A Study of Circulation, Water Masses and Mixing Processes in the Southeastern Mediterranean off the Egyptian Coast During Autumn

A total of 34 stations were sampled in the Egyptian Mediterranean continental shelf during autumn 1994. A sharp thermocline of about a quarter of a celsius degree per meter in a layer of about 50 m depth was formed. Correspondingly, a sharp pycnocline layer is found and acts as a barrier to vertical mixing and greatly allows for horizontal mixing. The average current speed of this area was about 30 cm/sec and it was mainly wind driven current at the surface. The Nile Delta Convergence Zone (NDCZ) covers a great area in front of the Nile Delta. This cyclonic gyre was detected earlier during summer season. Introduction The investigation of the circulation pattern is obviously of great importance to the problem of coastal transport of pollutant. Most pollutants are either floating on the sea surface or presents in a mixed state within the upper surface layer of the water. The transport of such pollutants is therefore affected by the surface circulation (Gerges, 1978). Although, many investigators study the circulation pattern in the southeastern Mediterranean sea especially in summer and winter time such as Morcos (1972); Sharaf ElDin (1972); Morcos and Hassan (1976); Sharaf El-Din and Karam (1976); Gerges (1976); Abdel-Moati and Said (1987); Said (1985, 1993); and Said and Eid (1994), but still little is known about the circulation pattern of the Egyptian Mediterranean waters. The aim of the present work is to investigate the circulation pattern during autumn season based on water masses, mixing processes and surface current on the continental shelf of the Egyptian coast. Data and Method of Analysis A cruise was conducted in early autumn (28 September – 8 October, 1994) to survey the Egyptian continental shelf of the Mediterranean using Sayed Maiza vessels. 3 J. KAU: Mar. Sci., vol. 10, pp. 3-15 (1419 A.H. / 1999 A.D.) Ebtessam E.E. Mohamed 4 Through this survey many parameters are measured such as physical parameters (temperature, salinity and current), chemical parameters (nutrients and hydrocarbons), bottom sediments, bottom fauna, phytoplankton, zooplankton, eggs and larvae and fish detection by ESP biosonic echo-sounder along seven meridional sections. These sections were taken at 27, 28, 29, 30, 31, 32 and 33oE. Five oceanographic stations were occupied in each section at depths of 10, 20, 50, 100 and 200 m (Fig. 1). The position of each location was determined by GPS Magnavox 4400 and GPS “Magellan” 1000. Along each station temperature and salinity values were measured using a “Sea-Bird Electronic” CTD. The data provided by Seabird CTD have a resolution of 0.03% dbar for pressures, 0.001oC for temperature and 0.0001 S/m for conductivity. Also, observations of current were made at the depth of 2m below sea surface using an Aanderaa recording current meter. Readings 15 to 30 minutes duration were obtained at every station. FIG. 1. Maps of stations sampled during autumn 1994. Horizontal distribution of temperature, salinity and density at the surface and 50 m depth A) Distribution of temperature Surface temperature in the area of investigation varies from about 28oC in the east to about 27oC in the west. The isotherms at the surface run nearly perpendicular to the coastline (Fig. 2-a). The horizontal surface gradient is stronger in the east (0.01oC/km) near Damietta than in the west near Alexandria (0.005oC/km); it is weak near Matrouh (0.003oC/km). Subsurface temperatures at the depth of 50 m show a high of 25oC in front of the Nile Delta and a low of 19oC in the west; it is 21oC in the East (Fig. 2-b). In the warm area in front of the Nile delta the horizontal temperature gradient reaches 0.06oC/km which is six times greater than at the sea surface. This area will be called Nile Delta Convergence Zone (NDCZ). On either sides of this zone the gradient weakens to about 0.01oC/km. A Study of Circulation, Water Masses and... 5 FIG. 2. Horizontal distribution of temperature along a) 1m depth, b) 50 m depth. B) Distribution of salinity Surface salinity varies between a high of 39.6 in the east and a low of 39.1 in the west, with an even lower salinity of 38.9 in the near shore waters of Damietta (Fig. 3-a). The horizontal salinity gradient at the surface is about 0.02 km–1 near Damietta compared with almost absent gradient in the west. Subsurface salinity at the depth of 50 m shows a high of 39.2 in front of the NDCZ (Fig. 3-b). Lower salinities of 38.8 and 38.6 occur in the east and west respectively. The resulting horizontal salinity gradient is strongest (0.007 km–1) near the delta. C) Distribution of density The distribution of density at the surface reflects the corresponding distributions of temperature and salinity. The isopycnals run nearly normal to the coast (Fig. 4-a). The isopycnal 25.9 σt represents the offshore waters. Lower values of 25.5 σt and 25.8 σt occur near coast in the east and west, respectively. The horizontal density gradient at the surface is strongest near Damietta, 0.01 σt/km. Ebtessam E.E. Mohamed 6 FIG. 3. Horizontal distribution of salinity along a) 1 m depth, b) 50 m depth. The distribution of density at the depth of 50 m (Fig 4-b), also reflects the corresponding distributions of temperature and salinity. Here, the isopycnals run away from the coast, i.e. nearly perpendicular to it. The distribution shows a high of 26 σt in front of the Nile Deltanear Damietta. A low of 25.8 σt occurs both in the east and west. The strongest horizontal gradient is 0.002 σt/km near Damietta. Distribution of temperature, salinity and density with depth A) Distribution of temperature Surface temperature ranges between 27oC and 28.5oC. The top 30 m of the water column is homogeneous in temperature, indicating vertical mixing. A thermocline layer occurs around the depth of 50 m. Temperature drops sharply from about 27oC at 30 m to about 18oC at 70 m depth, with a vertical gradient of 0.23oC/m. Below the thermocline layer temperature drops gently with a rate of 0.02oC/m reaching 15.5oC at the depth of 200 m (Fig. 5-a). A Study of Circulation, Water Masses and... 7 FIG. 4. Horizontal distribution of density along a) 1 m depth, b) 50 m depth. B) Distribution of salinity Surface salinity ranges between about 38.9 and 39.6. The top 30 m is nearly homogeneous indicating vertical mixing. The layer of the water column between 30 m and 70 m (corresponding to the thermocline) shows an overall decrease with depth from about 39.2 to about 38.7, that is, a gradient of –0.01 m–1 (Fig. 5-b). In addition to this rapid fluctuations of up to 0.5 m–1 may be due to the existence of large scale horizontal eddies, makes salinity extremes of 38 and 40.2 in this layer. Below 70 m depth, the salinity increase gently with depth to about 39 at the depth of 100 m, reaching 39.1 at the depth of 200 m. C) Distribution of density Surface density (anomaly) ranges between 25.5 σ1 and 26 σ1. The top 30 m layer is homogeneous (Fig. 5). Pycnocline occurs in the layer between 30 m and 70 m depths. The density anomaly increases from about 26 σ1 at 30 m depth to about 28 σ1 at 70 m, that is, a gradient of 0.05 σ1/m is calculated. In this layer, some of the large salinity fluctuations bring about corresponding fluctuations in density of 0.5 σ1/m. Below the depth of 70 m the density increases gently from about 28 σ1 to 29 σ1 at 200 m depth. Ebtessam E.E. Mohamed 8 FIG. 5. Vertical profile of temperature, salinity and density a) on shore water and, b) offshore water. A Study of Circulation, Water Masses and... 9