Pakistan Journal of Marine Sciences, Vol.6(1&2), 1-12, 1997.
THE SEASONAL VARIATIONS OF SEA LEVEL DUE TO DENSITY VARIATIONS IN THE ARABIAN GULF AND GULF OF OMAN
El-Gindy, A.A. and F.M. Eid Oceanography Department, Faculty of Science, Alexandria University, Alexandria, Egypt.
ABSTRACT: From the distribution of oceanographic data (temperature and salinity) in both Arabian Gulf and Gulf of Oman, the steric components (thermal, haline and steric heights) are calculated for the upper 50m layer during different seasons. The anaiysis reveals relevant evidence, that temperature variations (thermal component) play a role in the fluctuations of sea level within the investigated area. The salinity variations (haline component) is only significant near the entrance. The sea level variations due to density (steric component) is low during Winter and Spring and high during Summer and Autumn. The steric height is always lower in the northern and central regions of Arabian Gulf and higher in eastern region of Arabian Gulf and in the Gulf of Oman, i.e. the surface water must flow from the Gulf of Oman to the Arabian Gulf. The steric sea level gradient around the Strait of Hormuz are 0.04 cm/km in Winter, 0.04 cm/km in Spring, 0.025 cm/km in Summer and 0.014cm/km in Autumn.
KEY WORDS: Thermal, haline, steric, sea level, Arabian and Oman gulfs.
INTRODUCTION
The Arabian Gulf is nearly a closed ocean area surrounded by land masses. High mountains extend along its eastern side and low d,esert areas are found in the west. It is connected to the deep Gulf of Oman through the Strait of Hormuz which is a siltless passage. The rate of evaporation in this area exceeds precipitation. Consequently, the area is considered as a negative estuary. Figure 1 shows the investigated area and its bathymetry. Sea level variations are affected by tidal motion, wind stress, atmospheric pressure gradients, currents and the water density distribution, i.e. steric effect (Lisitzin and Pattullo, 1961). Tidal sea level changes have been well studied (Defant, 1961; Le Provost, 1984; Ewans-Roberts, 1979; Sharaf El-Din, 1988; William, 1984 and others). These studies indicated that the Gulf has a lengtp. from its head up to the Strait of Hormuz of about 850km, and a mean width 250m with a mean depth 50m. This allows for free oscillations that are nearly in resonance with the diurnal tide period (of about 22 hours period). Diurnal, semidiurnal as well as mixed oscillation are found in the. study area. The tidal ranges lie between 1.5m in the central part to 3.0m in the northwest, near Shatt Al Arab. The nontidal sea level changes and the response of the sea level to atmospheric pressure gradients have been investigated by Sharaf El-Din (1988) and El-Gindy (1991). The latter author used monthly mean sea level data taken in the period 1980-1987 at six sea level gauges fixed along the eastern coast of Saudi Arabia and atmospheric pressure data at three meteorological stations around this area. The descented barometric response of the sea level (i.e. 1 cm increase in sea level corresponds to about a lmb decrease in atmospheric pressure). The annual cycle of the sea level has an amplitude of lOcm to maximum observed value of l lcm. The difference between the observed mean sea level 2 Pakistan Journal of Marine Sciences, Vol.6(1&2), 1997.
~E ·40° 50" 52" 54° 56" 58"
SHATT·AL~ARAB
IRAN 0 250km 28°
SAUDI ARABIA
Fig.1. Arabian Gulf and its bathymetry.
