The Summer Circulation in the Gulf of Suez and Its Influence in the Red

AUGUST 2017 S O F I A N O S A N D J O H N S 2047

The Summer Circulation in the Gulf of Suez and Its Inﬂuence in the Red Sea Thermohaline Circulation

S. SOFIANOS Department of Environmental Physics, National and Kapodistrian University of Athens, Athens, Greece

W. E. JOHNS Department of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida

(Manuscript received 20 December 2016, in ﬁnal form 8 June 2017)

ABSTRACT

The Gulf of Suez is accepted as an important location for Red Sea Deep Water formation, but the circulation and exchange with the Red Sea around the year remains elusive. A summer cruise in the area gives the opportunity to investigate features of the summertime hydrological structure and exchange with the Red Sea. An inverse estuarine circulation and exchange with the Red Sea is evident. The topographic patterns of the gulf play an important role in the circulation. Two sills, one in midbasin and a second at the mouth of the gulf, inhibit the bottom flow, topographically trapping waters that were formed in the cold season. Although the water mass characteristics of the outflowing waters during the other seasons are not directly related to the deep waters, they can influence the water column structure of the northern Red Sea. A simple box model shows that their contribution can have a significant influence in the formation of the intermediate layer. A hypersaline (40.6 psu) but relatively warm (238C) water mass, originating in the Gulf of Suez, is detected at intermediate depths (100–150 m), with a strong signal in the western part of the Red Sea.

1. Introduction To the north, the gulf is connected to the Suez Canal. Although there is no significant communication be- The Gulf of Suez is located at the northern part of the tween the Mediterranean and the Red Sea through the Red Sea. It is a long and narrow gulf, without a typical channel, processes taking place in the channel may sill at its entrance (Fig. 1a). Because of its shape it is play a role in the stratification of the northern part of fairly isolated from the Red Sea general circulation the Gulf of Suez. features, exchanging waters through the mouth of the The mean annual air–sea fluxes (net heat flux, evap- gulf. It is also a shallow gulf (maximum depth about oration minus precipitation) over the region are Qnet 5 80 m), so that surface forcing can penetrate rapidly 2 2 58.5 W m 2 (out of the ocean) and E 2 P 5 2.21 m yr 1, through the whole water column, especially in winter. as derived from the closest point of the Comprehensive A topographic map derived from the General Bathy- Ocean–Atmosphere Data Set (COADS) climatology metric Chart of the Oceans (GEBCO; the GEBCO_ (27.58N, 34.58E) and corrected for large-scale biases 2014 Grid, version 20150318, www.gebco.net)at1/1208 over the Red Sea (Sofianos et al. 2002). Maximum heat is presented in Fig. 1b, and two areas in the middle and and freshwater loss take place during the winter. The mouth of the gulf are presented in Figs. 1c and 1d,re- Gulf of Suez is subject to strong evaporation throughout spectively. The topography is rather complicated and the year, and so it is expected to be a concentration two sills can be identified (Figs. 1b–d). One sill is lo- basin, converting relatively fresher surface waters of cated in the middle of the gulf (28.4128N, 33.1638E, Red Sea origin to waters of higher salinity. The ther- with sill depth 56 m) and a second one at the mouth of mohaline circulation of the gulf is characterized as a the gulf (27.6888N, 33.8888E, with sill depth 67 m). two-layered inverse estuarine system, with water en- tering from the Red Sea moving northward—even Corresponding author: S. Sofianos, sofi[email protected] against the prevailing northwesterly winds—and denser

DOI: 10.1175/JPO-D-16-0282.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/04/21 06:46 PM UTC 2048 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 47

