VARIABILITY
OFNo.
WESTERN
INDIAN2002
OCEAN CURRENTS
Western Indian Ocean J. Mar.
Sci. Vol. 1,
1, pp. 81–90,
81
© 2002 WIOMSA
Variability of Western Indian Ocean Currents
Peter N. Benny
Department of Physical Oceanography, Cochin University of Science & Technology, Cochin, India –682016
E-mail: [email protected]
Key words: western Indian Ocean, currents, circulation, Somali Current, dynamic topography,
variability
Abstract—In the study reported, an attempt was made to understand the intra-annual variability
of the western Indian Ocean circulation by estimating the monthly dynamic topography with
respect to 400db. The major currents in the western Indian Ocean are clearly depicted in the
topography. Among the currents, the Somali Current exhibits strong annual variability. Eddy
circulation is prominent in the northern part of the Somali Current during the southwest monsoon
period. Seasonal variability is also noticed in the North Equatorial Current. Slight spatial and
temporal changes are noticed in the South Equatorial Current and Equatorial Counter Current.
The Equatorial Jet flow occurs in the monsoon transition periods of May and November between
the equator and 3° South.
INTRODUCTION
The western Indian Ocean receives special
attention from oceanographers and meteorologists,
as it exhibits more dramatic seasonal variation than
the rest of the Indian Ocean and is an important
region of air–sea interaction. Intense upwelling
occurs during the south-west monsoon season in
the Somali area (Bruce, 1974; Packard, 1985). As
a result of this upwelling, cold surface water is
brought to the nearshore surface layers, which
spreads over extensive areas of the Arabian Sea.
Thus, the local climate and biological productivity
are much controlled by the oceanic processes.
The existence of a strong surface current during
the southwest monsoon flow parallel to the African
Coast in the northwestern Indian Ocean, close
inshore and towards the northeast has been long
known from the ship reports of Findlay (1866) and
Hoffmann (1886). Based on collection of ship logs,
Schott’s (1935) chart gives a clear picture of the
Somali Current.
During the southwest monsoon, the North
Equatorial Current, which was directed towards
the west during the northeast monsoon, reverses
its directions towards the east. This combines with
the eastward-flowing Equatorial Counter Current
and the whole eastward flow from 7 to 15 °N is
called the monsoon current. One component of the
South Equatorial Current turns to north and
supplies the Somali Current up the east coast of
Africa. The Somali Current is notable for its high
speeds of up to 200 cm/s and has a transport of
about 65 Sv, most of it in the upper 200 m (Pickard
and Emery, 1964). However, the current speed in
western tropical Indian Ocean is of the order of 75
to 100 cm/s (Bahulayan et al., 1997). Tomczak and
Godfrey (1994) have given a detailed account of
currents and circulation.
Several authors (Bruce, 1966; 1968; 1974;
1979; 1985; Leetmaa, 1972; 1982; Reverdin &
Fieux, 1987; Schott et al., 1990; Fischer et al.,
1996; Benny & Mizuno, 2000) have studied the
western Indian Ocean currents. Modelling efforts
were made by Cox (1970; 1979), Anderson et al.
(1976), Luther & O’Brien (1985), Das et al. (1987),
82
P. N. BENNY
Woodberry et al. (1989), Bhahulayen and Shaji
(1996) and Shaji et al. (1997), to simulate the
western Indian Ocean circulation. Satellite
observations of sea surface temperature, altimetery
and scatterometer wind data have been used in
studying the circulation and processes in the
western Indian Ocean (Perigaud & Minister, 1988;
Perigaud & Delecluse, 1989; 1992; 1993).
Considering the influence of the western Indian
Ocean on the climate of the neighbouring region,
efforts were made in the study reported here, to
elucidate the intra-annual variability of the Ocean’s
currents using available oceanographic information.
DATA SETS AND METHODS
The study was confined to the region between the
eastern African coast and 60 °E, 20 °S and 15 °N.
Historical data sets were used to produce the
dynamic topography with reference to 400db. The
available hydrographic observations (BT, XBT,
CTD and STD and Nansen casts) for the region
from NODC (National Oceanographic Data
Centre, Washington, New York), INODC (Indian
National Oceanographic Data Centre, Goa) and
data collected by National Research Institute of
Far Seas Fisheries, Japan were acquired and used
for this analysis.
