereby
confirmed to be attributed to two main factors: anthropogenic modifications,
especially the damming of the Nile River, and to local climatic changes. The
amount and type of information available to-date on the latter factor, calls for
further work to be carried out on this question.
References
Bethoux, J. P., B. Gentili, J. Raunet, and D. Tailliez. 1990. Warming trend in
the western Mediterranean deep water, Nature, 347, 660-662.
Lacombe, H., and Tchernia, P. 1960. Quelques traits generaux de I'hydrologie
Mediterranean- Cah. Oceanogr. , 12 (8), 527-547.
Millot, C. 2007. Intenational salinification of the Mediterranean inflow.
Geophysical
Research
Letters,
vol.
34,
Lxxxxx,
doi:10.1029/2007/GL031179.
Rohling, E.J., and H. Bryden. 1992. Man-induced salinity and temperature
increases in Mediterranean deep water, J. Geophys. Res. 97, 11,19111,981.
Said, M. A. and F. M. Eid. 1994. A quantitative analysis on the Egyptian
Mediterranean water. Acta Oceanol. Sin., 13, 2 , 203-212.
Said, M.A.; I. A. Maiyza; M. A. Hussain and A.A. Radwan 2007.
Characteristics of the Egyptian Mediterranean water masses during the
cold and warm winters. J. Acta Adriatica, 48 (2): 145-159.
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Temperature Anomaly (°C)
2.2
y = 0.0135x - 26.725
1.1
0.0
-1.1
-2.2
1958
(a)
1963
1968
1973
1978
1983
1988
1993
1998
2003
2008
Years
0.30
y = -0.0034x + 6.7302
Salinity Anomaly
0.20
0.10
0.00
-0.10
(b)
-0.20
-0.30
1958
1963
1968
1973
1978
1983
1988
1993
1998
2003
2008
Years
Temperature Anomaly (°C)
Fig (1): time series from 1958 to 2008 of (a) temperature and (b) salinity for the
Mediterranean surface waters.
2.2
y = 0.0117x - 23.097
1.1
0.0
-1.1
-2.2
1958
(a)
1963
1968
1973
1978
1983
1988
1993
1998
2003
2008
Years
0.10
Salinity Anomaly
y = -3E-05x + 0.0641
0.05
0.00
-0.05
-0.10
1958
(b)
1963
1968
1973
1978
1983
1988
1993
1998
2003
2008
Years
Fig (2): Time series from 1958 to 2008 of (a) temperature and (b) salinity for the
Atlantic waters.
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