Ekman Drift

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The monsoon currents in the north Indian Ocean

D. Shankar a, P. N. Vinayachandran b, A. S. Unnikrishnan a, and

S. R. Shetye a

aPhysical Oceanography Division, National Institute of Oceanography, Dona Paula,

Goa 403 004, India.

bCentre for Atmospheric and Oceanic Sciences, Indian Institute of Science,

Bangalore 560 012, India.

Abstract

The north Indian Ocean is distinguished by the presence of seasonally reversing currentsthat flow between the Bay of Bengal and the Arabian Sea. These currents are located be-

tween the equator and approximately 10

N. The Summer Monsoon Current (SMC) flows

eastward during the summer monsoon (MaySeptember) and the Winter Monsoon Cur-

rent (WMC) flows westward during the winter monsoon (NovemberFebruary), March

April and October being months of transition between these well-defined current systems.

We assemble data on ship drifts, winds and Ekman drift, geostrophic currents derive from

TOPEX/Poseidon sea-level anomalies, and hydrography to define the climatological cur-

rents in observations. An Oceanic General Circulation Model (OGCM) is used to simulate

the climatology of these currents and estimate transports, and numerical experiments with

a simpler model are used to investigate the processes that force these currents.

The ship drifts show that the monsoon currents extend over the entire basin, from the

Somali coast in the west to the Andaman Sea in the east. They do not, however, come into

being, or decay, over this entire region at a given time. Different parts of the currents form

at different times, and it is only in the mature phase that the currents exist as trans-basin

flows. The westward WMC first forms south of Sri Lanka in November and is initially

fed by the equatorward East India Coastal Current (EICC); the westward WMC in the

southern bay appears later. The WMC divides into two branches in the Arabian Sea, one

branch continuing to flow westward, and the other turningaround the Lakshadweep high off

southwest India to flow into the poleward West India Coastal Current (WICC). The SMC in

the Arabian Sea is a continuation of the Somali Current and the coastal current off Oman.

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It flows eastward and southeastward across the Arabian Sea and around the Lakshadweep

low off southwest India. It continues as the eastward SMC south of Sri Lanka. In the Bay

of Bengal, the SMC branches, one branch turning into the Bay of Bengal and the other

flowing eastward.

Ekman drift driven by the monsoon windsoverwhelms the geostrophic flow at the surface

in the western Arabian Sea. During the summer monsoon, Ekman drift dominates over most

of the Arabian Sea; it is only in the eastern Arabian Sea, in the eddies off Somalia, and in

the Bay of Bengal that the geostrophic current makes a significant contribution. During

the winter monsoon, geostrophy dominates, and Ekman drift modulates the geostrophic

current. The Ekman drift shows much less spatial structure than the geostrophic current.

Signatures of westward propagation of sea-level anomalies are evident in the altimeter data

in the regime of the monsoon currents.

The OGCM simulations show that Ekman drift dominates in a shallow surface layer

(about 20 m deep), but geostrophy dominates below this. The WMC is primarily a geo-

strophic current, with Ekman drift modulating it. The strong winds during the summer

monsoon ensure that Ekman drift dominates at the surface, leading to a more complexvertical structure in the SMC than in the WMC. At the surface, the SMC in the Arabian

Sea flows eastward and southeastward, feeding into the eastward SMC south of Sri Lanka.

This flow branches east of Sri Lanka, one branch flowing into the bay, the other continuing

to flow eastward. The geostrophic component of the SMC is a continuation of the Somali

Current. A part of the recirculation around the eddies off Somalia merges with the flow to

the west of the Lakshadweep low off southwest India to form a curving SMC that flows

into the eastward SMC south of Sri Lanka. The net transport due to the shallow monsoon

currents is due to both Ekman drift and geostrophic flow. The WMC (SMC) transports

7 Sv (

6 Sv) westward (eastward) in the top 100 m between 3-6

N at 80.5

E (south of

Sri Lanka) during the winter (summer) monsoon.

Numerical experiments with a 1 12-layer reduced-gravity model show that the dynamics

of the north Indian Ocean on seasonal time scales is explicable by linear wave theory. The

equatorial Rossby wave, the equatorial Kelvin wave, and the coastal Kelvin wave merge

the Arabian Sea, the Bay of Bengal, and the equatorial Indian Ocean into a single dynam-

ical entity, the north Indian Ocean, which must be modelled as a whole even to simulate

circulation in its parts. Circulation at any point is decided by both local forcing and remote

forcing, whose signals are carried by the equatorial and coastal waves. Superimposed on

the currents associated with these waves is the local Ekman drift. The geostrophic com-

ponent of the monsoon currents is forced by several processes. In the Bay of Bengal, the

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currents are forced by Ekman pumping and by the winds in the equatorial Indian Ocean.

To the west of Sri Lanka, in the eastern Arabian Sea, the major forcing is by winds along

the east and west coasts of India and Sri Lanka. Ekman pumping in the central Arabian Sea

and off the Somali coast are important processes in the central and western Arabian Sea,

with the Rossby waves radiated from the Indian west coast also playing a role. Thus, the

monsoon currents are actually composed of several parts, each of which is forced by one

or more processes, these processes acting in concert to produce the continuous monsoon

currents seen flowing across the breadth of the north Indian Ocean.

Key words: Tropical Oceanography. Summer Monsoon Current. Winter Monsoon

Current. Coastal Kelvin wave. Equatorial Kelvin wave. Equatorial Rossby wave. Arabian

Sea. Bay of Bengal.

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Contents

1 Introduction 12

1.1 Observational background 13

1.2 Theoretical background 17

2 Observations 23

2.1 Ekman drift 23

2.2 Geostrophic currents 31

2.3 The net flow at the surface 39

3 The monsoon currents in an OGCM 48

3.1 Numerical model 48

3.2 The model circulation 49

3.3 Transport estimates 56

4 Forcing mechanisms 62

4.1 The numerical model and the control run 62

4.2 Process solutions 68

4.3 Dynamics of the north Indian Ocean 86

5 Summary 91

Acknowledgements 95

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List of Figures

1 Schematic representation of the circulation in the Indian

Ocean during January (winter monsoon) and July (summer

monsoon). The abbreviations are as follows. SC, Somali Current;

EC, Equatorial Current; SMC, Summer Monsoon Current;

WMC, Winter Monsoon Current; EICC, East India Coastal

Current; WICC, West India Coastal Current; SCC, South

Equatorial Counter Current; EACC, East African Coastal Current;

SEC, South Equatorial Current; LH, Lakshadweep high; LL,

Lakshadweep low; GW, Great Whirl; and SH, Socotra high. 18

2 Wind stress (dyne cm 2) from the climatology of Hellerman and

Rosenstein (1983). 25

3 Surface Ekman drift (cm s 1

in the Indian Ocean. The drift is

computed from the wind-stress climatology of Hellerman and

Rosenstein (1983) and is based on the Ekman spiral formula. 26

4 Monthly climatology of sea-level anomalies (cm, left panel) and

geostrophic current (cm s 1, right panel) in the Indian Ocean.

Negative anomalies are indicated by dashed contours and the

contour interval is 5 cm. The sea-level anomalies and geostrophic

currents are derived from the TOPEX/Poseidon altimeter data for

19931997. 27

4 (continued) 28

4 (continued) 29

4 (continued) 30

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5 Longitude-time plots of the monthly climatology of

TOPEX/Poseidon sea-level anomalies (cm) at 10N (top

panel), 8N (middle panel), and 5

N (lower panel). Negative

anomalies are indicated by dashed contours and the contour

interval is 5 cm. Westward propagation is evident in both Arabian

Sea and Bay of Bengal at all latitudes. At 5 N, there is a break in

the signal at 80 5E (south of Sri Lanka), even though there is no

land barrier there. 36

6 Latitude-time plots of the monthly climatology of zonal

geostrophic current (cm s 1), derived from TOPEX/Poseidon

altimetry, at 805

E (south of Sri Lanka). Westward flow is

indicated by dashed contours and the contour interval is 5 cm s 1. 37

7 Depth-time plots of the meridionally averaged zonal geostrophic

current (cm s 1) derived from the climatologies of Levitus and

Boyer (1994) and Levitus, Burgett, and Boyer (1994). The left

(right) panel shows the current averaged over 0

3

N (3

6

N).

