Journal of Marine Research, 57, 885–907, 1999
Near-surface circulation and kinetic energy in the tropical
Indian Ocean derived from Lagrangian drifters
by S. S. C. Shenoi1, P. K. Saji1 and A. M. Almeida1
ABSTRACT
Trajectories of 412 satellite-tracked drifting buoys deployed in the tropical Indian Ocean have
been analyzed to document the surface circulation and kinetic energy Ž eld. Only drifters drogued at
15 m depth and having drag area ratio greater than 35 are used to estimate current velocities. Unlike
in earlier studies, the widening of the Equatorial Jet in the eastern equatorial Indian Ocean and the
westward  ow at the equator during July–August are apparent in the present data set. The comparison
of drifter data with the seasonal mean dynamic topography (0/1000 db) shows that the surface
circulation pattern inferred from dynamic topography does not always represent the surface currents
in the Indian Ocean. Both compare well for the South Equatorial Current, the Equatorial Counter
Current, and the southwestward current along the IndonesianIslands; they differ in the Bay of Bengal
during the southwest monsoon, but are similar during the northeast monsoon. Maps of mean and
eddy kinetic energy show maxima in the regions of western boundary currents and equatorial
currents and minima in the Arabian Sea, the Bay of Bengal, and south of 20°S.
1. Introduction
Surface circulation in the Indian Ocean is unique because of its response to the annually
reversing monsoon winds because of which the major currents in the Indian Ocean also
undergo variations on semiannual and annual time scales. The seasonality is more
prominent in the north than in the south. The major surface currents in the Indian Ocean are
summarized in the schematic in Figure 1. During the northern winter (i.e., during the
northeast monsoon), the surface current systems resemble the general circulation patterns
in the PaciŽ c and Atlantic oceans; the South Equatorial Current (SEC), the Equatorial
Counter Current (ECC), the North Equatorial Current (Northeast Monsoon Current (NMC)
hereafter), and the associated boundary currents are seen during this season. During the
northern summer (i.e., during the southwest monsoon), the surface currents do not
resemble the patterns seen in the other two oceans; the eastward Southwest Monsoon
Current (SMC) replaces the westward NMC and a northward  ow, the Somali Current
(SC), replaces the southward  ow along the coast of Somalia. At the equator, the scenario
is further complicated with the appearance of an eastward Equatorial Jet (EJ) during the
transition periods, April–May and November–December (Wyrtki, 1973).
1. Department of Physical Oceanography, National Institute of Oceanography, Dona Paula, Goa 403 004,
India. email: [email protected]
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Figure 1. Schematic of major surface currents in the Indian Ocean during (a) the northeast monsoon
and (b) the southwest monsoon. The major currents depicted are: South Equatorial Current (SEC),
Northeast Monsoon Current (NMC), Equatorial Counter Current (ECC), Equatorial Jet (EJ), East
African Coastal Current (EACC), Somali Current (SC), Southwest Monsoon Current (SMC), West
India Coastal Current (WICC), East India Coastal Current (EICC) and East Madagascar Current
(EMC). The EJ, though depicted in the schematic for winter, does not appear either during summer
or winter monsoon season; it appears during the transition period in April–May and November–
December. The thickness of the curve represents the relative magnitude of the current.
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Just as the open ocean currents, the boundary currents in this region also undergo
seasonal reversals (Shetye and Gouveia, 1998). The West India Coastal Current (WICC)
 ows poleward during winter (November–February) and  ows equatorward during summer (June–September). The East India Coastal Current (EICC)  ows poleward during
February–April and equatorward during November–December; during the southwest
monsoon, a weak poleward EICC appears in the south and an equatorward EICC appears in
the north.
The satellite-tracked drifting buoys are useful in tracking these space-time dependent
surface currents. Earlier, Reverdin et al. (1983), Shetye and Michael (1988), and Molinari
et al. (1990) used drifting buoy data to study the currents in the Indian Ocean. Molinari et
al. concluded that, in general, the mean annual surface circulation determined from buoys
is similar to that determined from the ship drift reports of Cutler and Swallow (1984) in the
regions where buoy measurements are numerous.
In this paper, we present a compilation of data on surface currents in the Indian Ocean
from 412 buoys and discuss the distributions of kinetic energy due to the mean  ow (EM )
and the temporally varying part (EE ). The temporally varying part, the eddy kinetic energy,
represents the variance of the velocity Ž eld due to  uctuations spanning a broad range of
time and space scales. The distribution of eddy kinetic energy (EE ) helps in identifying the
regions with the most energetic temporally varying  uctuations. Wyrtki et al. (1976)
presented the kinetic energy distributions over the world oceans using ship-drift data; since
then no similar estimates have been made for the tropical Indian Ocean. The kinetic energy
distributions presented here, therefore, reŽ ne those presented by Wyrtki et al. (1976).
Traditionally, the  ow Ž elds inferred from the distribution of density Ž elds are used to
represent the surface circulation patterns. In this paper, we compare the surface  ows
derived from buoys and dynamic height Ž elds. Not surprisingly, the comparison suggests
that the surface circulation pattern inferred from the dynamic heights need not always
represent the surface currents in the Indian Ocean.
