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  • View in gallery

    Topographic chart of the geographical location of the Red Sea, the Bab-el-Mandeb, and the Gulf of Aden. Light and dark blue depict shallow and deep ocean regions, respectively. The inset region of study is shown in more detail in Fig. 2.

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    Map of the western Gulf of Aden (the inset from Fig. 1), the region of the high-resolution hydrographic plume survey during REDSOX-I and -II. Color shading is from light blue (shallow) to dark blue (deep). The plume splits into two deep channels, the Northern and the Southern Channels, marked in yellow. The locations of across-plume sections (sections 14) as well as the location of moorings from the 1995–96 Bab-el-Mandeb experiment (A, D, E, F) are shown.

  • View in gallery

    Time series of unfiltered velocity data illustrating the strong temporal variability at the Bab-el-Mandeb and downstream in the Northern and Southern Channels. Velocities are rotated into the flow direction and positive velocities are directed outward from the Red Sea into the Gulf of Aden. The high-frequency tidal oscillations are apparent, as are the strong seasonal cycle and the variability on synoptic time scales of days to weeks. The tidal fluctuations about the mean can reach amplitudes of the same magnitude as the mean current.

  • View in gallery

    (top) Salinity section at the Perim Narrows, at the exit of Bab-el-Mandeb. The salinity is strongly stratified with very salty water (>39.5) emanating from the strait in the bottom layer and fresher Gulf of Aden Water (<36.5) in the surface layer. In the salty bottom layer, the isohalines are tilted to the western edge of the channel in agreement with geostrophy. Dotted lines depict station locations and data points, the corresponding station numbers from REDSOX-I are indicated. (bottom) Velocity section at the Bab-el-Mandeb. The velocities are rotated into the section and negative velocities depict outwardly directed flow. The Red Sea outflow is apparent as a strong velocity core in the bottom layer. The outflow velocities are banked against the western channel wall following the tilt of the isohalines.

  • View in gallery

    (top) Salinity section at section 2. High-salinity Red Sea Outflow Water can be observed in the deep channels and a local salinity maximum is found in an equilibrated intrusion layer above the Southern Channel. A thick layer of low-salinity Gulf of Aden Intermediate Water (≈35.5) can be observed in the eastern part of the section above the deep Northern Channel. Dotted lines depict station locations and data points, the corresponding station numbers from REDSOX-I are indicated. Solid black lines delineate the regions chosen for transport calculations of the individual plume branches. (bottom) Velocity section at section 2. The velocities are rotated into the section and negative velocities depict outwardly directed flow. The salinity maxima from the top figure are mirrored in the velocity maxima in the deep channels and the IL. Inwardly directed velocities above the deep NC show inflowing GAIW.

  • View in gallery

    As in Fig. 5 but for section 3. The deep channels are now divided by a 250-m ridge adjacent to the Northern Channel. The water in the deep NC does not show significant dilution by mixing and entrainment whereas the water in the Southern Channel is considerably less saline than at upstream sections. The intrusion layer is still present as a local salinity maximum above the SC.

  • View in gallery

    As in Fig. 5 but for section 4. The high-salinity signature of the Red Sea Outflow Water is still apparent in the deep channels at this section. The intrusion layer is much reduced but still present on the western edge of the section. The velocity distribution across the section only in part reflects the salinity distribution. Outflow velocities remain strong in the Northern Channel and are present in the western part of the Southern Channel as well as in the remnant of the IL. The velocity maximum in the NC is now banked against the northeastern channel wall.

  • View in gallery

    Transport in salinity classes for sections 14. At the Bab-el-Mandeb (section 1), the highest salinity class accounts for the largest transport. By section 2, there has been a dilution toward lower salinity classes, which continues toward section 3. By section 4, the highest salinity class has almost vanished. Transport is in Sverdrups. Total transport integrated across each section approximately doubles from the strait (0.29 Sv) to section 4 (0.56 Sv).

  • View in gallery

    Total transport (T), transport of pure Red Sea Outflow Water (TRS), and entrainment (TE) from the Bab-el-Mandeb to section 4. Total transport and entrainment increase downstream from the strait as expected. Error bars reflect the error associated with temporal variability in outflow transport and other uncertainties (see text for details).

  • View in gallery

    Percentage of transport of pure Red Sea Outflow Water within each plume branch across sections 24, illustrating the relative contributions of each pathway to the Red Sea outflow transport. Overall, the Northern Channel accounts for about one-half of the total transport, the Southern Channel carries about one-third, and the intrusion layer contributes the remainder (∼17%). Actual transport values are shown in parentheses (Sv).

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    Fig. A1. Semimajor axis component of the dominant tidal constituents in the region of the Bab-el-Mandeb and the Red Sea outflow. Tidal currents are strong in the strait and quickly decay downstream. The mooring location for moorings from the Bab-el-Mandeb experiment and the estimated tidal amplitude from harmonic analysis on the mooring data are shown (cm s−1). Triangle DEF was used to fit the model data to observations in order to estimate tidal corrections for LADCP velocity data. Contour lines are every 1 cm s−1 with the contour line crossing mooring E in both panels corresponding to 2 cm s−1 and increasing toward the strait.

  • View in gallery

    Fig. A2. Tidal velocities predicted by combining a tidal model and observational data in the region of the REDSOX plume survey. The curve shows through-section velocity; positive velocities are directed outward of the Red Sea. Stations occupied repeatedly over the tidal cycle during REDSOX-I are shown and illustrate that the tidal currents were moderate at time of station occupation but can reach considerably larger amplitudes. Stations 44, 49, and 55 coincide with section 2 and are located in the center of the Southern Channel. Time axis shows UTC time of station occupation. The tidal predictions were made hourly (asterisks) over a 25-h period.

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Transport and Entrainment in the Red Sea Outflow Plume

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  • 1 Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida
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Abstract

The Red Sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing. As it descends the continental slope in the western Gulf of Aden, it entrains significantly less-dense near-surface water, which itself varies on seasonal time scales. High-resolution hydrographic and direct velocity data collected during the 2001 Red Sea Outflow Experiment (REDSOX) are used herein to characterize and quantify the pathways of the Red Sea Outflow Water (RSOW) and the associated entrainment of Gulf of Aden Water. The outflow transport exhibits a maximum in winter of about 0.29 Sv (Sv ≡ 106 m3 s−1) at the exit of the Bab-el-Mandeb and approximately doubles to 0.56 Sv as it descends into the Gulf of Aden and entrains ambient water. In summer, the outflow is much weaker, reaching about 0.06 Sv at the strait and about 0.18 Sv downstream. The outflow plume divides into three distinct branches in winter, consisting of descending branches along two bathymetrically confined channels (the “Northern” and “Southern” channels, respectively), and an adjusted intrusion layer at shallower depths in the water column. Estimates of transport of “pure” Red Sea Outflow Water through salt flux conservation show the general partitioning of the outflow between the individual plumes, where the Northern Channel (NC) accounts for 52% of Red Sea Outflow Water, the Southern Channel (SC) carries 31%, and the intrusion layer (IL) the remaining 17%. The results also indicate that the transport of Red Sea Outflow Water is subject to considerable synoptic temporal variability that is unresolved by the present study.

Corresponding author address: Dr. William E. Johns, Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149. Email: wjohns@rsmas.miami.edu

Abstract

The Red Sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing. As it descends the continental slope in the western Gulf of Aden, it entrains significantly less-dense near-surface water, which itself varies on seasonal time scales. High-resolution hydrographic and direct velocity data collected during the 2001 Red Sea Outflow Experiment (REDSOX) are used herein to characterize and quantify the pathways of the Red Sea Outflow Water (RSOW) and the associated entrainment of Gulf of Aden Water. The outflow transport exhibits a maximum in winter of about 0.29 Sv (Sv ≡ 106 m3 s−1) at the exit of the Bab-el-Mandeb and approximately doubles to 0.56 Sv as it descends into the Gulf of Aden and entrains ambient water. In summer, the outflow is much weaker, reaching about 0.06 Sv at the strait and about 0.18 Sv downstream. The outflow plume divides into three distinct branches in winter, consisting of descending branches along two bathymetrically confined channels (the “Northern” and “Southern” channels, respectively), and an adjusted intrusion layer at shallower depths in the water column. Estimates of transport of “pure” Red Sea Outflow Water through salt flux conservation show the general partitioning of the outflow between the individual plumes, where the Northern Channel (NC) accounts for 52% of Red Sea Outflow Water, the Southern Channel (SC) carries 31%, and the intrusion layer (IL) the remaining 17%. The results also indicate that the transport of Red Sea Outflow Water is subject to considerable synoptic temporal variability that is unresolved by the present study.