The depth is in metres. Westward flow is indicated by dashed

contours and the contour interval is 5 cm s 1. 38

8 The net flow (NF) at the surface (cm s 1, left panel), computed as

the sum of Ekman drift (Figure 3) and geostrophic flow (Figure 4),

and ship drifts (cm s 1, right panel). The source for the ship drifts

are the Ocean Current Drifter Data CDROMs NODC-53 and

NODC-54 (NODC, US Department of Commerce, NOAA). 40

8 (continued) 41

8 (continued) 42

8 (continued) 43

9 OGCM currents (cm s 1) at 5 m. 50

10 OGCM currents (cm s 1) at 35 m. 51

11 OGCM currents (cm s 1) averaged over the top 50 m. 52

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12 Longitude-time plot of the OGCM meridional velocity (cm s 1)

at 8 N. Southward flows are indicated by dashed contours and the

contour interval is 5 cm s 1. 54

13 Longitude-time plot of the OGCM meridional velocity (cm s 1)

at 5N. Southward flows are indicated by dashed contours and

the contour interval is 5 cm s 1. Note the break in the westward

propagation at

80E. 55

14 Depth-time plot of the OGCM zonal current (cm s 1) at 80.5

E

(south of Sri Lanka). The upper (lower) panel shows the current

averaged over 36

N (03

N). Westward flows are indicated by

dashed contours and the contour interval is 5 cm s 1. 57

15 Latitude-time plots of the depth-integrated zonal current (m2 s 1).

The current is integrated over the top 100 m. Westward flows are

indicated by dashed contours and the contour interval is 10 m2 s 1. 60

16 Sea-level deviation from the initial surface (cm, left panel) and

upper-layer velocity (cm s 1, right panel) for the nonlinear

simulation. Negative sea level is indicated by dashed contours and

the contour interval is 5 cm. 65

16 (continued) 66

17 Longitude-time plot of sea-level deviation (cm) from the

reduced-gravity model (nonlinear simulation). Negative sea level

is indicated by dashed contours and the contour interval is 5 cm. 67

18 Sea-level deviation from the initial surface (cm, left panel) and

upper-layer velocity (cm s 1, right panel) for the linear simulation.

Negative sea level is indicated by dashed contours and the contourinterval is 5 cm. 69

18 (continued) 70

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19 Effect of winds along the western boundary of the Bay of Bengal

(Process WB). Sea-level deviation (cm, left panel) and upper-layer

velocity (cm s 1) is shown. Negative sea level is indicated by

dashed contours and the contour interval is 5 cm. 72

19 (continued) 73

20 Effect of winds along the eastern boundary of the Arabian Sea(Process EA). Sea-level deviation (cm, left panel) and upper-layer

velocity (cm s 1) is shown. Negative sea level is indicated by


20 (continued) 76

21 Effect of winds along the northern and western boundaries of the

Arabian Sea, except the Somali coast (Process WA). Upper-layer

velocity (cm s

1) is shown. 77

22 Effect of winds along the Somali coast (Process SA). Upper-layer

velocity (cm s 1) is shown. 79

23 Effect of alongshore winds in the north Indian Ocean (Process

CW). Sea-level deviation (cm, left panel) and upper-layer velocity

(cm s 1, right panel) are shown. Negative sea level is indicated by


23 (continued) 81

24 Effect of filtering out forcing by alongshore winds in the north

Indian Ocean (Process OP). Sea-level deviation (cm, left panel)

and upper-layer velocity (cm s 1, right panel) are shown. Negative

sea level is indicated by dashed contours and the contour interval

is 5 cm. 83

24 (continued) 84

25 Ekman pumping (m day 1), derived from the wind-stress

climatology of Hellerman and Rosenstein (1983). 85

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26 Schematic illustrating the dynamics of the north Indian Ocean.

The linear theoretical framework depicted here invokes the

equatorial Kelvin wave, the equatorial Rossby wave, and the

coastal Kelvin wave. These three waves merge the equatorial

Indian Ocean, the Bay of Bengal, and the Arabian Sea into a

single dynamical entity. The horizontal hatching indicates the

equatorial waveguide, which extends about 2.5

on either side of

the equator; the vertical hatching indicates the coastal waveguide.

The coastal Kelvin wave is trapped at the coast poleward of a

critical latitude; equatorward of this latitude, westward radiation

of energy is possible, and the coastal Kelvin wave is inseparable

from the westward propagating Rossby wave. The critical latitudes

for Rossby waves at annual (semiannual) period is 42

( 21);

hence, annual and semiannual Kelvin waves are inseparable from

westward propagating Rossby waves in the north Indian Ocean,

and energy leaks at these periods from the eastern boundary into

the open ocean (shown by arrows pointing out of the coastal

waveguide). Shetye (1998) and Shankar (1998) called this the

leaky waveguide of the north Indian Ocean. Energy is also

generated by Ekman pumping (shown by the closed circles) in

the interior of the basin, this signal also propagating westward as

Rossby waves. 87

27 Monthly-mean geostrophic current (cm s 1), derived from

TOPEX/Poseidon altimetry for January. The upper-left panel

shows the 1993-1997 climatology, and the other panels show

the geostrophic currents for the January of each of the years

19931997. 89

28 Monthly-mean geostrophic current (cm s

1

), derived fromTOPEX/Poseidon altimetry for July. The upper-left panel shows

the 1993-1997 climatology, and the other panels show the

geostrophic currents for the July of each of the years 19931997. 90

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29 Geostrophic currents (cm s 1) from TOPEX/Poseidon altimetry

for three cycles each during January (left panel) and July (right

panel) of 1993. The plots show that the GWMC and GSMC can

be traced even in individual TOPEX/Poseidon cycles, even though

the currents are more noisy and meander more than in climatology

(Figure 4) or in monthly averages (Figures 27 and 28). 93

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List of Tables

1 Nomenclature used for currents in this paper. Many of the currents

in the Indian Ocean have been referred to by different names in the

literature. These are listed here. The first column lists the name

and acronym used by us. The second column lists names and

acronyms used earlier; this column is blank if no other name isknown to have been used by other authors. The names used here

are based on the geographical location of the current, the common

practice. Except in the case of the monsoon currents, no allowance

has been made for a change in direction with season. 19

2 Currents and transports associated with the monsoon currents. The

direction is given in parentheses in the first column (N implies

northward flow, S southward, etc.) 20

2 (continued) 21

3 Observed and model zonal transports (in Sv; 1 Sv

106 m3 s 1) in

the top 300 m between 345 N and 5

52 N at 80.5

E (south of Sri

Lanka). Positive (negative) values indicate eastward (westward)

flow, and the values listed are averages over the period indicated.

All observations, except that marked (*), are for 1991; the marked

observation is for 1992. The observed transports are derived from

the direct current measurements of Schott, Reppin, Fischer, and

Quadfasel (1994). The model was forced by climatological wind

stress (Hellerman and Rosenstein, 1983). The last two values are

average model transports for the winter and summer monsoons,

respectively. 58

4 Zonal transport (in Sv; 1 Sv

106 m3 s 1) in the top 100 m in

the domain of the monsoon currents. Negative values indicate

westward flows and the transports are averages over the periodsmentioned. 61

5 Parameters for the 112-layer reduced-gravity model. 63

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1 Introduction

The winds over the Indian Ocean (see Figure 1) north of 10S reverse direction

twice during an year. Over the north Indian Ocean, they generally blow from the

southwest during MaySeptember and from the northeast during NovemberFeb-

ruary, MarchApril and October being the months of transition with weak winds.

In this paper, we refer to MaySeptember as the summer monsoon and November

February as the winter monsoon. The winds are much stronger during the summer

monsoon than during the winter monsoon. These seasonally reversing monsoon

winds over the north Indian Ocean force a seasonally reversing circulation in the

upper ocean.

The best studied of the seasonally reversing currents are the Somali Current (SC),

which flows poleward (equatorward) along the coast of Somalia during the summer

(winter) monsoon (see the reviews by Schott, 1983; Shetye and Gouveia, 1998;

Schott and McCreary, 2001, and the many references therein), and the current

along the equator (called Equatorial Current (EC) in this paper), where eastward

surface jets are observed during AprilMay and OctoberNovember (see, for ex-

ample, Wyrtki, 1973a; OBrien and Hurlburt, 1974; Jensen, 1993; Han, McCreary,

Anderson, and Mariano, 1999; Schott and McCreary, 2001, and the many refer-

ences therein). In the last decade, however, other coastal currents have also re-

ceived attention. These include the currents along the east coast of India, called the

East India Coastal Current (EICC) (Shetye, Shenoi, Gouveia, Michael, Sundar, andNampoothiri, 1991b; Shetye, Gouveia, Shenoi, Sundar, Michael, and Nampoothiri,

1993; Shetye, Gouveia, Shankar, Shenoi, Vinayachandran, Sundar, Michael, and

Nampoothiri, 1996; Shankar, McCreary, Han, and Shetye, 1996; McCreary, Kundu,

and Molinari, 1993; McCreary, Han, Shankar, and Shetye, 1996; Vinayachandran,

Shetye, Sengupta, and Gadgil, 1996; Shetye and Gouveia, 1998; Schott and Mc-

Creary, 2001), the current along the west coast of India, called the West India

Coastal Current (WICC) (Shetye, Gouveia, Shenoi, Sundar, Michael, Almeida, and

Santanam, 1990; Shetye, Gouveia, Shenoi, Michael, Sundar, Almeida, and San-tanam, 1991a; McCreary et al., 1993; Stramma, Fischer, and Schott, 1996; Shankar

and Shetye, 1997; Shetye and Gouveia, 1998), and the current along the Arabian-

Sea coast of Oman (McCreary et al., 1993; Flagg and Kim, 1998; Shetye and Gou-

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veia, 1998; Bohm, Morrison, Manghnani, Kim, and Flagg, 1999; Shi, Morrison,

Bohm, and Manghnani, 2000; Schott and McCreary, 2001). There have been no

observational studies of the coastal current along the eastern boundary of the Bay

of Bengal. Apart from these coastal currents, the most significant large-scale cur-

rents known in the north Indian Ocean are the open-ocean, seasonally reversing

monsoon currents. During the summer monsoon, the monsoon current flows east-

ward as a continuous current from the western Arabian Sea to the Bay of Bengal;

during the winter monsoon, it flows westward, from the eastern boundary of the bay

to the western Arabian Sea (see the schematic in Figure 1). We call these currents

the Summer Monsoon Current (SMC) and Winter Monsoon Current (WMC), re-

spectively. It is these currents, which transfer water masses between the two highly

dissimilar arms of the north Indian Ocean, the Bay of Bengal and the Arabian Sea,

that form the subject of this paper.

