| dc.description.abstract | The low-level cross Equatorial Jet (EALLJ)
overlies a narrow current of fast moving water,
which also reverses its direction between
summer and winter seasons. This is known as
the Somali Current.
The Somali current is a remarkable
response of the seasonally reversing circulation
in the northern part of the western Indian
Ocean. It flows northwards in summer, but
reverses direction in winter. The Somali
current is closely linked with strong coastal
upwelling near gONe This creates a narrow
strip of low sea surface temperatures off
Somali coast where the temperature may be as
o low as 15 C.
~his study attempts to establish the
main characteristic features of the Somali
current observed throughout the year over the
northwest Indian Ocean. To achieve this
objective we first establish the monthly
mean wind flow patterns, and compute the curl and
divergence of the mean wind stress.
Then, numerically evaluate the streamfunction
using a simple steady-state barotropic model
which is derived from a simplified vorticity
equation. Finally, we evaluate the intensity
of upwelling from the divergent part of the
oceanic circulation by developing a simple
model from a simplified divergence equation.
The wind data taken near the coast of
East Africa by commercial ships for a period
of five years were used in the study. This
wind observation was on daily basis and
located along the ship routes.
The monthly mean features of the Somali
current are presented and discussed under the
four well-known seasons over our region of
interest: the winter monsoon season (December-
February), the pre-summer monsoon season
(March-April), the summer monsoon season
(May-September) and the pre-winter monsoon
(October-November).
During winter, the northeasterly flow
along the East African coast is evident, with
increasing anticyclonic curvature south of
the equator. The air current intensifies and
reaches maximum intensity in January.
Negative vorticity is found to the right and
positive vorticity to the left of the mean
speed maximum in the northeasterly flow. The
northeasterly flow is a diffluent one with
maximum divergence of order of about
1.0 x 10-5 S-1 . Our simple barotropic model
clearly reproduced the winter Somali Current,
which attained its peak in January. A zone of
downwelling is observed to occur off the coast
of Somalia and upwelling south of the equator
during winter season.
Pre-summer monsoon is a transition period
from the winter season to summer monsoon
season. The transition between winter and
summer is gradual. It is not an abrupt and
sudden reversal in the mean wind direction.
This season·is characterised by the retreat of
the winter monsoon and the establishment of
the summer monsoon current in April. Small
zones of convergence are observed along the
coast of East Africa and also in the equatorial
region by April, depicting a well-defined
convergence zone (ITCZ) between northerly and
southerly air current. During the period, the
ocean circulation is weak. However, the
winter Somali Current is seen to prevail still
in March, but it is replaced by summer Somali
Current in April, which is well defined along
o 0 the coast between 4 Sand 8 N. By April,
upwelling off the coast of East Africa is
evident in the northern hemisphere.
In summer, strong cross equatorial f Low
originating from the south (in the vicinity
of Malagasy) in form of southeasterlysouthwesterly
flow, intensifies progressively
and reaches maximum intensity in July (- 14 m/s).
The summer monsoon flow is, like the winter
monsoon flow, highly diffluent north of the
equator, and is characterised by positive
vorticity to the left and negative vorticity to
the right of the mean speed maximum. The
sUIT@er Somali Current is well - defined along
the coast in summer. The current intensifies
as from May and reaches its peak in July, when
speeds in excess of 3.0 mls have been computed.
Ovr model reproduces the southern gyre found
in the equatorial region (between SOS and SON
approximately) . But, it doesn't reproduce the
northern gyre ('great whirl'), which is
believed to exist during the season between
o 0 5 Nand 12 N (near Socotra). It is thought
to be caused by the westward propagation of
baroclinic modes towards the coast. Therefore,
the absence of this northern (anticyclonic)
gyre, in our model results, we feel, provides an
indirect evidence suggesting the propagation
of the baroclinic modes. The zone of upwelling
in the northern hemisphere is observed to
intensify progressively as from May and reaches
its peak in July. However,a zone of downwelling
is also observed to occur off the south coast
(southern hemisphere) throughout the season.
The pre-winter monsoon season, like the
pre-summer monsoon, is gradual. The transition
between the summer and winter season takes
place in November. Confluence is observed
along the coast in November due to the onshore
flow and diffluence elsewhere in the interior
of the ocean. By November, the winter Somali
Current is clearly evident along the coast,
but is weak. Upwelling observed during the
summer season is replaced by downwelling in the
northern hemisphere during the pre-winter se | en |
