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Investigating changes in the Atlantic Waters characteristics along the Egyptian Mediterranean Coast

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. Introduction.

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Investigating changes in the Atlantic Waters characteristics along the Egyptian Mediterranean Coast

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  1. 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

  2. Introduction 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, representing the signature of their Atlantic origin.

  3. Introduction Millot (2007), using an autonomous CTD set at 80 m depth on the Moroccan shelf to monitor the inflowing AW during the period 2003-2007, found that the AW has encountered considerable salinification at the rate of about 0.05/yr, i.e. ~0.2 in the 4-year period of observation, together with consequent densification (~0.03 kgm-3/yr in the same period, i.e. 0.12 kgm-3). The much larger warming (~0.3°C/dec) of the AW was found off the coast of Spain (Pascual et al., 1995).

  4. Aim of the present work 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

  5. Area of Investigation The investigated area which is the Egyptian Mediterranean coast lies between longitudes 25°30'E and 34°E and extends northward to the 33°N latitude. Its surface area is about 154,840 km2, with an estimated water volume of about 225 km3. The most striking feature of this area is the presence of different water masses which converge and mix. These are: i- the surface water mass of high salinity; ii- the subsurface water mass of minimum salinity and maximum oxygen, which is of Atlantic origin and extends between 50-150 m; iii- the intermediate water mass of maximum of salinity that extends below 150 m to about 300-400 m depth, and iv- the deep Eastern Mediterranean waters.

  6. Area of investigation

  7. Sources of Data 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. The grid points with missing data were filled by interpolation of the surrounding values. Winter was represented by data collected during the period from January to March, while summer was represented by data collected from July to September. To seek better quality of hydrographic data, a few observations have been rejected because of their poor quality, perhaps due to personal, instrumental, and/or location errors.

  8. Results and Discussion Long-term (1912-1971) time series of data on the Nile River discharge into the Mediterranean before and after the construction of Aswan High Dam in 1964, showed that the average yearly discharge before damming was of the order of 62 km3. The summer of 1964 witnessed the last normal Nile flood, which was exceptionally high and reached 63.73 km3. From 1965 onward, the Nile discharge remarkably decreased to a yearly average of 12.75 km3 for the 7-year period following the damming (1965-1971), with a total discharge of only 4.10 km3 in 1971. Recent records show that the average yearly discharge of the Nile River from 1966 to 2007, i.e. for the last 42 successive years, amounted to only 3.92 km3, representing about 8% of the average value for the period prior to 1965.

  9. Results and Discussion The anomaly of the Nile water discharge through Rosetta Branch indicated that the yearly values during the last three decades are less than the average yearly discharge.

  10. Results and Discussion Moreover, the annual cycle of the discharge has also changed. The discharge usually occurred from July or August until December or January, with the maximum discharge observed during September/October, representing about 25 to 30% of the total discharge. At present, the discharge is only through Rosetta, and the maximum is recorded in winter months. About 65% of the total annual discharge flows into the sea during the three months of December, January and February

  11. Characteristics of the Atlantic waters off the Egyptian coast • 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°30` and 34°00`E longitudes

  12. Winter Season The surface water temperature varied between 16.6 and 18.5°C, with slightly colder or warmer spots. The surface salinity changes between 38.6 and 39.3, with a general trend of increasing eastwards. The surface salinity changes between 38.6 and 39.3, with a general trend of increasing eastwards. The feature of the surface salinity distribution is the presence of a nucleus of salinity >39.0 that lies between longitudes 27-29°E. Which characterized by low temperature (16.6°C) and high density 28.7t, which coincides with the location of the well-recognized gyre known as MersaMatruh gyre.

  13. Winter Season The vertical distribution of the water temperature in the upper 200 m layer of this section shows a great uniformity in temperature in the western part of the study area which could be attributed to a severe cooling at the sea surface in winter. Salinity values increase eastwards and show also great homogeneity, obviously due to vertical mixing 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° to 17°C, salinity maximum in the range of 38.90 - >39.10 and corresponding density values of 28.5-28.9 t

  14. Summer Season The surface water temperature varied between 22 and 28°C, except in an area with slightly cold water. This is the area of the above- mentioned MersaMatruh gyre which lies between longitudes 27° and 29°E. The gyre area is characterized by low values of water temperature (22-25°C), salinity (39.10-39.20) and high density (26.4-27 t).

  15. Summer Season Three water masses could be observed : 1-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. 2-The subsurface water mass with temperature values of 16 to 22°C and minimum salinity (<38.60-38.80) of Atlantic origin, and occupies the 50-150m layer. 3-The Levantine intermediate water mass (LIW) of temperature <16°C and maximum of salinity (38.90-39.10) is clearly identified.

  16. MarsaMatrouh Gyre • . The MersaMatruh gyre has been given different names such as “The Egyptian anticyclonic gyre” by Said (1984, 1990), “The Egypt high” by Brenner (1988) and “MersaMatruh gyre” by Ozsoy et al. (1988). • The MersaMatruh gyre is characterized by: 1- The anticyclonic circulation from surface to the 500m depth during the winter and summer seasons . 2- The gyre splits into two centers at the 50 and 100m levels. 3- Below these levels, the gyre is intensified and split into multiple centers. The eddy centers are shifted horizontally with depth. 4- At the 500m level, the gyre could be observed during both the seasons. The geostrophic current velocity at the edge of the MersaMatruh gyre varied between 12.5 and 29.1 cm.sec-1 in winter and between 6.5 and 13.1 cm.sec-1 in summer .

  17. Time series summary plot for long-term comparison of water temperature and salinity for the Mediterranean surface waters along the Egyptian Coast indicated that, 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°C/dec and 0.067/dec respectively.

  18. 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°C/dec for the last 20 years. Meantime, the salinity anomaly of AW was positive indicating a salinity trend of 0.035/dec.

  19. Conclusions (1) 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.

  20. Conclusions (2) Three water masses could now be observed in summer, namely, the surface water mass in the upper 30-50 m of temperature 22-28°C and salinity 38.8-39.20, the subsurface water mass of temperature 16-22°C and minimum salinity (<38.60-38.80), which is of Atlantic origin and occupies the 50-150 m layer, and the Levantine intermediate water mass (LIW) of temperature <16°C and maximum of salinity (38.90-39.10).

  21. Conclusions (3) Temperature and salinity anomalies of the surface water indicated increasing trends in the last 20 years that reached 0.62°C/dec and 0.067/dec respectively. For the Atlantic waters, the rate of increase was 0.56°C/dec for the temperature and 0.035/dec for the salinity.

  22. Conclusions (4) 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.

  23. Thank you

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