1.1 Observational background

The existence of seasonally reversing currents in the Arabian Sea has been known

for long (see Warren, 1966, for references to medieval Arab sources), but the first

comprehensive study of the circulation in the Indian Ocean was made based on the

hydrographic surveys conducted during the International Indian Ocean Expedition

(IIOE) during 19591965. The IIOE led to a large number of papers, most of which,

as noted above, were devoted to the Somali Current. Though the monsoon currents

have not received as much attention, their importance to the circulation in the north

Indian Ocean was recognized early.

The first major description of the currents followed soon after the IIOE (Duing,

1970; Wyrtki, 1971, 1973b). Based on these hydrographic data, Wyrtki (1973b)

highlighted what he called the seasonally changing monsoon gyre as a gyre un-

like those found in the other oceans. In his scheme of circulation, the monsoon gyre

during the winter monsoon consists of the westward North Equatorial Current, a

southward flow off the Somali coast, and the Equatorial Counter Current, whichruns east between the equator and 8

S across the entire width of the ocean. During

the summer monsoon, his monsoon gyre consists of the northern portions of the

South Equatorial Current, which now extends almost to the equator, the strong So-

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mali Current flowing north as a western boundary current, and the monsoon current,

into which the Counter Current has merged. There is no standard nomenclature for

the monsoon currents. The SMC has been called Southwest Monsoon Current (or

Drift) or the Indian Monsoon Current or just Monsoon Current, and the WMC has

been called the Northeast Monsoon Current or the North Equatorial Current. In

this paper, we shall stick to the terminology used above, i.e., Summer and Win-

ter Monsoon Current, following a growing tendency among meteorologists to use

the terms summer monsoon and winter monsoon (Sulochana Gadgil, personal

communication, 2000). The nomenclature used in this paper is given in Table 1.

Wyrtki (1973b) noted that the circulation in the Indian Ocean is complex; the win-

ter monsoon gyre did not close cleanly in the east, with most of the flow from the

South Equatorial Counter Current (SCC) flowing into the South Equatorial Cur-

rent (SEC), and a strong branch of the WMC turned north to flow along the Indian

west coast, transporting low-salinity water from the Bay of Bengal into the eastern

Arabian Sea. The circulation during the winter monsoon was shallow compared to

that during the summer monsoon, when intense upwelling was observed in sev-

eral places and the circulation penetrated deeper, affecting the movement of water

masses below the thermocline, especially in the western Arabian Sea. The com-

plexity of the circulation represented by the hydrographic data was seen in the large

number of eddies (Duing, 1970; Wyrtki, 1973b), which were found to be connected

intimately to the dynamics of the monsoon gyre. The most vigorous of these eddies

lay about 300 km offshore of the Somali coast; large parts of the Somali Current

were recirculated around this eddy, the Great Whirl.

A different picture emerges from the ship-drift data (Defant, 1961; Cutler and Swal-

low, 1984; Rao, Molinari, and Festa, 1989) or surface-drifter data (Molinari, Ol-

son, and Reverdin, 1990; Shenoi, Saji, and Almeida, 1999a), which tend to show

broad eastward (westward) or southeastward flows across the Arabian Sea during

the summer (winter) monsoon. Hastenrath and Greischar (1991) used ship drifts,

hydrography, and Ekman drift computed from wind-stress climatologies to study

the monsoon currents in the Arabian Sea. They concluded that the monsoon cur-rents are essentially Ekman drifts forced by the monsoon winds, the geostrophic

contribution to these flows being negligible. Shenoi et al. (1999a) compared hy-

drography based on the climatologies of Levitus and Boyer (1994) and Levitus

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et al. (1994) to current estimates from surface drifters, and concluded that the role

of geostrophic flows in representing the surface flows varies both geographically

and seasonally. The agreement between the drifter data and hydrography was worst

during the summer monsoon, when the winds are strong; at this time, the drifters

showed southeastward flows all over the Arabian Sea, unlike in hydrography. The

dynamic heights, however, do capture the drifter movement in the eastern Arabian

Sea during the winter monsoon.

Hydrographic data, however, were also used in later studies (Bruce, Johnson, and

Kindle, 1994; Bruce, Kindle, Kantha, Kerling, and Bailey, 1998; Donguy and Mey-

ers, 1995; Murty, Sarma, Rao, and Murty, 1992; Murty, Sarma, Lambata, Gopalakr-

ishna, Pednekar, Rao, Luis, Kaka, and Rao, 2000; Gopalakrishna, Pednekar, and

Murty, 1996; Vinayachandran, Masumoto, Mikawa, and Yamagata, 1999a), which

showed strong geostrophic flows and transports associated with the monsoon cur-

rents. These estimates (Table 2) yield current strengths of 40 cm s 1 and trans-

ports of 10 106 m3s

1 in the upper 4001000 m, which implies that the geo-

strophic flows associated with the monsoon currents are not small, even if they are

weaker than the surface Ekman flows in some regions during some seasons. The

geostrophic flows estimated by Hastenrath and Greischar (1991) are weak proba-

bly owing to the averaging they did to obtain climatological currents and transports

in a region; in contrast, the studies mentioned above usually used hydrographic data

from individual cruises. The hydrographic data show that the monsoon currents are

not found in the same location during a season or across different years; for ex-

ample, Vinayachandran et al. (1999a) showed that the SMC in the Bay of Bengal

intensifies and shifts westward as the summer monsoon progresses.

Despite these differences, all the observations using hydrography, ship drifts,

and surface drifters show that the monsoon currents flow across the breadth of

the north Indian Ocean. The branches of the SMC and WMC that flow around the

Lakshadweep high and low in the southeastern Arabian Sea (McCreary et al., 1993;

Bruce et al., 1994; Shankar and Shetye, 1997) link the circulations in the Arabian

Sea and the Bay of Bengal (Figure 1). The SMC flows eastward south of Sri Lankaand into the bay. It is fed by a flow from the southwest near the equator and by the

flow around the Lakshadweep low. East of Sri Lanka, the SMC flows northeastward

into the Bay of Bengal. A part, however, appears to flow southeastward and crosses

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the equator near Sumatra in the surface-drifter data (Shenoi et al., 1999a); recent

hydrographic data (Unnikrishnan, Murty, Babu, Gopinathan, and Charyulu, 2001)

also show that the SMC between 8088E flows close to the equator and even to its

south. The SMC transports high-salinity water (Arabian Sea High Salinity Water)

into the bay (Murty et al., 1992; Gopalakrishna et al., 1996). The WMC flows west-

ward south of Sri Lanka, where it divides into two branches, one flowing westward

into the southern Arabian Sea, and the other flowing around the Lakshadweep high

into the WICC. The WMC transports low-salinity water (Bay of Bengal Water)

into the eastern Arabian Sea, where it is entrained into the Lakshadweep high and

spread along the Indian west coast by the WICC (Bruce et al., 1994; Han, 1999;

Shenoi, Shankar, and Shetye, 1999b; Shankar and Shetye, 1999).

The passage between Sri Lanka and the equator is therefore significant because

the monsoon currents have to flow through it, making it the one location where

the monsoon currents are geographically frozen, relatively speaking, unlike in the

open ocean, where they meander a lot. It is also here, south of Sri Lanka, that

the monsoon currents attain their maximum strength (Duing, 1970), probably be-

cause the currents are squeezed through a relatively narrow bottleneck. Hence, it

is not surprising that the only direct current measurements of the monsoon cur-

rents have been made between Sri Lanka and the equator along 8030 E (Schott

et al., 1994; Reppin, Schott, Fischer, and Quadfasel, 1999). The current-meter and

ADCP (Acoustic Doppler Current Profiler) observations (Schott et al., 1994; Rep-

pin et al., 1999) show that the SMC and WMC transport

10

106 m3s 1 in

the upper 300 m. These direct measurements also confirm the observation in hy-

drography that the monsoon currents are shallow, with most of the variation being

restricted to the upper 100 m. The moored array shows upward phase propagation,

implying downward propagation of energy. Most striking is the difference between

the Equatorial Current and the monsoon currents, even though both flow together

in the same bottleneck. The Equatorial Current includes a large semiannual har-

monic, unlike the monsoon currents, which are dominated by the annual harmonic;

thus, the Equatorial Current reverses direction four times an year, but the monsoon

currents reverse direction twice. Superimposed on these seasonal changes are largeintraseasonal oscillations.

Though the observations differ in their presentation of the monsoon-current sys-

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tem in the north Indian Ocean, they show that the open-ocean currents in the north

Indian Ocean extend all across the basin, reverse direction with season, and are rel-

atively shallow compared to the deep western boundary current off Somalia. Given

that there are many different interpretations of the monsoon currents (contrast, for

example, Hastenrath and Greischar (1991) with Wyrtki (1973b)), it is not surprising

that more than one hypothesis exists to explain the observations.

