340
V. Barale and M. Gade
The basin’s location in a hot and arid climate zone, where evaporation far exceeds
precipitation, and where fluvial runoff is negligible, results in a negative hydrological
budget (Morcos 1970). This deficit is compensated by a net water inflow from the Gulf
of Aden, via the Strait of Bab-el-Mandeb (Eshel and Naik 1997). The input of fresher
water from the Gulf of Aden and high evaporation rate result in a gradual northward
increase in salinity (in excess of 40 pps, in the northern basin) and density (Sofianos
et al. 2002). Conversely, SST increases from north to south, only to decrease again
in the southermost part of the basin.
The Red Sea surface currents are somewhat irregular and can vary significantly
on a seasonal scale. In the northern part of the basin, thermohaline forcing generates
a cyclonic circulation all around the year, in contrast to the prevailing wind forcing
(Quadfasel and Baudner 1993). The central part of the Red Sea, again around 20
◦ N, is
dominated by recurring or persistent anticyclonic eddies. The formation mechanisms
and temporal variability of which are still unclear, but which are thought to be an
indication of a very strong response of the Red Sea to atmospheric forcing (Sofianos
and Johns 2003). In the southern part, the circulation is very complex (due to the
exchanges with the Gulf of Aden, the peculiar bathymetry with wide continental
shelves on both sides of the basin, and the high space/time variability of atmospheric
forcing) and results in strong mixing of the different water masses involved (Sofianos
and Johns 2007).
In general, two distinct seasonal situations prevail. During the NE monsoon
(winter), the direction of net surface transport is northward. Following the typical inverse-estuarine circulation of concentration basins, fresher surface water enters
the Red Sea from the Gulf of Aden. It moves northward, mixing and sinking in
the northern basin, and then returning south as a saltier subsurface current that ultimately outflows at depth over the Hanish Sill (Smeed 2000). During the SW monsoon
(summer), the direction of net surface transport is southward. A three-layer system
develops in the Bab-el-Mandeb, as a consequence of opposing wind stress and thermohaline forces, composed of Red Sea water outflowing at the surface and at depth,
and of Gulf of Aden intermediate water, upwelled by the monsoon winds, inflowing
between these two layers (Sofianos and Johns 2007). In between these different seasonal states, transitional periods occur around April and October, when the winds
reverse direction (in the southern basin), surface currents are weakest and variable,
and the flow in the Bab-el-Mandeb reaches a minimum (Patzert 1974; Murray and
Johns 1997).
In the Red Sea, the water column permanent stratification, only somewhat weaker
in winter than in summer, is the main factor hindering nutrient renewal from deeper
layers, particularly in the northern part of the basin. The persistent thermocline
(and halocline), extending from about 50 m to 300 m of depth, isolates a homogeneous deep layer with uniform salinity of 40.6 pps (Woelk and Quadfasel 1996).
Consequently, the biological production of surface waters depends on the nutrient
concentration found in the upper water column, and in particular on the nutrients
received through the surface (winter) or intermediate (summer) water flow from
the Gulf of Aden and the ensuing northward overall drift (Edwards 1987). Vertical
mixing induced by winter surface cooling, due to a negative heat flux at the air-sea
V. Barale and M. Gade
The basin’s location in a hot and arid climate zone, where evaporation far exceeds
precipitation, and where fluvial runoff is negligible, results in a negative hydrological
budget (Morcos 1970). This deficit is compensated by a net water inflow from the Gulf
of Aden, via the Strait of Bab-el-Mandeb (Eshel and Naik 1997). The input of fresher
water from the Gulf of Aden and high evaporation rate result in a gradual northward
increase in salinity (in excess of 40 pps, in the northern basin) and density (Sofianos
et al. 2002). Conversely, SST increases from north to south, only to decrease again
in the southermost part of the basin.
The Red Sea surface currents are somewhat irregular and can vary significantly
on a seasonal scale. In the northern part of the basin, thermohaline forcing generates
a cyclonic circulation all around the year, in contrast to the prevailing wind forcing
(Quadfasel and Baudner 1993). The central part of the Red Sea, again around 20
◦ N, is
dominated by recurring or persistent anticyclonic eddies. The formation mechanisms
and temporal variability of which are still unclear, but which are thought to be an
indication of a very strong response of the Red Sea to atmospheric forcing (Sofianos
and Johns 2003). In the southern part, the circulation is very complex (due to the
exchanges with the Gulf of Aden, the peculiar bathymetry with wide continental
shelves on both sides of the basin, and the high space/time variability of atmospheric
forcing) and results in strong mixing of the different water masses involved (Sofianos
and Johns 2007).
In general, two distinct seasonal situations prevail. During the NE monsoon
(winter), the direction of net surface transport is northward. Following the typical inverse-estuarine circulation of concentration basins, fresher surface water enters
the Red Sea from the Gulf of Aden. It moves northward, mixing and sinking in
the northern basin, and then returning south as a saltier subsurface current that ultimately outflows at depth over the Hanish Sill (Smeed 2000). During the SW monsoon
(summer), the direction of net surface transport is southward. A three-layer system
develops in the Bab-el-Mandeb, as a consequence of opposing wind stress and thermohaline forces, composed of Red Sea water outflowing at the surface and at depth,
and of Gulf of Aden intermediate water, upwelled by the monsoon winds, inflowing
between these two layers (Sofianos and Johns 2007). In between these different seasonal states, transitional periods occur around April and October, when the winds
reverse direction (in the southern basin), surface currents are weakest and variable,
and the flow in the Bab-el-Mandeb reaches a minimum (Patzert 1974; Murray and
Johns 1997).
In the Red Sea, the water column permanent stratification, only somewhat weaker
in winter than in summer, is the main factor hindering nutrient renewal from deeper
layers, particularly in the northern part of the basin. The persistent thermocline
(and halocline), extending from about 50 m to 300 m of depth, isolates a homogeneous deep layer with uniform salinity of 40.6 pps (Woelk and Quadfasel 1996).
Consequently, the biological production of surface waters depends on the nutrient
concentration found in the upper water column, and in particular on the nutrients
received through the surface (winter) or intermediate (summer) water flow from
the Gulf of Aden and the ensuing northward overall drift (Edwards 1987). Vertical
mixing induced by winter surface cooling, due to a negative heat flux at the air-sea