Ever since the existence of an intimate
relationship between the field of mass and field of
flow was established, dynamic topographies
(geopotential topographies) have been used to
study the circulation of the oceans.
The anomaly of the dynamic depth (∆D) is
given by
∆D = 0 ∫ p δ dP
Where δ is the specific volume anomaly and P
is the pressure in decibars.
The first step involved is the use of in situ
temperature and salinity at observed depths to
compute the specific volume anomaly. For all
integrations the depth in metres was taken to be
numerically equivalent to pressure in decibars.
The temperature-salinity profiles number about
3200 for the study region. Employing the
temperature-salinity profiles alone, it is not
possible to give an adequate description of annual
cycle of dynamic height. Hence, we tried to include
the temperature/depth data also to estimate the
dynamic height employing the correlation between
heat content and dynamic height.
The heat content (H)= 0∫z⌠CpTdz
where ⌠ is the density, Cp is the specific heat
at constant pressure, T is the in situ temperature
and z is depth.
The correlation between the heat content and
dynamic height with reference to 400 m was
determined using the hydrographic data.
Significant correlation was obtained between
dynamic height and heat content for the study
region.
The linear fit equation of correlation is
Y= – 0.251916 + 0.0751593 X
where Y is the dynamic height, X is the heat
content, correlation coefficient = 0.95 and standard
deviation = 0.02.
The temperature profiles aggregate to about
10,000 for the study region and only observations
extending to 400 m depths were considered for this
study. Also, observations with no data for more
than two consecutive standard depths (0, 10, 20,
30, 50, 75, 100, 125, 150, 200, 250, 300 and 400
m) were rejected.
Thus, using the correlation dynamic height was
also estimated from the temperature/depth data and
the monthly dynamic topography maps were
prepared. Figure 1 shows the location of stations
including temperature-salinity as well as the
temperature profiles used for the estimation of
dynamic height (steric height).
RESULTS
The long-term mean monthly dynamic topography
of the western Indian Ocean is presented in Figs
2–13.
January
The long term mean dynamic topography exhibits
strong gradient in steric height in the northern,
southern and eastern parts of the study region
VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS
83
20
10
Latitude
AFRICA
0
-10
-20
35
40
45
50
55
60
Longitude
Fig. 1. Location of stations used for dynamic height estimation
(Fig. 2). The equatorial zone shows dull circulation
where the steric height is less than 1 dynamic
metre. Along the Somali Coast a weak southward
flow is noticed. The Equatorial Counter Current is
weak and is around the equator. The northern part
is marked with counter-rotating eddies between 10
and 15 °N. The strong flow of the South Equatorial
Current is between 5 and 15 °S. Part of the South
Equatorial Current turns south at the Madagascar
Coast. An anticyclonic eddy circulation is evident
between Africa and Madagascar.
February
Slight changes in dynamic height distribution are
visible in this month (Fig. 3). The steric height is
increased at the equatorial zone, 5 °N – 5 °S. The
flow along the Somali Coast is towards south itself.
The eddy circulation is weaker at the northern part
than it is in January. To the south, the South
Equatorial Current is more extended into the higher
latitudes, up to 18 °S. The Equatorial Counter
Current is much stronger and is between 2 and 7
°S. Thus, the dynamic topography depicts that the
whole western Indian Ocean is dynamically active
in this month.
March
The steric height distribution in March (Fig. 4)
shows slight increase in height, but the gradient is
weakened in the northern Indian Ocean compared
to February. An offshore current displaces the
southward flow along the Somali Coast. The
conspicuous feature noticed in this month is the
presence of the North Equatorial Current in
between 0 and 5 °N. Counter-rotating eddies are
present in the northern part. To the south, the South
Equatorial Current and the Equatorial Counter
Current keeps the same structure as in February.
Strong anticyclonic circulation is present between
Africa and Madagascar.