1.2 Theoretical background

Though the existence of the monsoon currents in observations has been known

for long, the mechanism leading to their formation has been understood only dur-

ing the last decade. Early ideas attributed the monsoon currents as seen in ship

drifts to direct Ekman forcing by the monsoon winds (Defant, 1961; Hastenrath

and Greischar, 1991), and as seen in hydrography to the local curl of wind stress

(Murty et al., 1992). The north Indian Ocean is essentially a tropical basin with

its northern boundary located south of 25N. The pioneering work of Matsuno

(1966), Moore (1968), and Lighthill (1969) showed that baroclinic waves propa-

gate fast in the tropics, and it is now appreciated that the open-ocean, equatorial,

and coastal currents in the north Indian Ocean, all of which reverse seasonally, are

manifestations of direct forcing (Ekman drift) by the monsoon winds, and of equa-

torial and coastal long, baroclinic waves generated by the seasonal winds (Cane,

1980; Potemra, Luther, and OBrien, 1991; Yu, OBrien, and Yang, 1991; Peri-

gaud and Delecluse, 1992; McCreary et al., 1993; Shankar et al., 1996; McCreary

et al., 1996; Vinayachandranet al., 1996; Shankar andShetye, 1997; Shankar, 1998;

Vinayachandran and Yamagata, 1998; Han, 1999; Han et al., 1999; Shankar, 2000).

The small size of the basin implies that these waves can traverse the basin in a few

months. This is unique to the north Indian Ocean.

The framework that has evolved in the last decade suggests a unity of dynamics in

the north Indian Ocean. The equatorial Rossby wave, the equatorial Kelvin wave,

and the coastal Kelvin waves merge the Arabian Sea, the Bay of Bengal, and theequatorial Indian Ocean into a single dynamical entity, the north Indian Ocean,

which must be modelled as a whole even to simulate circulation in its parts. Cir-

culation at any point is decided by both local forcing and remote forcing, whose

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40E 50E 60E 70E 80E 90E 100E

10S

0

10N

20N

30N

SCC

SEC

EACC

SC

SMC

WICC

EICC

ECSMC

SMC

LH

SEC

SCC

EC

EACC

SC

WICC

EICC

WMCWMC

Schematic of circulation in the Indian Ocean

GW

SHLL

July

IndiaOman

Somalia

Sri Lanka

ArabianSea

Bay ofBengal

AndamanSea

Sumatra

40E 50E 60E 70E 80E 90E 100E

10S

0

10N

20N

30N

January

IndiaOman

Somalia

Sri Lanka

ArabianSea

Bay ofBengal

AndamanSea

Sumatra

Fig. 1. Schematic representation of the circulation in the Indian Ocean during January (win-

ter monsoon) and July (summer monsoon). The abbreviations are as follows. SC, Somali

Current; EC, Equatorial Current; SMC, Summer Monsoon Current; WMC, Winter Mon-

soon Current; EICC, East India Coastal Current; WICC, West India Coastal Current; SCC,

South Equatorial Counter Current; EACC, East African Coastal Current; SEC, South Equa-

torial Current; LH, Lakshadweep high; LL, Lakshadweep low; GW, Great Whirl; and SH,

Socotra high.

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Name of current (acronym) Other commonly used names

(acronyms)

Winter Monsoon Current (WMC) Northeast Monsoon Current (NMC),

North Equatorial Current (NEC)

Summer Monsoon Current (SMC) Southwest Monsoon Current (SMC),

Indian Monsoon Current (IMC),

Monsoon Current

Equatorial Current (EC) Equatorial jet or Wyrtki jet (when

flowing eastward)

South Equatorial Counter Current

(SCC)

South Equatorial Counter Current

(SCC), Equatorial Counter Current

(ECC)

Somali Current (SC)

West India Coastal Current (WICC)

East India Coastal Current (EICC)

Table 1

Nomenclature used for currents in this paper. Many of the currents in the Indian Ocean have

been referred to by different names in the literature. These are listed here. The first column

lists the name and acronym used by us. The second column lists names and acronyms used

earlier; this column is blank if no other name is known to have been used by other authors.

The names used here are based on the geographical location of the current, the common

practice. Except in the case of the monsoon currents, no allowance has been made for a

change in direction with season.

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Cur-

rent/Transport

and direction

Location and period Remarks

9 Sv (E) 78

E, 35

N; June 1992. Geostrophic transport with respect to

400 m, based on XBT data with salinity

from T-S relation based on climatology(Levitus, 1982); Source Murty et al.

(2000).

14 Sv (E) 80

E, 35

N; July 1992. As above.

9.3 Sv (E) 68

E, 47

N; July 1995. As above.

6 Sv (E) 68

E, 26

N;

September 1993.

As above.

14 Sv (W) 7279

E, 68

N;

February 1993.

As above.

78 Sv (W) 25

N; January 1996,February 1993,

March 1992.

As above, but average values for thegiven months over three different

transects.

13 Sv (W) 6

N, east of Sri Lanka;

JanuaryFebruary

climatology

Geostrophic transport with respect to

400 m, based on TOGA XBT data for

19851989, with salinity from T-S

relation based on climatology (Levitus,

1982). Source Donguy and Meyers

(1995).

11 Sv (NE) West of Lakshadweep

high (see Figure 1);JanuaryFebruary

climatology.

As above.

19 Sv (N) 10

N, 6670

E; Winter

monsoon, 1965.

Geostrophic transport with respect to

1000 m, evaluated from IIOE data;

Atlantis II, Cruise 15. West of

Lakshadweep high. Source Bruce et al.

(1994).

15 Sv (S) 10

N, 6772

E;

Summer monsoon, 1963.

As above, but for IIOE data from

Atlantis II, Cruise 8.

35 cm s 1,5.2 Sv (S)

8

N, 72

10 75

E;August 1993.

Current from ADCP; geostrophictransport with respect to 1000 m from

hydrography. West of Lakshadweep low

(see Figure 1). Source Stramma et al.

(1996).

Table 2

Currents and transports associated with the monsoon currents. The direction is given in

parentheses in the first column (N implies northward flow, S southward, etc.)

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Cur-

rent/Transport

and direction

Location and period Remarks

8 Sv (E), with

a peak transport

of 24 Sv

80

30E; Summer

monsoon, 1991.

Transport estimated from current-meter

moorings and ADCP. Moorings located

south of Sri Lanka between 5

39N and

4

10N (transport estimated over top

300 m between 3

45 N and 5

52 N).

Source Schott et al. (1994).

12 Sv, 10 Sv (W),

with a peak

transport of

25 Sv

80

30E; Winter

monsoon, 1991, 1992.

As above. The first value is for 1991,

the second for 1992.

40 cm s

1 (N) 8789

E, 14

N; Summer

monsoon, 1984.

Maximum of geostrophic current

between 50100 m depth, estimated

from hydrographic data with respect to

1000 m. Source Murty et al. (1992).

40 cm s

1,

17 Sv (N)

8789

E, 11

N; July

1993.

Geostrophic current and transport with

respect to 1000 m, estimated using

hydrographic data. Source

Gopalakrishna et al. (1996).

40 cm s

1

,14 Sv (N) 8789

E, 12

N; August1991. As above.

40 cm s 1,

12 Sv (N)

8185

E, 6

N; July

climatology.

Geostrophic current and transport with

respect to 400 m, estimated from

TOGA XBT data during 19851996.

The current is restricted to the top

200 m and moves westward as the

season progresses. Source

Vinayachandran et al. (1999a).

Table 2

(continued)

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signals are carried by the equatorial and coastal waves. This is seen in numerical

simulations using both layered models and multi-level general circulation models.

The layered models, in particular, emphasize the quasi-geostrophic dynamics that

leads to the eddies and meanders so typical of hydrographic observations of the

monsoon currents. In these models, the monsoon currents appear as the fronts of

Rossby waves. For example, McCreary et al. (1993) and Shankar and Shetye (1997)

emphasized the role of Rossby-wave radiation in forcing the Lakshadweep high and

low, which are intimately connected to the monsoon currents seen in the hydrogra-

phy of the southeastern Arabian Sea, and McCreary et al. (1993) and Vinayachan-

dran et al. (1999a) showed that the westward movement of the SMC across the bay

was a result of Rossby-wave radiation from the eastern bay and the generation of

Rossby waves by Ekman pumping in the interior of the bay.

Notwithstanding the success of numerical models in simulating the circulation in

the north Indian Ocean, there remain several unanswered questions, especially with

respect to the monsoon currents. The nature of currents associated with Rossbywaves is strikingly different in places from the observed ship-drift and surface-

drifter data, this being more true of the SMC during the summer monsoon. Yet, all

authors generally claim success for their respective models, attributing the differ-

ences between simulations and observations to Ekman flow. Given that both ship

drifts and surface drifters show a circulation that differs significantly from that

seen in hydrography, a pertinent question is: what really are the monsoon currents?

How do we describe them, and what are the causes for their existence? It is these

questions that we seek to answer in this paper, and we begin with the commonlyaccepted definition of the monsoon currents as the open-ocean, seasonal currents

that link the circulations in the Arabian Sea and the Bay of Bengal.

In section 2, we assemble observations on ship-drifts, Ekman drift estimated from

winds, geostrophic currents computed from sea-level anomalies obtained from sate-

llite altimetry, and hydrography to define the surface circulation associated with

the monsoon currents. Numerical simulations with an Oceanic General Circulation

Model follow in section 3. In section 4, we use a 1 12-layer reduced-gravity model

to analyze the forcing mechanisms. Section 5 concludes the paper.