2. The data set
Satellite tracked Lagrangian drifting buoys have become an integral part of surface
current measurement. Over the last two decades, several drifters have been deployed in the
Indian Ocean for the measurement of surface currents. For this study, we have assembled a
data set by accumulating data from 412 drifting buoys from three sources (archives of
National Institute of Oceanography, NOAA/AOML/DAC, and R. Molinari). The data span
a period of 22 years from 1976 to August 1998, with no data during 1978. The archived
buoy data consist of an unevenly spaced time series of position Ž xes (often 1 to 8
observations per day per buoy) determined by the CLS Argos system with an accuracy of
approximately 6 300 m (Anonymous, 1990). Following Hansen and Poulain (1996) the
archived positions were Ž rst edited to remove the dubious positions. The trajectories of the
buoys (north of 25°S) plotted from all the buoys show the data availability and their
distribution over the tropical Indian Ocean (Fig. 2). The number of observations in the
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Figure 2. Trajectories of 412 satellite-tracked drifting buoys deployed from 1976 through August
1998 during various observationalprograms. No data are available for 1978.
equatorial and southern Indian Ocean are higher than in the Arabian Sea and the Bay of
Bengal because the buoys there have a greater chance of survival. In a smaller basin like
the Arabian Sea or the Bay of Bengal, the operational period of the buoy is reduced
drastically due to grounding.
Most of the buoys used in this study had a similar design, a spherical surface  otation
unit, a tether, and a holey sock drogue as sea anchor centered at 15 m below the surface.
The rest had a different design, an inverted cone attached with window shade drogue
centered at 30 m or 10 m below the water level. The water-following capability of any
mixed layer drifter depends critically upon the drag area ratio, R, which is the ratio of the
drag of the drogue area to the drag of the submerged  oat and tether. Niller and Paduan
(1995) show that the FGGE type drifters, with window shade drogues of drag area ratios
10–12, have signiŽ cant differences in water-following capabilities in comparison with that
of the later designs. The most important design parameter that affects the water-following
capability of a buoy is the drag area ratio and the most important environmental factor that
affects the slip is the wind (Niiler et al., 1995). This implies that combining the buoys
having different water-following capabilities will introduce errors in the estimation of
surface currents. Hence, to construct a data set from drifters having similar water following
capabilities, we avoided the drifters with R , 35. All buoys deployed prior to 1985 and a
few buoys deployed after this period did not satisfy this condition; for the remaining 321
buoys, R ranged between 35 and 42. The removal of the buoys with R , 35 affected the
coverage slightly; this effect was mainly in the western equatorial Indian Ocean.
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889
All buoys have a drogue attached when they are deployed. A drogue ON/OFF sensor
(submergence sensor) Ž tted to the buoy monitors its presence. Occasionally, the drogue
gets detached from the buoy. Its loss affects the velocity measurement because it alters (i)
the drag area ratio and hence the drift characteristics, (ii) the level at which the velocity is
determined, and (iii) the effects of local winds and waves on the buoy. If concurrent wind
data are available, empirical formulations (for example Poulain et al., 1996) can be used to
correct for the effect of winds on undrogued data. The absence of concurrent wind data
prevented us from applying this correction. Instead, to avoid using the affected data, we
excluded the undrogued data by examining the submergence values reported by the buoy.
The removal of undrogued data resulted in a further data loss of 13%. When the undrogued
data were retained, the number of observations increased in some bins. Even though the
monthly mean vectors estimated in a 2° 3 2° bin did not differ signiŽ cantly from the results
obtained using only drogued drifters, the standard errors on monthly means were generally
higher.
By attaching current meters to the drogue Niiler et al. (1995) measured the slippage of
drogues through water caused by winds. Based on the measurements, they suggested the
model
Us 5
a
R
Uw
to correct for the wind slippage vector Us (cm s2 1 ) in the direction of wind velocity vector
Uw (m s2 1 ). The constant a 5 7 6 0.7 was obtained from a least-squares Ž t of measured
slip and wind speed. R is the drag area ratio of the drifter involved. In the tropical Indian
Ocean, generally, the wind speeds varied between 8 m s2 1 and 15 m s2 1 during the summer
monsoon when the winds are strong. Assuming a wind speed of 15 m s2 1, the wind
slippage estimated for a buoy of R 5 42 is 2.5 cm s2 1. Thus the error introduced by the
wind-induced slippage in the individual surface velocity vector will not exceed 2.5 cm s2 1.
The buoy positions, retained after the above quality checks, were then interpolated to
generate four equally-spaced values per day and to compute the drifter speed and direction.
Finally, a Ž ve-day Gaussian Ž lter was applied to the data to reduce the effects of very
high-frequency motions.
3. Drifter trajectories
The rapidly changing surface currents in the Indian Ocean are best described by
following individual buoy trajectories. Sketched in Figure 3, for example are, the currents a
buoy encountered during its one-year lifetime. The buoy was released near the equator at
50°E on 15 October 1981 (Fig. 3a). On its release, the buoy drifted slowly toward the east
and then started moving faster under the in uence of the winter EJ that developed in
November. In November, under the in uence of the EJ, the buoy traveled about 4000 km
over 30 days. By mid-December when the EJ collapsed, the drifter was at 82°E. At this
point the drifter turned westward and drifted along the equator till 68°E. In late January,
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Figure 3. Trajectories of buoys with ARGOS ID (a) 1885 and (b) 21855. The date of deployment is
indicated at the beginning of the trajectory (hollow circle). Filled circles indicate the beginning of a
month.