48.0 50,0 56.0 MO 411.0 !!Q.O 52.0 :14.0 53.D !180 52.0 54.0
JO.O J0,0 JQ.O J&BllllJ'Y JO.O April Fobrauuy lilly Kar•h lll.O 211.0 211.0 28.0 .;. .,,• t 20.0 .. 211.0 211.0 .. 28.0 .;• .;" 24.0 24.0 24.0 24.0
no 22.0 51W22.0 2i.0 B.O 411.0 50.0 52.0 54.0 51.0 4 50.0 52.0 54.0 !16.0 ~ 411.0 50.0 !12.0 M.O lll.O 580 411.0 ~.o ~ !14.0 SllJI llllD
lO.O .lO.O ·~O .»J:l July Sopua!il 24.0 24.0 24.0 22.0 IJM .22.0 U0·2:!.0 48.0 !IO.O 52.0 114.0 5~0 u.o.a.o ~.o 52.0 511.0 511.£) lMf. d09. ~~· Fig.2. Location of hydro graphic stations used in the present study. El-Gindy et al.: Seasonal variations of sea level 3 and the calculated sea level using a linear statistical model that related monthly mean sea level and atmospheric pressure gradient was found to be positiv~ from June to December and negative from January to April or May. This sea level deviation could be due to the steric effect. Lisitzin and Pattullu (1961) discussed the principal causes of sea level fluctuation emphasizing the relative contribution of steric (thermal and haline) and atmospheric pressure components. This study is a first attempt to estimate the steric effect on sea level in the Arabian Gulf and the Gulf of Oman. DATA AND METHODS The. hydrographic data collected on various cruises during the period from 1933 to 1992 are used in this analysis, table 1. The data are grouped to represent the seasonal changes where Winter season includes January, February and March; Spring season includes April and May; Summer includes July, August and September and Autumn includes November and December (Fig.2). Using the above temperature and salinity data, the values at nodal points were calculated for each season, at depths 0, 10, 20, 30, 40 and 50 meters, El-Gindy (1993). The gridded data was used in the calculation of the steric component of the sea level. For any depth, dT and dS designate the seasonal departures in temperature and salinity from their averages Ti and S1. For small values of dT and dS the corresponding departures in specific volume are given by: aa aa da = a (TI , S 1, P) - a (T, S, P) at -- )dT a~ -- )dS + ______ aa aa where (C!T )and (as) are to be evaluated at T 1(P) and S 1(P). The "thermal" and "haline" departures are thus determined by: p? aa Zt=g·1 J (----) dTdP Pa CiT Po (la, Zs =g·' J( ----) dS dP Pa CiS and the added effect of the two terms (the total steric departures) is: Po Za=g·1 JdadP Pa Here Zt, Zs and Za are thermal, haline and steric departures from mean sea level, respectively; g is the acceleration of gravity; Pa is the atmospheric pressure, p0 the 4 Pakistan Journal of Marine Sciences, Vol.6(1&2), 1997. Table 1: Sources of hydrographic data used in the present study. Month Year Country No. of casts Total No. of casts January 1961 USA 103 121 1969 UK 3 1987 QATAR 15 February 1960 USA 13 270 1961 USA 30 1967 UK 36 1977 USA 29 1984 QATAR 14 1987 QATAR 28 1992 USA 120 March 1961 USA 9 68 1965 USA 2 1965 GERMANY 38 1967 UK 5 1969 UK 5 1984 QATAR 9 April 1950 USA 1 59 1965 GERMANY 45 1986 QATAR 13 May 1961 FRANCE 24 165 1966 USSR 4 1967 UK 41 1992 USA 75 June July 1968 USSR 36 54 1984 QATAR 18 August 1968 USSR 102 102 September 1962 INDIA 4 106 1968 USSR 32 1985 QATAR 26 1986 QATAR 44 October November 1933 UK 5 49 1963 USA 5 1968 UK 25 1983 QATAR 3 1984 QATAR 11 December 1933 UK 1 17 1949 USA 3 ·1984 QATAR 13 El-Gindy et al.: Seasonal variations of sea level 5 pressure (depth) to which the integration has to be carried out. In the present work we choose p0 =50 decibar (as the mean depth of the Arabian Gulf). RESULTS AND DISCUSSION Steric components in Arabian Gulf and Gulf of Oman i-.Winter: The horizontal distribution of therffial, haline and total steric departures from mean sea level during winter are shown in figure 3. The fluctuation of sea level due to thermal effect varied between -1.7 and -6.8 cm within Arabian Gulf and between - 0.6 and -2.8 cm within the Gulf of Oman. This is because the water temperature within the Arabian Gulf is much smaller