Corresponding author address: Dr. William E. Johns, Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149. Email: wjohns@rsmas.miami.edu

1. Introduction

The deep water masses of the World Ocean and several of the intermediate waters are formed in the marginal and polar seas. Dense overflows from these marginal seas play a crucial role in the global thermohaline circulation and thus closely link the ocean to climate dynamics. In the northern Indian Ocean, the two main intermediate water masses come from the Red Sea and the Persian Gulf (Beal et al. 2000). The Red Sea outflow is fundamentally different from the marginal sea outflows of the Atlantic in that it exhibits strong seasonal variability in outflow transport due to the effect of monsoon winds and seasonal variability in buoyancy forcing in the region (Sofianos and Johns 2002). It flows over a shallow sill and enters the ocean at low latitudes. Red Sea Water (RSW) originates in the northern Red Sea due to an excess of evaporation over precipitation (Morcos 1970; Sofianos et al. 2002). It enters the Gulf of Aden in the northwestern Indian Ocean as a salty, dense gravity current through the shallow Bab-el-Mandeb (Fig. 1). The Red Sea outflow enters the ocean at a western boundary and it has a multilevel structure (Murray and Johns 1997), both features it shares with the Denmark Strait overflow (Girton et al. 2001).

As the Red Sea Outflow Water (RSOW) descends the continental slope into the western Gulf of Aden, it entrains significantly less-dense near-surface water, which itself varies on seasonal time scales. The outflow transport reaches a maximum in winter, with average transports around 0.6 Sv (Sv ≡ 106 m3 s−1) in the Bab-el-Mandeb (Murray and Johns 1997). In winter, significant transport variability has been observed in the Bab-el-Mandeb, where the transport can vary from 0.25 to 0.7 Sv on time scales from several days to a month (Murray and Johns 1997). The descending outflow plume is guided by the bathymetry in the western Gulf of Aden and it divides into different branches as it follows two well-defined bathymetric channels. The outflow plumes separate from the bottom and are injected into the Tadjura rift, where they produce several distinct salinity maxima (Bower et al. 2005). It appears that, about 20% of the time, during transport surges as have been observed during the stronger winter outflow (Murray and Johns 1997), the outflow plume may be dense enough to sink to the bottom of the deep rift (Bower et al. 2005). After it enters the Tadjura rift, the RSOW is advected by the local circulation and mixed further as it spreads southeastward along the coast of Africa and into the Gulf of Aden. In summer, the outflow is much weaker and the separation depth of the plume and the properties of the product waters are different than in the wintertime (Peters and Johns 2005; Bower et al. 2005).

The signature of the mixed RSOW can be traced as a middepth salinity maximum throughout much of the western Indian Ocean and as far south as the Agulhas Current. It has been shown that the salty RSW injected into the Indian Ocean through the Gulf of Aden is exported into the South Atlantic and hence the global thermohaline circulation via the Agulhas Current (Beal et al. 2000).

The data used in the present study were collected during the Red Sea Outflow Experiment (REDSOX), a joint effort between the Rosenstiel School of Marine and Atmospheric Science (RSMAS) and Woods Hole Oceanographic Institution (WHOI). REDSOX was conducted in 2001 in the Red Sea plume region in the western Gulf of Aden (Fig. 2). The objective of REDSOX was to provide a comprehensive description of the pathways, structure, and variability of the descending outflow plumes from the Red Sea and a better understanding of mixing and spreading processes in dense overflows.

Our main goals in this paper are

  • (i) to describe the flow structure and water property changes along the descending Red Sea outflow plumes in the western Gulf of Aden;
  • (ii) to quantify the mixing and entrainment in the outflow plume; and
  • (iii) to determine the partitioning of the RSOW along various plume branches.

In this paper, we will focus on the results from the winter survey, the season of maximum outflow and formation of the main product waters of the Red Sea outflow. The summer outflow transports and flow structure of the plume pathways differ considerably from the winter regime and will be described briefly.

2. Background

Early surveys of the area of the Red Sea overflow (Siedler 1969; Vercelli 1925, 1927) provide evidence that the exchange through the Bab-el-Mandeb is highly seasonal, with maximum exchange occurring in winter. Using indirect estimates of the transport of RSW through the Bab-el-Mandeb, these studies suggest an annual mean outflow of 0.33 Sv (Siedler 1968), reaching approximately 0.6 Sv in winter and dropping to nearly zero in late summer (Patzert 1974). Siedler (1968) was the first to observe the classical two-layer exchange flow during the winter period (October–May). In summer (June–September), the northwesterly winds drive a three-layer exchange, consisting of a thin surface outflow from the Red Sea, an inflowing layer of Gulf of Aden thermocline water, and a much weaker outflowing deep layer (Maillard and Soliman 1986).

The Bab-el-Mandeb experiment at the southern end of the Red Sea during 1995–96 involving moored acoustic Doppler current profilers (ADCPs) and temperature–salinity (TS) sensors (Fig. 2) revealed the annual cycle of the exchange in detail (Murray and Johns 1997; Sofianos et al. 2002). The inflow and outflow transports of the exchange flow were well resolved and showed a very repeatable transition between a two-layer regime in winter and a three-layer regime in summer over the 18 months of continuous observation. From that study, the annual mean outflow of RSW through the strait was estimated to be 0.37 Sv at Perim, with maximum exchange reaching 0.7 Sv in February and average summer outflow of ∼0.05 Sv (Murray and Johns 1997; Sofianos et al. 2002). Early surveys of the area of the Red Sea overflow (Siedler 1969) as well as more recent studies (Fedorov and Meschanov 1988) have shown that the descending outflow plume splits into two branches as it flows into the Gulf of Aden. One branch flows along a narrow “Northern Channel” (NC) off the southern coast of Yemen and the other branch follows a wider “Southern Channel” (SC) off the coast of Djibouti (Fig. 2). These results were confirmed in the Bab-el-Mandeb experiment. Figure 2 shows the location of moorings deployed in the outflow plume during the Bab-el-Mandeb study: A is an array of ADCP moorings in the Perim Narrows and D, E, and F are current-meter moorings, downstream in the plume. Both E and F are located about midway down the two channels. Time series of unfiltered along-channel velocity at mooring sites E and F illustrate the seasonal variability as well as the variability on shorter time scales, from tidal to synoptic (Fig. 3). The downslope flow in both channels follows the general cycle of the Bab-el-Mandeb outflow. The velocities in the NC are higher than in the SC, and after periods of low outflow that occur during the summer, the downslope flow in the SC “lags” the flow in the NC. This can, at least in part, be explained by a topographic saddle that separates the two channels at their bifurcation point (latitude 12.51°, longitude 43.45°; Fig. 2), which suggests that the outflow needs to reach a threshold thickness to spill over into the SC. In the strait, the tidal fluctuations can reach amplitudes of the same magnitude as the mean current, suggesting the tides may significantly impact the mean flow. The amplitude of the tidal signal quickly decays downstream of the strait, and the signal remains stronger in the SC than in the NC. Apart from the daily tides, the seasonal variability is clearly the dominant cycle in the outflow, which also shows considerable variability on synoptic time scales. This synoptic variability often appears to have a scale of about 7 days, the duration of the “transport surges” observed in winter (Murray and Johns 1997).

