1 The African Marginal and Enclosed Seas: An Overview
19
and metal-rich brines. More than 20 Deeps have been discovered (e.g. the Atlantis II
Deep, Discovery Deep, Oceanographer Deep), containing abundant metal-bearing
sediments and salt deposits
6 . The seafloor rises in the southern region toward the
Bab-el-Mandeb Strait, just 137 m deep at the Hanish Sill. The two gulfs in the
northern region are similar in shape, but their bottom topography is not. The Gulf
of Suez is shallower and has a relatively flat bottom, with depths ranging from 55 to
73 m. The Gulf of Aqaba, instead, comprises a deep basin – separated by a submarine
sill into two pits, both deeper than 1,000 m – bordered by a narrow shelf.
Desert or semi-desert areas, with no major freshwater inflow, surround the Red
Sea. Given the absence of rivers and permanent streams, occasional runoff is due
only to seasonal torrents (wadis). In this arid region, extremely high temperatures
characterize the weather, particularly in summer, increasing from the northern to
the southern area (which is considered to be among the hottest in the world). Air
temperatures range from 6 to 39
◦ C at the Suez Canal and from 13 to 42
◦ C along the
coast of the Arabian Peninsula. Rainfall is extremely sparse and occurs usually in the
form of short, local showers, often associated with thunderstorms and occasionally
with dust storms. The annual rainfall over the Red Sea region is extremely low,
averaging about 110 mm per year (but only half of that, about 60 mm per year, for
the Red Sea itself and its coastal zones).
The Red Sea climate is characterized by two distinct seasons, under the influence
of the northeast monsoon, in winter, and the southwest monsoon, in summer, respectively. Monsoon winds occur because of the differential heating between landmasses
and ocean surface. The local geographic and climatic conditions, leading to a low
freshwater input and a high evaporation rate, render the Red Sea one of the hottest and
saltiest bodies of seawater in the world. The average surface temperature in summer
ranges from 26
◦ C in the north to 30
◦ C in the south, with variations of only 2
◦ C in
winter. In general, surface temperature declines toward Bab-el-Mandeb, due to exchanges with the Gulf of Aden, and gradually decreases toward the northern region.
Deeper waters are stable throughout the region, so that below 300 m the temperature
is constantly between 21 and 22
◦ C. Salinity is also high, due to the combination of
high evaporation, low precipitation and lack of river outflow. It ranges between 36
and 39 psu, and is usually lower in the southern region, again due to the exchanges
with the Gulf of Aden. Water renewal in the Red Sea is relatively slow, as the exchange with the outer ocean takes approximately 6 years for the 200 m layer above
the thermocline, but approximately 200 years for the entire sea.
The Red Sea circulation follows an anti-estuarine pattern, with relatively fresher
water entering the basin above an outflowing saltier, denser layer. The fresher Gulf
of Aden Surface Water enters the Red Sea via the Bab-el-Mandeb. Within the basin,
this water is subject to the aforementioned excess of evaporation over precipitation
and runoff, estimated to be 2.06 ± 0.22 m year
−1 . The net evaporation drives the
formation of Red Sea Water in the northern part of the basin. This northern water
6 Sometime during the Paleocene (Tertiary) period, the Bab-el-Mandeb is thought to have closed
– either because of volcanic eruptions at Perim Island or due to sea level lowering during the Ice
Ages – causing the early Red Sea to evaporate, and creating the salt beds.
19
and metal-rich brines. More than 20 Deeps have been discovered (e.g. the Atlantis II
Deep, Discovery Deep, Oceanographer Deep), containing abundant metal-bearing
sediments and salt deposits
6 . The seafloor rises in the southern region toward the
Bab-el-Mandeb Strait, just 137 m deep at the Hanish Sill. The two gulfs in the
northern region are similar in shape, but their bottom topography is not. The Gulf
of Suez is shallower and has a relatively flat bottom, with depths ranging from 55 to
73 m. The Gulf of Aqaba, instead, comprises a deep basin – separated by a submarine
sill into two pits, both deeper than 1,000 m – bordered by a narrow shelf.
Desert or semi-desert areas, with no major freshwater inflow, surround the Red
Sea. Given the absence of rivers and permanent streams, occasional runoff is due
only to seasonal torrents (wadis). In this arid region, extremely high temperatures
characterize the weather, particularly in summer, increasing from the northern to
the southern area (which is considered to be among the hottest in the world). Air
temperatures range from 6 to 39
◦ C at the Suez Canal and from 13 to 42
◦ C along the
coast of the Arabian Peninsula. Rainfall is extremely sparse and occurs usually in the
form of short, local showers, often associated with thunderstorms and occasionally
with dust storms. The annual rainfall over the Red Sea region is extremely low,
averaging about 110 mm per year (but only half of that, about 60 mm per year, for
the Red Sea itself and its coastal zones).
The Red Sea climate is characterized by two distinct seasons, under the influence
of the northeast monsoon, in winter, and the southwest monsoon, in summer, respectively. Monsoon winds occur because of the differential heating between landmasses
and ocean surface. The local geographic and climatic conditions, leading to a low
freshwater input and a high evaporation rate, render the Red Sea one of the hottest and
saltiest bodies of seawater in the world. The average surface temperature in summer
ranges from 26
◦ C in the north to 30
◦ C in the south, with variations of only 2
◦ C in
winter. In general, surface temperature declines toward Bab-el-Mandeb, due to exchanges with the Gulf of Aden, and gradually decreases toward the northern region.
Deeper waters are stable throughout the region, so that below 300 m the temperature
is constantly between 21 and 22
◦ C. Salinity is also high, due to the combination of
high evaporation, low precipitation and lack of river outflow. It ranges between 36
and 39 psu, and is usually lower in the southern region, again due to the exchanges
with the Gulf of Aden. Water renewal in the Red Sea is relatively slow, as the exchange with the outer ocean takes approximately 6 years for the 200 m layer above
the thermocline, but approximately 200 years for the entire sea.
The Red Sea circulation follows an anti-estuarine pattern, with relatively fresher
water entering the basin above an outflowing saltier, denser layer. The fresher Gulf
of Aden Surface Water enters the Red Sea via the Bab-el-Mandeb. Within the basin,
this water is subject to the aforementioned excess of evaporation over precipitation
and runoff, estimated to be 2.06 ± 0.22 m year
−1 . The net evaporation drives the
formation of Red Sea Water in the northern part of the basin. This northern water
6 Sometime during the Paleocene (Tertiary) period, the Bab-el-Mandeb is thought to have closed
– either because of volcanic eruptions at Perim Island or due to sea level lowering during the Ice
Ages – causing the early Red Sea to evaporate, and creating the salt beds.