J. K.A. V.: Mar. Sci., Vol. 3, pp. 3-9 (1412 A.H./1992 A.D.
The Effect of Meteorological Forcing on the Flushing of Shuaiba Lagoon on the Eastern Coast of the Red Sea
F. AHMAD and S.A.R. SULTAN
Faculty of Marine Science, King Abdulaziz University, Jeddah, Saudi Arabia
ABSTRACT. The exchange of water between Shuaiba lagoon and the Red Sea, forced by the local winds, plays a variable but sometimes dominant role in the flushing of the lagoon. The tidal exchange is greatly affected by the force and direction of the wind, caused by the large diurnal differences in local heating. The seasonal mean sea level variations do not have a sig- nificant effect on the flushing of this lagoon.
KEY WORDS. Flushing, Coastal Lagoon, Wind, Red Sea.
Introduction
The exchange of water between coastal lagoons and the sea occurs in response to a variety of forces. In addition to the tides, meteorological forces, especially the wind, playa variable but sometimes dominant role in some cases, depending on local tides and characteristics wind speeds. In the Red Sea the tides are essentially oscillatory and semi diurnal (Morcos 1970). Shuaiba lagoon (Fig. 1) lies very close to the nodal zone and the tidal range is small (Mo~ley 1975 and Sultan and Ahmad 1990).
The Red Sea experiences seasonal reversible winds. The strong variation in the wind system between the summer and winter monsoon is the major factor controlling mean sea level in the Red Sea. During winter the SSE winds drive the surface waters of the Gulf of Aden northward into the Red Sea. This inflow of water exceeds water loss due to evaporation and the outflowing subsurface current. The result is the in- crease in mean sea level. In summer the mean sea level is lower as the NNW winds blowing the length of the Red Sea, drive the water into the Gulf of Aden (Morley
1975).
The Effect of Meteorological Forcing. 5
The coastal areas experience day time sea breezes and nocturnal land breezes caused by the large qiurnal differences in local heating. These local diurnal changes of the wind greatly alter the in and outflows over a tidal cycle. In an earlier paper the flushing of the Shuaiba lagoon was computed (Sultan and Ahmad 1990). In this paper the relative contribution of local meteorological wind in the flushing of the Shuaiba lagoon is calculated.
Non.;tidal Flushing
Apart from the periodic tidal intrusion of sea water in the lagoon, there often exists a non-tidal flux of sea water through the entrance forced by the meteorological forces, especially the wind stress. These non-tidal fluxes do not show the periodicity of the tidal discharges, but may be thought of as a random superposition of oscillating discharges of different periods and amplitudes. For a given velocity amplitude, .the displacement of water mass is inversely proportional to the frequency. Subtidal fre- quencies may play an important role in the displacement (Smith 1977, 1978) some- times having greater influence than the tides. Shuaiba is a region of characteristically low tidal amplitudes and strong variable winds caused by the large diurnal differ- ences in local heating. Therefore the non-tidal component due to meteorological forces may be significant.
Similar to the tidal prism, the non-tidal prism V 0 can be represented by V,,=SUT
where U T S
is the non-tidal velocity,
is the time scale of the non-tidal velocities, is the cross-sectional area of the inlet.
Seasonal Variation in the Non-tidal Flushing
The seasonal variability in the non-tidal component of flushing of a lagoon is re- lated to the non-tidal sea level fluctuations. It is the variance of the sea level and its correlated time scale which are of importance for seasonal fluctuations of non-tidal flushing.
An estimation of seasonal variation in U can be made from the non-tidal sea level variation. If L is the length of the lagoon and h a measure of sea level variance and H is the depth of the lagoon then:
UT=
For Shuaiba lagoon L = .7 km, H = 1.5 m, h = 12cm/month (Morley 1975). This gives a seasonal variation in the non-tidal velocity of about 0.02 cm/sec.
As the seasonal variation in the non-tidal velocity is very small, the seasonal mean sea level variations will not have a significant effect on the flushing.
The Effect of Meteorological Forcing
width of the channel and the relative contribution of local wind in the flushing of the lagoon are given in Table 2.
TABLE 2. The average in or outflow velocities, the computed residual velocities at the entrance and their relative contribution in the flushing.
Results and Conclusion
There are no systematic variations in the residual velocities obtained from 12.5 hours or 25 hours records. The relative contribution of local wind in the flushing of the lagoon on various trips varies from 12 to 97 percent during the period of this study. The tidal in and outflows are considerably altered by the force and direction of the wind and then the exchange appear to be the result of wind stress forcing water directly into or out of the Shuaiba lagoon. As Shuaiba is an area with characteristi- cally low amplitude tides, the relative importance of wind effect increases. Neither the spring-neap cycle of the tide nor the seasonal variability of the mean sea level in the Red Sea have significant effect on the flushing of this lagoon.
The study suggests that fluctuations in flushing are significant compared to the tidal values. Meteorological exchanges may sometimes be more important than the tidal exchanges.
Acknowledgement
This study was financed by a research grant No. 015/1409 from the King Abdulaziz University, Jeddah, Saudi Arabia. Thanks to Mr. Ahmad Azzoghd for typing the manuscript.
The Effect of Meteorological Forcing. 9
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