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 circu- lation 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 Unauthenticated | Downloaded 10/04/21 06:46 PM UTC AUGUST 2017 S O F I A N O S A N D J O H N S 2049 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 Unauthenticated | Downloaded 10/04/21 06:46 PM UTC 2050 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 47 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.