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2 Observations

To define the climatological monsoon-current system and the associated circulation

in the north Indian Ocean, we use climatological wind-stress data to estimate sur-

face Ekman drift, and satellite altimeter data from TOPEX/Poseidon to estimate the

geostrophic contribution to the surface currents. The Ekman drift and geostrophic

current, and the net surface current due to them are compared to surface currentsrepresented by ship drifts. We show that defining the monsoon-current system re-

quires more than one observational method because each method accentuates cer-

tain aspects of the flow field, thereby emphasizing a particular view of the surface

currents.

2.1 Ekman drift

The Ekman drift is computed using the Ekman spiral method. The surface Ekman

drift flows at 45

to the right (left) of the wind in the northern (southern) hemisphere

and its magnitude is given by (Pond and Pickard, 1983)

VE

A

f

12

(1)

where is the magnitude of the wind stress, A is the vertical eddy diffusivity and

f

is the magnitude of the Coriolis parameter. We use A 10 2 m2s

1 and from the

wind-stress climatology of Hellerman and Rosenstein (1983) (Figure 2) to obtain

the monthly climatology of the surface Ekman drift in the Indian Ocean north of

10 S (Figure 3), excluding the region within 2.5 of the equator, where (1) does not

apply.

The winter monsoon sets in during November, and the Ekman drift reverses direc-

tion to flow westward in the Arabian Sea and the Bay of Bengal. The drift is weak

in the eastern Arabian Sea and the eastern Bay of Bengal. The winter monsoon

strengthens in December; so does the Ekman drift. The magnitude of the drift is 15 cm s

1 in the western bay and 25 cm s 1 in the western Arabian Sea. The

winter monsoon peaks in January, with northeasterly winds over most of the north

Indian Ocean, and the surface Ekman drift is westward. Current strengths approach

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20 cm s 1 south of Sri Lanka and 30 cm s

1 in the southwestern Arabian Sea.

Ekman drift is weak in the eastern Arabian Sea and northern Bay of Bengal. The

winter monsoon weakens in February; so does the Ekman drift. During March

April, the months of transition between the winter and summer monsoons, Ekman

flow is weak, except in the northern bay in April, where it has reversed direction

since January to flow eastward. In both basins, a weak anticyclonic gyre is seen

during March.

With the onset of the summer monsoon in May, the winds begin to blow from the

southwest over most of the north Indian Ocean. The Ekman drift reverses to flow

eastward over most of the Arabian Sea; it is southeastward in the eastern Arabian

Sea, where the winds blow more from the west. In the bay, Ekman drift is east-

ward, except in the north and the west, where it tends to be oriented parallel to

the coast. The current strengths are now

25 cm s 1 southeast of Sri Lanka and

off the Somali coast. The surface Ekman drift strengthens all over the north In-

dian Ocean in June. It is eastward and southeastward in the Arabian Sea, with a

slight anticyclonic tendency; the drift is eastward in the bay. The direction remains

the same through JuneSeptember, but the Ekman drift peaks in July, when cur-

rent strengths approach

40 cm s 1 in parts of the bay and

100 cm s

1 off the

Somali coast. October is the month of transition between the summer and winter

monsoons, with weak winds all over the north Indian Ocean. The Ekman drift is

weaker than 5 cm s 1 over most of the basin, with currents of

20 cm s

1 seen

only southeast of Sri Lanka.

During the summer monsoon, the Ekman drift is strong in the western and central

Arabian Sea and south of Sri Lanka; it is relatively weak in the eastern parts of the

Arabian Sea and Bay of Bengal. During the winter monsoon, the spatial variation

in magnitude is much less. Most striking is the spatial uniformity of the Ekman

drift in comparison with the eddy-like circulations seen in hydrography (Duing,

1970; Wyrtki, 1971; Murty et al., 1992). It strengthens and weakens almost all overthe north Indian Ocean at the same time, in harmony with the seasonally reversing

winds. This lack of spatial structure in the Ekman drift implies that geostrophy must

make a significant contribution to the surface current in the north Indian Ocean.

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Fig. 2. Wind stress (dyne cm

2) from the climatology of Hellerman and Rosenstein (1983).

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Fig. 3. Surface Ekman drift (cm s

1 in the Indian Ocean. The drift is computed from the

wind-stress climatology of Hellerman and Rosenstein (1983) and is based on the Ekman

spiral formula.

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Fig. 4. Monthly climatology of sea-level anomalies (cm, left panel) and geostrophic current

(cm s 1, right panel) in the Indian Ocean. Negative anomalies are indicated by dashed

contours and the contour interval is 5 cm. The sea-level anomalies and geostrophic currents

are derived from the TOPEX/Poseidon altimeter data for 19931997.

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Fig. 4. (continued)

28


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Fig. 4. (continued)

29


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Fig. 4. (continued)

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2.2 Geostrophic currents

Climatological hydrographic data (Levitus and Boyer, 1994; Levitus et al., 1994)

are incapable of resolving, in both space and time, the geostrophic component of the

rapidly changing monsoon circulation of the north Indian Ocean. Hence, to com-

pute geostrophic currents, we use the sea-level anomalies from TOPEX/Poseidon

altimetry, which are available on a 0 25 0 25 grid (Le Traon, Gaspar, Bouys-

sel, and Makhmara, 1995; Le Traon and Ogor, 1998; Le Traon, Nadal, and Ducet,

1998). We construct a monthly climatology of sea-level anomalies using the 10-

day repeat-cycle data for 19931997, and use this climatology to compute surface

geostrophic currents in the Indian Ocean (Figure 4), excluding the region within

2.5

of the equator. Though the period of averaging is small for making a climatol-

ogy, it is based on the best data available, and throws light on the monsoon-current

system. Unlike the Ekman drift, which shows little spatial structure, the surface

geostrophic flow is dominated by eddies. The geostrophic monsoon currents do not

form, or decay, across the basin all at once. Instead, patches of the currents appear

or decay at different times. The monsoon currents can be traced as continuous trans-

basin flows only in their mature phase. At other times, incipient or relic patches are

identifiable in the surface geostrophic flow.

By November, with the onset of the winter monsoon, the geostrophic SMC (GSMC)

breaks into separate currents in the Arabian Sea and the Bay of Bengal, this split

into two relic currents being caused by the continuity of the flow along the coast ofthe Indian subcontinent, and by the relentless westward propagation of the sea-level

anomalies associated with the GSMC. In the Arabian Sea, the relic GSMC is re-

stricted west of 72E, and it appears as a geostrophic flow around a low in sea level;

this low has propagated westward from the Indian coast, where it appeared during

the summer monsoon as the Lakshadweep low. The relic GSMC flows southwest-

ward to the west of the low and eastward to its south. In the bay, the relic GSMC

flows northeastward as the eastern arm of a geostrophic flow around a low in sea

level off the east coast of India; its western arm is the equatorward EICC. Thegeostrophic WMC (GWMC) first appears during November as a westward flow

south of Sri Lanka. This incipient GWMC is fed by the equatorward EICC and it

feeds, in turn, the poleward WICC.

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The relic GSMC continues to shift westward, and by December, it is restricted to

the west of 65E in the Arabian Sea. The sea-level low abuts the Somali coast, and

the southwestward GSMC to its west is now synonymous with the equatorward

Somali Current. In the bay, what remains of GSMC is barely recognizable even

as a relic. In December, the GWMC is fed by the EICC, but it also appears in the

southern bay as a weak westward flow southeast of Sri Lanka. It flows west beyond

70E before turning to flow around the Lakshadweep high, which forms by this

time off southwest India and Sri Lanka.

By January, the relic GSMC is restricted to a minor eastward flow west of 55E, and

the most significant geostrophic flows in the north Indian Ocean are the GWMC,

the WICC, and the recirculations around eddies in western Arabian Sea and west-

ern Bay of Bengal. The EICC reverses to flow poleward off Sri Lanka, and the

GWMC now appears as a westward flow across the southern bay at

6N; it flows

southwestward in the eastern bay. The GWMC flows westward halfway across the

southern Arabian Sea at 5 N, where it turns to flow northeastward around a high

in sea level and into the WICC. A branch of the GWMC, however, turns to flow

around the Lakshadweep high and into the WICC. The western high is distinct from

the Lakshadweep high and retains its identity within a region of high sea level even

later in the season. Thus, the GWMC flows westward to the south of the sea-level

high and eastward to its north.

Westward propagation of the sea-level anomalies is evident in the north Indian

Ocean, and the relic GSMC finally disappears in February. The southwestward

GWMC in the eastern bay has shifted west since January and is located

93E.

The sea-level highs in the southern Arabian Sea have also spread and shifted west-

ward, with the result that a continuous geostrophic flow exists around a sea-level

high in the southern Arabian Sea. The GWMC flows westward (eastward) to the

south (north) of the high. The eastward GWMC, as earlier in the season, feeds a

current parallel to the Indian west coast; this poleward current was the WICC in

January, but has since shifted offshore.

Westward propagation of the sea-level anomalies continues during March. In the

Bay of Bengal, the southwestward GWMC is located at 83 E and the GWMC no

longer exists in the eastern bay. This part of the GWMC is coupled to a distorted

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anticyclonic gyre in the western bay. The sea-level high in the southern Arabian

Sea extends across the basin. The GWMC flows around the high, and at the west-

ern boundary, it is synonymous with the poleward Somali Current between 46N.

