Circulation in the Gulf of Aqaba (Red Sea) During Winter–Spring

Journal of Oceanography, Vol. 62, pp. 219 to 225, 2006

Short Contribution

Circulation in the Gulf of Aqaba (Red Sea) during WinterÐSpring

1 2 2 RIYAD MANASRAH *, H. ULI LASS and WOLFGANG FENNEL

1Marine Science Station, P.O. Box 195, 77110-Aqaba, Jordan 2Baltic Sea Research Institute, Seestr. 15, D-18119 Warnemuende, Germany

(Received 4 November 2004; in revised form 1 December 2005; accepted 1 December 2005)

This study is devoted to oceanographic features of the semi-enclosed Gulf of Aqaba, Keywords: Red Sea. The data were recorded in winterÐspring 1999 on the R/V Meteor cruise leg ⋅ Circulation, 44/2. Temperature and salinity profiles were measured at six positions (IÐVI). The ⋅ eddy current, ⋅ shipboard NarrowBand Acoustic Doppler Current Profiler (NB ADCP) 150 kHz con- deep convection, ⋅ tinuously recorded current profiles down to 350 m en route. The research revealed Gulf of Aqaba, ⋅ Red Sea. that the current near the Strait of Tiran front (position VI) represents a semidiurnal signal of an internal tide wave (~12 h period; 0.2 msÐ1 amplitude) that might be generated by the barotropic tide at the sill of the Strait. A sequence of cyclonic and anti- cyclonic eddy pairs is found along the axis of the Gulf of Aqaba during winter-spring seasons. These sub-mesoscale signals are dominant above the main thermocline and might be caused by wind forcing and the narrowness of the Gulf; it might remain in other seasons with different dimensions in relation to the depth of thermocline. The total diameter of each pair was twice the baroclinic Rossby radius (R ≈ 10 km). A single anti-cyclonic eddy was observed in the upper 300 m in the northern tip of the Gulf with a diameter of about 5Ð8 km.

1. Introduction The Gulf of Aqaba is considered to be a unique wa- The Red Sea (Fig. 1(a)) can be considered as a mini- ter body in terms of its importance as a major source of ature world ocean. It features a unique combination of intermediate and deep water formation in Red Sea and in being relatively small size with a rather complex terms of high salinity waters (40Ð41 PSU) compared with bathymetry. It displays several characteristics of global other seas (Plaehn et al., 2002; Manasrah et al., 2004). ocean, such as the role of convective and subductive wa- Water exchange between the Red Sea and the Gulf ter mass formation in maintaining meridional were represented in two-layer and three-layer schemes thermohaline overturning, air-sea interactions leading to for winter and summer seasons, respectively (Klinker et deep-water formation events, interactions with adjacent al., 1976). The average residence time of the water in the semi-enclosed basins, and the significance of small scale Gulf was reported to be about one year (Klinker et al., mixing processes (Eshel et al., 1994; Eshel and Naik, 1976; Paldor and Anati, 1979; Hulings, 1979). 1996). Consistent seasonal current trends along the west The Gulf of Aqaba is the eastern segment of the V- coast of the northern tip of the Gulf were detected during shaped northern extension of the Red Sea (Fig. 1). It is 1988Ð1991 (Genin and Paldor, 1998). A southward cur- located in the subtropical arid area between 28°Ð29°30′ rent along the west coast was observed most of the year, N and 34°30′Ð35°E, it is about 180 km long and has a with a short period (NovemberÐJanuary) of northward maximum width of 25 km. The width decreases at the flow and an abrupt reversal in early February. Berman et northern tip to about 5 km (Hall and Ben-Avraham, 1978; al. (2000) concluded, based on a wind-driven circulation Ben-Avraham et al., 1979; Hulings, 1989). models that the circulation in the Gulf is made up of a series of permanent gyres oriented along its axis, which are influenced by the topography and depth of thermocline * Corresponding author. E-mail: [email protected] (~250 m) in terms of the location and diameter of these Copyright © The Oceanographic Society of Japan. gyres. (Berman et al., 2000).