when it reached near 70°E, the buoy started drifting northwestward because of the NMC,
which developed north of the equator during January. In March, the buoy drifted along the
coast of Africa with the northward Somali Current, which sets in in early March, much
before the onset of the southwest monsoon. The buoy followed the northward SC till
mid-May and later drifted toward the coast of Arabia. In mid-May, the buoy turned
eastward under the in uence of the SMC that began to build up during May. After drifting
eastward with the SMC, for two and half months, the buoy shifted into the equatorward
WICC off the Indian west coast in early August. The WICC moved the buoy farther south
(south of Sri Lanka) till it re-joined the eastward SMC. When the buoy reached near 87°E
in September, it was pushed into the Bay of Bengal by a branch of the SMC that turns north
near 87°E; later the buoy was caught in a clockwise eddy till it joined the EICC that  owed
equatorward in November. The EICC carried the buoy around Sri Lanka and the NMC
brought it back into the Arabian Sea in December.
Similarly, the trajectory shown in Figure 3b depicts the history of a buoy deployed in the
Arabian Sea. This buoy, released on 12 May 1995 in the Arabian Sea at around 12°N,
reached the SEC (at 15°S) within a year. Initially, the buoy moved southeast under the
in uence of the SMC, joining the eastward EJ in November, and Ž nally drifted into the
SEC after traveling along the coast of the Indonesian island of Sumatra. After it entered the
SEC, the buoy moved westward till it stopped transmitting.
The life-history of these two buoys shows how variable the circulation in the Indian
Ocean is; most current systems, especially those in the north and equatorial Indian Ocean,
reverse with season. The above description captures this variation in a Lagrangian frame
work. In the following section, we describe the variability in a Eulerian frame work.
Before doing this, however, we examine the geographical distribution of surface current
velocities by segregating the drifter speeds into four categories: low speeds (, 10 cm s2 1 ),
medium speeds (10 to 50 cm s2 1 ), high speeds (50 to 100 cm s2 1 ) and very high speeds
(. 100 cm s2 1 ). Of the 2,86,965 six-hourly velocity observations from 319 buoys, the low
speeds accounted for 18% of the observations with no geographic preference. The medium
velocities, which accounted for 74% of the observations, also showed no preference for
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Shenoi et al.: Indian Ocean circulation
891
speciŽ c geographic location. However, the high speeds which accounted for about 7% of
the total observations were concentrated in the equatorial and western boundary regions.
Similarly, most of the observations with speeds greater than 100 cm s2 1 (about 1%) were
observed only in the equatorial region. From the trajectories shown in Figure 2 and from
the above, it appears that majority (about 74%) of the mesoscale motions in the tropical
Indian Ocean have speeds in the range of 10–50 cm s2 1.
4. Surface current vectors
To describe the surface circulation pattern in a Eulerian frame work, the distribution of
mean velocity vectors on a 2° 3 2° spatial grid and monthly time scale are prepared from
319 buoys. Due to the reasons discussed in Section 2, only the position Ž xes from drogued
buoys having R . 35 are used for this analysis. The grids containing less than 12
observations in a month are also excluded. The resulting mean Ž eld was then spatially
smoothed using a binomial smoother. The velocity variance within the bins include spatial
variability at scales smaller than 200 km and temporal variability with time scales smaller
than 30 days. From the trajectory plot (Fig. 2) it is obvious that the horizontal scales vary
from a few tens of kilometers to basin-scale motions. Further, as a drawback of the
nonuniform Lagrangian sampling, signiŽ cant aliasing can occur between the spatial and
temporal variability. Where eddies are strong and the mean is weak, the mean vectors may
not be very reliable. Given the nonuniformly distributed observations in the 2° 3 2° bins
we conclude that the present data set is not well suited for the study of variability in the
Indian Ocean on time scales less than a month. However, as it will become apparent from
the following, they can be used to describe the major currents in the Indian Ocean on
seasonal time scales. Figure 4 shows the distribution of monthly mean velocity vectors;
they include many more observations than those presented by Molinari et al. (1990).
Figure 5 shows the monthly mean vectors in a 2° 3 2° grid and their standard deviations
for two selected months, January and July; a majority of the vectors show low standard
deviation. In general, the standard deviations are high for the grids having a smaller
number of observations. In such cases, more observations per box would give a more stable
vector.
A general discussion on the surface currents in the Indian Ocean is available in Molinari
et al. (1990). They characterize the circulation pattern in the tropical Indian Ocean as two
primary gyres extending across the basin, one on either side of the equator, the southern
gyre rotating clockwise and the northern gyre rotating anti-clockwise. Though the
additional data provides a better picture of the surface circulation pattern in the Indian
Ocean, in general, the circulation pattern that emerges from this analysis is not very
different from that discussed by Molinari et al.
a. Southern gyre
The clockwise rotating southern tropical gyre appears as a permanent feature of the
surface circulation in the Indian Ocean (Fig. 4). The SEC, observed as a broad westward
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Figure 4. Monthly mean surface current vectors based on a 2°-square analysis of surface drifting
buoys. Missing vectors indicate data voids.
