Residual Currents in Coastal Waters Near Jeddah Desalination Plants, Red Sea
ALAA M.A. AL-BARAKATI
Faculty of Marine Science,
King Abdulaziz University, Jeddah, Saudi Arabia E-mail: [email protected]
ABSTRACT. Data from current meters deployed at 5m and 30m depths, from February to August 2003 in the coastal water near desalination plants Jeddah, were analyzed to investigate the residual currents.
During this period the residual currents were mainly directed to the south both near the surface and at 30m depths. The residual currents are relatively strong during the month of June and are weak in Au- gust, with a velocity of about 2 cm.s–1. The currents are the strongest around middle of May with a velocity of about 87 cm.s–1. Although during the transition period (May, June) the residual currents are ex- pected to be weak but the observations are otherwise. It seems that the local winds have a significant role in controlling the residuals at this location and needs further investigation.
Introduction
The Red Sea stretches from the strait of Bab Al-Mandab in the south to the Sinai Peninsula in the north, a distance of about 1900km. It is separated from the deep waters of the Gulf of Aden and Indian Ocean by the narrow and shallow Strait of Bab Al-Mandab. The climate is arid with high evaporation. This configuration is mainly responsible for the general hydrographic properties of the Red Sea.
Wind Regime over the Red Sea
In the northern part of the Red Sea (north of 19ºN) the winds are from the north-west throughout the year (Edwards and Head, 1987), while in the south- ern part (south of 19ºN) the winds are mainly controlled by the monsoon system
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over the Arabian Sea. From June to September the north-westerly winds are predominant in the southern part as well and from October to May the southern part of the Red Sea is influenced by the south-easterly winds. In other words, the wind system over the southern part of the Red Sea reverses twice during a year (Morcos, 1970; Quadfasel and Baudner, 1993).
From October to December the weak north-westerly winds (~ 2.4-4.4 m.s–1) in the northern part and the strong south-easterly winds (~ 6.7-9.3 m.s–1) in the southern part converge almost at the middle of the Red Sea (19ºN) and create the intermediate zone. This intermediate zone moves toward the south until June when the entire length of the Red Sea is dominated by the north-westerly winds (Patzert, 1974).
Water Circulation in the Red Sea
Due to the arid climate and the changing wind regime there are two main forces involved in the general circulation of the Red Sea: The wind system and the density differences. An excess of evaporation over precipitation of more than 2m.year–1 leads to near surface salinities higher than 42PSU in the north and together with cooling towards the north, drives a thermohaline flow in the upper and deep layers (Maury, 1855; Wyrtki, 1972; Manins, 1973; Eshel et al., 1994). The atmospheric forcing, at least in the southern part, is dominated by bi-annually reversing monsoon winds that give rise to seasonally changing pat- terns of the upper layer circulation (Siedler, 1969; Patzert, 1974). The relative importance of these mechanisms has been much debated. The nature of circula- tion at any time depends on which forcing agent, density gradient or wind, having a predominant effect or more generally on the relative effectiveness of each.
During winter the Red Sea is influenced by two wind systems. In the south- ern region the strong south-easterly winds are dominant to about 19ºN. These winds cause the surface water to flow in a north-westerly direction through the Straight of Bab Al-Mandab, with a mean flow speed of between 15-20 cm.s–1. At about 26ºN, momentum carries the surface wind-driven current in a north- easterly direction against the weak opposing north-westerly wind, which domi- nates the northern region of the Red Sea (Bethoux, 1988; Shapiro and Mescha- nov, 1991).
During summer the north-westerly winds dominate the entire Red Sea caus- ing the mean surface flows to the south-east, toward the Gulf of Aden through the Strait of Bab Al-Mandab. Maximum velocities in excess of 20cm.s–1 are found in this area in July. In early June the direction of the synoptic wind changes from south-easterly to north-westerly causing a reverse in the flow
direction in the southern regions of the Red Sea. Generally, changes in the flow direction are approximately in phase with the changes in the direction of the synoptic wind. However, in early September, the change in the surface water flow direction lags one month behind the change in the direction of the winds.
(Patzert, 1974; Maillard and Soliman, 1986; Souvermezoglou et al., 1989). This is a considerably longer period than would normally be attributed to momentum effects and barotropic change. It is evidently linked to a combination of baro- tropic and baroclinic factors.
The upper layer circulation is considered to be mainly due to wind forcing.
However, Neumann and McGill (1962) and Phillips (1966) argued that, the cir- culation at the upper layer could be entirely attributed to the thermohaline forc- ing. As the surface inflow of warm water from the Gulf of Aden moves north- ward it cools, largely due to the high evaporation in the northern region, and becomes more saline. As a result of the salinity increase the seawater becomes denser and sinks to form a cool high salinity outflow of subsurface water. Using the results of a model that allowed for the conservation of water masses, Eshel et al. (1994) argued that the Red Sea circulation is due primarily to thermoha- line forcing. These authors considered wind forcing to be secondary because in winter the surface water flows against the synoptic wind direction in the north- ern part of the Red Sea. Earlier, Patzert (1974) had presented a similar explana- tion but had limited the scope of his work to the surface water circulation in winter and spring.