FIG. 1. (a) Map and locations of the station occupied in the Gulf of Suez and the northern Red Sea, (b) Gulf of Suez bathymetry, and the topographic characteristics in the area of the two sills: (c) middle of the gulf and (d) mouth of the gulf. water exiting the gulf (Morcos 1970; Murray and mixing with ambient waters at the mouth of the gulf Babcock 1982; El-Sabh and Beltagy 1983). The Gulf of sink to replenish the deep reservoir of the Red Sea Suez is generally accepted as an important source of (Woelk and Quadfasel 1996; Cember 1988; Maillard dense, hypersaline water to the Red Sea (Woelk and 1974; Wyrtki 1974). It is not known if this process Quadfasel 1996; Cember 1988; Maillard 1974; takes place every year (Woelk and Quadfasel 1996). Wyrtki 1974). The ﬂux of RSDW into the Red Sea derived by direct The deep waters of the Red Sea (deeper than 600 m) observations (Maillard 1974)isO(0.04) Sv (1 Sv [ 2 are renewed through sinking of the deep water 106 m3 s 1) on an annual basis. Another O(0.02) Sv is formed in the gulfs of Suez and Aqaba (Plähn et al. contributed by the Gulf of Aqaba (Biton and Gildor 2002; Woelk and Quadfasel 1996; Cember 1988; 2011). During the rest of the year, the heat loss from Maillard 1974; Wyrtki 1974). The time of this forma- the gulf’s surface is relatively weak and the waters tion as well as the amount of Red Sea Deep Water cannot become dense enough to produce RSDW. The (RSDW) produced is not well known. It is generally circulation and stratiﬁcation in the Gulf of Suez as believed that it is during the peak of the winter sea- well as its interaction with the open Red Sea during son, when intense evaporation and surface cooling the summer season, remain unexplored. In this paper take place, that relatively small amounts of extremely we use summer hydrographic data from the Gulf dense water are produced in the gulf, which after of Suez and the northern Red Sea to investigate

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FIG. 2. The T, S, su, and O2 transects along the main axis of the Gulf of Suez. whether dense waters are also exported from the Gulf 3. Results of Suez in summer and what their fate is in the Red In the summer of 2001, the Gulf of Suez has the typical Sea basin. inverse-estuarine system stratification pattern (Fig. 2), with an overturning cell bringing relatively buoyant water (su 5 2. Data 23 26.5 kg m ,wheresu is the potential density anomaly) During August 2001, a hydrographic survey was car- into the gulf, which, subject to intense evaporation, be- ried out in the Red Sea, including the Gulf of Suez, comes denser and outflows from the mouth, achieving 2 aboard the R/V Maurice Ewing as part of a project by potential density around 27.5–28 kg m 3. The bottom wa- the University of Miami’s Rosenstiel School of Marine ters at the northern part of the gulf are isolated and of high and Atmospheric Science (Sofianos and Johns 2007). density and low oxygen concentration values. This water The stations collected in the northern Red Sea and the mass must have been trapped in the area probably from Gulf of Suez are shown in Fig. 1a. At all stations, pres- the dense water formation that took place in the winter sure, temperature, salinity, and dissolved oxygen were season, as already observed and suggested by Wyrtki collected using a SeaBird 9plus CTD system. Water (1974). In the middle of the gulf, the very dense waters samples for the calibration of the salinity and dissolved appear trapped due to the presence of the midgulf sill. The oxygen were collected at about one-third of these sta- summer two-layer circulation pattern seems to take place tions. The survey covered stations in the Red Sea and on the top of the layer of very dense water, while the O2 along the main axis of the Gulf of Suez, following its distribution suggests that the water column is well venti- deeper parts (Fig. 1a). The spacing of the station along lateddownto;45 m, and thus the temperature gradient the axis of the gulf varies from about 20 km in northern between 25 and 45 m in the northern Gulf indicates vertical and middle parts of the basin to a few kilometers close to mixing with the remnant winter water. Lower temperature the mouth. and oxygen waters are also found close to the bottom in the

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FIG.3.Au/S diagram of the NODC data (black symbols) with ‘‘x’’ for the winter values (December–April) and ‘‘o’’ for the summer values (June–October) compared with the cruise data (with the color indicating the stations presented in Fig. 2). southern part of the basin. Their presence can be associ- values observed during winter. Waters of density be- 2 ated with the effect of the mouth sill and some leakage tween 28 and 29 kg m 3 are observed in this part of the of the wintertime water trapped in the north Gulf of Suez Gulf of Suez even in the summer season, indicating that that had undergone mixing with surface waters. they are trapped by the topographic features of the gulf. The temperature and salinity characteristics measured during the cruise in the Gulf of Suez were 4. Contribution to the Red Sea intermediate waters compared to historical observations. The National Oceanographic Data Center (NODC) dataset covering Although the density of the bottom waters at the the period 1889–90 was used to extract the historical mouth of the gulf during the time of the cruise is not high temperature and salinity characteristics during winter enough to produce RSDW, it is still much higher than (December–March) and summer (May–October) (Fig. 3). that of northern Red Sea surface waters, and the out- The temperature and salinity characteristics measured ﬂow, after entraining ambient water, sinks below the during the cruise (Fig. 3, color dots associated with the mixed layer. Thus, the observations indicate a warm stations) represent the warmest conditions during the season contribution of the Gulf of Suez to the year of the Gulf of Suez water column. It can be seen that, despite the latitudinal trend in salinity, the observed range of salinity values is rather constant throughout the year. On the other hand, temperature has a large seasonal variability, especially in the northern part of the gulf, and can get as low as ;168C at the end of winter (February–March), when heat loss be- comes maximal and the deep-water formation takes place. The southern part of the Gulf of Suez is characterized by lower salinity values, with the largest gradient located between the midgulf region and the mouth. Temperature and salinities observed during the cruise in the northern part of the gulf are similar to the highest FIG. 4. A box model of the intermediate Red Sea layer.