 ow between 8°S and 16°S, acts as the southern boundary of this gyre. Near the
northeastern tip of Madagascar, a major portion of this current  ows westward, while the
other portion  ows south along the east coast of Madagascar as the East Madagascar
Current (EMC). The branch that  ows past the northern tip of Madagascar turns northward
and feeds the boundary current along east Africa, the EACC. Later, the EACC turns
eastward and joins the ECC (axis around 3 to 5°S), which serves as the northern boundary
of the southern tropical gyre. The surface currents in the region between the SEC and ECC
are noisy and are often Ž lled with recirculation cells. One such recirculation cell can be
seen in June between 65°E–75°E and another can be seen in February centered at 10°S and
60°E. Woodbery et al. (1989) note that the shear zone between SEC and ECC is Ž lled with
westward propagating Rossby waves that are re ected by the Seychelles-Mauritius Ridge
along 60°E. It is likely that the noisy nature of the currents and the recirculation cells noted
above are a manifestation of the westward propagating Rossby waves and their re ections
from the Seychelles-Mauritius Ridge. To the south of the SEC, only the northern portion of
the southwestward  ow due to the anticyclonic subtropical gyre is visible.
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893
Figure 4.–(Continued)
During April–May and again during November, an eastward EJ (with speeds higher than
50 cm s2 1 ) develops at the equator. At that time, the ECC becomes indistinguishable from
the EJ; the EJ serves as the northern boundary of the southern tropical gyre. The eastward
jet, forced by the near-equatorial westerlies is stronger in the east than in the west. As it
accelerates from west to east, the width of the jet also increases from about 4° at 70°E to
about 7° at 90°E. At around 90°E, the apparent width of the jet increases because of the
veering off of the vectors; the vectors in the northern side of the jet veer toward the north
and the vectors on the southern side of the jet veer toward the south.
In July–August, no well-organized eastward  ow is observed along the equator to act as
the northern boundary of the southern gyre (Fig. 4g–h). The vectors during this period
suggest a narrow westward  ow, stronger in August to the west of 80°E. There is no
indication of this westward  ow near the equator in the ship-drifts of Cutler and Swallow
(1984); the model runs of McCreary et al. (1993), however, report a westward  ow in the
eastern equatorial region during August.
In the east, a southeastward current that  ows along the Indonesian coast acts as the
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eastern boundary of the southern gyre; it is stronger when the EJ exists and is weaker
during December–January.
b. Northern gyre
During January to March a gyre is also seen on the northern side of the equator, but it is
not as well deŽ ned as the southern gyre (Fig. 4a–c). The westward NMC (axis around
5°N), which forms during December and peaks in February (30–40 cm s2 1 ), acts as the
northern boundary of this gyre till April. The vectors in January clearly show the turning of
this  ow toward the south near the Somali coast. The southward  ow (southward SC)
meets the northward EACC at about 5°S and the two  ow together into the ECC. In April,
an anticyclonic eddy appears near the Somali coast at 5°N; it is similar to the Great Whirl
described by Duing et al. (1980). Simultaneously, the northward SC also gets established
along the coast of Somalia. In April, the ECC together with the eastward EJ acts as the
southern boundary of the northern tropical gyre. On the eastern side, except during
February, very few buoys are available to document the  ows.
With the arrival of southwest monsoon winds over the North Indian Ocean in June, the
northern gyre disintegrates rapidly. The NMC vanishes and is replaced by the eastward
SMC that persists till October. Associated with the SMC, southeastward  ows develop all
over the Arabian Sea, often extending to the equator. In September, a part of the SMC
enters the Bay of Bengal while the other branch continues to  ow eastward. On the western
side, the SC strengthens further and continues to  ow northward.
The available observations along the west coast of India show a poleward current during
January–March and an equatorward current during April–September. In the interior of the
Arabian Sea the  ows, which are similar to those reported in Cutler and Swallow (1984),
are westward during December–February and eastward during March–April. During
June–September they intensify,  ow toward the southeast and, feed the SMC. Along the
coast of Arabia the  ow is southward during December–January and is northward during
May–September.
Very few observations are available in the coastal regions of the Bay of Bengal. The
available observations show a clockwise gyre during January–May and an anticlockwise
gyre during November–December. The observations during the rest of the year do not
suggest such gyres.
In summary, the surface current vectors derived from a compilation of the surface
drifting buoy data appear to be adequate to depict the major currents in the Indian Ocean on
a seasonal time scale. It is obvious that the surface currents in the tropical Indian Ocean are
highly seasonal and have strong annual and semi-annual periodicities.
c. Annual and semi-annual harmonics
To quantify the annual and semi-annual signals of surface circulation, the monthly mean
data were subjected to harmonic analysis using a least-square Ž t. The gaps in buoy data
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Shenoi et al.: Indian Ocean circulation
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Figure 5. Monthly mean surface current vectors with standard deviation ellipses (a) for January and
(b) for July.
time series (mostly a gap of one month) were not Ž lled as the scheme adopted can handle
the data gaps.