The REDSOX surveys investigated in detail the splitting of the outflow plume and showed that the two channels carry different amounts of outflow waters and allow the formation of different product waters (Bower et al. 2005; Peters and Johns 2005; Peters et al. 2005). The Northern Channel, with its more confined topography, carries very high salinity water downslope and appears to allow less mixing and entrainment of ambient water masses than the broader Southern Channel (Peters et al. 2005; Peters and Johns 2005; Özgökmen et al. 2003). A third branch of the RSOW is also found in winter, which reaches neutral density at ∼150 m and forms a thermocline intrusion into the Gulf of Aden. This water is formed in the interfacial layer in the strait where RSOW is in direct contact with and mixes with Gulf of Aden Surface Water (GASW) flowing into the strait. The mixing product of RSOW and GASW that flows into the Gulf of Aden at the very top of the outflow plume is lighter than the Gulf of Aden Intermediate Water (GAIW) and equilibrates above the bottom plume at intermediate depths. We refer to this as the RSOW “intrusion layer” (IL).

3. Data and methods

The data used in the present study were collected during two cruises in 2001, one in the winter (REDSOX-I) and one in the summer (REDSOX-II), timed to coincide with the climatological periods of maximum and minimum deep outflows, respectively. During each cruise, both a high-resolution plume survey of the near field of the outflow (Fig. 2) as well as a survey throughout the Gulf of Aden to capture the far field of the outflow were conducted. High-resolution hydrographic and direct current surveys of the western Gulf of Aden between the strait of Bab-el-Mandeb and the Tadjura rift were conducted. On the first cruise, aboard the R/V Knorr, stations were taken from 14 February to 11 March 2001. Stations on the second cruise, aboard the R/V Maurice Ewing, were taken between 21 August and 9 September 2001. A total of 238 (227) conductivity–temperature–depth (CTD) and direct velocity stations during the winter cruise (summer cruise) were occupied to describe the three-dimensional water property distributions as well as the circulation characteristics. Direct measurements of turbulent mixing were made using bottom-mounted ADCP moorings to study the bottom stress and mixing processes at a few selected locations in the descending plumes (Peters et al. 2005; Peters and Johns 2005). Figure 1 shows the geography of the study area and the location of the high-resolution “plume study” (inset) in the western Gulf of Aden. The high-resolution plume survey consists of about 100 casts per cruise. Figure 2 shows the station locations in the region of the plume survey. The bathymetry shown in Figs. 1 and 2 was constructed by merging a high-resolution multibeam bathymetric dataset (courtesy of S. Swift at Woods Hole Oceanographic Institution and P. Huchon of Geosciences Azur, Villefranche-sur-Mer, France) covering the two channels and the Tadjura rift with the 2′-resolution GTOPO2 dataset in regions that were unresolved.

During the high-resolution plume survey, sections across and along the two outflow channels were taken with stations spaced as close as 2 km apart along section 2 and as far as 10 km apart along section 4 (Fig. 2). These sections were spaced to provide a high-resolution view of the descending outflow plume. Section 3 was chosen to coincide with the location of moorings from the Bab-el-Mandeb study. The Bab-el-Mandeb moorings had been deployed in the deepest parts of the two outflow channels to capture the behavior of the axes of the outflow. A third section, section 4, was taken between section 3 and the Tadjura rift, before the outflow water separates from the bottom and equilibrates in the deep rift. Another section was taken in the rift, to observe the final product waters of the Red Sea outflow plumes and their equilibrium depths (Bower et al. 2005). Observations in the area of the plume survey were done over a time period of 6 days (winter) and 5 days (summer). The overall plume survey can therefore not be considered synoptic with respect to time scales of variability observed in the outflow. It was, however, conducted in a relatively short period of time (order 1 week). The temporal variability in the outflow occurs on a range of time scales from tidal to synoptic (days to weeks) to seasonal (Fig. 3). Nevertheless, the basic structure of the outflow plume remains continuous (Peters et al. 2005; Peters and Johns 2005), such that our results imply a reasonable representation of the transport and mixing processes in the plume.

The observations, providing simultaneous temperature, salinity, and velocity profiles, were done using a package combining a CTD probe and a lowered acoustic Doppler current profiler (LADCP) mounted on a SeaBird Electronics carousel with eight 1.5 L Niskin bottles. The CTD/LADCP package consisted of a 911+ CTD system (SeaBird, Inc.) and a downward-looking 300-KHz broadband “Workhorse” ADCP (RD Instruments, Inc.). The CTD/LADCP package was lowered at a rate of 30 m min−1 from the surface to 10 m of depth and at 60 m min−1 to within 10 m of the bottom. An altimeter mounted on the package allowed monitoring of the position of the package relative to the bottom. CTD processing and calibration are described in Johns et al. (2001). The accuracy of the temperature and salinity measurements are estimated to be approximately 0.001°C and 0.0015 psu, respectively. Measurement and processing routines for the LADCP were developed to optimize the observation of dense plumes flowing along the ocean floor. The typical uncertainty in the LADCP velocities is on the order of 0.03 m s−1 (Peters et al. 2005).

In this paper, we present CTD/LADCP data from the winter survey for a detailed description of the maximum outflow regime. The winter is the season of maximum exchange through the Bab-el-Mandeb and the main product waters of the Red Sea outflow are formed at this time. Therefore, we focus our investigation of the plume transports and local entrainment processes on the winter regime and only briefly describe the summer regime.

a. Tidal corrections

The tidal signal is strong in the narrow Bab-el-Mandeb through which the RSOW is entering the Gulf of Aden but decays farther down in the outflow (Fig. 3). However, the tidal signal can partly contaminate the velocity measurements and affect any subsequent calculations, such as transport calculations, which are an important tool for estimating bulk entrainment. Therefore, in order to quantify transport and entrainment, the velocity data needed to be corrected for tidal aliasing. The REDSOX LADCP velocity data used in this study were “detided” by combining the output from a tidal model (Jarosz et al. 2005) and harmonic analysis on the Bab-el-Mandeb mooring data. The tidal current amplitudes from the model were fit to the mooring data in the triangle between the moorings D, E, and F. This allowed the prediction of tidal currents for each station location within this area. Tidal corrections were performed at Perim where the results from the harmonic analysis on mooring A could be used directly and for sections 2 and 3 using the above method. Section 4 was determined to be too far downstream to require the use of tidal corrections since the tidal amplitudes at this location are negligibly small, and none were applied. The reader is referred to the appendix for a more detailed description of the tidal regime in the region and the implementation of tidal corrections in our velocity measurements.

b. Transport calculations

This section will briefly discuss the methods used in this study to estimate transports. As explained above, several sections across the outflow plume were taken, with stations spaced apart from as little as 2 km at section 2 to as much as 10 km at section 4 (Fig. 2). To obtain a smooth representation of the salinity and velocity fields, the data were gridded using the Generic Mapping Tools (GMT) “surface” command, on a 0.5 km (x, distance) by 5 m (z, depth) rectangular grid. To have a high level of control over the gridding of the data in the “bottom triangles” between adjacent stations, the value at 20 m above the bottom in the velocity profiles was interpolated between the stations at horizontal grid resolution (0.5 km) and included as additional points in the (xz) grid. This procedure is believed to result in the most realistic near-bottom velocity fields. To account for the frictional decay of velocity in the bottom boundary layer, a correction was applied to the gridded profiles, letting them decay linearly to zero in the last 20 m of the water column. Investigation of the available velocity profiles that extend to less than 20 m above the bottom showed this to be a valid choice.

Transports were calculated for each of the three different plume branches and broken down into salinity “classes” within each branch. Salinity classes were chosen in 0.5-psu increments spanning the range of the RSOW plume salinities, from 36.5 to 40 psu. Hence, in all the results referring to “total” transport of a given plume, the range of salinities includes only values above 36.5 psu. This was done to capture the Red Sea outflow to the greatest degree possible, without including flows associated with the lower salinity Gulf of Aden Water. For comparison, calculations were performed extending this range down to 35.5 psu, as well as truncating it at 37.5 psu. The results from these calculations showed that 36.5 psu was the best choice for the upper limit of the RSOW. On certain sections where repeat stations were available [sections 2 and 3 in winter; section 1 in summer (not shown)], the average transport value from the realizations—obtained by using the different stations to obtain the gridded fields and transports—was used. This was the most conservative approach given the temporal variability of the outflow and the fact that at least in the winter case the stations across the sections were not always taken consecutively.