General idea regarding Red Sea circulation is available but the detailed infor- mation about local areas is lacking. The coastal circulation is sometimes domi- nated by local topography, wind pattern and tides in the area. Therefore, it is important to investigate the residual currents pattern for any coastal zone man- agement plans.
The Kingdom of Saudi Arabia made unparalleled progresses over the past few decades. Urban centers have grown fast with increased fresh water demand.
The Jeddah Desalination Plants discharge back to the aquatic environment, therefore, the study of residual currents in the area is of significant importance.
Current Meters Data and Analysis
Current meters were deployed at a station (21º32'N 39º06'E) close to Jeddah Desalination Plants (Fig. 1). The I-shaped mooring line was equipped with two Valeport 105 current meters at 5m (surface) and 30m depth levels. The meters were set for 20 minutes recording interval and the duration of the data is from 1st February to 31st August 2003.
FIG. 1. A map of the area, showing the Red Sea in the inset.
In the analysis of the current meter records, numerical filters are used either to remove unwanted frequencies from the observations or to isolate a particular band of frequencies so that the filtered data can be analyzed smoothly. There- fore, the filtering involves the carrying out of certain operations on the observa- tions to create a new sequence of numbers so that the spectrum of the new se- quence satisfies the requirement.
Goodin (1972) discusses the type of filters used for the analysis of current data and recommends “αn” type of filters. These filters involve the summation of n consecutive observations of a time series and their products are particularly useful in tidal and current data analysis.
Most common low pass filters are of the type:
For a 20 min sampling interval Goodin (1972) recommends a filter xxxxxx, txxxyxxxxpe xx for smoothing. This filter was used for smoothing and decimat- ing the observations from a 20 min to 1 h sampling interval. Different filters have been suggested to obtain the daily residuals from the smoothed hourly data. Ahmad et al. (1995) showed that in most circumstances, arithmetic aver- aging over 25 h of smoothed hourly data provides a reliable estimate of the resi- dential drift in the Red Sea.
From the 20 m values of speed (V) and direction (θ), the u (eastward) and v (northward) components were smoothed by the filter xxxxxx. Thereafter, the 25 h mean filter was applied to the smoothed hourly values of u, v components.
The resulting u– , v– values were then used to get daily residual speed and direc- tion:
The plots of the daily residuals are shown in Fig. 2-8.
Results and Discussion
Daily residual current velocities at the surface and 30m depth for the months of February to August show that, the currents at both the surface and 30m depth are relatively stronger during the month of June and are weakest in August, with
α αn n α αn n
n n or
n n
+ +
+ +
2 1 2
2 1
1 2 1
( ) ( )
type
n n
n n
α α2 2
2 2
+
+
( )
α α32 5
3 52⋅ .
V u v
u v
= +
=
2 2
θ tan–1
α α32
2 5
3 5. ,
FIG. 2. Daily average of current velocities at surface and 30m depth during February, 2003.
FIG. 3. Daily average of current velocities at surface and 30m depth during March, 2003.
FIG. 4. Daily average of current velocities at surface and 30m depth during April, 2003.
FIG. 5. Daily average of current velocities at surface and 30m depth during May, 2003.
FIG. 6. Daily average of current velocities at surface and 30m depth during June, 2003.
FIG. 7. Daily average of current velocities at surface and 30m depth during July, 2003.
FIG. 8. Daily average of current velocities at surface and 30m depth during August, 2003.
a velocity of about 2 cm.s–1. However, the residual currents are strongest around middle of May with velocity of about 87 cm.s–1. The current direction during the period of study almost varies from southeast to southwest.
The wind system over the Red Sea, during the winter season is mainly north- westerly in the northern part and southeasterly in the southern part of the Red Sea.
The wind direction changes during the summer season and the entire Red Sea is influenced by the northwesterly winds. Therefore, over the year, there are two transition periods. The location of Jeddah is almost in the middle region of the Red Sea where the minimum wind speed is expected during the transition period.
However, during May and June months, the recorded current velocities are higher compared to the other months. These relatively higher current velocities, during May and June, may be due to local winds and need to have a further investigation.
Conclusion
Two current meters were deployed at 5m and 30m depths near Jeddah Desali- nation Plants from February to August 2003. Daily averaged residual current ve- locities were calculated by using Godin’s (1972) method for smoothing and deci- mating the data to hourly intervals. The residual currents were then obtained by the application of 25 h mean filter. At both depths (5m and 30m) the current di- rections mainly vary from southeast to southwest. Maximum current speeds at 5m and 30m were recorded during May and June respectively. Minimum current speed was observed at 5m during May and at 30m in August.
Acknowledgement. Sincere thanks to King Abdulaziz City for Science and Technology (KACST) for financial support and to the Faculty of Marine Sci- ences, King Abdulaziz University (KAU) for making available their facilities.
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