Figure 6 shows the amplitude and phase of the annual and semi-annual harmonic of the
zonal component. The harmonics of the meridional component are not shown as the signals
are low except near the boundaries. The annual signal in the zonal component is larger
north of 10°S and is lower south of 10°S. The SEC exhibits a clear annual signal with a
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Journal of Marine Research
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Figure 6. Amplitude and phase of the (a) annual and (b) semi-annual harmonic of zonal velocity
component computed from drifting buoys. The phase, measured clockwise from north, is zero on 1
January (see inset). For the annual harmonic 360° covers one year. For semi-annual harmonic 360°
covers six months (1 January to 30 June/1 July to 31 December). The length of the stick is
proportional to the amplitude.
peak of about 20 cm s2 1 in April. In the western Arabian Sea, the annual signal peaks
during July–August. At the equator, in the east, the annual signal peaks in July and in the
west it peaks in August. In general, the patterns are similar to those presented by Molinari
et al. (1990) except for the clear annual signal that emerges for the SEC from the present
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Shenoi et al.: Indian Ocean circulation
897
analysis. Woodberry et al. (1989) note that the seasonal cycle in the southern hemisphere
trades generates an annual Rossby wave in the ocean, which manifests as an annual cycle
in the SEC. The present analysis clearly shows this annual signal embedded in the SEC. It
also shows an appreciable annual signal (10 to 30 cm s2 1 ) for the surface currents in the
Arabian Sea and the Bay of Bengal, which is not seen in Molinari et al. (1990) perhaps due
to poor data coverage.
The semi-annual harmonic shows a maximum at the equator (Fig. 6b), the peaks
coinciding with the occurrence of an equatorial jet that appears in April–May and in
November. The semi-annual signals are also appreciable in the western Arabian Sea and
southern Bay of Bengal, but they are weaker than the annual signals. For semi-annual
harmonics there is general agreement with the results presented by Molinari et al. (1990).
However, there are differences in the phases reported in the southern Bay of Bengal and in
the regions south of Sri Lanka and the southern tip of India. For these regions, the present
results compare well with harmonics estimated from ship drifts by Molinari et al. (1990).
5. Kinetic energy of the  ow Ž eld
a. Kinetic energy of mean  ow
The kinetic energy of the  ow Ž eld was estimated from the surface current vectors
described above. The distribution of the kinetic energy of the mean surface circulation is
shown in Figure 7a. Assuming that there are a total of N velocity measurements from all
drifters in a given box, the vector mean velocities were computed as
ui j 5
vi j 5
o
1
N
N k5
1
o
1
ui j
N
N k5
1
vi j.
The mean kinetic energy (EM ), or the kinetic energy of the mean  ow, is deŽ ned as
EMi j 5
u i j2 1
v ij 2
2
.
The computed EM Ž eld was smoothed using a three-point two-dimensional binomial
smoother. Given the uneven distribution of velocity vectors (Fig. 4), the distribution of EM
presented here is subject to large uncertainties. However, since the present data set is the
most extensive of this type ever assembled, it provides one of the best estimates of these
Ž elds currently available. In general, the highest values of EM are associated with strong
persistent  ows, the meridional  ows in the western basin, and the zonal SEC. A ridge of
EM (. 200 cm2 s2 2 ) coincides with the region of the SEC. The maximum value of
1000 cm2 s2 2 observed in the Somali Basin is associated with the strong SC that exceeds
100 cm s2 1 during May–September. The values of EM are also high (greater than
100 cm2 s2 2 ) along the equator, where the EJ appears twice an year. To the west of the SEC
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Figure 7. Distribution of kinetic energy per unit mass based on a 2°-square analysis of surface
velocity Ž eld. (a) The kinetic energy of the mean  ow and (b) the eddy kinetic energy. A Ž ve point
binomial smoothing was done prior to contouring.
ridge, a bulge of high EM (400 cm2 s2 2 ) extends eastward (off East Africa) along 10°S;
associated with the strengthening of the SEC when it  ows past the northern tip of
Madagascar, where the buoys moved with speeds . 50 cm s2 1. South of 20°S, the EM
values did not exceed 100 cm2 s2 2. In the Arabian Sea and also in the Bay of Bengal, EM
did not exceed 100 cm2 s2 2; the lowest values of EM are observed in mid-Arabian Sea and
mid-Bay-of-Bengal.
The EM estimates have also been compared with the global maps of EM presented by
Wyrtki et al. (1976) using ship drifts. There is qualitative agreement between the two
distributions. Both report maxima in the western basin and minima in the Arabian Sea and
Shenoi et al.: Indian Ocean circulation
1999]
899
the Bay of Bengal. The ridges of high EM in the equatorial and the SEC regions and the
drop in EM to the south of 20°S are well seen in both the analysis. There are, however, large
quantitative differences between the two distributions. The buoy-derived EM are at least
twice those derived from ship drifts. For example, the buoy-derived EM in the equatorial
region are in the range 100–200 cm2 s2 2 and those values derived from ship drifts are in the
range 50–100 cm2 s2 2. Similarly, in the western basin, the ship-drift-derived EM do not
exceed 500 cm2 s2 2, whereas the drifter derived EM reach 1000 cm2 s2 2. In the Arabian Sea
and the Bay of Bengal also, the drifting-buoy-derived EM are twice the ship-drift-derived
EM. For the southern hemisphere, Patterson (1985) also reports a qualitative agreement
between the EM derived from drifting buoys and those derived from the ship drifts by
Wyrtki et al. (1976). Quantitatively, however, his buoy-derived values of EM too were 2–4
times larger than those of Wyrtki et al. Such differences in the magnitude of EM are reported
to be due to the coarse spatial resolution (5° grid in Wyrtki et al.) and the errors inherent in
the ship-drift measurements. The coarse resolution tends to smooth the curves in the path
of the mean  ow, which leads to systematically lower values of EM (Patterson, 1985). The
way the ship drift observations are made, each ship-drift vector represents a value averaged
over a period of 24 hours, equivalent to a distance of about 400 km. Hence, the ship-drift
estimates assigned to a 5° box contain information from a larger box extending up to
200 km beyond the boundary of the 5° box. This makes the effective averaging box size as
large as 9° instead 5° (Patterson, 1985). Conversely, due to the frequent observations of
buoy movements, the buoy drifts contain no information beyond the boxes selected for
averaging. Hence, it is likely that the EM values estimated from ship drifts are substantially
lower.