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TABLE 1. Transport and salinity values used in the box model. comparable to deep-water formation rate (Maillard 1974) that takes place during the winter season. Parameter Description Value To quantify the influence of the water masses out- Qo Outflow transport 0.36 Sv flowing from the Gulf of Suez to the main thermohaline QD Deep layer contribution to the 0.06 Sv intermediate layer cell of the Red Sea, related to the intermediate layer that QG Gulf of Suez contributing transport 0.03 Sv forms the Red Sea Overflow Water (RSOW) layer So Outflow salinity 39.8 (Sofianos and Johns 2003), a simple box model was SD Deep layer salinity 40.55 constructed (Fig. 4), depicting the main water (Q) and SG Gulf of Suez contributing salinity 40.6 salt (S) fluxes of the intermediate layer. It includes the SS Surface layer salinity at the southern end 39.8 of the basin RSOW outflow (Qo and So), mixing with the surface (QS SN Surface layer salinity at the northern end 40.25 with SS and SN noting the surface layer salinity at the two of the basin ends of the basin) and bottom layers (QD and SD; all the deep water is finally entrained by the intermediate layer or contributing to the outflow), a RSOW formation rate intermediate layers of the Red Sea. Using the salinity (QF), and the Gulf of Suez contribution to the inter- profile of Fig. 2 at the mouth of the gulf (40.61 and 40.88 mediate layer (QG and SG). The latter is switched on and for the surface and lower layer, respectively), a summer off, in order to estimate its contribution. The values 2 evaporation rate of 2 m yr 1 (based on COADS data) (Table 1) were selected from available observations with a surface area of 6760 km2, and assuming a quasi- (Sofianos et al. 2002; Sofianos and Johns 2007) and from steady state, where water volume and salt is conserved, the estimated summer contribution of the Gulf of Suez. according to the Knudsen (1900) equations, we can es- The volume and salt conservation must satisfy the timate the water mass outflow from the gulf at a rate of following equations: about 0.06 Sv. If we assume that the summer type con- å 5 å 5 Qi 0 and QiSi 0. tribution takes place during the months that the net heat i i flux is small or positive toward the ocean (May–October, i.e., six months), we can estimate the yearly averaged Leaving the formation rate (QF)astheunknown contribution rate at about 0.03 Sv. This amount is parameter, we find that without the summer contribution

FIG. 5. Salinity vs depth at speciﬁc stations of the northern Red Sea showing a salinity maximum at depths between 100 and 150 m (and weaker maxima at smaller depths).

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FIG.6.Au/S diagram of the northern Red Sea stations (ST18–ST32). The red square represents the salinity maximum at depths 100–150 m. of the Gulf of Suez the northern Red Sea subduction Interestingly, the maximum bottom densities near the 23 rate is about 0.2 Sv, while when including it the forma- mouth of the gulf are about su 5 28.0 kg m (stations tion rate reduces by about 20% (0.16 Sv). To estimate 13 and 14; Fig. 3), lower than the density at the core of this 23 the sensitivity of this result to the selection of the maximum salinity intruding layer (su ; 28.2 kg m ; parameters used in the box-model calculations, we Fig. 6). Therefore, it is likely that these waters come applied a range of values in the warm season outflow from the wintertime deep waters in the gulf somewhat rate from the Gulf of Suez and its salinity. A 650% earlier in the year, before midsummer, when our change of the outflow rate results in an about 10% survey took place. After this intrusion layer is formed, change of reduction, while a 60.5 change in the salinity it can circulate around the northern basin by the cy- of the outflowing waters results in an about 4% change clonic circulation observed in the region. The pres- of the reduction. ence of the high salinity signal in the western An interesting feature, shown in Figs. 5 and 6,isa boundary of the northern Red Sea indicates a pathway salinity maximum at depths 100–150 m in several sta- around the cyclonic circulation pattern, which is tions of the northern Red Sea, from just outside the consistent with previous studies (Zhai et al. 2015). mouth of the Gulf of Suez up to station 32, which is These saline waters can be trapped in the region, located over 260 km away from the gulf. This very where the RSOW formation takes place during winter saline layer penetrates the Red Sea water column (Sofianos and Johns 2003). between the RSOW layer and the RSDW layer and is most prominent in the vicinity of the gulf and closer to 5. Conclusions the western coast, while there is no clear signal of this water mass near the center of the basin. The Gulf of The summer circulation of the Gulf of Suez consists of Suez is the only source of hypersaline waters close to an overturning cell, where concentration of waters of the region. The Gulf of Aqaba stratification during the Red Sea origin takes place. Although summertime me- same season has a salinity minimum at the respective teorological conditions are not favorable for RSDW levels (Biton and Gildor 2011)andsurfacesalinityof formation, the Gulf of Suez influences the Red Sea the northern Red Sea is significantly lower. The thermohaline circulation also by providing a source of thickness of the layer and the salinity maximum are intermediate waters that have a significant contribution also larger outside the mouth of the Gulf of Suez. to the formation of the intermediate layer of the basin.

