M. A. Said+, M. A. Gerges, I. A. maiyza, M. A. Hussien and A. A. Radwan

The study analyzed a long series of temperature, salinity and σ_t 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 16.6-18.5^oC in winter, and between 22-28^oC 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 σ_t.

Temperature and salinity anomalies indicated increasing trends for both temperature and salinity that reached 0.62^oC/dec and 0.067/dec, respectively for the Mediterranean surface waters. For the Atlantic water, the trends were 0.56^oC/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.

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.

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.

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.

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 50-150 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 (1988-2008), 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 0.56^oC/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 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.