TITLE:
Effects of Monsoon Winds and Topographical Features on the Vertical Thermohaline and Biogeochemical Structure in the Gulf of Tadjourah (Djibouti)
AUTHORS:
Youssouf Moussa Omar, Laurent Memery, Xavier Carton, Abdourahman Daher, Eric Duvielbourg
KEYWORDS:
Thermohaline, Biogeochemical Structures, Mixed Layer Depth, Thermocline, Monsoon Wind, Topography
JOURNAL NAME:
Open Journal of Marine Science,
Vol.6 No.3,
July
28,
2016
ABSTRACT: 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
chlorophyllaconcentrations 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 chlorophyllaconcentrations were observed.