219 29.5 Potential temperature ( C) Salinity (PSU) I 30 20.5 21 21.5 22 40.2 40.4 40.6 40.8 Gulf of Aqaba 0 0 (b) II 29.2 (a) -200 -200 25 R III

E -400 -400 28.9 (m) D

IV -600 -600

Depth 20 S

E 28.6 -800 Position I -800 Position I Latitude (N) V A Position VI Position VI -1000 -1000

15 28.3 VI Fig. 2. Average potential temperature (°C) and salinity (PSU) profiles at position VI during March 2nd 01:55ÐMarch 3rd 07:53 1999. R/V Meteor cruise 44/2. 28 35 40 34.4 34.7 35 Longitude (E) Longitude (E)

Fig. 1. (a) Map of the study area and (b) locations of the positions (IÐVI) in the Gulf of Aqaba. Bottom depth of posi- -55 tions I, II, III, IV, V and VI are 600, 850, 840, 1400, 1200 -45 -1 and 850 m, respectively. 0.2 ms

-105

-155 The physical properties of the Gulf of Aqaba have (m) Depth been studied by several authors who tried to describe and -205 understand the dynamics and thermodynamics of the water masses (e.g., Genin and Paldor, 1998; Berman et al., 2000; Plaehn et al., 2002; Manasrah, 2002; Manasrah et -255 al., 2004). Plaehn et al. (2002) studied the importance of 0 2.4 4.8 7.2 9.6 12 14.4 16.8 19.2 21.6 24 26.4 28.8 31.2 nd the Gulf of Aqaba for the formation of bottom water in Hours (after March 2 1999 00:00 ) the Red Sea. The authors applied a simple box model to Fig. 3. Current vector distribution at selected depth levels at estimate the range of possible Gulf of Aqaba and Gulf of position VI during March 2nd 01:55ÐMarch 3rd 07:53 1999. Suez water outflow ratios, which could produce the ob- R/V Meteor cruise 44/2. served chlorofluorocarbon component (CFC-12) satura- tion ratio at the bottom (below 900 m depth) of the northern Red Sea. The results showed that the Gulf of Aqaba waters contribute at least 1.5 times as much as the Gulf 5thÐ7th. The area of observation covered the Gulf, where of Suez waters to the bottom water formation in the Red six positions (IÐVI) were separated by 25Ð30 km along Sea. However, more studies are needed for a better un- the axis of the Gulf (Fig. 1(b)). derstanding of the circulation in response to the driving forces of the Gulf. 2.2 Field data This study therefore aims at a better understanding Meteorological data as well as seawater temperature of the general circulation in the Gulf of Aqaba during the and salinity profiles and current records down to 350 m mixing period (winterÐspring 1999 by R/V Meteor cruise depth were continuously recorded en route. 44/2). Currents were recorded using a shipboard Acoustic Doppler Current Profiler 150 kHz (ADCP) covering a 2. Materials and Methods range of about 350 m at a bin length of 8 m and a pulse length of 16 m. The number of bins was 60. The profiles 2.1 Study area were averaged over 2 minutes. A GLONASS/GPS receiver The fieldwork in the Gulf of Aqaba was done from was used to correct for the ship’s motion and a three-di- February 21st to March 7th 1999 by the R/V Meteor cruise mensional GPS receiver (ADU) was used for highly ac- leg 44/2. The Gulf was sampled four times on February curate heading measurements in order to correct the 21stÐ23rd, February 25thÐ27th, March 1stÐ3rd and March Schuler oscillation of the gyro-compass.

220 R. Manasrah et al. -1 Current direction ( ) Current velocity (ms ) 0 60 120 180 240 300 360 00.10.20.3 0 0 (a) (b) -50 -50

-100 -100

-150 -150

-200

Depth (m) Depth -200

-250 -250

-300 -300

-350 -350

Fig. 4. Profile of the mean current speed (msÐ1) and mean direction (°) at position VI during March 2nd 01:55ÐMarch 3rd 07:53 1999. R/V Meteor cruise 44/2.