b. Eddy kinetic energy
The distribution of eddy kinetic energy (EE ) due to the perturbations of the surface
velocity about the mean  ow is presented in Figure 7b. EE per unit mass was calculated as
EEi j 5
1
2N
3o
N
k5 1
(uki j 2
ui j ) 1
2
o
N
k5 1
(vki j 2
vi j ) 2
4
where the sum is computed over all velocity measurements in a box. EE (Fig. 7b) is much
higher than EM (Fig. 7a), indicating that most of the kinetic energy of the surface  ows are
in the eddy Ž eld. EE contains a part of the low frequency oscillation as well as a part of the
seasonal cycle. A harmonic analysis of the EE Ž eld shows that the contribution of the
average seasonal cycle is as high as 75%. The annual harmonic accounts for about 40%, the
semi-annual cycle for about 25%, and the quarter annual cycle for about 10%; the balance
is due to shorter time scales and interannual variations. EE is highest in the western
boundary and equatorial currents. Qualitatively, both EM and EE show similar distributions
with maxima in the western basin and minima in the Arabian Sea, in the Bay of Bengal, and
south of 20°S.
As with EM, a comparison of the EE estimated here and those estimated by Wyrtki et al.
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(1976) shows that they agree qualitatively, but not quantitatively. Both show highest eddy
energy in the western basin and lowest eddy energy in the Arabian Sea and in the Bay of
Bengal.Again, both estimates show high EE at the equator. One quantitative difference is in
the magnitude of EE at the equator, where the drifting buoys show values greater than
1200 cm2 s2 2. Another difference is in the magnitude of EE in the Arabian Sea and the Bay
of Bengal, where Wyrtki et al. (1976) report 600 cm2 s2 2 and the drifting buoy estimates
are around 200 cm2 s2 2. On an average, the estimates from drifting buoys and ship-drifts
made by Wyrtki et al. (1976) differ by about 200 cm2 s2 2. For example, the 1000 cm2 s2 2
contour that connects the tip of Africa and Sri Lanka in the ship-drift estimate corresponds
to the 800 cm2 s2 2 contour in the drifting-buoy estimate; similarly, the 600 cm2 s2 2 contour
in the ship-drift estimate corresponds to the 400 cm2 s2 2 contour in the drifting-buoy
estimate. This difference of approximately 200 cm2 s2 2 was also noted by Richardson
(1983) in the western North Atlantic. He attributed it to an error of about 20 cm s2 1 in the
ship-drifts, which translates to an error of 200 cm2 s2 2 in eddy kinetic energy.
6. Comparison of drifter trajectories and dynamic topography
Traditionally, the  ow Ž eld inferred from dynamic topography is used to represent the
surface circulation. In this section we examine how far the dynamic topography Ž elds are
useful in representing the surface  ows in the Indian Ocean. For want of monthly
hydrographic data to compute dynamic topography in the Indian Ocean, the seasonal data
compiled by Levitus et al. (1994) is used. Hence, it is important to note that the
comparisons made are valid only for a season and lower periods are not resolved. A
qualitative comparison between seasonal dynamic topography (0/1000 db) and seasonal
mean surface drift vectors is shown in Figure 8. The computation of geostrophic velocities
is avoided as they are sensitive to the selection of reference level.
The comparisons show that the representation of surface  ows with dynamic topography
are not always correct. The role of the geostrophic  ows in representing the surface  ows
varies both geographically and seasonally. For example, the drifter vectors and isolines of
dynamic heights are in good agreement in the regions of SEC, ECC and the southward
current along the coast of Indonesia during January–March (Fig. 8a). In the NMC, they
agree to the east of 70°E and disagree to the west, where the surface current vectors cross
the dynamic height contours. The anticyclonic gyre in the dynamic height Ž eld in the Bay
of Bengal during January–March is also seen in the buoy-derived vectors. The surface
current vectors in the southeastern Arabian Sea also agree with the geostrophic  ows,
though they are somewhat different in the western regions. Similarly, the drifter vectors
and isolines of dynamic topography for July–September (Fig. 8b) agree well in the regions
of SEC and the southeastward current along the Indonesian Islands. For the SMC, they
agree well in the Arabian Sea, but cross the dynamic height contours east of 75°E. The drift
vectors in the northeastward SC also do not appear to follow the geostrophic Ž eld closely.
In the Bay of Bengal also the surface current vectors do not follow the dynamic height
contours during July–September.
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901
Figure 8. Seasonal mean surface current vectors realized from satellite-tracked drifters superimposed on the seasonal mean dynamic topography (0/1000 db) from the climatology of Levitus et
al. (1994). (a) January–February–March and (b) July–August–September.
The vectors derived from the drifters represent the total near-surface  ow comprising the
geostrophic currents, the Ekman currents, and the Stoke drift. Hence, a good agreement
between the drifter-vectors and dynamic heights is possible only when the geostrophic
currents dominate the surface  ows.