than that within the Gulf of Oman, leading to a gradient in steric sea level from the Gulf of Oman to the Arabian Gulf. A different result is obtained from analysis of haline departures, which varied between -7.8 and 5.0 cm in the Arabian Gulf and between 3.0 and 7.6 cm in the Gulf of Oman. Negative haline departures (low sea level) are found in the northern and central regions of the Arabian Gulf, while positive haline departures (high sea level) occur in the east~i;n part close to the entrance. The. latter is associated with high salinity within the Arabian Gulf and it decreases rapidly near the entrance. In the Gulf of Oman the haline he1ght is positive, i.e. it bas a low salinity which leads to a rise of sea level. Total steric heights reflect the magnitudes of the thermal and haline heights as seen from figure 3. The steric height varied between -14 cm in the northwest of the Arabian . Gulf and 2 cm near the Strait of Hormuz, and between 4 cm and 6 cm within the Gulf of Oman. Sea level is depressed in northern and central parts of the Arabian Gulf due to the low temperature and high salinity (high density), and is elevated close to the Strait of Hormuz and within the Gulf of Oman due to the higher temperature and lower salinity (i.e. generally lower density). It is also noticed that the steric height is higher near the Iranian side than near the Arabian side with differences of about 4-5 cm. By comparing the values of thermal and haline departures with the total steric effects, we can observe that: 1) the steric height in Arabian Gulf is large and the contribution of both thermal and haline heights are important, and 2) in the Gulf of Oman the steric height is mainly due to haline departures (i.e., temperature is less important). ii· Spring: Figure 4 show the horizontal distributions of steric components for the investigated area during Spring. Thermal departure in the Arabian Gulf is generally negative, and varied between -0.5 and -6.9 cm. The sea level decrease is much deeper in the innermost portion, NW of the Gulf, and it gradually increases toward the Strait of Hormuz. In the Gulf of Oman, the thermal height is positive and changed between 0 and 2.9 cm. The lower values are found at the extreme northeastern corner of the region,. while the higher values are observed in the south. Haline departure in the Arabian Gulf fluctuates between -6.3 and 4.9 cm. The negative haline values are observed in the northern aqd central regions. Positive haline consequences are observed in the eastern part of the Gulf, near the Strait of Hormuz. This distribution of haline heights indicate that: 1) high salinity sea water is found in the 6 Pakistan Journal of Marine Sciences, Vol.6(1&2), 1997. w.u :'.>U.U 52.0 54.0 56.0 58.0 60.0 62.0 °E ~0.0 .30.0 Zt Winter z 28.0 2B.O Q +-' 0 _J 26.0 26.0 24.0 24.0 ~~ 22.0 22.0 30.0 30.0 Zs Winter z 28.0 28.0 0 ...... _g 28.0 .26.0 24-.D 24.0 22.0. 22.0 JO.O 30.0 Zo Winter 2.8.0 0z 26.0 .... 0 -' 26.0 26.0 24-.0 24.0 22.0 22.0 4a.O 50.0 52.0 54.0 56.0 58.0 so.o 62.0 Fig.3. Horizontal distribution of steric components (in cm) during Winter. Zt =thermal, Zs =haline, and Za = total steric departures. El-Gindy et al.: Seasonal variations of sea level 7 ~.Q 5Q,Q 52.0 54.0 56,Q 56.0 60,Q 62.0 Of!; 30.0 .30.0 Zt Spring z 28.0 0 28.0 -1-' 0 . ...J 26.0 26.0 2-4-.0 24.0 22.0 22.0 .30.0 30.0 Zs Spring z 28.0 () 28.0 +" _g 26.0 26.0 24.0 24.0 I) 22.0 .:22.0 30.0 .30.0 Za Spring z 28.0 25.0 Q ...... c __I 26.0 26.0 24.0 24.0 22.0 '---..J.--b.....l.-'--"-__,___._...__.___,__....__~....._--1 22.0 48.D so.o s2.o M.o ss.o sito so.a s2.o Fig.4. Horizontal distribution of steric components (in cm) during Spring. Zt =thermal, Zs= haline, and Za. =total steric departures. 