Salinity and temperature profiles were generated at I 52 stations by the CTD (Conductivity, Temperature, and Depth meter) Neil Brown Mark IIIb that was attached to a 24-bottle 10 L General Oceanic rosette water sampler. s1 s2 s3 s4s5 Four of the bottles were equipped with deep-sea revers- 29.5 ing electronic thermometers from SIS. When employed, 40 m 100 m the rosette was lowered and heaved at a speed of 0.5 29.47 Ð1 Ð1 -1 ms in the upper 100 m and at a speed of 1 ms at greater 0.2 ms depths. 29.44 Calibrations of the pressure and temperature sensors were done prior to the cruise at the Institut für 29.41 29.5 Meereskunde (IFM), Kiel University. During the cruise, 150 m 200 m thermometer readings were used to check the laboratory 29.47 calibration of the temperature sensor. Salinity samples, typically three per profile, were analyzed after the cruise 29.44 Latitude (N) Latitude using an Autosal Salinometer in the IFM. 29.41 29.5 250 m 300 m 3. Results 29.47 3.1 Weather and meteorological conditions during the 29.44 cruise During the cruise the weather conditions in the Gulf 29.41 of Aqaba were good, with poor cloudiness. The winds 34.86 34.9 34.94 34.98 34.86 34.9 34.94 34.98 Longitude (E) blew daily from northerly directions at speeds between Ð1 18 and 26 knots (1 knot = 1.51 ms ). Additionally Fig. 5. Distribution of the horizontal current vectors between orographic- and katabatic effects forced the wind speed stations I, s1, s2, s3, s4, and s5 at selected depth levels dur- up to 40 knots in some regions of the Gulf of Aqaba, es- ing March 5th 01:40ÐMarch 6th 16:00 1999. R/V Meteor pecially in the Strait of Tiran. The average wind speed cruise 44/2. during the cruise was about 20 knots from northerly directions with a frequency of 92%. Due to the short fetch of about 100 km, the maximum wave amplitude was about 1.5 meter. The temperatures dropped to 13°C in the morn- the afternoon with wind speeds up to 8 knots from south- ing, which is about 8°C lower than the sea surface tem- erly directions (Brauner, 2000). perature, rising to 22°C in the afternoon, which equals the sea surface temperature. Only in the last three days of 3.2 Potential temperature and salinity profiles the cruise did the wind become calm due to a low pres- The typical vertical profiles of the average potential sure gradient in the area. A sea breeze then developed in temperature (°C) and salinity (PSU) (Fig. 2) reveal the

Circulation in the Gulf of Aqaba (Red Sea) during WinterÐSpring 221 Fig. 6. Longitudinal section of the current components (msÐ1) along the axis of the Gulf of Aqaba during the R/V Meteor cruise 44/2.

differences in mixing and stratification conditions be- of potential temperature and salinity revealed no signifi- tween the northern (position I) and southern (position VI) cant difference of potential temperature (P = 0.339) be- parts of the Gulf of Aqaba during the study period. In the tween the two positions, whereas the salinity values at upper 600 m, a difference in the potential temperature position I were significantly higher than that at position and salinity at both positions was observed. In the upper VI (P < 0.0001). 250 m, well-mixed waters (range: 21.31Ð21.34°C, 40.66Ð 40.67 PSU; standard deviation: 0.007°C, 0.003 PSU) were 3.3 Rossby radius (R) dominant at position I and weak stratification (range: The Rossby radius is the fundamental horizontal 21.46Ð21.87°C, 40.30Ð40.56 PSU; standard deviation: length scale in fluids that are affected by both gravity 0.095°C, 0.060 PSU) was measured at position VI. At and rotation. In a homogeneous layer of fluid the depths 250Ð600 m in the water column, salinity remained barotropic Rossby radius R is given by R = c/f, where c is homogenous (range: 40.65Ð40.67 PSU; standard devia- the gravity wave propagation velocity gH , g the gravi- tion: 0.006 PSU) but a thermocline was dominant (range: tational acceleration, H the water depth, and f the Coriolis ° ° 20.63Ð21.31 C; standard deviation: 0.230 C) at position parameter. I, whereas the weak stratification was continual (range: In a stratified fluid the baroclinic Rossby radius is ° 20.66Ð21.39 C, 40.56Ð40.67 PSU; standard deviation: computed similarly, except that c is now the wave speed ° 0.205 C, 0.031 PSU) at position VI. of the n-th baroclinic mode as would be found in a nor- Statistical comparison (one-way ANOVA test with mal mode decomposition of the system. The baroclinic significance level 5%) between positions I and VI in terms radius is a natural scale in the ocean associated with