84
P. N. BENNY
15
15
0.75
0.90
0.90
10
0.95
1.00
1.00
10
1.00
0.95
5
5
1.00
0
0.95
1.20
0.95
-5
Latitude
Latitude
0.95
1.20
1.00
0
0.95
-5
0.90
1.00
-10
-10
0.95
1.20
-15
1.20
-15
1.20
1.20
-20
40
1.20
1.10
-20
45
55
50
Longitude
40
60
Fig. 2. Dynamic topography (January)
45
55
50
Longitude
60
Fig. 3. Dynamic topography (February)
15
15
0.90
1.00
0.95
1.00
1.00
10
0.95
1.05
10
1.00
1.05
1.05
5
1.10
1.05
1.05
0
1.00
1.05
-5
0.95
Latitude
Latitude
5
0
1.00
-5
0.95
0.90
0.90 1.00
-10
1.10
1.15
-15
0.90
-10
1.10
1.15
1.10
-15
1.10
1.20
1.00
-20
40
45
50
Longitude
Fig. 4. Dynamic topography (March)
55
60
-20
0.90
40
45
50
Longitude
Fig. 5. Dynamic topography (April)
1.20
55
60
VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS
85
April
July
During April the steric height is further increased
in the northern part of the study region and strong
meridional gradients are developed (Fig. 5). The
North Equatorial Current (0–5 °N) reaches up to
the African Coast and turns southward. No coastal
current is observed off Somali where the offshore
flow prevails. In the south, the Equatorial Counter
Current is more shifted to the lower latitudes
between 2 and 5 °S compared to the month of
March. A slight shift in South Equatorial Current
towards the north is also between 40 and 45 °E.
The anticyclonic eddy is diffused and a clockwise
flow is present in the southwest region from 15–
20 °S.
In July, tremendous changes have taken place in
the northern part of the western Indian Ocean (Fig.
8). Along the Somali Coast a strong northward flow
has developed and branches at 10 °N, one branch
initiating an anticlockwise eddy and the other
continuing its northeastern flow towards the Gulf
region. Strong counter-rotating eddy circulation is
embedded in the northeastern part. Thus, very
complex but strong dynamic conditions prevail in
the western Indian Ocean between 3 and 15 °N. A
narrow band of eastward and westward flow is also
evident around the equator in this month. The
circulation pattern remains the same as in the
month of June in the southern part except at the
southwest end, where an anticyclonic circulation
is noticed instead of the cyclonic pattern of the
earlier months.
May
In May, slight changes are observed in the flow
pattern (Fig. 6). The meridional steric gradients in
the northern part are diminished. A northwardtending coastal current starts in the northern end
of Somalia and it seems the coastal current is part
of an anticyclonic eddy. The conspicuous feature
noticed during this month is that the Equatorial
Counter Current is modified by strong jet-like
eastward flow just below the equator (0–3 °S). The
South Equatorial Current is more widened and
takes a northward turn between 40 and 45 °E. An
anticyclonic eddy circulation is occurring between
Africa and Madagascar.
June
The dynamic topography for the month of June is
marked by the reversal of the Somali Current (Fig.
7). The eastward Equatorial Jet flow observed in
the earlier period is weakened. The South
Equatorial Current is more widened into the lower
latitudes (between 7 and 17 °S) and turns northeast
at the African coast. Near the equator an opposing
flow pattern is noticed, thus the eastward flow in
the southern part and westward flow in the northern
part. An anticlockwise circulation is strengthened
between Africa and Madagascar.
August
The conspicuous change observed during August
is the weakening of the circulation in the northern
region (Fig. 9). The northward Somali Current and
the East Arabian Current are present but much
weakened. As in the previous month the Somali
Current branches at 10 °N. Counter-rotating eddies
are also noticed. The South Equatorial Current and
the Equatorial Counter Current are stronger
between 2 and 15 °S. A cyclonic circulation is
observed at the southwest end. A strong
anticyclonic eddy is noticed between Africa and
Madagascar (10–15 °S).
September
A northward-flowing current is visible in the
northern end off Somalia, but no coastal flow is
observed in the southern part (Fig. 10). Part of
northward flow is turned southward and joins the
anticyclonic eddy circulation, whereas the other
component continues as the East Arabian Current.
The eddy circulation at the northeastern part is
weakened but the one between 5 and 10 °N is still
strong. Little change in circulation is observed in
the south. The Equatorial Counter Current is more
shifted towards the equator. The eddy circulation
is strong between Africa and Madagascar.
