139736eo.pdf (20MB) - Japan Oceanographic Data Center

Several two-dimensional models of tides in the Gulf already exist (for complete references,see EL-SABH, 1984; LE PROVOST, 1984). All of these models either have been used or are capableof predicting tidal currents as well as heights, but there is almost a complete absence of systematiccurrent surveys to measure tidal current in detail to provide empirical verification. IMCOS Marine(HIBBERT, 1980) claims to have developed a verified model to predict currents from tide heightpredictions applicable to any part of the Gulf.More recently, MURTY and EL-SABH (1985) studied the age of tides in the Gulf (the age oftides is a term used to denote the interval between the time of new or full moon and the time of the localspring tide). As can be seen in Figure 10, the age of the semi-diurnal tide in the Gulf varies between-35 to 76 hours, with the major variations occurring in areas where the semi-diurnal tide has anamphidromic point. Higher values are found in the shallow area between Bahrain and United ArabEmirates. On the other hand, all ages for the diurnal tide show negative values which vary between -4and -92 hours; the lower value is found in the southern part of the Strait of Hormuz while highervalues occur in the area where the amphidromic point for the diurnal tide occurs. In general, higherages occur in shallow areas and near the amphidromic points, associated with a major topographicfeature.RESIDUAL CIRCULATIONVery little information exists describing the residual current field in the Gulf, the most recenteffort are those by HUNTER (1984) and GALT et al. (1984). It has been known for a long time thatevaporation exceeds precipitation in the Gulf, and so the more saline dense water should sink and passout of the Strait of Hormuz, giving rise to a compensating surface flow of less dense water into theGulf. The effect of the Earth's rotation would deflect these flows to the right, giving a surface flowwest and northwest along the Iranian coast, and a deep flow to the southeast and east along the coastsof Saudi Arabia and the United Arab Emirates (the deep flow would be constrained further to theselatter coastlines adjacent to the shallow sea areas of high evaporation). This circulation patternundoubtedly is modified by forcing of wind and atmospheric pressure, but it generally has beensupported by observations of ship drift. Evidence for this anti-clockwise residual circulation in theGulf, driven predominantly by evaporation (giving rise to horizontal density and pressure gradients),has been described by several authors (see HUNTER, 1984, for details).The northwestern part of the Gulf is undoubtedly influenced by the freshwater inflow of theTigris, the Euphrates and the Karun (entering the Gulf via the Shatt Al-Arab). This inflow appears tobe deflected to the right by Coriolis force to form a river plume of width approximately 20 km, flowingalong the Iraqi coast into Kuwait waters (MATHEWS et al., 1979). Freshwater inflow has beenestimated by GRASSHOFF (1976) to vary between 5 and 100 km3 yr-1. A more realistic estimatemade by HARTMANN et al. (1971) of 37 km3 yr-l represents only about 9% water loss fromevaporation. Since precipitation is light in the region, consideration of the differential flow through theStrait of Hormuz is crucial to the water balance in the Gulf.On the basis of a geostrophic force balance across the Gulf and a frictional balance along theGulf, HUNTER (1982) indicated the circulation schematically by the diagram shown in Figure 11.Evaporation in the shallow areas of the southwestern and southern Gulf leads to a sinking of densewater that is deflected to the right by Coriolis force, to flow out at the bottom of the Strait of Hormuz.This flow is compensated by a surface inflow along the Iranian coast. This circulation will bemodified by the effect of surface wind stress, especially in shallow areas where the effect of densityforcing wil be small.GALT et al. (1984) numerically simulated the steady state surface circulation in the Gulf fromthe combined effects of wind forcing and excess of evaporation (Fig. 12). One major feature of thecirculation is a current moving to the southeast along the western shore. This first develops insouthern Kuwait and then moves south along Saudi Arabia, gaining strength as it moves. By the timeit reaches Bahrain, some of it moves south along the western shore of Bahrain, but most of the flow isbathymetrically steered to the south and east towards the northern tip of Qatar where some smallfraction of it rounds the Qatar coast and moves into the eastern basin of the Gulf. Some re-circulationnorth of Qatar also can be seen. The maximum speed associated with this current is about one-halfknot along the southern coast of Saudi Arabia and north of Bahrain and Qatar. A second major featureof the derived current pattern is coastal flow along the eastern shore or the southwestern coast of Iran.240