22
K. WYRTKI:
explained by DtlING (1970) as the westernmost of a system of eddies in the Arabian Sea
which is intimately connected with the dynamics of the monsoon gyre. On the basis of
direct current measurements, SWALLOW and BRUCE (1966) have shown that during the SW
monsoon transports in the Somali Current in the upper 200 m increase from 30 megatons
sec- 1 at 3
0
S to more than 50 megatons sec- 1 at 8 0 S. The total geostrophic transport is
about 65 megatons sec-I, demonstrating that most of the high speed flow is concentrated
in the surface layer.
The upwelling along the coast is most intense between SON and 11 0 N, where the
entire warm surface layer is removed and subsurface water with temperatures well
below 20 0 C reaches the sea surface (WARREN, STOMMEL and SWALLOW, 1966).
P04 3 - - P concentrations reach more than 10 ,ug-at 1- 1 and N03 - - N concentrations
more than 10 ,ug-at 1- 1 in contrast to values of 0.2 and 0.5, respectively, in the offshore areas. Water with such high nutrient concentrations is otherwise found at
150 m depth. When the Somali Current leaves the coast near lIoN and turns east, the
cool upwelled water continues to follow for some hundred kilometers. In the north, the
upwelling region is terminated by a flow of warm surface water out of the Gulf of Aden,
forming a strong temperature front. This warm surface water coming from the Gulf of
Aden and flowing east into the central Arabian Sea also separates the Somali upwelling
from the upwelling along the coast of Arabia.
Strong winds also blow parallel to the coast of Arabia east of 55 0 E during the SE
monsoon, and cause upwelling. This upwelling is different from the Somali upwelling, as
no strong current develops parallel to the coast. In volume it may even be stronger than the
Somali upwelling, and also the nutrient enrichment is more intense with maximum values
in excess of 1.5 ,ug-at 1- 1 for P04 3- - P and a larger area affected by higher concentrations
of both P0 4 3 - - P and N0 3 - - N.
During the SW monsoon weak upwelling may also develop under favorable conditions
along some parts of the east coast of India, but does not seem to have very noticeable
effects. Along the west coast of India subsurface water comes very close to the surface, and
the 20
0
C isotherm rises to less than 50 m depth in July and August as a result of the
baroclinic adjustment of the water structure in the Arabian Sea to the anticyclonic
monsoon circulation. When this circulation becomes very strong, cool water may locally
appear at the sea surface and be taken as upwelling. Although this water is rich in
nutrients, it is also extremely depleted in O2 and may cause adverse biological effects, as
discussed by BANSE (1968).
Two different surface water masses are formed in the northern Indian Ocean, the highsalinity water of the Arabian Sea and the low-salinity water of the Bay of Bengal; these
are caused by the excess of either evaporation or precipitation, which is intensified by the
large runoff into the Bay of Bengal. The low-salinity water of the Bay of Bengal flows
during the NE monsoon south of Ceylon to the west, with one branch continuing westward along 50 N, and the other northwestward along the coast of India. During the SW
monsoon it flows to the SE along the coast of Sumatra where its salinity is further reduced
by the high rainfall in this region. This low-salinity tropical surface water then flows westward in the northern portions of the South Equatorial Current, and can be followed near
10° S as a surface-salinity minimum extending all the way to Africa. The salinity of this
water is kept low by the strong rainfall in the Intertropical Convergence Zone during the
NE monsoon season.
High-salinity surface water is formed by the strong excess of evaporation in the central
K. WYRTKI:
explained by DtlING (1970) as the westernmost of a system of eddies in the Arabian Sea
which is intimately connected with the dynamics of the monsoon gyre. On the basis of
direct current measurements, SWALLOW and BRUCE (1966) have shown that during the SW
monsoon transports in the Somali Current in the upper 200 m increase from 30 megatons
sec- 1 at 3
0
S to more than 50 megatons sec- 1 at 8 0 S. The total geostrophic transport is
about 65 megatons sec-I, demonstrating that most of the high speed flow is concentrated
in the surface layer.
The upwelling along the coast is most intense between SON and 11 0 N, where the
entire warm surface layer is removed and subsurface water with temperatures well
below 20 0 C reaches the sea surface (WARREN, STOMMEL and SWALLOW, 1966).
P04 3 - - P concentrations reach more than 10 ,ug-at 1- 1 and N03 - - N concentrations
more than 10 ,ug-at 1- 1 in contrast to values of 0.2 and 0.5, respectively, in the offshore areas. Water with such high nutrient concentrations is otherwise found at
150 m depth. When the Somali Current leaves the coast near lIoN and turns east, the
cool upwelled water continues to follow for some hundred kilometers. In the north, the
upwelling region is terminated by a flow of warm surface water out of the Gulf of Aden,
forming a strong temperature front. This warm surface water coming from the Gulf of
Aden and flowing east into the central Arabian Sea also separates the Somali upwelling
from the upwelling along the coast of Arabia.
Strong winds also blow parallel to the coast of Arabia east of 55 0 E during the SE
monsoon, and cause upwelling. This upwelling is different from the Somali upwelling, as
no strong current develops parallel to the coast. In volume it may even be stronger than the
Somali upwelling, and also the nutrient enrichment is more intense with maximum values
in excess of 1.5 ,ug-at 1- 1 for P04 3- - P and a larger area affected by higher concentrations
of both P0 4 3 - - P and N0 3 - - N.
During the SW monsoon weak upwelling may also develop under favorable conditions
along some parts of the east coast of India, but does not seem to have very noticeable
effects. Along the west coast of India subsurface water comes very close to the surface, and
the 20
0
C isotherm rises to less than 50 m depth in July and August as a result of the
baroclinic adjustment of the water structure in the Arabian Sea to the anticyclonic
monsoon circulation. When this circulation becomes very strong, cool water may locally
appear at the sea surface and be taken as upwelling. Although this water is rich in
nutrients, it is also extremely depleted in O2 and may cause adverse biological effects, as
discussed by BANSE (1968).
Two different surface water masses are formed in the northern Indian Ocean, the highsalinity water of the Arabian Sea and the low-salinity water of the Bay of Bengal; these
are caused by the excess of either evaporation or precipitation, which is intensified by the
large runoff into the Bay of Bengal. The low-salinity water of the Bay of Bengal flows
during the NE monsoon south of Ceylon to the west, with one branch continuing westward along 50 N, and the other northwestward along the coast of India. During the SW
monsoon it flows to the SE along the coast of Sumatra where its salinity is further reduced
by the high rainfall in this region. This low-salinity tropical surface water then flows westward in the northern portions of the South Equatorial Current, and can be followed near
10° S as a surface-salinity minimum extending all the way to Africa. The salinity of this
water is kept low by the strong rainfall in the Intertropical Convergence Zone during the
NE monsoon season.
High-salinity surface water is formed by the strong excess of evaporation in the central