Journal of Oceanography, Vol. 60, pp. 205 to 218, 2004
⋅ Air-sea interaction,
⋅ coastal circulation,
⋅ water masses,
⋅ primary produc-
⋅ eastern Arabian
* Corresponding author. E-mail: email@example.com
Present address: National Center for Antarctic & Ocean Research,
Govt. of India, Headland Sada, Vasco-da-Gama, Goa 403804, India.
Copyright © The Oceanographic Society of Japan.
Air-Sea Interaction, Coastal Circulation and Primary
Production in the Eastern Arabian Sea: A Review
ALVARINHO J. LUIS* and HIROSHI KAWAMURA
Center for Atmospheric and Oceanic Studies, Faculty of Science, Tohoku University,
Sendai 980-8578, Japan
(Received 14 November 2002; in revised form 21 April 2003; accepted 19 May 2003)
Air-sea interaction, coastal circulation and primary production exhibit an annual
cycle in the eastern Arabian Sea (AS). During June to September, strong southwest-
erly winds (4~9 m s–1) promote sea surface cooling through surface heat loss and
vertical mixing in the central AS and force the West India Coastal Current
equatorward. Positive wind stress curl induced by the Findlater jet facilitates Ekman
pumping in the northern AS, and equatorward-directed alongshore wind stress in-
duces upwelling which lowers sea surface temperature by about 2.5°C (compared to
the offshore value) along the southwestern shelf of India and enhances phytoplankton
concentration by more than 70% as compared to that in the central AS. During win-
ter monsoon, from November to March, dry and weak northeasterly winds (2–6
m s–1) from the Indo-China continent enhance convective cooling of the upper ocean
and deepen the mixed layer by more than 80 m, thereby increasing the vertical flux of
nutrients in the photic layer which promotes wintertime phytoplankton blooms in the
northern AS. The primary production rate integrated for photic layer and surface
chlorophyll-a estimated from the Coastal Zone Color Scanner, both averaged for the
entire western India shelf, increases from winter to summer monsoon from 24 to 70
g C m–2month and from 9 to 24 mg m–2, respectively. Remotely-forced coastal Kelvin
waves from the Bay of Bengal propagate into the coastal AS, which modulate circula-
tion pattern along the western India shelf; these Kelvin waves in turn radiate Rossby
waves which reverse the circulation in the Lakshadweep Sea semiannually. This re-
view leads us to the conclusion that seasonal monsoon forcing and remotely forced
waves modulate the circulation and primary production in the eastern AS.
ing an intense low-level jet (Findlater, 1977) over the
central AS. In response to these winds a clockwise circu-
lation evolves in the AS (Wyrtki, 1971; Schott, 1983;
Swallow, 1984). The equatorward eastern boundary of this
anticyclonic circulation is known as the West India
Coastal Current (WICC) (Shetye et al., 1990). To the south
of Sri Lanka, the WICC merges with the eastward flow-
ing Southwest Monsoon Current (SMC) which bends
around the Sri Lankan coast and flows poleward into the
Bay of Bengal (Fig. 1(a)).
During the winter or northeast monsoon, generally
from November to February, the winds blow from the
northeast. These winds force the Northeast Monsoon
Current (NMC) toward the west (Fig. 1(b)). Part of this
flow bifurcates at the southwest Indian coast and flows
poleward to form the WICC (Fig. 1(b)) (Shetye et al.,
One of the highest primary productions among the
world oceans occurs in the Arabian Sea (AS) due to a
variety of physical processes introduced by the semian-
nual reversal of monsoon winds (Qasim, 1982; Banse,
1987). This basin, which forms the western flank of the
north Indian Ocean, offers a striking example of wind-
driven ocean circulation (Wyrtki, 1973). During the south-
west or summer monsoon, generally from June through
September, strong winds blow from the southwest form-
206A. J. Luis and H. Kawamura
1991). This poleward flow, which occurs in opposition to
the wind field, is facilitated by a density gradient along
the west coast of India (WCI) (Shetye and Shenoi, 1988).
Reduced solar radiation and high evaporative cooling in
the northern AS lead to high salinity water (>36 ppt), while
low salinity water occupies the region to the south of the
Indian tip (IT). This establishes a density gradient which
leads to the poleward flow of the WICC. During the
intermonsoon period, from March to April and October
to November, weak, highly variable wind regimes (2~3
m s–1) occur in the AS (Hastenrath and Lamb, 1979a) and
the basin surface circulation dissipates (Cutler and
Swallow, 1984). The southwest monsoon period is char-
acterized by sea surface cooling in the central AS
(Colborn, 1976; Düing and Leetmaa, 1980) due to sur-
face heat loss and entrainment (Hastenrath and Lamb,
1979b; Shetye, 1986; Bauer et al., 1991). While a posi-
tive wind stress curl to the north of the Findlater jet pro-
motes Ekman pumping during the summer monsoon, con-
vective cooling during the winter monsoon leads to
densification and downwelling in the northern AS.
In this review we highlight the studies carried out
along the eastern AS, extending from the Pakistan coast
(25.5°N) to south of Sri Lanka (5.5°N) (dashed line in
Fig. 1(a)), and identify “gray” areas that need further at-
tention. The purpose of emphasizing on only the eastern
AS is that recent statistics indicate that of the annual fish
catch, which ranges from 2.2 to 2.8 million tonnes (Cen-
tral Marine Fisheries Research Institute, 1995), about 73%
of the catch originates from the west coast of India due to
a high primary production (PP) in this sector of the AS.
The continental shelf, which we identify by the 500-m
contour in Fig. 1(a), is narrow (50 km) south of Karachi,
widens to ~350 km off the Gulf of Cambay and gradually
tapers towards south to ~120 km at the south Indian tip
At this point, it is relevant to note that the dynamic
processes in the eastern AS are induced by local and re-
mote forcing. During the summer monsoon, offshore di-
vergence of the alongshore wind stress component leads
to coastal upwelling and sea surface temperature (SST)
cooling (Shetye et al., 1985; Muraleedharan and Kumar,
1996; Naidu et al., 1999; Luis and Kawamura, 2002a, b).
Strong summer monsoon winds enhance evaporative cool-
ing in the central AS (McCreary and Kundu, 1989). On
the other hand, wind jets in the equatorial Indian Ocean,
between 5°S to 5°N, excite equatorial Kelvin waves
which, on reflection from the eastern boundary of the Bay
Fig. 1. (a) Geography of the northern Arabian Sea. This review focuses on the region identified by a thick dashed line. Schemat-
ics of summer-monsoon circulation are superimposed. Ekman pumping region in the northern Arabian Sea is highlighted in
yellow tone. Coastal upwelling promoted by divergence of alongshore wind stress component is indicated in green tone.
Current branches indicated are the Ras al Hadd Jet (RHJ), Lakshadweep Low (LL), West India Coastal Current (WICC),
Southwest Monsoon Current (SMC), Sri Lanka Dome (SD) and East India Coastal Current (EICC). The boxes (a), (b), (c) and
(d) are referred in Figs. 7 and 10. The Findlater Jet and wind direction are indicated by bold gray arrows. (b) As in (a), but for
winter monsoon. Convective cooling region is shown in yellow tone. Additional abbreviations shown are: Lakshadweep High
(LH) and Northeast Monsoon Current (NMC).
Air-Sea Interaction, Coastal Circulation and PP in the Eastern Arabian Sea: A Review217
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