In the absence of concurrent wind data, an estimate of the contribution from the Ekman
drifts is not possible. However, for a qualitative assessment, an estimate of the Ekman drift
was made based on the seasonal mean winds, in 2.5° 3 2.5° squares, available from the
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NCEP/NCAR reanalysis program (Kalnay et al., 1996). Using the NCEP/NCAR winds the
Ekman surface  ow, VE, was computed following Pond and Pickard (1986) as
VE 5
t 0r
2 1
w (A *
f * )2
0.5
,
where t 0 5 r aCDWW is the surface wind stress, r w the density of water, A the vertical eddy
viscosity coefficient, f the Coriolis parameter and W the wind speed at 10 m. For
computing VE, we used r a 5 1.25 kg m2 3, r w 5 103 kg m2 3, A 5 102 2 m2 3 s2 1, and CD 5
2 3 102 3. The direction of Ekman surface current vector was at a 45° angle to the right/left
of the surface wind vector in the northern/southern hemisphere. The selection of the values
for the constants is unimportant as the VE are used only for qualitative comparison.
The seasonal mean Ekman drift (Fig. 9), not computed within 1.25° of the equator, is
weaker where there is a good match between the observed surface drifts and the geostrophic
currents envisaged from dynamic topography. In the Bay of Bengal, the Ekman surface
drifts are weaker during January–March (Fig. 9a); during this period the buoy-derived
surface current vectors follow the dynamic height contours that depict the anticyclonic
gyre. The Ekman drifts are stronger during July–September when there is a mismatch
between the buoy-derived surface currents and the dynamic height contours. During the
southwest monsoon (June–September), McCreary et al. (1996) also noted that the Ekman
drifts dominate in the Bay of Bengal. Similarly, in the region of the NMC during
January–March, the Ekman drifts are weaker in the east, but they are stronger west of 60°E.
In this case a mismatch between the buoy-derived surface current vectors and dynamic
height contours was noticed to the west of 70°E.
Using ship drifts as observed surface currents, Hastenrath and Greischar (1991) have
discussed the individual contributions of geostrophic currents and Ekman drifts to the
major surface currents in the Indian Ocean. They suggest a dominant geostrophic
component for SEC, South Equatorial CounterCurrent (SCC), and Eastward Equatorial Jet
(EEJ), and a dominant Ekman component for the westward NorthEast Monsoon current
(NEM), the eastward SouthWest Monsoon current (SWM), the northward East African
Current (EAC) and the northeastward Somali Current (SCN). Hastenrath and Greischar’s
SEC, SCC, EEJ, NEM, SWM, EAC and SCN correspond to our SEC, ECC, EJ, NMC,
SMC, EACC and northward SC, respectively. Our analysis also shows the dominance of
geostrophic  ows for the SEC and ECC and the dominance of Ekman drift for the SC.
Hastenrath and Greischar deŽ ne a box bounded between 8°N and 10°N latitudes and 60°E
and 80°E longitudes for the computation of representative mean velocity components for
the NMC and SMC. In this box, the present analysis also suggests a lack of agreement
between the geostrophic  ow Ž eld and surface vectors in the NMC and the SMC. However,
outside the box deŽ ned by Hastenrath and Greischar, the present analysis shows a good
match between geostrophic  ows and the drifting-buoy-derived vectors for the NMC and
SMC; for example, the match is very good inside the Arabian Sea for SMC during
June–September. Similarly, the match is good for the NMC to the east of 75°E. In general,
1999]
Shenoi et al.: Indian Ocean circulation
903
Figure 9. Seasonal mean Ekman surface currents estimated from the monthly mean climatology of
NCEP/NCAR reanalysis surface winds. The computations were not done within 1.25° of the
equator. (a) January–February–March and (b) July–August–September.
the present analysis and the results from Hastenrath and Greischar (1991) agree well for all
major currents in the Indian Ocean.
7. Summary and discussions
The trajectories of 412 satellite-tracked, free-drifting surface buoys were analyzed. The
analysis shows a good agreement between the monthly mean surface circulation and the
features of large-scale circulation. Trajectory plots of individual buoys together with the
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monthly mean vectors shows that the surface circulation in the tropical Indian Ocean hosts
a series of mesoscale variability. The typical speed of the surface current in the tropical
Indian Ocean is below 50 cm s2 1 (92% of the individual estimates of surface drift speeds);
a few vectors (1%) exceed 100 cm s2 1.
The major currents revealed by the drifter data are summarized in Table 1. The maps of
monthly mean vectors reŽ ne the earlier maps presented by Molinari et al. (1990). Both data
sets characterize the circulation pattern in the tropical Indian Ocean as two primary gyres
extending across the basin, a southern gyre rotating clockwise and a northern gyre rotating
anticlockwise. However, some features of large-scale circulation not depicted clearly in the
earlier analysis are apparent in the present analysis. For example, the widening and
branching of EJ in the east, the branching of SMC near 87°E, the westward  ow at the
equator during July–August, the westward  ow in the interior Arabian Sea during
December–February, the EMC, etc. From the buoy trajectories (see Fig. 3a) and monthly
mean vectors (see Fig. 4), it is clear that the SC north of 5°N turns toward northeast in
March well before the winds reverse; this is in agreement with hydrographic observations
made during 1979 (Leetmaa et al., 1982) and model simulations (McCreary et al., 1993).