The eastward GWMC to the north of the high is also fed by an equatorward So-

mali Current. To the north of the eastward GWMC is a low in sea level, and the

southwestward flow to its north feeds the equatorward Somali Current.

The circulation during April is dominated by eddies. The GWMC in the bay has

shifted westward and it now intersects the east coast of Sri Lanka; this breaks the

GWMC into separate currents in the bay and the Arabian Sea. The relic GWMC in

the bay is the eastern arm of an anticyclonic gyre. The relic GWMC in the Arabian

Sea has weakened and meanders more, but its spatial structure is similar to that in

March.

During May, the relic GWMC in the bay flows southwestward from the central

bay to Sri Lanka. In the Arabian Sea, it appears primarily as a westward flow at

7

N and is now fed by the equatorward WICC. The sea-level highs are now well

offshore, and the eastward flow to their north is weaker and meanders more than

in April. An eastward flow appears in the southern bay between 48

N; it flows

right across the basin, from the eastern boundary to Sri Lanka. This is the incipient

GSMC, and it appears first in the bay even as the summer monsoon sets in.

In June, the relic GWMC is restricted to the central and western bay, flowing south-

westward from (90

E, 16

N) to the northern tip of Sri Lanka. In the Arabian Sea,the relic GWMC is traceable as a meander to the south and north of the sea-level

highs in the southwest of the basin. The eastward relic of the GWMC, and the

southeastward flow to the east of the high can be considered the incipient GSMC

in the western and central Arabian Sea because the sea-level highs that propagated

westward across the Arabian Sea lose their identity in the eddies off Somalia dur-

ing July, when the anticyclonic geostrophic flow around them is part of the GSMC.

By June, the GSMC is evident in the bay, flowing northeastward from the southern

tip of Sri Lanka to (90

E, 14

N); a branch of the GSMC also recirculates arounda low in sea level to the east of Sri Lanka. The GSMC in the bay is pushed west-

ward by the westward movement of a sea-level high from the eastern equatorial

Indian Ocean. The GSMC also appears in the Arabian Sea as a flow around the

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Lakshadweep low off southwest India, and it is fed by the equatorward WICC. In

the southern Arabian Sea, there is eastward flow at 5N between 6575

E, merging

into the GSMC south of Sri Lanka. There is, however, no clear link yet between the

GSMC in the eastern Arabian Sea and the incipient GSMC (or relic GWMC) in the

west.

The GSMC shifts westward in the bay during July, pushed by the sea-level high

to its east. One branch appears as a northeastward flow from the southern tip of

Sri Lanka almost to the northeastern boundary of the bay. Around 87E, another

branch of the GSMC flows due north and feeds into the relic GWMC, which has

shifted westward and now appears as the eastern arm of an anticyclonic circulation

around a high in sea level in the western bay. The GSMC south of Sri Lanka peaks

in July with current strengths reaching 60 cm s 1. It is fed by a flow around the

Lakshadweep low, this being a branch of the GSMC that is fed by the WICC, and

by an eastward flow across the southern Arabian Sea. This eastward GSMC in the

south is the continuation of the GSMC in the western and central Arabian Sea; this

branch of the GSMC flows geostrophically around the sea-level high (low) in the

western (eastern) Arabian Sea. Though the GSMC now exists as a continuous trans-

basin current from the northern limits of the Somali Current to the southern tip of

Sri Lanka and into the Bay of Bengal, the current consists of two branches, and the

maximum inflow into the GSMC south of Sri Lanka comes from the branch that

flows around the Lakshadweep low, rather than from the eastward GSMC across

the southern Arabian Sea.

Westward propagation continues in the Bay of Bengal, and the GSMC is detached

from the eastern boundary by August. It can be traced as a continuous current from

the southern tip of Sri Lanka to the northern bay, though there are cyclonic eddies

associated with it east of Sri Lanka and in the central bay. In the Arabian Sea,

the Lakshadweep low propagates westward, taking the GSMC and WICC with it;

the coastal current off the Indian west coast collapses. Westward propagation of

the sea-level highs in the western Arabian Sea compresses them and strengthens

the geostrophic flow around them. Except for this part of the basin, the GSMC isweaker in August than in July.

With the monsoon winds weakening during September, so does the GSMC, ex-

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cept in the western bay and, to a lesser extent, in the western Arabian Sea, where

westward propagation compresses the sea-level lows and highs and strengthens

the geostrophic flow associated with them. The Lakshadweep low is now detached

from the coast and a part of the GSMC recirculates around it. The sea-level high

that propagated westward across the southern bay is now almost at the southern tip

of Sri Lanka, and the GSMC is squeezed against the coast. There is geostrophic

flow around this high, and a westward current exists to its south, flowing from the

Andaman Sea to south of Sri Lanka.

The sea-level high that pushed the GSMC westward through the bay abuts the Sri

Lankan coast in October, breaking the connection between the GSMC in the bay

and the Arabian Sea. In the bay, the relic GSMC flows from the east coast of Sri

Lanka into the northcentral bay, and it is associated with, and is fed by the recircu-

lations around, sea-level lows in the western bay. The EICC reverses in October to

flow equatorward as the western arm of these cyclonic geostrophic flows. The Lak-

shadweep low is now well offshore and the WICC also reverses to flow poleward

off southwest India. The eddies and sea-level highs in the western Arabian Sea be-

gin disintegrating, but the relic GSMC is still traceable as a distinct southwestward

current in the western Arabian Sea; this relic current turns to flow eastward in the

southern Arabian Sea.

Longitude-time plots of the sea-level anomalies (Figure 5) clearly show westward

propagation at all latitudes, with the speed of propagation decreasing with increas-

ing latitude. Similar westward propagation has been noted earlier in hydrogra-phy (Kumar and Unnikrishnan, 1995; Unnikrishnan, Kumar, and Navelkar, 1997;

Rao, 1998) and altimetry (Perigaud and Delecluse, 1992), and has been attributed

to westward propagating Rossby waves. Most striking, however, is the break in

the westward propagating signal at 805

E, south of Sri Lanka, throughout the

year, even though the GSMC and GWMC form continuous currents from the Ara-

bian Sea to the Bay of Bengal. Latitude-time plots of the zonal component of the

geostrophic current at 805

E (Figure 6) show that the signal south of Sri Lanka

is dominated by the annual harmonic north of

3

N, and by the semiannual har-monic in the vicinity of the equator; this is also seen in the zonal geostrophic cur-

rent derived from the climatologies of Levitus and Boyer (1994) and Levitus et al.

(1994) (Figure 7). This is in agreement with direct current measurements in this

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Fig. 5. Longitude-time plots of the monthly climatology of TOPEX/Poseidon sea-level

anomalies (cm) at 10

N (top panel), 8

N (middle panel), and 5

N (lower panel). Negative

anomalies are indicated by dashed contours and the contour interval is 5 cm. Westward

propagation is evident in both Arabian Sea and Bay of Bengal at all latitudes. At 5

N, there

is a break in the signal at 805

E (south of Sri Lanka), even though there is no land barrier

there.

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Fig. 6. Latitude-time plots of the monthly climatology of zonal geostrophic current

(cm s

1), derived from TOPEX/Poseidon altimetry, at 805

E (south of Sri Lanka). West-

ward flow is indicated by dashed contours and the contour interval is 5 cm s 1.

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Fig. 7. Depth-time plots of the meridionally averaged zonal geostrophic current (cm s

1)

derived from the climatologies of Levitus and Boyer (1994) and Levitus et al. (1994). The

left (right) panel shows the current averaged over 0

3

N (3

6

N). The depth is in metres.

Westward flow is indicated by dashed contours and the contour interval is 5 cm s

1.

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region (Schott et al., 1994; Reppin et al., 1999), and suggests that the monsoon-

current system south of Sri Lanka is distinct from the Equatorial Current farther

south, even though it is difficult to distinguish between these two regimes during,

say, DecemberFebruary, when the WMC flows westward, as does the Equatorial

Current. The core of the climatological GWMC and GSMC south of Sri Lanka is

not at the coast, but is separated from it. The core of the GWMC is at 5 N during

NovemberDecember and at 4 5N during JanuaryFebruary. The core of the

GSMC is at

475

N during JuneJuly and at

5

N during AugustSeptember.

The geostrophic currents derived from altimetry show that excepting the GSMC

and the GWMC in the vicinity of Sri Lanka, strong geostrophic flows in the north

Indian Ocean are associated either with coastal currents, or with recirculations

around eddies or highs and lows in sea level. These flows, however, are not weak.

For example, south of Sri Lanka, the GWMC (GSMC) attains speeds of 50cms 1

during NovemberJanuary (JuneSeptember). These magnitudes are comparable to

the Ekman drifts, except in the western Arabian Sea. Hence, we expect geostrophyto make a significant contribution to the surface circulation in the north Indian

Ocean.

2.3 The net flow at the surface

In the preceding subsections, we presented the monthly climatologies of Ekman

drift and geostrophic flow at the surface. The sum of these two components consti-

tutes, barring a residual, the net flow (NF) at the surface (Hastenrath and Greischar,

1991). We compare this estimated surface current with ship drifts (from the Ocean

Current Drifter Data CDROMs NODC-53 and NODC-54, NODC, US Department

of Commerce, NOAA) in Figure 8.

