Physical Oceanography of the Indian Ocean
21
Somalia most of its water turns south, crosses the Equator and forms the Equatorial
Countercurrent. The southward flow off Somalia is strongest in December, but lasts
until March. In November and December the countercurrent flows along both sides of
the Equator, extending from about 3
0
N to 5
0
S. Later, from January through April, the
countercurrent is entirely south of the Equator and its southern boundary is near 10 0 S.
It should be noted that some of the water of the countercurrent at its western origin is
drawn from the South Equatorial Current. At its eastern end only some of the water
returns north into the monsoon drift, while a large part continues to the SE as the Java
Coastal Current or turns directly into the South Equatorial Current. The proximity of·
this current to the Equator makes geostrophic computations rather uncertain and
therefore does not allow a numerical evaluation of the transports of the various branches.
During the NE monsoon season an Equatorial Undercurrrent is also developed in the
Indian Ocean, as measured by TAFT and KNAUSS (1967). This undercurrent is, however,
rather weak and appears variable and sometimes connected with the surface countercurrent.
In general it can be said that during the NE monsoon, which is a rather gentle
phenomenon, the oceanic circulation is also only moderately strong developed. The
surface flow does not appear to penetrate much beyond the thermocline and no striking
upwelling areas develop. Some shoaling of the thermocline, which under favorable wind
conditions may occasionally result in weak upwelling, seems to occur only along the
eastern shore of the Andaman Sea and in the northern part of the Arabian Sea off
Karachi.
With the onset of the SW monsoon the circulation changes drastically. In April the NE
monsoon current collapses, water starts to flow north along Somalia, and by May almost
everywhere north of the Equator water starts flowing east. In July the Somali Current has
reached its greatest strength, the monsoon current flowing east everywhere to the north
of the Equator has formed and the countercurrent has shifted north and joined the
monsoon current. The South Equatorial Current does not appear to shift much to the
north, but it becomes stronger, and most of its water turns north into the Somali Current.
Off Sumatra the monsoon current crosses the Equator and turns into the South Equatorial
Current. These 3 currents, monsoon current, South Equatorial Current and Somali
Current, form a very strong wind driven gyre in the equatorial Indian Ocean. The associated current speeds are stronger than those during the NE monsoon period. This circulation is maintained throughout September, and only in October does it start to break
down and is slowly replaced by the NE monsoon circulation.
The more vigorous atmospheric and oceanic circulation during the SW monsoon
causes not only the development of a strong western boundary current, the Somali
Current, but also intense upwelling in several places. The circulation appears to penetrate
deeper, affecting the movements of water masses below the thermocline. The formation
of the Somali Current as a strong western boundary current of the monsoon gyre causes
a strong baroclinic adjustment of the structure along the coast of Somalia. This baroclinic
structure is noticeable as deep as 1000 m, but is especially pronounced in the upper 400 m.
The strong winds parallel to the coast intensify the baroclinicity in the upper layer and
cause, in addition, strong upwelling along the coast. Large parts of the Somali Current
are recirculated in an intense eddy, the center of which is about 300 km offshore. This
elongated eliptical eddy stretches for about 1000 km parallel to the coast, and is
21
Somalia most of its water turns south, crosses the Equator and forms the Equatorial
Countercurrent. The southward flow off Somalia is strongest in December, but lasts
until March. In November and December the countercurrent flows along both sides of
the Equator, extending from about 3
0
N to 5
0
S. Later, from January through April, the
countercurrent is entirely south of the Equator and its southern boundary is near 10 0 S.
It should be noted that some of the water of the countercurrent at its western origin is
drawn from the South Equatorial Current. At its eastern end only some of the water
returns north into the monsoon drift, while a large part continues to the SE as the Java
Coastal Current or turns directly into the South Equatorial Current. The proximity of·
this current to the Equator makes geostrophic computations rather uncertain and
therefore does not allow a numerical evaluation of the transports of the various branches.
During the NE monsoon season an Equatorial Undercurrrent is also developed in the
Indian Ocean, as measured by TAFT and KNAUSS (1967). This undercurrent is, however,
rather weak and appears variable and sometimes connected with the surface countercurrent.
In general it can be said that during the NE monsoon, which is a rather gentle
phenomenon, the oceanic circulation is also only moderately strong developed. The
surface flow does not appear to penetrate much beyond the thermocline and no striking
upwelling areas develop. Some shoaling of the thermocline, which under favorable wind
conditions may occasionally result in weak upwelling, seems to occur only along the
eastern shore of the Andaman Sea and in the northern part of the Arabian Sea off
Karachi.
With the onset of the SW monsoon the circulation changes drastically. In April the NE
monsoon current collapses, water starts to flow north along Somalia, and by May almost
everywhere north of the Equator water starts flowing east. In July the Somali Current has
reached its greatest strength, the monsoon current flowing east everywhere to the north
of the Equator has formed and the countercurrent has shifted north and joined the
monsoon current. The South Equatorial Current does not appear to shift much to the
north, but it becomes stronger, and most of its water turns north into the Somali Current.
Off Sumatra the monsoon current crosses the Equator and turns into the South Equatorial
Current. These 3 currents, monsoon current, South Equatorial Current and Somali
Current, form a very strong wind driven gyre in the equatorial Indian Ocean. The associated current speeds are stronger than those during the NE monsoon period. This circulation is maintained throughout September, and only in October does it start to break
down and is slowly replaced by the NE monsoon circulation.
The more vigorous atmospheric and oceanic circulation during the SW monsoon
causes not only the development of a strong western boundary current, the Somali
Current, but also intense upwelling in several places. The circulation appears to penetrate
deeper, affecting the movements of water masses below the thermocline. The formation
of the Somali Current as a strong western boundary current of the monsoon gyre causes
a strong baroclinic adjustment of the structure along the coast of Somalia. This baroclinic
structure is noticeable as deep as 1000 m, but is especially pronounced in the upper 400 m.
The strong winds parallel to the coast intensify the baroclinicity in the upper layer and
cause, in addition, strong upwelling along the coast. Large parts of the Somali Current
are recirculated in an intense eddy, the center of which is about 300 km offshore. This
elongated eliptical eddy stretches for about 1000 km parallel to the coast, and is