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The contribution of this water mass to the total RSOW Knudsen, M., 1900: Ein hydrographischer Lehrsatz. Ann. Hydrogr. formation is estimated to about 20%, using a simple box Maritimen Meteor., 28, 316–320. model of the layer. Two topographic constraints, in the Maillard, C., 1974: Eaux intermediaires et formation d’eau profonde en Mer Rouge. L’océanographie Physique de la Mer middle and mouth of the gulf, trap wintertime waters in Rouge, CNEXO, 105–133. the northern and southern parts of the gulf that act as Morcos, S. A., 1970: Physical and chemical oceanography of the reservoirs of dense water. This dense water can form the Red Sea. Oceanogr. Mar. Biol. Annu. Rev., 8, 73–202. core of a layer that intrudes into the northern Red Sea at Murray, S. P., and A. L. Babcock, 1982: Observations of two-layer intermediate levels (100–150 m). The presence of this circulation in the Gulf of Suez. Eos, Trans. Amer. Geophys. Union, 63, 1013. layer is more prominent at the exit of the gulf and the Plähn, O., B. Baschek, T. H. Badewien, M. Walter, and M. Rhein, western coast of the northern Red Sea. 2002: Importance of the Gulf of Aqaba for the formation of bottom water in the Red Sea. J. Geophys. Res., 107, 3108, Acknowledgments. This work was supported by doi:10.1029/2000JC000342. Office of Naval Research under Contracts N00014-95- Sofianos, S. S., and W. E. Johns, 2003: An oceanic general circulation model (OGCM) investigation of the Red Sea 1-0025 and N00014-03-10319. We acknowledge the circulation. 2. The three-dimensional circulation in the excellent support provided by the Ocean Technology Red Sea. J. Geophys. Res., 108, 3066, doi:10.1029/ Group of the Rosenstiel School of Marine and Atmo- 2001JC001185. spheric Sciences, University of Miami, in acquiring the ——, and ——, 2007: Observations of the summer Red Sea 112 observations. circulation. J. Geophys. Res., , C06025, doi:10.1029/ 2006JC003886. ——, ——, and S. P. Murray, 2002: Heat and freshwater bud- REFERENCES gets in the Red Sea from direct observations at Bab el Mandeb. Deep-Sea Res. II, 49, 1323–1340, doi:10.1016/ Biton, E., and H. Gildor, 2011: The general circulation of the S0967-0645(01)00164-3. Gulf of Aqaba (Gulf of Eilat) revisited: The interplay Woelk, S., and D. Quadfasel, 1996: Renewal of deep water in between the exchange flow through the Straits of Tiran and the Red Sea during 1982–1987. J. Geophys. Res., 101, surface fluxes. J. Geophys. Res., 116, C08020, doi:10.1029/ 18 155–18 165, doi:10.1029/96JC01148. 2010JC006860. Wyrtki, K., 1974: On the deep circulation of the Red Sea. Cember, R., 1988: On the sources, formation, and circulation of L’océanographie Physique de la Mer Rouge, CNEXO, Red Sea deep water. J. Geophys. Res., 93, 8175–8191, 55–90. doi:10.1029/JC093iC07p08175. Zhai, P., A. S. Bower, W. M. Smethie, and L. J. Pratt, 2015: For- El-Sabh, M. I., and A. I. Beltagy, 1983: Hydrography and chemistry mation and spreading of Red Sea outflow water in the Red of the Gulf of Suez during September 1966. Bull. Inst. Sea. J. Geophys. Res. Oceans, 120, 6542–6563, doi:10.1002/ Oceanogr. Fish., 9, 78–82. 2015JC010751.

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