The NF matches the ship drifts well in the regime of the monsoon currents. Dur-

ing November, geostrophy dominates in the Bay of Bengal and along the coast of

the Indian subcontinent. The equatorward EICC, the westward WMC south of SriLanka, and the poleward WICC form a continuous current in both NF and ship

drifts; the latter, however, are stronger than the former. Elsewhere, the picture is

more confused. Both geostrophy and Ekman drift, however, combine to keep alive

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Fig. 8. The net flow (NF) at the surface (cm s

1, left panel), computed as the sum of Ekman

drift (Figure 3) and geostrophic flow (Figure 4), and ship drifts (cm s

1, right panel). The

source for the ship drifts are the Ocean Current Drifter Data CDROMs NODC-53 and

NODC-54 (NODC, US Department of Commerce, NOAA).

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Fig. 8. (continued)

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Fig. 8. (continued)

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Fig. 8. (continued)

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a weak eastward flow, a relic of the SMC, in the southern bay. Unlike in October,

this current does not extend to Sri Lanka. The relic SMC in the Arabian Sea appears

as a southwestward flow from ( 65E, 12

N) to ( 55

E, 4

N), where it turns to

flow eastward.

Ekman drift strengthens in December, but it dominates geostrophy only in the

northern Arabian Sea, where geostrophic currents are negligible. The drift, how-

ever, is crucial for producing good agreement between the NF and ship drifts. It

combines with geostrophy to produce a strong westward WMC in the southern

bay. The Ekman drift accentuates the westward geostrophic flow to the north of the

sea-level low in southwestern Arabian Sea (Figure 4), and attenuates the eastward

GSMC to its south; as a consequence, the WMC appears farther north in western

Arabian Sea compared to eastern Arabian Sea. It branches around 65E, one branch

flowing around the Lakshadweep high, the other continuing to flow westward. The

latter branch of the WMC is synonymous with the relic SMC to the north of the sea-

level low. It is the Ekman drift that enables the WMC to extend from the easternbay to the western Arabian Sea as early as December, before the winter monsoon

peaks.

The NF in January is similar to that in December, but it is stronger. Both geostrophy

and Ekman drift contribute to the WMC in the bay. In the Arabian Sea, the Ekman

drift is responsible for extending the WMC west of 60

E, but geostrophic flow

around the sea-level highs dominates in the east. There is gentle westward drift

across the northern Arabian Sea, where geostrophic flow is weak. The NF is in

excellent agreement with ship drifts both in the domain of the WMC and outside it,

even though the ship drifts are noisier in the Arabian Sea.

During February, Ekman drift is weak, and the NF is dominated by eddies, result-

ing in relatively poor agreement with ship drifts outside the domain of the strong

WMC. Owing to the dominance of geostrophy, the WMC is almost identical to the

GWMC.

This dominance of geostrophy continues through to April. During March, Ekmandrift contributes only in the southwestern Arabian Sea, where it accentuates the

westward WMC to the south of the sea-level highs and attenuates the eastward

WMC to their north. As in February, there is good agreement between the NF and

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ship drifts only in the domain of the WMC. The major discrepancy occurs in the

bay, where ship drifts show a basin-wide anticyclonic gyre, but the NF shows a

distorted anticyclonic gyre restricted to the western bay. The poleward EICC is

also stronger in the ship drifts.

The winds are weakest in April, and the WMC is identical to the GWMC. The

GWMC, as described earlier, appears in three distinct parts: a southwestward relic

in the western bay, a meandering westward flow south of a high in sea level in the

southern Arabian Sea, and a meandering eastward flow to the north of the high sea

level. The two relics of the WMC in the Arabian Sea are connected by a poleward

Somali Current. This is in agreement with the ship drifts, in which too relics of

the WMC appear as three separate currents; the only difference is that the Somali

Current flows poleward all along the coast in the ship drifts, but not in the NF. The

EICC continues to flow poleward in both ship drifts and NF, but the latter exhibits

a richer structure.

The SMC appears in the bay in both ship drifts and NF during May, but it is broader

in the latter because of the strong Ekman drift across the southern bay. The SMC

can be traced as a continuous current from the northern limit of the poleward Somali

Current to the eastern bay in both data sets. A primarily geostrophic WICC feeds

into the SMC. Ekman drift and geostrophic flow combine to produce the SMC in

the Arabian Sea, Ekman drift dominating in the west and geostrophic flow in the

east. In the NF, these two components combine to form a curving flow across the

Arabian Sea, but this current flows zonally across the basin at 10N in the ship

drifts.

Ekman drift swamps geostrophic flow when the summer monsoon winds strengthen

in June. This is more so in the Arabian Sea, where the currents cross the isolines of

sea level, and a little less in the bay, where weaker winds compared to the Arabian

Sea (Figure 2) combine with the multiplicity of eddies (Figure 4) to make the NF

different from the uniform eastward Ekman drift. Though the ship drifts are a little

noisier, the dominance of Ekman drift is seen in them too. In June, therefore, the

SMC appears as an eastward or southeastward flow in the Arabian Sea.

The summer-monsoon winds peak in July; so does the Ekman drift. Geostrophy

makes a significant contribution to the surface current only in the bay and south of

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Sri Lanka. Again, there is good agreement between the NF and ship drifts, but the

latter appear to be affected somewhat less by the direct Ekman forcing.

Apart from a weakening of the SMC owing to slightly weaker winds, the NF and

ship drifts during August are as in July.

The summer-monsoon winds continue to weaken through September, making geo-

strophy more relevant to the surface flow field. In the Arabian Sea, anticycloniccurvature of the Ekman flow increases compared to August, and Ekman drift dom-

inates, except in the southeast, where geostrophic flow around the Lakshadweep

low is important, and in the vicinity of the Great Whirl off Somalia, where there is

strong geostrophic flow around a high in sea level. In the bay, Ekman drift domi-

nates in the south, forcing broad eastward SMC, but geostrophy is strong enough to

make a part of the flow turn northeastward into the central bay. In the rest of the bay,

Ekman drift is weaker, and geostrophic flow around eddies dominates. Once again,

there is good agreement between the NF and ship drifts, except in the western bay;

the EICC is equatorward in the ship drifts, but the NF presents an eddy-dominated

flow field.

October being a month of transition, Ekman drift is negligible, except in the south-

ern bay and to the southwest of Sri Lanka. Nevertheless, it combines with geostro-

phy to sustain the SMC as a continuous current from the central Arabian Sea to the

eastern bay. The SMC is split in the bay, a separate relic existing as a geostrophic

northeastward flow from eastern Sri Lanka to the central bay. These features are

also evident in the ship drifts, except that the relic SMC is farther east. The ship

drifts also show strong currents in the western Arabian Sea, but this is not seen in

the NF.

The Ekman drift eliminates the westward propagation seen in the zonal geostrophic

currents (Figure 5). It is dominated by the annual harmonic south of Sri Lanka, and

therefore, though weaker than the geostrophic flow here, accentuates the annual

signal in the NF. Ekman drift dominates the surface circulation in the north In-

dian Ocean during the summer monsoon, with geostrophic flow being significantonly south of Sri Lanka, around the eddies off Somalia, and in the Bay of Bengal.

Geostrophic flow and Ekman drift combine to produce the surface flow depicted

in the ship-drift data during the winter monsoon. During the transitions between

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the monsoons, in MarchApril and October, the geostrophic flow is still strong, es-

pecially along the Indian coasts and in the domain of the monsoon currents, and

it dominates the circulation. Thus, both Ekman drift and geostrophic currents are

important components of the surface circulation associated with the monsoon cur-

rents.

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3 The monsoon currents in an OGCM

The foregoing description of the monsoon currents based on observations shows

that both Ekman drift and geostrophy contribute to the surficial circulation in the

north Indian Ocean. The former decays rapidly with depth, while the latter is ex-

pected to be significant even in the subsurface layers. To ascertain the vertical struc-

ture of the monsoon currents and to make estimates of the transports due to them,

we use a multi-level Ocean General Circulation Model (OGCM) to simulate the

circulation associated with the monsoon currents.

3.1 Numerical model

The OGCM is based on the Modular Ocean Model (Pacanowski, 1996). The model

domain covers the tropical Indian Ocean (30S30

N, 30115

E). The model has

realistic coastline and topography based on the ETOPO5 data set. The horizontal

resolution is 033

0

33

and there are 25 levels in the vertical, of which 8 are in

the top 100 m. Horizontal eddy viscosity and diffusivity are 2 107 cm2 s 1 and

107 cm2 s 1 respectively, and vertical mixing is parameterized using the scheme of

Pacanowski and Philander (1981). The model is spun up for 5 years from a state of

rest and climatological temperature (Levitus and Boyer, 1994) and salinity (Levituset al., 1994) using the wind-stress climatology of Hellerman and Rosenstein (1983).

The model reproduces the monsoon circulation in the Indian Ocean reasonably

well. Vinayachandran et al. (1999a) compared the SMC along 6

N in a similar

model with TOPEX/Poseidon altimetry and geostrophic currents. They found that

the model SMC near Sri Lanka compares well with that derived from TOPEX/Po-

seidon altimetry, though the core of the SMC east of Sri Lanka was weaker in the

model compared to that derived from XBT data. Vinayachandran, Saji, and Yama-gata (1999b) used the model to investigate the unusual conditions in the equatorial

Indian Ocean in 1994 and noted that the model Equatorial Current is consistent

with direct current measurements (Reppin et al., 1999).

