The vertical thermohaline and biogeochemical structures of the upper layer (0 - 200 m) were studied in the Gulf of Tadjourah using high-resolution hydrographic data collected in July-August 2013, September 2013 and February 2014. During summer, the superficial layer consisted of the mixed layer (ML) extending to a depth of about 20 - 30 m followed by the thermocline located between 30 and 50 m depth. The ML was thicker in the west and the southeast where the thermal gradient and chlorophyll a concentrations were particularly high. During September, this stratification persisted but the ML became warmer and saltier and the thermocline moved slightly deeper. In February, the ML extended to about 120 m, and the thermocline was less pronounced. A comparison of the directly measured currents to the wind induced Ekman current and to geostrophic velocity profiles revealed that the thermohaline and the biogeochemical features in summer were related to the southwest monsoon (SWM). The SWM drives surface water from the Gulf of Tadjourah to the Gulf of Aden and thus induces westward intrusion of the high salinity thermocline water from the Gulf of Aden; this near surface flow mixes surface waters in the extreme west of the Gulf of Tajourah. In contrast, the northeast monsoon (NEM), predominant in winter, brings cold water toward the Gulf of Tadjourah and thickens the ML through convective mixing. Our study shows that the SWM plays a crucial role in the stratification of the water column during summer but bathymetry influences its effects. The bowl-shape of the basin and its elongated slope in the west enhance the upwelling in this area where negative sea surface temperature anomalies and high chlorophyll a concentrations were observed.
The Gulf of Tadjourah (GOT hereafter) is located at the western end of the Gulf of Aden and to the southeast of the Bab Al-Mandeb Strait which connects the Red Sea to the Indian Ocean. Extending over about 4500 km2, this semi-enclosed sea is an extension of the rift of Tadjourah (
From both social and economic perspectives, the GOT plays a crucial role as a local fishing area. It is also vital in securing food to coastal communities of the surrounding arid lands of Djibouti. In the context of current concerns regarding the effects of global warming, vulnerable marine habitats and their resources may be detrimentally impacted and therefore, it is essential to establish a fishing policy based on scientific knowledge of the ecosystem and its potential for exploitation. Surprisingly, the oceanographic data are scarceand the area remains poorly studied.
Recently, the main patterns of temporal and spatial variability at the surface in the GOT were identified using satellite historical data on sea surface temperature (SST) from AVHRR MetopA radiometer and a merged product on ocean color from Modis/Meris sensors (Glob Colour/ST-ACRI), [
The limited available data on the water column in the GOT originate exclusively from research vessels passing through Djibouti on their way to study other ocean areas. Thus, during pelagic fish sampling, Myklevoll [
The questions addressed in this paper are: 1) How is the water column structured during summer time and how does it evolve throughout the year? 2) How does the vertical structure contribute to the observed variability of SST and CHL-a? These questions are dealt with using newly collected data in the GOT. These successively collected data as well as new analysis methods [
The hydrologic data were collected successively during three particular periods: July-August (summer), September 2013 (transitional period), and February 2014 (winter) at eight stations in the GOT (see
current measurements, a Seaguard sensor, with an ADCP (Acoustic Doppler Current Profiler) was used. At each station, the two instruments were used simultaneously and data were recorded both during descent and ascent of the probes. Two or three immersions were carried out at each station. Thus, at least four vertical profiles were obtained for each parameter and at each station. For two probes, data specifications are presented in
To assess the quality of the data, the standard deviation was computed from the total number of profiles. Thus, values exceeding more than three standard deviations (3σ) were systematically removed. Each profile was subsequently checked for an inversion in depth which may have been generated by boat instability at the sea surface. All the data were smoothed by 5-points moving average and interpolated between the surface and 200 m.
The vertical structure of the upper layer (0 - 200 m) was studied by means of data profile analysis. Firstly, the thermal vertical structure was examined using temperature gradient profile analysis. In contrast to the commonly used gradient criterion based on a predetermined threshold value [
Thus, from the surface downward, the part of the profile where the temperature gradient remains below
The Mean Gradient Method (MGM) was then compared to two objective methods for determining mixed layer depth from the profile data: the Maximum Angle Method (MAM) and the Curvature Method (CM). The MAM is based on an angle between two successive vectors along the profile (
Measured parameter | Range | Accuracy | Resolution |
---|---|---|---|
Temperature (˚C) | −5˚C to 35˚C | ±0.01˚C | 0.001˚C |
Conductivity (mS/cm) | 0 - 100 mS/cm | ±0.001 mS/cm | 0.0001 - 0.01 mS/cm |
Depth (m) | 0 - 200 m | ±0.003 - 0.004 m | 0.001 m |
Dissolved Oxygen (mg/l) | ±0.1 mg/l | 0.01 mg/l | |
Chlorophyll a concentration | 0 - 280 µg/l | - | 0.01 μg/l |
Current velocity (m/s) | 0 - 300 m | 1% | 0.01 m/s |
Current direction (˚) | 0 - 300 m | ±5˚ | 0.01˚ |
Subsequently, the vertical variation of salinity, CHL-a concentration (fluorescence), DO, currents and nutrients were studied exclusively using the individual profile data and cross-section analysis.
