J.K.A.
U.: Mar. Sci.. Vol. 2, pp. 3-17 (1411A.H./1991 A.D.
Geostrophic Currents Along the Eastern Coast of the Central Red Sea
F. AHMAD and S.A.R. SULTAN Faculty of Marine Science, King Abdulaziz University,
leddah, Saudi A,abia
ABSTRACf. Geostrophic computations of currents along the eastern coast of the Red Sea between 21 ° -24°N show that the suriace circulation is mainly governed by the seasonal wind system. During November 1977 and De- cember 1982 geostrophic currents were directed in a north-northwest direc- tion under the influence of the south-southeasterly winds of the southern half of the Red Sea. In March 1982 the currents were weak and variable ex"
cept at the northernmost section where the current had reversed to south- southeast under the influence of the north-northwesterly winds. In May 1978 geostrophic currents were in a south-southeasterly direction. The cur- rent meter data during May 1978 also show that the current direction is the same as obtained from geostrophic computations.
Intr~uction
The Red Sea stretches from the straits of Bab-el- Mandab in the south to the Sinai Peninsula in the north. It is situated in an Arid zone where evaporation greatly ex- ceeds precipitation (Morcos1970). Consequently the large excess of evaporation over precipitation has been thought to control the circulation (Maury 1855;
Neumann & McGill 1962; Phillips 1966). However, Thompson (1939 a, b), Siedler (1969) and Patzert ~1974) considered wind as the primary factor for generating the flow. The wind system ove.the Red Sea is subjected to the seasonal reversible winds {~.1orcos .1970). Northward of about 20oN, the prevailing wind is north-northwest all thf'. year round. In the southern Red Sea (southward of 200N) the wind blows from
3
F. Ahmad andS.A.R. Sultan
the same direction as in the northern Red Sea during summer (May to September).
During winter (October to April) the winds change to south-southeast, consequently the surface water of the Gulf of Aden is driven. iIito the Red Sea. In. summer months the winds and mean surface currents are directed towards the Gulf of Aden. During transition periods when winds of the southern Red Sea change direction, monthly mean surface currents are weak and varia.ble (Phillips 1966, P!Jtzert 1974). The inves- tigations of various workers in the northern Red Sea is summarized.
Between 1.9° and:25°Nthe mean surface flow IS north-northwest against the wind and currentS with velocities up to 0.42 mls occur in the middle part of the Red Sea
(DHI Handbuch 1978). North of 25° to 26°N a weak flow is to the south due to the north-northwest winds (Barlow 1934, KNMI Atlas 1949, Boisvert 1966). Bibik (1968) showed a large cyclonic eddy between 24°N and 28°N fipm the winter hydrog- raphic data. Morcos and Soliman (1973) showed that there exists a north-northwest surface current in Jan.-Feb. near the coast of Saudi Arabia between 25°N and 27°N Maillard's (1971) geostrophic analysis suggests that the currents during February 1963 were variable and generally across the sea. Wassef et al. (1983) studied theoret- ically the circulation in the Red Sea under the various conditions of bottom topog- raphy and wind stress. The circulation was marked by gyres giving rise to cross cur- rents. Edwards (1987) remarks that "The general features of the Red Sea currents are their: weakness and variability".
Oceanographic Data and Analysis. .
The temperature and salinity data at two pairs of stations were obtained from the Saudi-Sudanese Commission for the Exploitation of the Red Sea Resources (Eig.l).
These data were collected during their Environmental Survey Programme 1977-78.
A digita(CfO probe was used to measure conductivity ratio (j: 0.0004), tempera- ture (j: 0.02°C) and pressure (j: 1%). Stations 6 and 7 (Fig. I) were occupied in November 1.977 while data at stations 22 and 23 (Fig. 1) were collected in May 1978.
In addition, data collected during March and December 1982 by Marine Physics De- partment of the Faculty of Marine Science, King Abdulaziz University at three pairs of stations J/4; R1R4and Q1Q4were_alsoused, (MeshaI1984) (Fig. 1). See also Table~
The temperature and salinity values at discrete depth intervals are given in tables 2 t04.
The seasonal variations in the temperature structure of the Red Sea below about 200-300 m are very small (P;itzert 1972). The salinity and potential density plots pic- ture accurately the" spatial and temporal changes in temperature structure. The geopotential anomalies were calculated relative to 250 m depth.. This consideration for the reference level was basedo~ the availability of temperature and salinity data with depth. The error in the computation of geopotential anomaly is directly propor- tional to the error in the determination ofsp~cific volume anomaly and to the dis- tance from the reference level to the given isolJar:It however requires that the refer- encesurface be located in the no motion layer. Retaining this require~ent and con- sidering the accuracy of the dynamic method the error in the computed geopotential
Geostrophic Currents Along the Eastern.
