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concentration is primarily induced by upwelled water with rich nutrients
through Ekman pumping.
In the nearshore area to southeast Vietnam, the strong southwesterly
wind stress (Fig. 2(B)) parallel to the coastline induces strong offshore
Ekman transport, leading to coastal upwelling with low SST. In this region,
well-defined coastal upwelling can be also identified in the SST image
(Fig. 2(D)). By the intense Ekman upwelling associated with the strong
influence of southwesterly winds, the upwelling east of Vietnam in the
southwestern SCS may lead to conspicuous localized cooling of the area.
The findings of Tang
et al.
9
,
16
and Xie
et al.
6
also revealed similar condi-
tions in the same area under investigations. High Chl-
a
concentration may
be a response to the upwelling in the coastal area. The band of high Chl-
a
concentration over the near-shore region in south Vietnam (south of PRB,
in Fig. 1) coincides with the region of strong wind (Fig. 2(B)), with the
band of low SST parallel to the coastline (Fig. 2(D)). Overall, by the mech-
anism of coastal upwelling in the near-shore area, offshore Ekman pumping
transports cold water from the bottom to the surface and from the coast to
offshore and thereby could induce the continuous supply of nutrients, low
temperature and high Chl-
a
concentration.
In addition, in nearshore regions, another important factor for the sus-
tenance of high Chl-
a
is attributed to the advantageous position of being
close to the discharge waters of the Mekong River, which brings nutri-
ents from upstream. Earlier investigations
19
revealed that there is a strong
northeastward coastal current during the southwest monsoon, originat-
ing from shallow water in the southwest part of the SCS and the maxi-
mum runoff from the Mekong river along the Vietnamese coast (available:
http://www.na.unep.net). This current turns into the northeast basin of
the SCS near 11
◦
N, forming an offshore jet in the northeast direction.
5
As
a consequence, these conditions may also support a wide band of Chl-
a
in the coastal waters southeast of Vietnam and lead to a high Chl-
a
jet
protruding into northeastern basin near PRB, as shown in Fig. 2(A).
5. Conclusions
This study revealed a high Chl-
a
concentrations in the western SCS during
summer, also showing a larger spatial variation in the west than in the
east owing to differences in oceanic conditions, especially the wind stress
and Ekman pumping velocity. The high Chl-
a
concentration and the Chl-
a
jet in the western SCS are related to upwelling induced by southwesterly