86
P. N. BENNY
15
15
0.90
1.00
0.95
0.90
1.05
10
0.95
10
1.05
5
5
1.05
Latitude
Latitude
1.00
0
1.00
-5
0.85
0
0.95
0.90
-5
0.95
-10
1.15
-15
40
1.15
1.10
1.05
-15
1.15
1.05
-20
0.85
-10
1.20
45
55
50
Longitude
0.95
60
0.95
40
45
55
50
Longitude
60
Fig. 7. Dynamic topography (June)
Fig. 6. Dynamic topography (May)
15
-20
15
0.90
0.80
0.75
0.90
1.05
10
1.05
10
0.85
1.00
1.10
5
5
1.00
1.05
0
0.95
0.90
-5
1.05
Latitude
Latitude
0.90
0
1.00
1.00
-5
0.85
0.85
-10
-10
1.15
-15
0.95
1.15
-15
1.10
1.10
1.20
-20
40
45
50
Longitude
Fig. 8. Dynamic topography (July)
55
1.05
1.10
1.15
60
-20
1.15
1.00
40
45
50
Longitude
Fig. 9. Dynamic topography (August)
55
60
VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS
15
0.70
0.95
0.80
0.75
15
0.80
0.85
10
1.00
1.10
5
87
10
1.05
1.10
1.05
5
Latitude
Latitude
0.95
0
1.00
0.95
-5
0.90
1.00
0
0.95
-5
0.90
0.90
-10
-10
1.00
1.15
1.10
1.20
-15
-15
1.15
1.15
1.10
1.00
-20
40
45
50
Longitude
55
60
Fig. 10. Dynamic topography (September)
15
0.80
-20
40
45
50
Longitude
55
Fig. 11. Dynamic topography (October)
1.05
15
0.85
0.95
1.00
0.80
10
1.00
5
1.00
0.90
Latitude
0.95
0.95
-5
0.95
0.95
5
Latitude
1.00
10
0
60
0.85
0
0.95
0.90
-5
0.85
0.95
0.90
0.95
-10
1.20
-15
-10
1.15
1.15
-15
1.10
1.20
-20
40
0.95
1.20
1.10
45
50
Longitude
55
Fig. 12. Dynamic topography (November)
1.20
1.15
1.15
60
-20
1.20
1.05
40
45
50
Longitude
55
Fig. 13. Dynamic topography (December)
60
88
P. N. BENNY
October
DISCUSSION AND CONCLUSION
In October the northeastward-flowing Somali Current
beyond 7 °N turns eastward as it advances (Fig. 11).
Thus, the flow is nearly zonal in the northern region.
The anticyclonic eddy is still stronger and enlarged.
The South Equatorial Current is between 5 and 12
°S, thus its latitudinal extent is reduced. The
Equatorial Counter Current occurs between 0 and 3
°S. The anticyclonic eddy circulation between Africa
and Madagascar is weakened compared to the earlier
months. Southward flow prevails in the southwestern
end.
The annual cycle of currents in the western Indian
Ocean is clearly demonstrated in the long-term
average monthly dynamic topography maps. The
major currents identified are the South Equatorial
Current, North Equatorial Current, Equatorial
Counter Current, Somali Current and East Arabian
Current. The Equatorial Jet is noticed during the
monsoon transition periods (Wyrtki, 1971).
Among the currents the Somali Current shows
strong variability during the annual period.
Southward flow prevails along the Somali coast
during January and February. This is the general
flow pattern in the northwest Indian Ocean during
the northeast monsoon. Numerical simulation of
Shaji et al. (1997) shows that even at 10 m depth
the flow fields are generally towards the south.
Field studies reveal that during winter the current
will turn offshore and merge with the eastwardflowing South Equatorial Counter Current (Düing
et al., 1978).
The analysis indicates that the change in the
circulation pattern off Somali begins in March and
that during this month the meridional gradient is
higher, the coastal flow is weaker and zonal
circulation stronger. Schott et al. (1990) accounted
the inflow into the coast during March–April in
the Somali region. In mid-April the Somali Current
weakens and the East African Coastal Current
advances northward. May is marked by the
northward flow pattern of the Somali Current.
Along the Somali coast the steric heights greatly
increase indicating that the current is attaining
greater strength and the actual change in direction
of the coastal flow is taking place in the form of
an anticyclonic gyre in the Somali Basin in this
month.
