20
V. Barale
flows southwards as a dense, cool, salty layer, under which resides a relatively
stagnant layer of still denser Red Sea Deep Water, formed during the winter months
in the Gulf of Suez and Gulf of Aqaba. The Red Sea thermohaline circulation is
modified in the upper two layers (Gulf of Aden Surface Water and Red Sea Water)
by the action of the wind field and rotation, which helps generate a system of gyres,
eddies and boundary currents.
In the Red Sea, detailed current data are lacking, partially because currents are
weak and variable, both spatially and temporally. Such space and time variations are
essentially wind-driven. In summer, the prevailing winds drive surface waters south,
whereas in winter the mean flow is reversed. Over the full annual cycle, the net value
of the latter predominates, resulting in an overall drift to the northern end of the Red
Sea, where evaporation generates the Red Sea Water, which then moves southward,
and ultimately leaves the basin at intermediate depth.
Unlike that of many other marginal seas, the Red Sea overflow is highly seasonal
7 ,
due to the monsoon winds and to variations in buoyancy fluxes. In the colder months
(November to May), a two-layer system dominates the exchange over the Hanish Sill:
Gulf of Aden Surface Water enters the basin above the Red Sea Water layer (while
the underlying Red Sea Deep Water leaves the basin at a much slower rate, via a
combination of mixing into the Red Sea Water above it and of Bernoulli aspiration
8 ,
without significantly altering the two-layer exchange). In the warmer months (June
to October), the Gulf of Aden Intermediate Water, present under the Gulf of Aden
Surface Water, is upwelled in the Gulf of Aden by the southwest monsoon winds.
As the Gulf of Aden Intermediate Water upper interface rises, it penetrates into the
southern Red Sea as an intermediate layer. The intrusion can last for a period of
3 months and is mixed into the upper layer, inducing an occasional Gulf of Aden
Surface Water flow reversal.
Tides are small in the Red Sea, ranging from 0.6 m in the north, near the Gulf of
Suez, to 0.9 m in the south, near the Gulf of Aden, but fluctuating between 0.2 and
0.3 m away from the nodal point. The Central Basin is therefore almost tideless, and
thus annual water level changes appear to be more significant than tides. In spite of
the small tidal range, a thin sheet of water can inundate extensive near-coastal areas
and lagoons during high tide, especially along the coast of the Arabian Peninsula.
The Red Sea has low productivity, due to a strong nutrient limitation, given the
absence of major continental inflows and the presence of a permanent thermocline,
which inhibits benthic nutrients from mixing into surface waters, where most primary
production occurs. The main nutrient input is from the Indian Ocean, through the
Gulf of Aden and the southern part of the basin. In spite of these conditions, the Red
Sea constitutes a rich and diverse ecosystem. More than 1100 species of fish have
been recorded, with about 10 % of these being endemic. This rich diversity is in part
7 Transport varies from a winter maximum of 0.6 Sv to a summer minimum of 0.05 Sv.
8 The high velocities of the Red Sea outflow in the bab el Mandeb can provide a Bernoulli suction
that enables the Red Sea Deep Water to flow up and over the sill, out into the Gulf of Aden. If the
speed along the upper streamline is great enough, in the stratified outflow, the lower streamline will
be able to rise above the depth of the sill.
V. Barale
flows southwards as a dense, cool, salty layer, under which resides a relatively
stagnant layer of still denser Red Sea Deep Water, formed during the winter months
in the Gulf of Suez and Gulf of Aqaba. The Red Sea thermohaline circulation is
modified in the upper two layers (Gulf of Aden Surface Water and Red Sea Water)
by the action of the wind field and rotation, which helps generate a system of gyres,
eddies and boundary currents.
In the Red Sea, detailed current data are lacking, partially because currents are
weak and variable, both spatially and temporally. Such space and time variations are
essentially wind-driven. In summer, the prevailing winds drive surface waters south,
whereas in winter the mean flow is reversed. Over the full annual cycle, the net value
of the latter predominates, resulting in an overall drift to the northern end of the Red
Sea, where evaporation generates the Red Sea Water, which then moves southward,
and ultimately leaves the basin at intermediate depth.
Unlike that of many other marginal seas, the Red Sea overflow is highly seasonal
7 ,
due to the monsoon winds and to variations in buoyancy fluxes. In the colder months
(November to May), a two-layer system dominates the exchange over the Hanish Sill:
Gulf of Aden Surface Water enters the basin above the Red Sea Water layer (while
the underlying Red Sea Deep Water leaves the basin at a much slower rate, via a
combination of mixing into the Red Sea Water above it and of Bernoulli aspiration
8 ,
without significantly altering the two-layer exchange). In the warmer months (June
to October), the Gulf of Aden Intermediate Water, present under the Gulf of Aden
Surface Water, is upwelled in the Gulf of Aden by the southwest monsoon winds.
As the Gulf of Aden Intermediate Water upper interface rises, it penetrates into the
southern Red Sea as an intermediate layer. The intrusion can last for a period of
3 months and is mixed into the upper layer, inducing an occasional Gulf of Aden
Surface Water flow reversal.
Tides are small in the Red Sea, ranging from 0.6 m in the north, near the Gulf of
Suez, to 0.9 m in the south, near the Gulf of Aden, but fluctuating between 0.2 and
0.3 m away from the nodal point. The Central Basin is therefore almost tideless, and
thus annual water level changes appear to be more significant than tides. In spite of
the small tidal range, a thin sheet of water can inundate extensive near-coastal areas
and lagoons during high tide, especially along the coast of the Arabian Peninsula.
The Red Sea has low productivity, due to a strong nutrient limitation, given the
absence of major continental inflows and the presence of a permanent thermocline,
which inhibits benthic nutrients from mixing into surface waters, where most primary
production occurs. The main nutrient input is from the Indian Ocean, through the
Gulf of Aden and the southern part of the basin. In spite of these conditions, the Red
Sea constitutes a rich and diverse ecosystem. More than 1100 species of fish have
been recorded, with about 10 % of these being endemic. This rich diversity is in part
7 Transport varies from a winter maximum of 0.6 Sv to a summer minimum of 0.05 Sv.
8 The high velocities of the Red Sea outflow in the bab el Mandeb can provide a Bernoulli suction
that enables the Red Sea Deep Water to flow up and over the sill, out into the Gulf of Aden. If the
speed along the upper streamline is great enough, in the stratified outflow, the lower streamline will
be able to rise above the depth of the sill.