4. Results

a. Overview of the plume structure

1) The outflow from Bab-el-Mandeb—Section 1

Sections taken across the Bab-el-Mandeb near Perim show that the salinity structure is stratified with a salinity reaching up to 39.8 psu in a 25–30-m bottom mixed layer (Fig. 4, top). The salinity of the Red Sea Outflow Water entering the strait at Hanish sill, the upstream end of the strait, about 150 km northwest of Perim, is about 0.5 psu higher, around 40.3 psu (Murray and Johns 1997; Sofianos et al. 2002). Therefore, some mixing has occurred within the strait. At Perim, the velocity field is highly vertically sheared with a core outflow velocity maximum of up to −1.6 m s−1 (negative sign depicts outflow) at about 150-m depth on the western side of the channel (Fig. 4, bottom). The velocities shown were tidally corrected to eliminate tidal aliasing from the data, which is especially important within the strait.

Overlying the high-salinity RSOW is a relatively fresh surface mixed layer of Gulf of Aden Surface Water that is flowing into the Red Sea. The velocity shows a clear separation between the surface inflow and the deep outflow of RSOW. The salinity contours are banked up on the west side of the channel, and the outflow is more concentrated there, consistent with a rotational intensification of the outflow on the western side of the channel. As the outflow continues on its path out of the strait and down the continental slope it very soon encounters topographic features that guide and shape its way into the Gulf of Aden (Fig. 2). The primary path of the outflow is along a narrow canyon that continues as a narrow, deep channel almost directly west–east before it turns south and opens into the Tadjura rift about 140 km downstream of Perim. This path of the outflow has been termed the Northern Channel (Peters et al. 2005; Peters and Johns 2005; Bower et al. 2005). This channel is very confined topographically, which laterally constrains the plume and gives the mixing behavior in this channel a more two-dimensional character (Özgökmen et al. 2003; Peters et al. 2005; Peters and Johns 2005). This path of the outflow is separated from the western part of the continental slope by a small topographic feature where the outflow spills over a saddle into the Southern Channel, the “secondary” path of the outflow plume. The Southern Channel is a broad channel that puts less geometrical constraints on the entrainment of Gulf of Aden Surface and Intermediate Waters into the plume as it leads into the Tadjura rift.

2) Sections across the descending plumes

(i) The upper slope—Section 2

Section 2 crosses both outflow channels about 25–30 km downstream of the point where the outflow bifurcates. Here, the plume has been divided up into three different branches: the Northern Channel plume, the Southern Channel plume, and a gravitationally adjusted intrusion layer above the deep SC. The NC carries high-salinity RSOW, which appears to have been diluted very little since its exit from the strait at Perim (Fig. 5, top). The CTD profile at the deepest point shows a maximum salinity of 39.7 psu in the bottom mixed layer, very close to the value of 39.8 psu at Perim. Associated with this salinity maximum, there is a velocity maximum at the bottom of the NC, reaching maximum outflow velocities of up to 0.9 m s−1 (Fig. 5, bottom). Interestingly, at shallower depths of 200–300 m, just overlying the plume, the velocity changes direction and there is an inflow of GAIW, characterized by a salinity around 35.5 psu, with velocities of about 0.3 m s−1. The SC also carries high-salinity water at depth at a maximum value of 39.5 psu, but the velocity remains much lower, reaching only about half the magnitude of the velocity in the deep NC, 0.4 m s−1. In the water column above the deep SC, the salinity reaches a local minimum of about 36.5 psu at 190–200 m, then increases again to values around 38 psu at 150 m. This second salinity maximum depicts the core of the IL, the third branch of the plume in winter. This IL branch of the plume reaches eastward to about 18 km across the section where the flow changes direction and there is an inflow of low-salinity GAIW, indicative of a local recirculation or possibly an eddy.

(ii) Midslope—Section 3

Section 3 was taken approximately 15 km downstream of section 2, across both channels, now spanning a width of 50 km. The location of this section coincides with the location of the current meter moorings from the 1995 Bab-el-Mandeb study (sites E and F in Fig. 2), which were located in the middle of the NC and the SC, respectively. Both the salinity and the velocity fields show that the NC still carries water with a strong RSOW signature (Fig. 6). The salinity profile in the center of the NC shows a bottom mixed layer of RSOW with salinities of up to 39.6 psu. It has been shown before that the mixing in the NC plume is concentrated within an interfacial layer above the high-salinity bottom plume (Peters et al. 2005; Peters and Johns 2005). The bottom layer of high-salinity RSOW remains fairly isolated from the mixing in the interfacial layer and this allows for the transport of high-salinity water far downstream in the channel (Özgökmen et al. 2003; Peters et al. 2005; Peters and Johns 2005). The outflow velocity reaches about 0.7 m s−1 magnitude, close to the previously observed maximum wintertime speed of 0.8 m s−1 seen at mooring site E (Murray and Johns 1997). The bottom plume in the SC has broadened along with the channel width and its dilution is indicative of more mixing and entrainment of fresher ambient water than in the NC. The velocities in the SC are about 0.2 m s−1 less in magnitude than in the NC. The deep SC plume appears to be losing momentum as it entrains fresher water and spreads out on the banks of the SC. The IL branch is clearly apparent in this section in the salinity as well as in the velocity field. Its core salinity and velocity are lower than in the bottom plumes, reaching salinity values of about 38 psu and outflow velocities of about 0.2 m s−1. As in section 2, inflowing GAIW appears on the eastern side of the cross section just above the outflowing RSOW. The high-salinity RSOW is strongly banked against the southern side of the channel wall in the NC (which is especially apparent in the velocity field), while between 100 and 400 m depth there is a core of GAIW inflow of 0.2 m s−1 flowing up the channel. In the deep NC, the vertical structure of this inflow and its low salinity suggest GAIW is feeding the entrainment into the deep plume. The inflow at shallower depths, 200 m and above, is likely to be detached from the dynamics of the lower plume and could be part of a local recirculation or eddy.

(iii) Lower slope—Section 4

Section 4 is located just before the plumes exit the two channels and flow out over the Tadjura rift. The IL branch is now reduced to a small remnant on the far western side of the section (Fig. 7). There is a large core of GAIW throughout most of the intermediate depth of the section with a very low velocity signature in direction of both outflow and inflow. The bottom plumes in both channels still carry high-salinity water, with values as high as 39.5 psu in the NC, and 39.2 psu in the SC. The velocities in the SC show very little remaining outflow velocity ≤0.2 m s−1, whereas in the NC, the outflow velocities still reach values of up to 0.6 m s−1. In the NC, the core of the outflow velocity has shifted from being banked against the southern side of the NC (the right-hand wall facing downstream) in section 3 to hugging the opposite channel wall by section 4. The counterflow from the Gulf of Aden that is believed to feed the entrainment also shifts sides. This occurs in a region where the channel changes direction and curves toward the south, an almost 90° turn from its previous west–east orientation. This suggests that centrifugal forces on the plume may overcome the Coriolis forces tending to bank the flow on the right-hand side of the channel. The radius of curvature of the NC at this location, Rc, can be estimated at around 20–25 km. The velocity has a magnitude of about 0.6 m s−1, and f, the Coriolis parameter at this location (about 12°N), is approximately 0.3 × 10−4 s−1, which leads to a value of u/fRc ∼ 1. The centrifugal force will become dominant as u/fRc > 1, suggesting that inertia may indeed drive the flow to shift to the left-hand channel wall.

(iv) The Red Sea outflow in summer

While the main focus of this study is the winter regime of the Red Sea outflow, when the main product water masses of the RSOW are formed, we briefly describe the summer outflow conditions here for completeness.

