298
Chapter 10: The Indian Ocean
reach to about 500–600 m (Schott, 1983), turn offshore at the origin of the SECC. North
of Cape Guardafui, along the coast of Oman, flow at this season is weak and variable,
although there is significant transport into the Gulf of Aden and the Red Sea (Halim,
1984). On the evidence of satellite chlorophyll images, there is significant upwelling
along the eastern Omani coastline during winter, although this appears to be somewhat
intermittent: I can find no record of this having been investigated. It is perhaps a simple
case of offshore transport of surface water and its replacement from below.
With the onset of the Southwest Monsoon winds of boreal summer, response and
spin-up of northeast flow in the Somali Current occurs within a few weeks. The current responds to progressive step increases in wind strength as the monsoon evolves in
a series of surges, and upwelling is rapidly induced at several locations on the Somali
and Omani coastlines by processes specific to each (Swallow, 1984). The first indication
of the reversal of the monsoon is the onset in March of onshore winds off southern
Somalia that cause the onshore flow to shift to about 5
N instead of at Cape Guardafui
(the prominent tip of the Horn of Africa) and to diverge into northward and southward
coastal currents. In April, southwest winds begin to blow along the coast south of the
equator and within a few days a northward coastal current develops from there to at
least 5
N, where offshore divergence occurs (Schott, 1983) and a wedge of cold upwelled
water develops along the offshore flow around the north side of a large gyre. In May, the
main onset of the Southwest Monsoon occurs quite suddenly: the response of the current
is to turn offshore a little further to the south (3–4
N), creating a cold wedge at this latitude. In June the alongshore winds extend to northern Somalia with strong anticyclonic
curl offshore of the Findlater Jet, and within about 14 days this situation induces an
anticyclonic gyre, the “Great Whirl,” south of Socotra and Cape Guardafui (Swallow and
Fieux, 1982).
Finally, in August, this gyre intensifies, even as the offshore motion and cold wedge
of surface water located at 3–4
N moves farther north and comes to lie on the southern
side of the “Great Whirl.” A second clockwise gyre is spun up under the influence of the
Findlater Jet to the east of Socotra. The Great Whirl is associated with strong upwelling at
the coast so that offshore transport around the northern side of the gyre forms a second
cold wedge at the surface; the cool water is subsequently entrained into the Socotra
gyre. The actual locations of the centers of these two gyres have considerable interannual
variability that is apparently due not entirely to variability in the wind field, but also to
an internal source in the chaotic nature of the dynamic circulation of the Arabian Sea
(Wirth et al., 2002).
Further northeast still, flow of the Somali Current (though weaker than off Somalia)
continues along the coast around the head of the Arabian Sea to the Indian subcontinent
(Swallow, 1984; Schott, 1983). Thus, the Somali Current itself, which has attracted so
much attention for its rapid reversal, is actually only part of a much larger boundary
current system along which coastal divergence occurs in several places.
The Findlater Jet of the Southwest Monsoon winds leaves the coast over Cape
Guardafui and continues northeasterly toward Pakistan, well offshore from the Arabian
coast (Brock and McLain, 1992). The area of cyclonic wind curl to the north of this
jet creates significant Ekman suction off the Omani continental shelf, whereas the anticyclonic curl field to the southeast creates Ekman pumping and deepens the mixed
layer (Lee, 2000). The total upwelling region induced by Ekman wind-stress curl off
Arabia is very large: 1000 km along the coast and 400 km offshore through which pass
8 × 10
6 m
−3 sec
−1 at the 50-m level which is equivalent to a vertical velocity of 1–2 m d
−1
(Swallow, 1984). The innermost 30–40 km of upwelling occurs as a direct response to
coastal wind effects, especially off Ras Madraka and at the Kuria Muria islands, and brings
cooler water (242
C) to the surface than farther offshore. The most prominent upwelling
feature is the offshore jet that develops at Ras al Hadd, the prominent cape at the eastern
Chapter 10: The Indian Ocean
reach to about 500–600 m (Schott, 1983), turn offshore at the origin of the SECC. North
of Cape Guardafui, along the coast of Oman, flow at this season is weak and variable,
although there is significant transport into the Gulf of Aden and the Red Sea (Halim,
1984). On the evidence of satellite chlorophyll images, there is significant upwelling
along the eastern Omani coastline during winter, although this appears to be somewhat
intermittent: I can find no record of this having been investigated. It is perhaps a simple
case of offshore transport of surface water and its replacement from below.