In the equatorial region, in general, the surface  ows are eastward, except during
July–August, when a narrow, weak westward  ow appeared at the equator; a similar
current is seen in the model simulation of McCreary et al. (1993). Two processes, local
wind curl and equatorially trapped Rossby waves, are the likely contributors to these
currents (McCreary et al., 1993). The eastward EJ that appeared during April–May and
again in November branched near 90°E; a major branch turned toward southward along the
west coast of Sumatra and the other branch turned northward. The southward branch
ultimately feeds the SEC and the northward branch contributes to the coastal current along
the eastern rim of the Bay of Bengal; this is consistent with the model solutions of Yu et al.
(1992) and McCreary et al. (1993), who attribute this to equatorial Kelvin waves
propagating eastward in the equatorial waveguide. These waves re ect at the eastern
boundary; a part of the re ected wave propagates northward along the eastern rim of the
bay, setting up a northward (southward) current along the eastern rim if the Kelvin waves
have a downwelling (upwelling) phase.
The onset of the southwest monsoon winds over the north Indian Ocean (in May) replace
the westward NMC with the eastward SMC. Associated with the SMC, everywhere in the
Arabian Sea between 7°N and 22°N, the surface currents are southeastward. This implies
that the water carried into the Arabian Sea during the southwest monsoon along its western
boundary (by the northeastward SC) is removed immediately by the surface currents.
The surface velocities were also used to infer the distribution of kinetic energy of the
 ow Ž eld. Most of the kinetic energy of the surface circulation is in the eddy Ž eld (ranging
from 200–1200 cm2 s2 2 ). The mean kinetic energy (EM ) per unit mass ranges from
20–200 cm2 s2 2. From the distribution of EE, it appears that the western boundary along the
coast of Africa act as a source and the northern (Arabian Sea and Bay of Bengal) and the
regions south of 20°S act as sinks. The proximity of the equatorial wave guide allows the
1999]
Shenoi et al.: Indian Ocean circulation
905
Table 1. Major surface currents in the Tropical Indian Ocean.
Name of Current
South Equatorial
Current (SEC)
East Madagascar
Current (EMC)
East Africa Coastal
Current (EACC)
Equatorial Counter
Current (ECC)
Equatorial Jet (EJ)
Northeast Monsoon
Current (NMC)
Southwest Monsoon
Current (SMC)
Somali Current (SC)
West India Coastal
Current (WICC)
East India Coastal
Current (EICC)
Description
A westward  ow between 8°S and 16°S
extending from 95°E to 50°E. Exists
throughout the year.
A southward  ow, fed by the SEC, along
the east coast of Madagascar. Exists
throughout the year.
A northward  ow, fed by the SEC, along
the east coast of Africa. Exists
throughout the year.
An eastward  ow to the south (between
3°S to 5°S) of the equator. Absent
during August.
An intense eastward current that appears at
the equator twice a year (during
April–May and November).
A westward  ow to the north of the
equator, with its axis around 5°N; exists
only during December–April.
An eastward  ow to the north of the
equator, with its axis around 5°N.
Develops in June, associated with the
southwest monsoon winds over the
north Indian Ocean, and persists till
October.
Flows southward along the coast of
Somalia during December–February and
 ows northward during March–September.
The current along the west coast of India
 ows during November–March poleward and equatorward during April–
September.
The current along the east coast of India,
 ows during February–April poleward
and equatorward during November–
December. During the southwest monsoon a weak poleward  ow develops in
the south (south of 15°N) and an equatorward  ow develops in the north.
Remarks
Also known as South
Equatorial Counter
Current (SECC)
Also known as
Wyrtki Jet
Also known as North
Equatorial Current
(NEC)
Also known as
Indian Monsoon
Current (IMC)
energy to propagate away from the western boundary in the form of equatorial Kelvin
waves and mixed Rossby waves. This is consistent with the model results of Yu et al.
(1992) and McCreary et al. (1993), which suggest that the propagation of energy into the
Bay of Bengal is in the form of Kelvin waves along its eastern rim.
906
Journal of Marine Research
[57, 6
The near-surface circulation depicted by the drifters was compared with the geostrophic
component of the circulation computed from the seasonal mean Ž elds of dynamic
topography. The comparisons corroborate the Ž ndings of Hastenrath and Greischar (1991),
who estimated the individual contributions due to geostrophic and Ekman components in
the Indian Ocean; the surface circulation in the Indian Ocean realized from dynamic
topography need not always represent the surface currents.
To conclude, this analysis has provided a composite overview of the kinematics of the
surface circulation of the tropical Indian Ocean. While reŽ ning the earlier observations, the
analysis also provides observational evidence for model solutions. There is a paucity of
observations in the east along the eastern rim of the Bay of Bengal and in the coastal
regions of the Arabian Sea. To complete the picture, more buoy deployments are needed in
these areas.
Acknowledgments. We are grateful to Robert Molinari, and NOAA/AOML/DAC, Miami, Florida
for generously providing the drifter data from their archives. The Department of Ocean Development, New Delhi provided the Ž nancial support through a project grant. We thank L. V. G. Rao,
Project Coordinator, for support and encouragement. The softwares FERRET, GMT, and netCDF
were used extensively. D. Shankar provided the computer code for harmonic analysis. This is NIO
contribution no. 3494.
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