During summer, the salinity of the outflow layer is much lower, reaching salinity maxima of only around 39 psu (not shown). The exchange in the strait has changed from a two-layer structure with deep outflow and surface inflow to a three layer structure with an intermediate layer of inflowing GAIW (Murray and Johns 1997). The outflow velocity in the bottom layer is also much lower than in the winter case, reaching only about 0.5 m s−1. The plume separates into the NC and SC branches; however, the IL is absent in the summer plume. This is because GAIW flows all the way through the strait into the southern Red Sea in summer, and therefore the outflowing RSOW mixes only with GAIW during its passage through the strait. The product water in the interfacial layer of the outflow plume is always denser than the GAIW and therefore does not separate in the Gulf of Aden. At section 2, the velocities in the bottom plume in the NC are only about half of the values reached in winter and the SC shows very little outflow velocity. By section 3, the NC shows very small outflow velocity and the SC outflow is essentially shut down. The general structure of salinity and velocity in the NC are, however, similar to that observed in winter, the banking of the higher salinity and outflow velocity on the south side of the channel. By section 4, the bottom layer exhibits very weak outflow velocities, and the velocity and salinity fields suggest that the plumes are beginning to equilibrate and separate from the bottom in several small lenses.

b. Transport calculations in the plume

In this section, we will present results of transport calculations in the plume for the winter regime. Transports have been calculated for the outflow in the Bab-el-Mandeb as well as for the downstream sections 24 and the individual plume branches (NC, SC, and IL). Table 1 summarizes the results for the four sections and shows results obtained for the individual plume branches. Section 1 was taken at Perim, the exit of the Bab-el-Mandeb and can be taken as representative of the initial state for the RSOW exiting the strait. Calculations result in a total RSOW transport value of 0.29 Sv at Perim (Table 1). The bulk of the transport is concentrated on the western side of the channel, coinciding with the velocity maximum that is banked against the western edge of the strait. The transport in salinity classes (Fig. 8) shows inward transport in the lowest salinity class 36.5–37 psu. This inflowing water carries a salinity signature higher than the GAIW (35.5) and also slightly higher than the GASW (36.25). Most likely this inflow consists of GASW that has experienced some limited mixing with the upper interface of the RSOW plume, as well as some “pure” GASW. The transport values in the intermediate salinity classes, 37–39.5 psu, range between about 0.02 and 0.04 Sv. All intermediate salinity classes show similar transport with a marked increase only toward the two highest salinity classes. Since the RSOW has not yet been substantially diluted by entraining fresher water, it comes as no surprise that a large fraction of the RSOW transport at this section is concentrated in the highest salinity class >39.5 psu (Fig. 8).

In comparison with climatology, the value observed for transport of RSOW in the strait during the REDSOX winter survey, 0.29 Sv, is atypically low. Previous studies (e.g., Murray and Johns 1997) show a typical winter Red Sea outflow of 0.6 Sv. However, these studies also show considerable synoptic variability in the Red Sea outflow at the Bab-el-Mandeb, varying from as little as 0.25 Sv to 0.7 Sv during winter (Murray and Johns 1997). The time scales of this synoptic variability range from days to weeks (Fig. 3) and there appears to be a close relation to fluctuations in along-channel wind forcing and possibly coastally trapped waves (Murray and Johns 1997). It is likely the REDSOX-I plume survey with its duration of around 6 days fell in a period of such low outflow conditions. In fact, Peters et al. (2005) show that there was considerable variability in the strength of the outflow in the NC during the REDSOX-I plume survey. Unfortunately, no continuous wind observations are available in the strait to verify this assumption. Furthermore, the outflow may also be subject to variability on interannual time scales (Smeed 2004).

Reaching section 2, the outflow has covered a distance of about 70 km from the strait and encountered steep bottom slopes (Fig. 2). Mixing intensifies as the plumes accelerate down the steeper slopes, and it can therefore be expected that enhanced entrainment has occurred. The total transport has increased to 0.35 Sv (Fig. 8) and, as shown earlier, the plume has split up into three distinct branches: the NC plume, the SC plume, and the IL plume. The transport is divided between the southern plumes (deep plume and IL) and the NC plume in about equal parts (Table 1). The NC carries most of its water in the highest salinity class, whereas in the SC deep plume, the transport occurs exclusively in the intermediate and lower salinity classes (not shown). This can in part be expected: the water that enters the deep SC—over the topographic saddle where the channels split—is not of the densest RSOW carried at the bottom of the NC. Furthermore, the broader channel geometry is likely to play a role and allow greater dilution of RSOW by mixing and entrainment in the SC.

The NC at section 3 is still very constrained by its narrow topography, whereas the SC has experienced some increase in bottom slope and the channel has opened into a broader bathymetry. The overall transport across the section has increased to 0.42 Sv (Fig. 8). There is a clear partitioning of transport between the two channels. The SC, taken to include the IL plume, carries about two-thirds of the total plume transport at this section; the NC the remaining one-third (Table 1). This indicates that mixing and entrainment have diluted the water in the SC. Transports in salinity classes for the individual plume branches (not shown) show that mixing and entrainment of fresher Gulf of Aden Water result in considerably more dilution of the water in the SC and the IL plume than the water in the NC. Over the entire plume, the highest salinity water has been diluted toward lower salinity classes and there is a gain of transport in the classes with intermediate salinity (Fig. 8).

At section 4, the total outflow transport has increased to 0.56 Sv (Fig. 8). The NC plume begins to leave the laterally constraining topography of the NC and encounters steep bottom slopes shortly before reaching section 4 (Fig. 2). The channels now again carry close to equal amounts of the total plume transport (Table 1). This indicates that the NC plume has entrained significantly between sections 3 and 4. The highest salinity class has nearly vanished, again indicating significant entrainment in the NC with a large gain in the intermediate salinity classes, especially in the third highest salinity class 38.5–39 psu (Fig. 8). This salinity class spans the salinities observed in the Red Sea product waters equilibrated at intermediate depths in the Tadjura rift (Bower et al. 2005).

c. Transport of RSOW and entrainment of ambient water

To determine the partitioning of the total Red Sea Outflow Water transport into the various branches of the outflow and estimate the bulk entrainment of ambient Gulf of Aden Water, the equations for conservation of salt flux can be applied. Furthermore, this serves as a check on the robustness of the transport results. In a steady-state scenario, the total salt flux of RSOW exiting the strait would be conserved between the sections.

Conservation of salt flux in the plume can be expressed as
i1520-0485-37-4-819-e1
where T is the total transport of the outflow plume, S is the flux-weighted salinity of the outflow plume, TRS is the transport of pure RSOW, SRS is the salinity of pure RSOW, TE is the amount of water entrained into the plume, and SE is the average salinity of the entrained water. Additionally, conservation of mass requires:
i1520-0485-37-4-819-e2
We take as unknowns in the above equations TRS and TE, which can be solved for at each of the sections from the data. The salinity used for the RSOW, SRS, was chosen at 40.3 ± 0.2 psu, which is the mean salinity of the pure RSOW flowing over the sill at Hanish. This value is derived from 18 months of moored salinity time series collected at the Hanish sill in the Murray and Johns (1997) study (see also Sofianos et al. 2002). Note that this value is about 0.5 psu higher than the mean salinity (39.8) of the RSOW observed in the bottom averaged layer at Perim, indicating that some dilution of the RSOW has occurred within the strait over the 150-km distance between Hanish and Perim. By assuming SRS to be the value at Hanish at the upstream end of the strait, we are able to estimate the amount of entrainment (TE) that occurs in the strait as well as in the descending plume downstream of Perim. The salinity for the GAIW was chosen to be 35.5 psu. The plume at Perim is in contact and mixes with the GASW, which at 36.25 psu has a higher salinity than the GAIW. Downstream of Perim, the descending gravity plumes in the NC and SC only mix with GAIW, while the IL could mix with either GASW above or GAIW below. However, the IL does not show any systematic increase in transport due to entrainment downstream of section 2. Therefore, the GASW salinity value was used for SE in the calculations for section 1 at Perim, and that of the GAIW for the downstream sections, 2–4.