222 R. Manasrah et al. during March 2nd 01:55 to March 3rd 07:53 1999 (Fig. 29.6 -1 3) revealed that the mean current was vertically separated (a) 0.2 ms 29.4 into an eastward flow with a clockwise rotation of about 12 h period in the upper 100 m, and a north-westward 29.2 current carried with an internal tidal wave of about 12 h 29 period at depth 120Ð280 m in the water column. Profiles of the mean values of the current speed and direction (Fig. 28.8

Latitude (N) 4) exhibited an anticlockwise vertical rotation from 100° 28.6 at 35 m depth to 330° at 120 m depth, while below 120 m depth the current direction was mainly constant at about 28.4 315° (Fig. 4(a)). The mean current speed profile had an 28.2 0 24 48 72 96 120 144 168 192 216 240 264 288 312 interesting feature of sinusoidal distribution with an am- Hours (after 21/02/99 00:00) plitude of about 0.2 msÐ1 superimposed on the mean value 29.6 (b) (Fig. 4(b)). However, the amplitude of the clockwise tidal 29.4 rotation decreased with depth and seems not be present in the lower levels (155Ð255 m, Fig. 3). In the lower lev- 29.2 els only the magnitude of the current vectors varied with 29 the same period, but directions do not change. The current measurements in the northern Gulf were 28.8

Latitude (N) scanned at position I and south (~12 km apart) around 28.6 the central and eastern parts along stations s1, s2, s3, s4, and s5. An interpolation for 40, 100, 150, 200, 250, and 28.4 300 m depths was done in order to visualize the distribu- 28.2 0 24 48 72 96 120 144 168 192 216 240 264 288 312 tion of the horizontal current patterns (Fig. 5). The cur- Hours (after 21/02/99 00:00) rent in the upper 200 m in the western and eastern parts from the center of the northern Gulf was northeastward, Fig. 7. Time-latitude distribution of (a) current vectors of real while in the center it was southeastward. Consequently, measurements (Modified after Manasrah et al., 2004) and an anti-cyclonic circulation was observed around the cen- (b) a simulated pure semidiurnal clockwise rotating current tral part of the section in the upper 150 m. Below 200 m field along the axis of the Gulf of Aqaba at 105 m depth on repeated tracks during February 21stÐMarch 6th 1999. R/V to 300 m depth, the northeastward current still dominated Meteor cruise 44/2. in the western part from the centre, while the transition current from northeastward to southeastward can clearly be seen in the eastern part. Obviously, the northeastward and southeastward currents were parts of a relatively boundary phenomena such as boundary currents, fronts, larger anti-cyclonic circulation around the center. The and eddies. The first mode baroclinic radius is typically range and mean of the velocity in the upper 300 m were around 10Ð30 km in the ocean (Gill, 1982). Therefore the 0Ð0.30 and 0.06 msÐ1, respectively. The resulting local- baroclinic Rossby radii can be defined as Rn = cn/f. ized eddy had an apparent diameter of about 5 to 8 km The Rossby radii for the Gulf of Aqaba were calcu- (Fig. 5). lated based on the cruise CTD measurements at position In order to study the basin scale current pattern along VI (Figs. 1(b) and 2) in the southern part of the Gulf of the Gulf axis, four continuous transects were performed Aqaba, where the wave speeds of the first three modes within eight days, on February 26thÐ27th, February 28thÐ and corresponding Rossby radii were: March 1st, March 1stÐ2nd, and March 4thÐ5th 1999. The vertical distributions of the cross and along shore current Ð1 ≈ Ð1 ≈ Ð1 c1 = 0.76 ms ; R1 10 km, c2 = 0.38 ms ; R2 5.5 km, components (ms ) (Figs. 6(a)Ð(d)) showed that the most Ð1 ≈ c3 = 0.25 ms ; R3 3.6 km. significant feature was a wave-like variation of the velocity in the upper 250 m along the Gulf with a typical The baroclinic first mode Rossby radius, in agreement length scale of about 20 km. Furthermore, the distribu- with previous work (e.g., Berman et al., 2000), was R = tion of the cross and along shore current components (Fig. ≈ R1 10 km. 6) mostly showed a constant current at a specific latitude and time. Moreover, the current remained constant at a 3.4 Circulation in the Gulf of Aqaba during winterÐspring specific depth and latitude at different times. These find- Current vector plots at depths of 55, 105, 155, 205, ings therefore show that the tall sub-mesoscale signals and 255 m at position VI of about 30 h of measurements were dominant above the main thermocline.

