292
Chapter 10: The Indian Ocean
to prevailing wind stress. The circulation pattern is strongly influenced by the existence
of a shallow sill of only 110 m at the Straits of Bab-el-Mandeb. Excess of evaporation
over precipitation in the basin of about 2 m y
−1 forces a constant influx of surface
water through these straits from the Arabian Sea. Winter cooling to about 18
C at the
northern end of the Red Sea of water already having a salinity of 42‰ creates strong
deep convection cells in which the deep water (anomalously warm and saline at 215
C
and 40.6‰) of the Red Sea basin is formed (Dietrich et al., 1970). At the Straits of
Bab-el-Mendab, deep water flows out below the surface influx at a rate of about half that
of the Mediterranean outflow over the sill of the Straits of Gibraltar. Surface wind-driven
streams are generally weak and eddying, but flow occurs toward the south along the
whole length of the western coast of the Red Sea during the season of northerly, summer
winds; at other times the density-driven flow will cause surface drift contrary to these
winds, especially up the eastern coast. Thus, the general surface circulation is cyclonic.
The relative depth of the mixed layer is approximately the inverse of surface temperature, except where it is destroyed by winter mixing or tidal effects in shallow water. In
boreal spring, thermocline depths decrease, until the summer situation is reached, with
minimum depths of about 30 m on the eastern side of the rift. In the southern Red Sea,
the summer thermocline is sustained above the uplift of the weak permanent thermocline
forced by the very active flow of a cool-water core northward through the Straits of
Bab-el-Mandeb under the influence of the Southwest Monsoon. This shallow permanent
thermocline persists to about 22
N in the Red Sea. In the northern Red Sea, the summer
thermocline, at about 25 m, lies above the nearly isothermal deep water mass.
An important layer of oxygen deficiency (09–13 ml O 2 liter
−1 ) occurs at approximately 300–650 m throughout the Red Sea and is exceptional among such areas because
both temperature and salinity are relatively very high: ∼22
C and 40.5‰, respectively
(Weikert, 1984). This strange habitat will require (later) some discussion of the reaction
of zooplankton to its characteristics.
The monsoon reversal has only minor influence over the Red Sea, where the prevailing
winds are along the axis of the rift valley and from the north; during winter the wind is
reversed over the southern part so that a wind convergence occurs at 18–22
N (Edwards,
1987). Surface inflow at Bab-el-Mandeb is therefore stronger in winter than in summer.
At times of stronger than usual northerlies, the water column at these straits may comprise
three layers: a thin (40 m) surface, wind-driven outflow; a deep density-driven outflow;
and an intermediate low-salinity inflow. Interannual variability in inflow through Babel-Mandeb is also strong and can be detected by differential spreading of Arabian Sea
water (Ganssen and Kroon, 1991).
Arabian Gulf The shallow Arabian Gulf is oceanographically an extension of the surface water of the Arabian Sea, and there is no shallow sill between it and the Gulf of
Oman as there is between the Red Sea and the Gulf of Aden. Evaporation greatly exceeds
both precipitation and the input of river water from the Euphrates and Tigris, and salinity
reaches >50% in shallow water on the Arabian coast. A slow cyclonic circulation is maintained having low-energy regions toward the head of the Gulf; evaporation (<15 m y
−1 )
occurs in the southern bight lying to the east of the Qatar peninsula; the dense water
formed in this way sinks to the deeper regions of the Gulf.
In winter, a weak thermocline is briefly established in the outer part of the Arabian
Gulf at 30–40 m, but during the intense heating of the boreal summer an isothermal
layer effectively ceases to exist, with surface temperatures reaching 32
C above a thermal
gradient to bottom water of 22–24
C. Salinity reaches 40‰ along the Iranian coast and
off Arabia, where the outflow of dense bottom water occurs, especially in the deeper
channels.
Chapter 10: The Indian Ocean
to prevailing wind stress. The circulation pattern is strongly influenced by the existence
of a shallow sill of only 110 m at the Straits of Bab-el-Mandeb. Excess of evaporation
over precipitation in the basin of about 2 m y
−1 forces a constant influx of surface
water through these straits from the Arabian Sea. Winter cooling to about 18
C at the
northern end of the Red Sea of water already having a salinity of 42‰ creates strong
deep convection cells in which the deep water (anomalously warm and saline at 215
C
and 40.6‰) of the Red Sea basin is formed (Dietrich et al., 1970). At the Straits of
Bab-el-Mendab, deep water flows out below the surface influx at a rate of about half that
of the Mediterranean outflow over the sill of the Straits of Gibraltar. Surface wind-driven
streams are generally weak and eddying, but flow occurs toward the south along the
whole length of the western coast of the Red Sea during the season of northerly, summer
winds; at other times the density-driven flow will cause surface drift contrary to these
winds, especially up the eastern coast. Thus, the general surface circulation is cyclonic.
The relative depth of the mixed layer is approximately the inverse of surface temperature, except where it is destroyed by winter mixing or tidal effects in shallow water. In
boreal spring, thermocline depths decrease, until the summer situation is reached, with
minimum depths of about 30 m on the eastern side of the rift. In the southern Red Sea,
the summer thermocline is sustained above the uplift of the weak permanent thermocline
forced by the very active flow of a cool-water core northward through the Straits of
Bab-el-Mandeb under the influence of the Southwest Monsoon. This shallow permanent
thermocline persists to about 22
N in the Red Sea. In the northern Red Sea, the summer
thermocline, at about 25 m, lies above the nearly isothermal deep water mass.
An important layer of oxygen deficiency (09–13 ml O 2 liter
−1 ) occurs at approximately 300–650 m throughout the Red Sea and is exceptional among such areas because
both temperature and salinity are relatively very high: ∼22
C and 40.5‰, respectively
(Weikert, 1984). This strange habitat will require (later) some discussion of the reaction
of zooplankton to its characteristics.
The monsoon reversal has only minor influence over the Red Sea, where the prevailing
winds are along the axis of the rift valley and from the north; during winter the wind is
reversed over the southern part so that a wind convergence occurs at 18–22
N (Edwards,
1987). Surface inflow at Bab-el-Mandeb is therefore stronger in winter than in summer.
At times of stronger than usual northerlies, the water column at these straits may comprise
three layers: a thin (40 m) surface, wind-driven outflow; a deep density-driven outflow;
and an intermediate low-salinity inflow. Interannual variability in inflow through Babel-Mandeb is also strong and can be detected by differential spreading of Arabian Sea
water (Ganssen and Kroon, 1991).
Arabian Gulf The shallow Arabian Gulf is oceanographically an extension of the surface water of the Arabian Sea, and there is no shallow sill between it and the Gulf of
Oman as there is between the Red Sea and the Gulf of Aden. Evaporation greatly exceeds
both precipitation and the input of river water from the Euphrates and Tigris, and salinity
reaches >50% in shallow water on the Arabian coast. A slow cyclonic circulation is maintained having low-energy regions toward the head of the Gulf; evaporation (<15 m y
−1 )
occurs in the southern bight lying to the east of the Qatar peninsula; the dense water
formed in this way sinks to the deeper regions of the Gulf.
In winter, a weak thermocline is briefly established in the outer part of the Arabian
Gulf at 30–40 m, but during the intense heating of the boreal summer an isothermal
layer effectively ceases to exist, with surface temperatures reaching 32
C above a thermal
gradient to bottom water of 22–24
C. Salinity reaches 40‰ along the Iranian coast and
off Arabia, where the outflow of dense bottom water occurs, especially in the deeper
channels.