As noted above, in a steady state, the overall salt flux should be conserved between the sections. Hence, while the entrainment of Gulf of Aden Water should show some increase along the plume, causing an overall growth in the transport of the plume, the transport of pure RSOW would remain constant. Errors introduced by sampling uncertainties in the observational data, the methods of transport calculation, or temporal variability in the strength of the outflow during the period of the survey could all contribute to a lack of salt conservation. Quantitatively, it is likely that the former two errors are negligible when compared to the considerable uncertainties arising from synoptic temporal variability in the outflow.

Calculations of the entrainment TE and transport of pure RSOW TRS were made for the entire plume from section 1 in the strait to section 4 (Fig. 9). The range of errors due to temporal variability in the outflow was estimated based on the transport data in the Bab-el-Mandeb obtained from the Bab-el-Mandeb experiment (Murray and Johns 1997). The typical outflow variability in winter for a time period of 6 days (the time it took to complete the winter plume survey) is approximately ±0.15 Sv. This uncertainty was propagated in the above equations to obtain the error it would cause in the transport of RSOW as well as for entrainment and total plume transport. The uncertainty in the salinity of pure RSOW, SRS, is taken as ±0.20 psu, and the uncertainty for the mean plume salinity S and salinity of entrained water SE were chosen to be 0.25 and 0.5 psu, respectively. The errors introduced by these uncertainties are relatively small compared to the error induced by the transport variability.

As shown in Fig. 9 and Table 2, the transport of pure RSOW, TRS, remains close to constant (∼0.20 Sv) between sections 1 and 2, while the amount of entrainment of Gulf of Aden Water, TE, along with the total plume transport, T, increase. The transport of pure RSOW accounts for about two-thirds of the total plume transport at section 1. The transport of entrained water is therefore around 50% of TRS (or about 0.10 Sv), which is a result of entrainment of GASW into the interfacial layer at the top of the RSOW within the strait, between the Hanish sill and Perim. The transport of the IL at section 2 reaches about 0.08 Sv, suggesting that a large fraction of the RSOW mixed in the strait between Hanish and Perim is contained in the IL downstream of Perim. By section 2, the transport of entrained water, TE, has increased to about 0.15 Sv. There is only a small additional increase in TE toward section 3, ∼0.17 Sv. At these sections, the entrainment TE averages about 75% of TRS. At section 4, TE increases further to 0.24 Sv reaching about 80% of TRS. The overall transport rises from 0.29 Sv at Perim (section 1) to about double this value, around 0.56 Sv at section 4. Theoretically, TRS should be constant, however, TRS varies by up to 50% of its mean value (0.23 Sv), increasing in the downstream direction. The increase in TRS has to be attributed to errors affecting the analysis as described above, where nonsteady temporal variability is believed to be the main reason. Within the range of error introduced by temporal variability in outflow transport on time scales on the order of days to weeks (shown by the error bars in Fig. 9), the transport of RSOW through the four sections is not significantly different. Both the time span over which the sections were occupied as well as the advection time between sections fall within this range of synoptic variability. Advection time between the strait and section 4 is estimated at about 2448 h at the maximum observed outflow velocities of 0.5–1.0 m s−1.

d. Partitioning of the RSOW between the three outflow branches

The same calculations can be applied to the three individual outflow branches of the outflow plume for sections 24 (not shown). Despite the inconsistency in the values for the transport of RSOW, TRS, a general pattern of partitioning between the three plume branches in the overall RSOW transport becomes apparent (Fig. 10). Initially, at section 2, the NC accounts for about 54% of transport of RSOW across the section, the deep SC plume carries 30%, and the IL 16%. Hence, the transport of RSOW is roughly split in half between the northern and the southern pathways. The deep SC accounts for about double the transport of the IL. The relative contributions by the individual plume branches vary somewhat for section 3 and 4, as expected from the results presented earlier. Nonetheless, the general partitioning remains similar: close to half of the RSOW transport is accounted for in the NC, about one-third is carried by the deep SC, and the IL contributes around 17%. The contribution by the IL plume shows the most variability between sections. The effects of temporal variability are likely to exert stronger influence on this plume through varying shear and mixing intensities in the strait itself, thus possibly affecting the formation of the IL plume. Furthermore, the shallow depth of the IL plume subjects it to the effects of upper-ocean variability from which the deeper plumes in both channels remain isolated.

The overall pattern of partitioning of RSOW between the branches is reflected when taking the average of the three sections (Fig. 10, rightmost panel). About half (52%) of the RSOW exiting the Bab-el-Mandeb is contained in the NC plume, close to one-third (31%) is contained in the SC plume, and the remainder (∼17%) is contained in the IL. This is a fairly robust result and provides the first quantitative partitioning of the RSOW between the different plume branches.

In summer, the outflow is much weaker and intermediate inflowing layers of similar velocity amplitudes and salinities as the deep outflow make it harder to estimate net transports and to determine the main locations of mixing and entrainment. The partitioning between the plumes is different in summer owing to weaker outflow velocities and differing stratification regime in the Bab-el-Mandeb. There is a thin wind-driven surface layer of outflow from the Red Sea and the IL plume branch is absent. The plume transport emanating from the strait is small, O(0.06 Sv), and increases to approximately 0.18 Sv by section 4, therefore about tripling in transport through entrainment (though within errors not significantly different than the doubling of transport seen in the winter case). Further, our results indicate that the NC appears to carry about 75% of the outflow transport in summer, consistent with the thinner RSOW layer in summer being more confined to the deeper NC.

5. Summary and conclusions

The Red Sea outflow exhibits several features that distinguish it from other marginal seas outflows such as the Mediterranean or the Northern overflows. The Red Sea outflow plume is guided strongly by topography and splits into several individual plume branches, the Northern Channel plume, the Southern Channel plume, and in winter an intrusion layer overlying the SC. The outflow exhibits strong temporal variability, predominantly on seasonal time scales, but temporal variability on synoptic time scales has been observed and appears to cause considerable variability in transport of “pure” RSOW and hence, salt, into the Gulf of Aden.

The main product waters of the Red Sea outflow are formed in winter, the season of maximum outflow (Bower et al. 2005; Murray and Johns 1997). During the winter stratification regime, the Red Sea outflow plume separates not only into the two bathymetrically confined deep channels but an IL is formed above the deep SC at intermediate depths. These three plume branches each exhibit a different mixing behavior. The NC plume carries high-salinity RSOW down the continental slope for close to 100 km before additional dilution by mixing and entrainment occurs in the region between sections 3 and 4. The SC broadens earlier and mixing and entrainment become significant after section 2, within 70 km of the strait. The IL above the deep SC is formed in the strait during winter. Water from the upper layer of the Red Sea outflow plume mixes with GASW, characterized by a higher salinity than the GAIW, and the mixing product continues downstream as an adjusted intrusion layer at shallow depths above the SC. Except for possible baroclinic instabilities on its sides, the IL plume is not thought to entrain significantly after leaving the strait. Transport calculations show that the total outflow was relatively low during the REDSOX winter survey, 0.29 Sv at Perim, about half of the average wintertime transport reported in earlier observations (Murray and Johns 1997). Our results confirm the considerable seasonal variability in outflow transport in previous studies; in summer the transport at Perim reached only 0.06 Sv in our survey. Previous studies have shown that the summer outflow is very weak and temporal variability can cause it to shut off completely or even reverse intermittently.