The northward-flowing Somali Current is well
developed in June and is stronger in July. Two
anticyclonic eddies are prominent in the Somali
Basin. One between 4 and 8 °N and another
between 9 and 12 °N. This is in agreement with
Bruce’s (1973) observation of two anticyclonic
eddies along 6 and 10 °N. The northward-flowing
Somali Current branches at about 10 °N; one part
flows northeastward supplying to the East Arabian
Current while the other branch joins the
anticyclonic eddy circulation. The Somali Current
November
In November, the dynamic topography shows the
characteristics of the post monsoon period (Fig.
12). The northward flow of the Somali Current
ceases and a weak zonal flow prevails. The
anticyclonic eddy present in the earlier months has
also disappeared. The South Equatorial Current is
more shifted to lower latitudes near the African
Coast. Just below the equator a jet-like flow
appears. The anticyclonic eddy between Africa and
Madagascar is again strengthened and counterrotating eddies have appeared in the southwestern
end.
December
The dynamic topography of December is marked
with the northeast monsoon characteristics (Fig.
13). The flow along the Somali Coast is reversed
towards the south and is strong in the northern part
off Somali. Dull circulation prevails in the zone
between 0 and 10 °N. A complex circulation pattern
that is embedded with eddies has been occurring
in the northern region between 10 and 15 °N.
Another conspicuous change noticed is the
structure of South Equatorial Current, between 7
and 17 °S, which branches at about 45 °E, the major
part continuing towards south and the minor one
flowing north.
VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS
is still strong in August but its southern extent is
reduced. In September, the current is prominent
only in the northern part beyond 5 °N.
One important peculiarity of the Somali
Current is the anticyclonic gyres. The maps clearly
exhibit the generation, growth and decay of these
eddies. Surveys during INDEX program (1975–
1978) indicated that the large ‘Prime Eddy’
described by Bruce (1968) was formed in the
northern Somali basin between approximately 4
and 12 °N. After the onset of the southwest
monsoon in April–May, the current reverses
northward and generally develops two gyres
separated by two tongues of cold upwelled water
(Swallow & Fieux, 1982). This system strengthens
with time and reaches its maximum in summer
(Leetmaa et al. 1982). It usually migrates
northward (Evans & Brown, 1981) before
weakening and collapsing at the end of the
monsoon. Altimeter data show that in the Somali
area the ‘Great Whirl’ migrates northeastward very
rapidly (Perigaud & Minister, 1988).
The Somali Current is further weakened in
October and November, indicating clearly
northeast monsoon features. The coastal flow is
weakened and zonal flow is prominent. Onshore
and offshore flows occur in the southern and
northern part of the Somali Basin. The Somali
Basin has a southward flow during December.
Seasonal variability is also seen for the North
Equatorial Current, which is visible in the late
northeast monsoon in March–April. In April, the
current extends up to the African Coast, further
than in March. The North Equatorial Current is
not prominent in the western Indian Ocean during
the earlier months.
Much spatial variability is found in the South
Equatorial Current. The seasonal changes of the
South Equatorial Current is documented in Wyrtki
(1971) and Rao et al. (1989). It is interesting to
note that the axis of the current is inclined to the
latitude and shifts more to the north off Africa
during the summer monsoon. During winter, the
South Equatorial Current shifts more to the higher
latitudes.
The Equatorial Counter Current is present
south of the Equator between 2 and 5 °S and is
quite strong in the northeast monsoon period
(December–April). Slight spatial changes are
89
evident in its position. The Equatorial Jet (Wyrtki,
1973) occurs in the monsoon transition periods,
especially in May and November. The Equatorial
Jet is very strong in May from 45 °E eastwards
between 0 and 3 °S compared to the winter period.
Thus, western Indian Ocean currents show
spatial and temporal variations. Among the
currents, the Somali Current exhibits more
dramatic seasonal variation than any other current
in the region. The strongest change in seasonal
forcing cycle occurs during the onset of the summer
monsoon. The Somali Current flows northeasterly
and strong during southwest monsoon whereas it
is weak and southward during northeast monsoon.
In the monsoon transitional period the flow is
mainly zonal in the Somali Basin. The directional
change of the Somali Current takes place in a
clockwise direction. Spatial and temporal changes
are also noticed in the North Equatorial Current,
South Equatorial Current and in Equatorial Counter
Current. The Equatorial Jet is evident in May and
November.
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