Circulation in the Gulf of Aqaba (Red Sea) during WinterÐSpring 223 Fig. 8. Vertical section of the, (a) geostrophic velocity (msÐ1) referenced to 600 m depth calculated from positions IÐVI and (b) the time average of cross shore current component (msÐ1) measured simultaneously by the ADCP 150 kHz during the R/V Meteor cruise 44/2.

Moreover, the persistent flow pattern seems to be a rent field with the scale of the Rossby radius in the Gulf typical feature because it could be detected during all the calls into questions the geostrophic current calculations sections within 8 days. The cross and along shore current from stations with a distance larger than the baroclinic component results revealed six wave crests along the Gulf Rossby radius. The distance between the stations was axis with a wavelength of about ~20 km (Fig. 6), i.e., about 25Ð30 km; therefore, changes in currents between twice the baroclinic Rossby radius (R ≈ 10 km). the stations were not detected by the geostrophic method, In order to display a wider picture of the pattern of while the ADCP measurements made this current vari- these waves’ crests, we plotted the current vectors in a ability visible. time-latitude diagram (Fig. 7(a); Manasrah et al., 2004). A comparison of geostrophic currents derived from The current vector plots suggest the existence of a wave the stratification and the time average of the directly train, which looks like a chain of cyclonic and anti-cy- measured currents clearly revealed the sampling error in clonic eddy pairs. Furthermore, both slow transects (the the geostrophic current field due to inadequate station first three tracks were interrupted by CTD stations) and distance (Fig. 8). Besides the differences due to the large the fast transects had a similar sequence of changing flow station distance there was some agreement if the directly (Fig. 7(a); Manasrah et al., 2004). These eddy pairs did observed currents were averaged along the track. not match a simulated pure clockwise rotation of semidiurnal current field along the Gulf axis (Fig. 7), 4. Discussion which had a 12 h period and was fitted to the initial phase condition of the observed current. Although such a sim- 4.1 Current pattern in the Gulf of Aqaba during winterÐ ple slab-like tidal motion is an oversimplification, the spring changes of the currents were not basic tidal motion. This It is likely that the tidal waves in the southern Gulf supports the existence of eddy sequences in the Gulf of of Aqaba are generated by the internal tides at the sill of Aqaba. the Strait of Tiran, carried by the inflow waters from the The existence of the energetic variations of the cur- Red Sea. These tidal waves propagate northwestward into