Transports calculated for downstream sections across the plume show that, during winter, the outflow almost doubles before leaving the channels and descending into the Tadjura rift, reaching 0.56 Sv by section 4. Only about two-thirds of this transport increase can be unequivocally attributed to entrainment, owing to nonconservation of TRS, which is likely a result of transport variability of the plume. However, the increase in transport by approximately a factor of 2 is comparable to the Mediterranean outflow (Price et al. 1993). Partitioning the plume into the individual branches and repeating the transport calculations shows that the entrainment in the SC plume occurs mainly between sections 2 and 3, farther upstream than in the NC where mixing is limited until just before section 4. This confirms results from earlier studies (Peters et al. 2005; Peters and Johns 2005; Özgökmen et al. 2003). By section 2, the outflow appears approximately equally partitioned between the northern and the southern (deep plume and IL) pathways. The SC deep plume and the IL plume contain about equal parts of this transport. By section 3, the southern plumes carry two thirds of the outflow water, whereas little additional entrainment has occurred in the NC. By section 4, this ratio between the northern and southern plumes has evened out again and both account for equal amounts of outflow transport. In summer, the IL is absent and the NC carries more RSOW than the SC.

Estimates of the transport of pure Red Sea Outflow Water in the individual plume branches allow a determination of the relative contribution to RSOW transport by each of the three plume pathways. About half (52%) of the total RSOW is carried down the NC plume, whereas the deep SC and IL plume contain about 31% and 17%, respectively. The entrainment in the strait between the Hanish sill and Perim Narrows can be estimated from these results at about 0.10 Sv, and much of this water created by mixing in the strait appears to be contained in the IL downstream of Perim.

The Red Sea outflow is in contrast to the marginal sea outflows in the Atlantic, in that it enters the Indian Ocean at low latitudes, which lessens rotational effects on its outflow plume. Bower et al. (2000) found that the combination of low latitude and low outflow transport resulted in Ekman numbers O(1). This indicates that this outflow can be characterized as a frictional density current modified by rotation, rather than a geostrophic density current modified by friction (Bower et al. 2000). This results in a rapidly entraining plume, which unlike the Atlantic outflows does not follow the continental slope as a geostrophic density current. Using a streamtube model, Bower et al. (2000) found topographic effects to be dominant in determining the generation of Red Sea product waters with significantly different densities. The results of the present study underline the significance of topography; however, seasonal effects are even more significant in determining the characteristics of product waters (Bower et al. 2005). Despite its differences in dynamics compared with the Atlantic outflows, the Red Sea outflow exhibits similarities in structure to at least one of the Atlantic outflows, namely the Mediterranean outflow (Peters et al. 2005) in that it flows over a relatively shallow sill and entrains much less-dense upper ocean and thermocline waters.

Numerical simulations of the large-scale ocean circulation show that the strength of the thermohaline circulation is strongly sensitive to details in representation of overflows in ocean general circulation models (OGCMs). Such processes pose significant numerical and dynamical challenges (Price and Baringer 1994; Willebrand et al. 2001). The Red Sea outflow presents us with a very valuable case for model–data intercomparison leading to potential improvement of outflow models or parameterization of outflows in larger scale numerical models (e.g., OGCMs). The Red Sea product waters in the Tadjura rift get advected laterally through the Gulf of Aden and into the Indian Ocean where they can be traced as an intermediate salinity maximum as far south as the Mozambique Channel in the Indian Ocean (Beal et al. 2000). At least part of the high-salinity water associated with the front of Red Sea Water is believed to become trapped in the Agulhas Current and be fed into the South Atlantic. Beal et al. (2000) argue that all the salt that is injected into the Indian Ocean by the Red Sea in the western Gulf of Aden appears to be leaving the Indian Ocean via the Agulhas Current. This result closely ties the product waters of the Red Sea outflow to the global ocean circulation. The Red Sea outflow shows a number of features that make it very valuable for the investigation of the dynamics of marginal sea overflows in general. Especially the wide range of temporal variability, from seasonal to tidal, and its effects on mixing and entrainment in the outflow plume, are of considerable interest. Further synoptic observations of the Red Sea outflow plume as well as modeling studies investigating the effects of temporal variability on this outflow will be helpful in gaining a better understanding of the variability in the Red Sea outflow and its impact on mixing dynamics in the plume.

Acknowledgments

This work was supported by the U.S. National Science Foundation under Grants OCE 9819506 and OCE 0351116. We also acknowledge Grant OCE 0326648. We thank the REDSOX principal investigators (H. Peters, A. Bower, and D. Fratantoni) for their strong contribution to the program and we thank M. Baringer, K. Leaman, and T. Özgökmen for valuable comments and discussions on this work. The authors are grateful to the captains and crews of the R/V Knorr and R/V Maurice Ewing for their support of the REDSOX field program. We also gratefully acknowledge the many technical and research associates who participated in the collection and processing of the REDSOX datasets. Special thanks are given to S. Swift (WHOI) and P. Huchon (Geosciences Azur) for their assistance in making the French multibeam bathymetric data available for our use.

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APPENDIX

Tidal Corrections

The tidal velocities are strong in the narrow Bab-el-Mandeb through which the RSOW is entering the Gulf of Aden but they decay quickly farther down in the outflow. Nonetheless, the tidal signal will contaminate the velocity measurements and affect any subsequent calculations, such as transport calculations, which are an important tool for estimating entrainment. Therefore, in order to quantify transport and entrainment, the velocity data need to be corrected for tidal aliasing. Previous studies (Jarosz et al. 2005) show that the tides in the western Gulf of Aden are mixed. At Perim Narrows, the total tidal variance is about equally partitioned between the diurnal and semidiurnal bands, 50% and 46%, respectively (Jarosz et al. 2005). The K1 and M2 tides are the dominant constituents in their respective bands. Time series data from the Bab-el-Mandeb experiment were used in order to more quantitatively investigate the tides in this area. As previously described, during the 1995–96 Bab-el-Mandeb experiment (Murray and Johns 1997), ADCP and current meter moorings were deployed in the strait and at downstream locations in the plume, respectively. Figure 2 shows the locations of the deployed moorings at site A in the Perim Narrows, at site D just upstream of the splitting of the outflow plume, at site E in the Northern Channel and at site F in the Southern Channel. The data span a period from May 1995 to March 1996, on the first deployment, and from April 1996 to November 1996, on the second deployment.

There are two simple theories on how the tides could influence the overflow plume. The first hypothesis is based on the idea that, since the tidal signal is strong in the strait, at certain outflow velocities–reaching up to 0.8 m s−1 downstream in the NC at maximum outflow–advection could carry the tidal signal downstream through features (e.g., bores, hydraulic jumps, roll waves) propagating along the density interface between the plume and ambient water. Alternatively, in the absence of such features, the tidal influence downstream could be entirely controlled by the local astronomical forcing. Spectral analysis on the data at A, E, and F show the major peaks in the semidiurnal and diurnal frequency bands, as expected. The tidal signal is strong in the strait and weaker in the downstream channels. The tidal amplitudes are higher in the SC than in the narrow NC. The data at locations A, E, and F were filtered to reconstruct a time series of observed tidal velocities in the semidiurnal band (periods from 11.75 to 13 h) and the diurnal band (periods from 23 to 27.5 h). Complex demodulation of the tidal time series provided time varying amplitude and phase of the tidal signal. Amplitude ratios between the signal at the strait and downstream showed no conclusive and consistent relationship between tidal velocities in the strait and at the downstream moorings. Cross-correlation analysis was performed to provide a measure of the phase lag between the strait and the downstream locations. The time lags were found to be small, about half an hour. Any advective signal would take in the order of 24 h to propagate from the strait down to the moorings E or F, at maximum outflow velocities. The tidal signals in the downstream plume region were therefore considered to be entirely related to local tidal forcing.