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3.2 The model circulation

The flow in the first 3 model levels is dominated by Ekman flow, geostrophy dom-

inating below this. At each level, however, both contribute to the model flow field.

Hence, we present the model circulation at 5 m (Figure 9) and 35 m (Figure 10)

depths, comparing them with the Ekman drift (Figure 3) and geostrophic flow (Fig-

ure 4). Then we present the depth-averaged flow in the top 50 m (Figure 11), whichis more representative of the shallow monsoon currents than the surface flow alone.

As the model flow is a composite of both Ekman and geostrophic flows, there are

several additional features in the model flow at 5 m (Figure 9) compared to the

estimated Ekman drift (Figure 3). For example, unlike the estimated Ekman drift,

the model flow at 5 m contains strong coastal currents. These include the Somali

Current, the coastal current off Oman, the EICC, and the WICC. The EICC flows

equatorward during November and feeds the westward WMC south of Sri Lanka,

this flow standing out from the eastward jet at the equator. In the western Arabian

Sea, there is a westward drift at

9N; this is the relic SMC or the incipient WMC.

The trans-basin WMC is seen during JanuaryMarch. Most of the flow is westward

at

5N, but a branch of the WMC turns to flow around the Lakshadweep high

in the southwestern Arabian Sea; this branch of the WMC propagates westward.

Apart from the WMC, the outstanding feature during the winter monsoon is the

anticyclonic gyre in the bay. Thus, during the winter monsoon, when the winds are

relatively weak, geostrophy dominates even at the surface, Ekman drift modulating

the geostrophic currents.

Ekman drift dominates the model surface flow during the summer monsoon. This is

more so in the Arabian Sea, where the winds are strongest (Figure 2). Nevertheless,

the coastal currents, the intrusion of the SMC into the bay, and the flow around the

eddies off Somalia, which are due to geostrophy, are evident even at 5 m. The

model SMC south of Sri Lanka is fed by the general southeastward drift across the

Arabian Sea; unlike in the NF, the contribution from the flow due west is much less.

The model currents at 35 m (Figure 10) are dominated by geostrophy because

the Ekman drift decays exponentially with depth; this flow compares well with

the geostrophic current estimated from TOPEX/Poseidon altimetry (Figure 4). The

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Fig. 9. OGCM currents (cm s

1) at 5 m.

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1) at 35 m.

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1) averaged over the top 50 m.

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genesis and decay of the (G)WMC and (G)SMC follow the pattern established from

satellite altimetry. There does not, however, seem to be a clear link between the re-

circulation in the eddies off Somalia and the SMC south of Sri Lanka; the latter is

fed more by the branch of the SMC that flows around the Lakshadweep low, this

in turn being fed by the WICC and the flow across the central Arabian Sea. The

recirculation in the eddies off Somalia instead flows mostly into the eastward cur-

rent in the western equatorial Indian Ocean. This eastward current feeds the SMC

south of Sri Lanka in ship drifts (Figure 8) and in the simulations of McCreary

et al. (1993); in the OGCM, however, this flow crosses the equator, as it does in the

surface-drifter data (Shenoi et al., 1999a).

In the depth-averaged flow (DAF) in the top 50 m (Figure 11), the WMC appears

primarily as a geostrophic flow modulated by Ekman drift. During the summer

monsoon, the SMC in the Arabian Sea appears as a strong southeastward drift

modulated by geostrophy; in the bay, the SMC is primarily geostrophic. Unlike

in the 35 m flow field, the DAF around the eddies off Somalia merges with the Ek-

man drift to form a single broad southeastward SMC across the Arabian Sea; this

current forms the broad western arm of the flow around the Lakshadweep low off

southwest India, and feeds into the SMC south of Sri Lanka. A part of the recircu-

lation around the eddies off Somalia also feeds the eastward Equatorial Current in

the western Indian Ocean. Since geostrophy makes a significant contribution to the

DAF, it does not compare well with the NF (Figure 8) during the summer monsoon,

when the winds are strong and the Ekman drift swamps the geostrophic current at

the surface. At subsurface levels, however, geostrophic flow is significant, as is ev-

ident from the DAF in the OGCM. Hence, the comparison between the DAF and

the NF is good during the winter monsoon.

Westward propagation associated with Rossby waves forms an essential component

of the monsoon circulation in the north Indian Ocean, as seen in the longitude-time

plots of OGCM meridional velocity at 5N and 8

N (Figures 12 and 13). This prop-

agation is best seen in the flow at 35 m. In the bay, however, westward propagation

is seen at all depths over the entire year, indicating the dominance of geostrophythere. A prominent example is the westward propagation of the SMC (Figures 9

11); this, however, shows a break south of Sri Lanka (Figure 13), like the sea-level

anomalies in altimetry (Figure 5). In the Arabian Sea, westward propagation is

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Fig. 12. Longitude-time plot of the OGCM meridional velocity (cm s

1) at 8

N. Southward

flows are indicated by dashed contours and the contour interval is 5 cm s

1.

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Fig. 13. Longitude-time plot of the OGCM meridional velocity (cm s

1) at 5

N. Southward

flows are indicated by dashed contours and the contour interval is 5 cm s

1. Note the break

in the westward propagation at 80

E.

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seen at all depths during the winter monsoon; a prominent example is the westward

propagation of the WMC associated with the Lakshadweep high (Figures 911).

During the summer monsoon, the dominance of Ekman drift eliminates the Rossby

wave signal at 5 m, but westward propagation is seen at 35 m; this signal, however,

is weak, possibly because the strong eastward Ekman drift slows down the Rossby

waves (Vinayachandran and Yamagata, 1998).

3.3 Transport estimates

The monsoon currents are shallow, unlike the deep currents observed in the west-

ern boundary current off Somalia and the associated eddies during the summer

monsoon (see, for example, Figure 7 and Schott and McCreary (2001)). The shal-

lowness of the monsoon currents in the OGCM, especially the SMC, is seen in

the depth-time of the zonal current at 80.5E (Figure 14). The SMC was shown

by Vinayachandran et al. (1999a) to be trapped close to the surface because of

the downwelling Rossby wave propagating westward from the eastern boundary of

the equatorial Indian Ocean; this signature is seen in the westward flow below the

eastward SMC during the summer monsoon. Upward (downward) propagation of

phase (energy) is evident, indicating the existence of free propagating waves.

The heat advected by the SMC has been shown (Shenoi, Shankar, and Shetye, 2001)

to be important for the heat budget of the near-surface Arabian Sea. Given the large

difference in salinity between the Bay of Bengal and the Arabian Sea, the contri-

bution of the SMC and WMC to advection of salt is even greater. Since subsurface

observations of these seasonal, open-ocean currents are limited (see Table 2), nu-

merical models that simulate well the observed surface circulation are an important

source of information on the transports associated with them. Given the significant

contribution of Ekman flow to the depth-averaged flow in the top 50 m, a com-

parison between the model transport and estimates based on hydrography is not

meaningful. Hence, we choose to compare the model transports with those esti-

mated from direct current measurements south of Sri Lanka (Schott et al., 1994)

(Table 3).

Latitude-time plots of the depth-integrated zonal current (over the top 100 m) are

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Fig. 14. Depth-time plot of the OGCM zonal current (cm s

1) at 80.5

E (south of Sri

Lanka). The upper (lower) panel shows the current averaged over 36

N (03

N). West-

ward flows are indicated by dashed contours and the contour interval is 5 cm s

1.

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Period Observed Model

10 Jan to 15 Feb -12.8 -10

10 Jan to 15 Feb

-10.4 -10

1 Jun to 5 Jul 8.4 9.1

10 Jul to 15 Aug 4.1 6.2

1 Jun to 15 Aug 7.8 7.9

1 Nov to 30 Mar -7.1

1 May to 30 Sep 5.4

Table 3

Observed and model zonal transports (in Sv; 1 Sv

106 m3 s

1) in the top 300 m between

3

45

N and 5

52

N at 80.5

E (south of Sri Lanka). Positive (negative) values indicate east-ward (westward) flow, and the values listed are averages over the period indicated. All

observations, except that marked (*), are for 1991; the marked observation is for 1992.

The observed transports are derived from the direct current measurements of Schott et al.

(1994). The model was forced by climatological wind stress (Hellerman and Rosenstein,

1983). The last two values are average model transports for the winter and summer mon-

soons, respectively.

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shown in Figure 15. In the Arabian Sea (65E), the core of the westward (eastward)

WMC is at 4N (8

N); in the bay (85

E) and south of Sri Lanka (80.5

E), the core

of the westward WMC is at 5N. The WMC is relatively narrow and strong over

the entire basin. In contrast, the SMC stands out only south of Sri Lanka and in

the bay, where its eastward flow is seen to shift poleward with time, marking the

westward propagation of the northeastward SMC across the bay. In the Arabian Sea

at 65E, the depth-integrated SMC is weak, but broad, the eastward flow extending

from

1220N during the summer monsoon; the weak westward flow between

46

N and the weak eastward flow between 04

N show the complicated spatial

Ekman Drift

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