8 Pakistan Journal of Marine Sciences, Vol.6(1 &2), 1997. northern and central regions, and 2) low salinity is concentrated around the eastern region near the entrance. Within the Gulf of Oman, the haline departures are positive and have nearly identical values (about 7 cm). The homogeneity of salinity values in the region east of the Strait of Hormuz changes near the entrance of the Arabian Gulf where the haline height slightly decreases to 6 cm. Steric departure~ are relatively high, ranging oscillate in the Arabian Gulf between -12 cm in the northern part, to 4 cm near the entrance. In the Gulf of Oman, steric departures vary between 6cm (close to the Strait of Hormuz) and 10 cm (at the central and southern regions). The water density in the Arabian Gulf is higher than that in Oman Gulf, and generally the density decreases eastward toward the Gulf of Oman. As in winter observations, the contributions of both temperature and salinity are relevant in the Arabian Gulf. The salinity effect is more important in the Gulf of Oman. The spring gradients of steric sea level between Iranian and Arabian sides in the Arabian Gulf are less than those observed in winter with a maximum difference of about 3 cm. iii- Summer: During Summer, observed thermal departures from mean sea level (Fig.5) indicate higher sea level withi.n the investigated area. The contribution of temperature leads to a rise in the sea level in the Arabian Gulf by amounts ranging between 3.2 and 9 cm. In the Gulf of Oman, thermal departures ranged between -2 and 5.5 cm. Haline departures in the Arabian Gulf varied between -7 and 1 cm. In the Gulf of Oman, haline departures oscillate between 2 and 8 cm. Haline influences in both the Gulfs (except the area around the Strait of Hormuz) is homogenous. It is about -5.5 cm in the Arabian Gulf and about 7 cm in the Gulf of Oman. Steric height in the Arabian Gulf is relatively small due to the effect of the high temperature (i.e. positive thermal effects) and high salinity (i.e. negative haline effects). The steric height is depressed by about -2 cm in most northern part and raised by about 6 cm near the Strait of Honnuz as shown from Fig.(5). In the Gulf of Oman, steric height is large and positive (between 6 and 10 cm). Low salinity (positive haline effects) summed with positive therrrial effects yields large and positive val.ues of steric height. Generally, the steric height in the Gulf of Oman is dominated by salinity, as temperatures are less variable. iv-Autumn: Within the Arabian Gulf, thermal departures varied between -1 and 2 cm. Negative values are concentrated in the central region (Fig.6). Within the Gulf of ·Oman, the thermal heights ranged between 1 and 2.5cm (average value of about 2 cm). Temperature effects on the steric height are homogenous and raise sea level only by about 2 cm. In central region of the Arabian Gulf, the temperature effect depress the sea level by only about -0.5 cm. The haline departures are large, varying between -9 and 5 cm within the Arabian Gulf and between 6 and 8 cm within the Gulf of Oman. Salinity values within the area contribute to the wide fluctuations of sea level. In contrast average, sea level is depressed due to salinity effects by about -7 cm within most of the Arabian Gulf and raised by about 7 cm within much of the Oman Gulf. , The steric height (Fig.6) reflects the haline height. In the Arabian Gulf, the steric height range between -8 and 7 cm. The negative steric height occupied the northern and central regions, while positive steric values are found at the eastern edge of this Gulf. In El-Gindy et al.: Seasonal variations of sea level 9 48.0 50.0 52.0 5-4.0 56.0 58.0 80.0 62.0· 0 E 30.0 30.0 Zt Summer z 2.8.0 0 28.0 +-' 0 _J 2.6.0 25.0 24.0 24.0 22.0 22.0 30.0 30.0 Zs Summer 28.0 ·Oz 26.0 ~ 0 _J 26.0 28.0 24.0 24.0 30.0 .30.0 Za Summer z 28.0 0 26.0 +-' 0 _J 26.0 26.0 24.0 24.0 22.0 22.0 48.0 50.0 52.0 54.0 56.0 58.0 80.0 62.0 Fig.5. Horizontal distribution of steric components (in cm) during Summer. Zt =thermal, Zs = haline, and Za = total steric departures. 