Results from a regional tidal model, kindly provided by E. Jarosz, show a rapid decrease in barotropic tidal current amplitudes downstream from the strait (Fig. A1) and little phase variation across the region of interest (not shown). Harmonic analysis on the data at A, E, and F show a decay in tidal amplitude between these sites generally consistent with the Jarosz tide model. Based on the assumption that the local barotropic tides are dominant, another mooring was included in the analysis: mooring D, where the only valid data was from a current meter about 100 m off the bottom. In the strait itself, the ADCP velocity measurement at 30 m off the bottom was used, which is at the same depth off the bottom as the current meter moorings at E and F. To obtain tidal corrections over the domain of the plume, an approach was used that merges the results of the tidal model with the current meter data and the harmonic analysis. The tidal current amplitudes from the model were adjusted to fit the mooring data in the triangle between moorings D, E, and F. This allowed the prediction of tidal current amplitudes for each station location within this area. The fit for the phase was ambiguous and, based on our knowledge of limited phase differences between the moorings, uniform phase values across the area were chosen. At Perim, the results from the harmonic analysis were used to directly predict the tidal currents at the time of station occupations. Tidal corrections using the method described above were then performed for sections 2 and 3. Section 4 was determined to be too far downstream to require the use of tidal corrections since the tidal amplitudes at this location are negligibly small, and none were applied.

Repeat stations taken over a tidal cycle during REDSOX-I show that at the time of station occupation, the amplitudes of the tidal currents were moderate to minimal (Fig. A2). Figure A2 also illustrates that depending on the time of data observation, the range of tidal amplitudes can be significantly larger and hence tidal corrections are necessary in order to realistically quantify transports.

To give an idea of its importance, the maximal impact of tidal velocities on the plume transports can be roughly quantified by taking the (average) amplitudes of the tidal constituents from the harmonic analysis and multiplying it by the typical “area” (in xz space) of the plume. This yields a maximal tidal impact (at spring tide) for the transport at section 3 of 0.34 Sv for the SC and 0.06 Sv for the NC. In comparison, the tidally corrected versus nontidally corrected transports in our results show differences in transport of 0.16 Sv at section 3 for the SC and 0.03 Sv for the NC.

Fig. 1.
Fig. 1.

Topographic chart of the geographical location of the Red Sea, the Bab-el-Mandeb, and the Gulf of Aden. Light and dark blue depict shallow and deep ocean regions, respectively. The inset region of study is shown in more detail in Fig. 2.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 2.
Fig. 2.

Map of the western Gulf of Aden (the inset from Fig. 1), the region of the high-resolution hydrographic plume survey during REDSOX-I and -II. Color shading is from light blue (shallow) to dark blue (deep). The plume splits into two deep channels, the Northern and the Southern Channels, marked in yellow. The locations of across-plume sections (sections 14) as well as the location of moorings from the 1995–96 Bab-el-Mandeb experiment (A, D, E, F) are shown.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 3.
Fig. 3.

Time series of unfiltered velocity data illustrating the strong temporal variability at the Bab-el-Mandeb and downstream in the Northern and Southern Channels. Velocities are rotated into the flow direction and positive velocities are directed outward from the Red Sea into the Gulf of Aden. The high-frequency tidal oscillations are apparent, as are the strong seasonal cycle and the variability on synoptic time scales of days to weeks. The tidal fluctuations about the mean can reach amplitudes of the same magnitude as the mean current.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 4.
Fig. 4.

(top) Salinity section at the Perim Narrows, at the exit of Bab-el-Mandeb. The salinity is strongly stratified with very salty water (>39.5) emanating from the strait in the bottom layer and fresher Gulf of Aden Water (<36.5) in the surface layer. In the salty bottom layer, the isohalines are tilted to the western edge of the channel in agreement with geostrophy. Dotted lines depict station locations and data points, the corresponding station numbers from REDSOX-I are indicated. (bottom) Velocity section at the Bab-el-Mandeb. The velocities are rotated into the section and negative velocities depict outwardly directed flow. The Red Sea outflow is apparent as a strong velocity core in the bottom layer. The outflow velocities are banked against the western channel wall following the tilt of the isohalines.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 5.
Fig. 5.

(top) Salinity section at section 2. High-salinity Red Sea Outflow Water can be observed in the deep channels and a local salinity maximum is found in an equilibrated intrusion layer above the Southern Channel. A thick layer of low-salinity Gulf of Aden Intermediate Water (≈35.5) can be observed in the eastern part of the section above the deep Northern Channel. Dotted lines depict station locations and data points, the corresponding station numbers from REDSOX-I are indicated. Solid black lines delineate the regions chosen for transport calculations of the individual plume branches. (bottom) Velocity section at section 2. The velocities are rotated into the section and negative velocities depict outwardly directed flow. The salinity maxima from the top figure are mirrored in the velocity maxima in the deep channels and the IL. Inwardly directed velocities above the deep NC show inflowing GAIW.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 6.
Fig. 6.

As in Fig. 5 but for section 3. The deep channels are now divided by a 250-m ridge adjacent to the Northern Channel. The water in the deep NC does not show significant dilution by mixing and entrainment whereas the water in the Southern Channel is considerably less saline than at upstream sections. The intrusion layer is still present as a local salinity maximum above the SC.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 7.
Fig. 7.

As in Fig. 5 but for section 4. The high-salinity signature of the Red Sea Outflow Water is still apparent in the deep channels at this section. The intrusion layer is much reduced but still present on the western edge of the section. The velocity distribution across the section only in part reflects the salinity distribution. Outflow velocities remain strong in the Northern Channel and are present in the western part of the Southern Channel as well as in the remnant of the IL. The velocity maximum in the NC is now banked against the northeastern channel wall.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 8.
Fig. 8.

Transport in salinity classes for sections 14. At the Bab-el-Mandeb (section 1), the highest salinity class accounts for the largest transport. By section 2, there has been a dilution toward lower salinity classes, which continues toward section 3. By section 4, the highest salinity class has almost vanished. Transport is in Sverdrups. Total transport integrated across each section approximately doubles from the strait (0.29 Sv) to section 4 (0.56 Sv).

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 9.
Fig. 9.

Total transport (T), transport of pure Red Sea Outflow Water (TRS), and entrainment (TE) from the Bab-el-Mandeb to section 4. Total transport and entrainment increase downstream from the strait as expected. Error bars reflect the error associated with temporal variability in outflow transport and other uncertainties (see text for details).

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Fig. 10.
Fig. 10.

Percentage of transport of pure Red Sea Outflow Water within each plume branch across sections 24, illustrating the relative contributions of each pathway to the Red Sea outflow transport. Overall, the Northern Channel accounts for about one-half of the total transport, the Southern Channel carries about one-third, and the intrusion layer contributes the remainder (∼17%). Actual transport values are shown in parentheses (Sv).

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

i1520-0485-37-4-819-fa01

Fig. A1. Semimajor axis component of the dominant tidal constituents in the region of the Bab-el-Mandeb and the Red Sea outflow. Tidal currents are strong in the strait and quickly decay downstream. The mooring location for moorings from the Bab-el-Mandeb experiment and the estimated tidal amplitude from harmonic analysis on the mooring data are shown (cm s−1). Triangle DEF was used to fit the model data to observations in order to estimate tidal corrections for LADCP velocity data. Contour lines are every 1 cm s−1 with the contour line crossing mooring E in both panels corresponding to 2 cm s−1 and increasing toward the strait.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

i1520-0485-37-4-819-fa02

Fig. A2. Tidal velocities predicted by combining a tidal model and observational data in the region of the REDSOX plume survey. The curve shows through-section velocity; positive velocities are directed outward of the Red Sea. Stations occupied repeatedly over the tidal cycle during REDSOX-I are shown and illustrate that the tidal currents were moderate at time of station occupation but can reach considerably larger amplitudes. Stations 44, 49, and 55 coincide with section 2 and are located in the center of the Southern Channel. Time axis shows UTC time of station occupation. The tidal predictions were made hourly (asterisks) over a 25-h period.

Citation: Journal of Physical Oceanography 37, 4; 10.1175/JPO2993.1

Table 1.

Wintertime outflow transport calculated for sections across the Red Sea outflow plume at the Bab-el-Mandeb and several locations downstream. Transports are shown for the total plume and the individual plume branches.

Table 1.
Table 2.

Transport of pure Red Sea Outflow Water TRS, entrainment of Gulf of Aden Water TE, and total plume transport T through each of the four sections across the outflow plume.

Table 2.
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