With the onset of the Southwest Monsoon winds of boreal summer, response and
spin-up of northeast flow in the Somali Current occurs within a few weeks. The current responds to progressive step increases in wind strength as the monsoon evolves in
a series of surges, and upwelling is rapidly induced at several locations on the Somali
and Omani coastlines by processes specific to each (Swallow, 1984). The first indication
of the reversal of the monsoon is the onset in March of onshore winds off southern
Somalia that cause the onshore flow to shift to about 5
N instead of at Cape Guardafui
(the prominent tip of the Horn of Africa) and to diverge into northward and southward
coastal currents. In April, southwest winds begin to blow along the coast south of the
equator and within a few days a northward coastal current develops from there to at
least 5
N, where offshore divergence occurs (Schott, 1983) and a wedge of cold upwelled
water develops along the offshore flow around the north side of a large gyre. In May, the
main onset of the Southwest Monsoon occurs quite suddenly: the response of the current
is to turn offshore a little further to the south (3–4
N), creating a cold wedge at this latitude. In June the alongshore winds extend to northern Somalia with strong anticyclonic
curl offshore of the Findlater Jet, and within about 14 days this situation induces an
anticyclonic gyre, the “Great Whirl,” south of Socotra and Cape Guardafui (Swallow and
Fieux, 1982).
Finally, in August, this gyre intensifies, even as the offshore motion and cold wedge
of surface water located at 3–4
N moves farther north and comes to lie on the southern
side of the “Great Whirl.” A second clockwise gyre is spun up under the influence of the
Findlater Jet to the east of Socotra. The Great Whirl is associated with strong upwelling at
the coast so that offshore transport around the northern side of the gyre forms a second
cold wedge at the surface; the cool water is subsequently entrained into the Socotra
gyre. The actual locations of the centers of these two gyres have considerable interannual
variability that is apparently due not entirely to variability in the wind field, but also to
an internal source in the chaotic nature of the dynamic circulation of the Arabian Sea
(Wirth et al., 2002).
Further northeast still, flow of the Somali Current (though weaker than off Somalia)
continues along the coast around the head of the Arabian Sea to the Indian subcontinent
(Swallow, 1984; Schott, 1983). Thus, the Somali Current itself, which has attracted so
much attention for its rapid reversal, is actually only part of a much larger boundary
current system along which coastal divergence occurs in several places.
The Findlater Jet of the Southwest Monsoon winds leaves the coast over Cape
Guardafui and continues northeasterly toward Pakistan, well offshore from the Arabian
coast (Brock and McLain, 1992). The area of cyclonic wind curl to the north of this
jet creates significant Ekman suction off the Omani continental shelf, whereas the anticyclonic curl field to the southeast creates Ekman pumping and deepens the mixed
layer (Lee, 2000). The total upwelling region induced by Ekman wind-stress curl off
Arabia is very large: 1000 km along the coast and 400 km offshore through which pass
8 × 10
6 m
−3 sec
−1 at the 50-m level which is equivalent to a vertical velocity of 1–2 m d
−1
(Swallow, 1984). The innermost 30–40 km of upwelling occurs as a direct response to
coastal wind effects, especially off Ras Madraka and at the Kuria Muria islands, and brings
cooler water (242
C) to the surface than farther offshore. The most prominent upwelling
feature is the offshore jet that develops at Ras al Hadd, the prominent cape at the eastern