10 Pakistan Journal of Marine Sciences, Vol.6(1&2), 1997. "K!.U :::iu.u :::iL.U o+.u ~ti.U 56.0 60.0 62.0 u.l!, 30.0 30.0 Zt Autumn z 28.0 0 28.0 +' 0 _J 26.0 26.0 24.0 24.0 ~ 22.0 22.0 30.0 30.Q Zs Autumn 28.0 zQ 28.0 +'a ...J 26.0 26.0 24.0 24.0 22.0 ~o.o ~o.o Za Autumn 28.0 zc:> 26.0 +' -5 '26.0 26.0 24.0 24.Q 22.0 22.0 48.0 50.0 52..0 M.O 56.0 58.0 60.0 62.0 Fig.6. Horizontal distribution of steric components (in cm) during Autumn. Zt = thermal, Zs = haline, and Za = total steric departures. El-Gindy et al.: Seasonal variations of sea level 11 the Gulf of Oman; the steric height is positive and large, it varies between 8 and 10 cm. In general, the steric height is always positive in the Gulf of Oman and negative in .the inner portion of the Arabian Gulf. This difference in sea·level leads to force the Indian Ocean water mass to flow into the Arabian Gulf under the effect of water density. The results summarized in table 2 show that the haline effect is dominant in the Arabian Gulf (high salinity yields negative haline departures) while in the Gulf of Oman low salinity gives a positive haline heights during all seasons. Thermal effects are dominant in the Arabian Gulf except during Autumn. Low temperature during Winter and Spring give a negative thermal height. High temperature during summer gives a positive thermal heights. Table 2: Average seasonal steric components in the Arabian and Oman Gulfs as one area Area Season Zt Zs Za Arabian Gulf Winter -4.75 -5.15 -9.90 Spring -4.11 -4.07 -8.18 Summer 5.21 -4.82 0.39 Autumn 0.71 -6.16 -5.45 Gulf of Oman Winter -1.60 6.70 5.10 Spring 1.83 7.09 8.92 Summer 1.93 6.61 8.54 Autumn 1.79 6.94 8.73 CONCLUSION Seasonal steric components (thermal, haline and steric heights) were calculated in the Arabian Gulf and Oman Gulf for the layer relative to 50 m. Due to the contrast in salinity values within each of the Arabian Gulf (high salinity) and Oman Gulf (low salinity), haline heights play an important role in the variability of sea level in both Gulfs, particularly if they are considered as one ocean area. Sea level is depressed by salinity in the Arabian Gulf by (annual mean of) about 5 cm, and is raised in the Oman Gulf by (annual mean of) about 6.8 cm. The contribution of temperature variation on sea level fluctuations dominates the Arabian Gulf except during autumn. Temperature contributions are less important in the Oman Gulf. Thermal departures depress the sea level during winter and spring in the Arabian Gulf by an average amount of -4.5 cm, and only during winter in the Oman Gulf by values of about -1.6 cm. Total steric departures are relatively large. Sea level is depressed within the Arabian Gulf and raised within the Gulf of Oman. Considering the Arabian Gulf and Gulf of Oman as one area, the steric effect on sea level variations has a strong seasonal trend, where sea level is depressed during winter and spring and is raised during summer and autumn. The lowest average steric height 12 Pakistan Journal of Marine Sciences, Vol.6(1&2), 1997. (-3.Scm) is found during winter, and the highest average one (3.8 cm) is found during summer. The other two seasons can be considered as transition seasons. The sea level gradient across the Strait of Hormuz, towards the Arabian Gulf, is 0.04, 0.04, 0.025 and 0.014 cm/km in Winter, Spring, Summer and Autumn respectively. This supports the flow from the Gulf of Oman to the Arabian Gulf., REFERENCES Defont, A. 1961. Physical oceanography, Vol.2, Pergamon Press. Ewans-Robetis, D.J. 1979. Tides in the Persian Gulf, Consulting Engineer. El-Gindy, A.A.H. 1991. Sea level variations and their relations to the meteorological factors in the Arab Gulf area with stress on monthly means. International Hydrographic Review, Monaco, LXVIII (1): 109-125. El-Gindy, A.A.H. and M. Hegazi, 1993. Atlas on hydrographic condition in Arabian Gulf and Gulf of Oman (ROPME Sea Area). In press by Scientific and Applied Research Centre (SARC), Qatar University. Le Provost, C. 1984. Models for tides in the KAP region. UNESCO reports in Marine Science, 28.Pp.25-36. 0 Lisitzin, E. and J. Pattullo, 1961. 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