224 R. Manasrah et al. the Gulf. Monismith and Genin (2004) found that the References observed tidal current in the northern Gulf of Aqaba are Ben-Avraham, Z., G. Almarog and Z. Garfunkel (1979): the result of internal tides generated at the Strait of Tiran. Sediments and structure of the Gulf of Eilat (Aqaba)-north- They attribute the annual variation in currents to varia- ern Red Sea. Sedim. Geol., 23, 239Ð267. tions in generation and propagation associated with Berman, T., N. Paldor and S. Brenner (2000): Simulation of wind-driven circulation in the Gulf of Elat (Aqaba). J. Mar. changes in stratification strength and structure through- Sys., 26, 349Ð365. out the year. Jayne and Laurent (2001) reported that in- Brauner, R. (2000): Meteor - Berichte 00-3. Östliches ternal waves are generated by tidal flow over areas of Mittelmeer - Nördliches Rotes Meer 1999 Cruise, 44(6), 213. rough topography. While some internal wave energy ra- Eshel, G. and N. Naik (1996): Climatological coastal jet colli- diates away as low baroclinic modes, a fraction of the sion, intermediate water formation, and general circulation generated energy flux dissipates locally as turbulence of the Red Sea. J. Phys. Oceanogr., 27, 1233Ð1257. through internal wave breaking. On the other hand, Eshel, G., M. Cane and M. Blumenthal (1994): Modes of sub- Vlasenko et al. (2003) reported that intense, short inter- surface, intermediate, and deep water renewal in the Red nal waves comprise the basic input of the total internal Sea. J. Geophys. Res., 99(C8), 15,941Ð15,952. wave field. These waves are generated by tidal currents Genin, A. and N. Paldor (1998): Changes in the circulation and at sill breaks, are trapped by topography in the genera- current spectrum near the tip of the narrow, seasonally mixed Gulf of Elat (Aqaba). Israel. J. Earth Sci., 47, 87Ð92. tion area and grow by continuous feedback into large- Gill, A. E. (1982): Atmosphere-ocean Dynamics. Academic amplitude waves. As the tidal flow slackens they move Press, New York. upstream as freely propagating waves. Hall, J. and Z. Ben-Avraham (1978): New bathymetric map of The sequence of the observed eddy pairs in the Gulf the Gulf of Eilat (Aqaba). Tenth Int. Conf. Sedimentol., Je- of Aqaba during spring could be represented as tall sub- rusalem, 1, 285 (abst.). mesoscale signals that dominated above the main Hulings, N. C. (1979): Currents in the Jordan Gulf of Aqaba. thermocline. The possible mechanisms by which these Dirasat, 6, 21Ð31. sub-mesoscale signals are formed in the Gulf are wind Hulings, N. C. (1989): A Review of Marine Science Research forcing and narrowness of the Gulf, which is close to the in the Gulf of Aqaba. Marine Science Station. internal Rossby radius and would probably affect the spa- Jayne, S. R. and L. C. Laurent (2001): Parameterizing tidal dis- tial scale of the dynamic signals in the Gulf. Furthermore, sipation over rough topography. Geophys. Res. Lett., 28, 811Ð814. the sub-mesoscal signals might remain in other seasons Klinker, J., Z. Reiss, C. Kropach, I. Levanon, H. Harpaz, E. with different dimensions in relation to the depth of Halicz and G. Assaf, (1976): Observation on the circula- thermocline. Berman et al. (2000), using a numerical tion pattern in the Gulf of Aqaba, Red Sea. Israel. J. Earth model during winter, found that three eddies in the north- Sci., 25, 85Ð103. ern half of the Gulf are a typical feature; the northern- Manasrah, R. (2002): The general circulation and water masses most gyre is anti-cyclonic with 18 km diameter, while characteristics in the Gulf of Aqaba and northern Red Sea. during spring the northernmost gyre is cyclonic with 10 Meereswissenschaftliche Berichte (Marine Science Report), km diameter. 50, 1Ð120. Manasrah, R., M. Badran, H. U. Lass and W. Fennel (2004): Acknowledgements Circulation and winter deep-water formation in the north- This study was funded by the German Ministry of ern Red Sea. Oceanologia, 46(1), 5Ð23. Monismith, S. and A. Genin (2004): Tides and sea level in the Education and Research (BMBF) and the cruise was Gulf of Aqaba (Eilat). J. Geophys. Res., 109(C04015), funded by the Deutsche Forschungsgemeinschaft (Ger- doi:10.1029/2003JC002069. man Science Foundation) under the leadership of the Red Paldor, N. and D. A. Anati (1979): Seasonal variation of tem- Sea Program (RSP). We thank Prof. Dr. Gotthilf Hempel, perature and salinity in the Gulf of Elat (Aqaba). Deep-Sea Dr. Mohammad Badran and Dr. Claudio Richter for co- Res., 26, 661Ð672. ordinating this project, also Dr. Olaf Plaehn for provid- Plaehn, O., B. Baschek, T. Badewien, M. Walter and M. Rhein ing data of the Meteor cruise 44/2, Burkard Baschek, who (2002): Important of the Gulf of Aqaba for the formation of calibrated the ADCP-data, and Marten Walter, who was bottom water in the Red Sea. J. Geophys. Res., 107(C8), mainly responsible for the ADCP measurements on board. doi:10.1029/2000JC000342. The Marine Science Station (MSS) in Aqaba (Jordan) and Vlasenko, V., N. Stashchuk and K. Hutter (2003): Water ex- the Baltic Sea Research Institute (IOW) in Warnemünde change in narrow straits induced by tidally generated nonlinear internal waves. Geophys. Res. Abs., 5, 00877. (Germany) are gratefully acknowledged for the friendly research environment they provide.

Circulation in the Gulf of Aqaba (Red Sea) during WinterÐSpring 225