The Ekman surface current was estimated from wind stress gridded data at 0.125˚ × 0.125˚ resolution (source). At each grid point, the zonal and meridian components were computed using Equations (3) and (4) (see [
Az: coefficient of vertical viscosity;
At each station, the specific volume anomaly (α = 1/ρ) was computed from the sea surface to a depth of 200 m, as a function of temperature, salinity and pressure, using seawater state equations. Then, the differences of geopotential between two stations were integrated between reference pressure
Geostrophic current between stations B and A, separated by distance L, is given by :
The relation between the two will be examined thereafter.
1) Temperature
From
The first two zones, which can be distinguished clearly due to their thermal gradient, were identified as the ocean mixed layer and the thermocline. In order to characterize the mixed layer well and to quantify its thickness (MLD), the MGM and the above described two objective methods (MAM and CM) were applied to temperature profiles.
2) Salinity
As with temperature, salinity remains quasi-uniform in the first 20 meters. In contrast, the salinity profiles indicate strong fluctuations of the maxima and minima particularly between 30 and 50 m depth where the thermocline was identified. The observed inversions of density in the pycnocline appear to be due to these variations in salinity rather than temperature. With the exception of station E2, salinity decreases slowly below the pycnocline and then increases below 100 m as observed for temperature (
At all stations, CHL-a is low between the surface and about 20 m depth below which it increases and reaches maximum values between 20 and 35 m. These maxima are highest at stations C1 and W1 (6.96 and 4.13 mg/l, respectively,
The concentrations of nutrients (phosphate, ammonium, nitrite, and silicate) were measured every 10 m from the surface to 160 m depth at stations W1, C1, C2 and E2 during July-August 2013. The vertical distribution of each element is illustrated in
In order to examine how the water column structure described in section 3.1 evolves over time, the vertical profiles and sections during July-August 2013 were compared to those obtained during September and February 2014. As can be seen in
STATIONS | Mean gradient | THERMOCLINE | |||||
---|---|---|---|---|---|---|---|
(˚C/m) | CM | MAM | MGM | Zup | MaxGrD | Zlow | |
E01 | −0.04 | 5.5 | 5.6 | 5.67 | - | - | - |
E02 | −0.13 | 29.7 | 29.6 | 29.65 | 30.9 | 31.80 | 44.20 |
E03 | −0.18 | 15.62 | 16.3 | 15.39 | 31.3 | 34.30 | 40.50 |
E04 | −0.17 | 24.8 | 21.0 | 20.57 | 24.0 | 32.60 | 44.40 |
C01 | −0.12 | 10.0 | 6.0 | 6.86 | 22.7 | 38 | 43.10 |
C02 | −0.11 | 25.1 | 25.8 | 25.10 | 28.6 | 41 | 47.90 |
C03 | −0.16 | 16.6 | 23.3 | 16.30 | 24.10 | 33.7 | 38.70 |
W01 | −0.10 | 13.7 | 3.9 | 11.60 | 28.24 | 32 | 35.20 |
Stations | DCM (µg/l) | Depth (m) |
---|---|---|
E2 | 2.06 | 35.67 |
E3 | 3.1 | 24.03 |
E4 | 2.59 | 30.8 |
C2 | 1.89 | 28.34 |
C3 | 2.48 | 34.94 |
C1 | 6.96 | 21.57 |
W1 | 4.13 | 18.33 |
differences between the three periods. From July-August to September (red and blue curves, respectively), temperature has increased in the mixed layer and the thermocline has deepened by a few meters. This slight difference continues down to about 120 m depth. By February (green), temperature has decreased by about 5˚C and the vertical structure has completely changed. The temperature is homogeneous from the surface down to 120 m and the thermocline has disappeared. In terms of salinity, the mixed layer becomes saltier in September. However, in February salinity is greatly reduced.
The East-West sections of temperature and salinity give more detail on spatial and temporal evolution of the vertical thermohaline structure.
As illustrated in
July-August, all isotherms are declined from the west to the east, indicating upwelling in the west. Between 30 m and 60 m, isotherms 22˚C - 24˚C-are tightening, which corresponds to the above identified thermocline from the individual temperature profiles. In September, the upper layer is more stratified. The mixed layer is warmer (T > 30˚C), more homogeneous and extends to about 40 m depth. The thermocline is well pronounced and is located between 34 and 60 m depth. The salinity layer in the thermocline thickens and extends deeper to about 75 m depth. In February, the thermal structure is notably different. The temperature (26˚C - 26.5˚C) and salinity (37.2 psu) are lower and more homogeneous from surface to 120 m. A slight salinity gradient can be observed from the east to the west. The E-W section of DO show similar temporal and spatial evolution to those of temperature.