40 0
350
fl
N
\I'
r
"'Goo...
~
() 25 1250.,~ .,~ "
.,
-Yanbu EGYPT
Q 'J\~ .
~~
~~J3.
; .
Jetfdah.I..
III {<\
..,
20020U SUDAN
15 150
Beb r---'"
350 40U
FIG. Map of the Red Sea showing current meter (0) and STD stations (.).
6 F. Ahmad and S..A.R. Sultan
TABLE 1. Station positions, maximum depths and dates of data collection.
TABLE 2. Temperature, salinity, geostrophic anomalies and relative velocities at stations 6, 7,23 and 22 Geostrophic anomaly
D x 103
Relative velocity Station cm/s
No.6
T"C S%o
Station
No.7
T"C S%o
Station No.23
rc
S%o
Station No.22
rc
~ Depth
m StationrStation
I
StationI
S~ationI
StationIStationNo.6 No.7 No.13 No. 22 6-7 23-22
93 153 205 36 ~23
27.60 39.72 27,63 39.73 27.38 39.89 25.49 39.75 23.70 40.22 22.90 40.28 22.12 40.35 21.80 40.45 21.73 40.50
27.70 38.73 27.62 38.89 26.80 39.18 25.30 39.46 24.02 39.95 23.35 40.25 22.05 40.40 21.70 40.48 21.67 40.53
28.40 38.43 28.18 38.64 27.26 38.71 25.75 39.06 24.90 39.40 24.32 39.96 22.85 40.31 21.95 40.43
21.68
40.52
0 28.15 175 39.48
28.15 39.63 28.15 39.67 28.15 39.99 26.70 39.81 24.45 39.92 22.25 40.38 21.80 40.48 21.70 40.52
153 74 127 176 35 -21
10
135 32
-19
110 56 92
25
48 80 25 -15
69 9
50
15 .14
25 11 ~ 39
75
-18
-19
12 -14
100 -5
-12 -':2 -1
-24 -21 .-24
150
-15
-12
-16 -13 -2
200
0 0 0 0
0 0
250
F. Ahmad and S;A.R. Sui/all 8
anomalies may become comparableiothe actual geopotential anomalies if the refer- ence surfac,e is located at a greater depth. Irithis case one can sacrifice absolute geopotential anomalies in favour of exact computation and to place the reference level at a depth where computational errors will not influence the reliability of the re- sults significantly.. In previpus studies Maillard (1974) considered the reference level at 300 m while Morcos and Soliman (1973) constructed velocity profiles with a refer- ence level as about 250m. From the temperature and salinity data at each station AD
= !(8 .AP) (summation of values relative to the reference level) were computed and the values are given in tables 2 to 4 (~is the anomaly of the specific volume, AP corresponds to the depth intervals).
Finally, the velocities at each depth between th~ pairs of stations 6, 7; 22, 23; JI,J 4;
RI, R4and QI' Q4 were calculated from
10 (ADA -ADB)
V=-
2Q sin fJ L-where A and B label the stations..
L is the distance between the two stations expressed in meters. .Qis the angular speed of the earth and fJ is the average latitude for the two stations. The computed values of velocity between the pairs of stations are given in tables 2 to 4 and the plots are shown inFig.2to9.
FIG. 2. Variation of geostrophic velocity with depth between station 6 and 7~JringNovember 1977.
Geostrophic Currents Along the Eastern.
Vel.cm.s-1
-20 -10
-30 0
\
\ 50100
,!c 150 ;:
-
3
\.
200
250
300
FIG. 3. Variation of geostrophic velocity with depth between station 23 and 22 during May 1978.
FIG.. 4. Variation of geostrophic velocity with depth betweenstationJ\ and J4 during March 1982.
10 F. Ahmad and S.A.R. Sultan
FIG. 5. Variation of geostrophic velocity with depth between station J 1 and J 4 during December 1982.
FIG. 6. Variation ofgeo~trophic velocity with depth between station Rl and R4 during March 1982.
Geostrophic Currents Along the Eastern.
FIG. 7. Variation of geostrophic velocity with depth between station Rl and R4 during December 1982.
50
100 c
CD
!'.
J
150 ,3
200
FIG, 8. Vari~tion of geostrophic velocity with depth between station QI andQ4 during March 1982.
12 F. Ahmad and S.A.R. Sultan
40 Vel. cms-1
10 20 30
0
50
E.100 .c Q.
0 150Q)
200
250
FIG. 9. Variationofgeostrophic velocity with depth between station QI and Q4 during December 1982
The accuracy of conductivity ratio (:t 0.0004) and the temperature (:t 0.02°C) will introduce an approximate error of 0.015 (Tt units. 'the error in specific volume is about 95% of the error in the computation of density. This will introduce an error of...1.6
x 10-5 8 units in the specific volume anomaly calculations. With reference level as 250m the maximum average error in the geopotential anomalies will be about 5 per- cent.
The data at depth 33, 145 m from Aanderra current meter mooring (21°33'N, 38°51'£) collected by the Saudi-Sudanese Commission during their Environmental Survey Programme 1977-78 were analysed to compare the direct measurements with the geostrophiccalculations. The data nearest to the time of hydrographic survey is only for the first four days of May 1978. Eighty hours (00 hour 1st May to 07 hours 4th May 1978) speed and direction data, recorded every 20 minutes, were broken into u (east) and v (north) components and then smoothed. The spectrum of the current meter record consists of the .low frequency band which is less than 1 cycle per lunar day; the central band from 1 to 6 cycles per day and the high frequency band contain- ing frequencies higher than 6 cycles per day (Godin 1972). The residual currents were obtained by filtering the tidal currents and the transient flows associated with
storm surges by applying a 25 hour mean filter.
The progressive vector diagrams were constructed and are shown in Figs. 10 and 11. At 33 m depth the velocity is about 24 cm/s in a south-southeast direction for the first day and thenveers to southeast with a magnitude of about 12 cm/s. The velocity
Geostrophic Currents Along the Eastern. 13
at 145 m is 5 cm/s on the first day and increases to 9-12 cm/s. The dominant direction results to be south-southeast.
N
\51978 r
\.
\.
~ \.
\.
\. \
\.
""
~ '"
'" '"
FIG. to. Progressive vector diagram based on hourly values of the daily residuals at 33 111
14 F. Ahmad and S.A.R. Sui/an
Scale:5cm/sec=2cm
FIG. 11. Progressive vector diagram based on hourly values of the daily residuals at 145m
Geostrophic Currents Along the Eastern.
Results and Discussions
Computation of geostrophic currents during November 1977 (station 6, 7) and De~
cember 1982 (stations J /4; RIR4 and QIQ4) showed a north-northwesterly current at the surface (Fig. 2, 5, 7,9). The current velocities decreased with depth. During this time the wind over the northern half of the Red Sea is also from a north-northwest di- rection but south of 20oN .the prevailing wind is south-southeast. The currents are op- posite to the wind direction. It seems that this occurs under the influence of the strong south-southeast winds of the southern half of the Red Sea. Edwards (1987) concludes that during winter the strong inflow from the Gulf of Aden weakens as it moves northward but the northerly drift persists to the northern end of the basin.
During March 1982 the currents at stations (J /4 and R1R4) were weak and variable (Fig. 4 and 6). However at the northern stations (QIQ4) the sunace current had re- versed to south-southeasterly with a velocity of 20 cm/s (Fig. 8). The month of March is within the transition period when the winds in the southern Red Sea change from south-southeast in winter to north-northwest in summer. Therefore the currents are weak and variable.. At stations Q I' Q4 it had reversed under the influence of the pre., vailing north-northwest winds which blow all the year round in. the northern half of the Red Sea.
In May 1978 (station 23, 22) the currents were in a south-southeasterly direction, with a sunace velocity of 23 cm/s (Fig. 3), under the influence of the prevailing north- northwest winds. The progressive vector diagrams (Fig. 10 and 11) constructed from the current meter records show that there is a considerable variation in velocity on a time scale of the order of a day.
The circulation of the Red Sea is caused by the interaction between the wind on theI
sea surface and the varying thermohaline characteristics. Phillips (1966) theorized that the Red Sea circulation during winter and spring consists of a sunace inflow of warm water which as it moves north cools largely by evaporation and becomes more saline. It gradually becomes denser and sinks, returning south as a cooler, high saline, subsunace current. Although this description of winter-spring circulation is accurate, the circulation is not necessarily of convective origin. The winter-spring circulation is influenced by the wind since during winter these two causes reinforce each other. During summer the sunace current reverses to south-southeast under the influence of the north-northwest winds, which blow along the whole length of the Red Sea.
Acknowledgement
The cooperation of the "Sau9i-Sudanese Commission for the Exploitation of the Red Sea Resources"in providing part of the data is highly appreciated. 'l'he study was supported by a project No. 144/407 of the King Abdulaziz University, Jeddah,
Saudi Arabia.
16 F. Ahmad and S.A.R. Sultan
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(eds.), Hot brines and recent heavy metal deposits in the Red Sea, Springer Verlag, New York In~., pp.131-137.
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Geostrophic Currents Along the Eastern.
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