The biogeochemical structure also shows noticeable seasonal variation. During July-August the DCM is about 20 m beneath the surface and located in the western parts of the Gulf. In September, the higher chlorophyll patch extends eastwards but becomes more narrow and deeper (
not exceed 1.2 µg/l. The vertical structure shows for all nutrients, except ammonium, that the high concentrations are located at a deeper level than in summer (
During July-August 2013, currents were measured in the east (St. E2), south (St. C1) and west (St. W1) of the GOT.
westward at station E2. The maximum velocity (50 cm/s) is reached at about 50 m depth and the minimum at 150 m. In the south (St. C1), the current has the same direction except at 50 - 60 m where it reverses. In the west (St. W1), the current is westward within the first 30 m and eastward below this depth. Comparatively, the current velocity is high at the eastern station and lower at the western and southern stations.
1) Influences of wind induced surface current (Ekman derived current)
The monthly mean of Ekman derived current vectors superimposed to wind stress field were calculated for the area 11˚N - 14˚N and 42˚E - 44˚E (Appendix 1). In the GOT (framed area) currents are toward the South-East from July to September and in the opposite direction (North-West) from October to June. In order to examine the spatial variation of the wind stress inside the gulf, wind stress field from SeaWind sensor gridded data at 0.25˚ × 0.25˚ were linearly interpolated to 0.125˚ × 0.125˚. This revealed that the wind stress is strong at the extreme west of the gulf during July and decreases in August (
The zonal and meridional components of the geostrophic velocity were estimated from the hydrographic data collected at stations E2 and E4 during July-August. A vertical profile of the geostrophic current was computed at each depth relative to the reference level fixed at 150 m using Equation (4) with L = 10,000 m (distance between two stations) and the Coriolis parameter at latitude 11˚40: f = 2.88 × 10−5 s−1. The directly measured current at this level is lower or close to zero (see
Before this study, only limited data were available on the vertical structure of the water column in the Gulf of
Tadjourah. In this work, thermohaline and biogeochemical properties of the upper layer (0 - 200 m) were studied using new hydrographic data from three investigations carried out during particular months: July-August (summer), September 2013 (transitional period) and February 2014 (middle of winter when the sea surface temperature is the lowest).
During July-August, vertical temperature profiles revealed a stratified structure of three main layers. This structure was also observed for salinity, CHL-a concentration and DO profiles. The first striking characteristic of the water column during summer is its spatial variability. The thermally more or less homogeneous area (mixed layer) extends from the surface to about 30 m depth, but varies from one station to another. This variation is statistically significant because the difference of the mixed layer depth (
ature profiles
The spatial differences between stations indicate the heterogeneity of the mixed layer and appear to be related essentially to wind stress, which in turn is influenced by the topographical features. Indeed, the southwest winds which predominate during July-August affect the western part of the gulf more significantly than the eastern part (see
The measured current at station E2 during July-August clearly indicates westward inflow, which is increased at station E2 (maximum velocity at 50 m depth). This current becomes weaker in the south (St. C1) and in the west (St. W1). So, thermocline water is injected into the gulf by advection in the northeastern part and then propagates toward the west. In the extreme west, upwelling brings the thermocline water to the surface where it mixes with the surface water. This is well illustrated by the East-West temperature and DO sections (
In addition, the thermocline water is characterized by high salinity (S > 38). The East-West salinity section shows that the saltier layer extends from the east to the west (
During September, the mixed layer deepens slightly and becomes thermally more homogeneous but the stratification into three main layers persists. September is a transition period during which winds are very weak. Hence, the increase of temperature and salinity could be related to solar heating of the surface water. In contrast, the deepening of the mixed layer and the thermocline in February is caused by NE monsoon winds which induce surface water flow toward the GOT (convergence). As the NE wind is cold, it cools down the surface water and induces deep convective mixing.
The monsoon winds and their seasonal reversal are the most important forces which govern surface circulation in the west of the Gulf of Aden [
Moreover, because of the presence of islands (Musha & Maskali) at the entrance of the gulf, the near surface water containing thermocline enters the Gulf predominantly at the northeastern part, between stations E2 and E4, where the strongest westward current was observed. The high salinity of the near surface water is possibly due to the mixing with Red Sea water before entering into the GOT. This interpretation is consistent with Obura’s observations [
This work was supported financially by the “Centre d’Etude et de Recherche de Djibouti (CERD)”. We are grateful to Dr Mohamed JALLUDIN and to Dr Abdulrahman DAHER for their support and encouragement
during this study. Many thanks to the Djiboutian coast guard for their logistic support during the sampling campaign. We acknowledge LEMAR (Marine Environmental Sciences Laboratory) and LPO (Ocean Physics Laboratory) and “Université de Djibouti” for technical support and for providing measuring instruments. We also thank all those who contributed to this work, in particular Dr Sharon WOOLSEY for helpful comments.
Youssouf Moussa Omar,Laurent Memery,Xavier Carton,Abdourahman Daher,Eric Duvielbourg, (2016) Effects of Monsoon Winds and Topographical Features on the Vertical Thermohaline and Biogeochemical Structure in the Gulf of Tadjourah (Djibouti). Open Journal of Marine Science,06,440-455. doi: 10.4236/ojms.2016.63037
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