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the southwesterly winds. The axis of this wind seems to divide the SCS into
two parts, with Ekman upwelling and downwelling in the north and south
areas, respectively (Fig. 2(B)). The Ekman Pumping image (Fig. 2(C)) dis-
plays a strong upwelling tendency in the west basin southeast of Vietnam.
3.3.
Spatial variability of SST
Figure 2(D) indicates that high temperatures prevail in most of the SCS
(
>
29.5
◦
C), while in the western SCS, low temperatures are obvious, in
the shape of ellipse with a short radius of approximately 300 km to the
southeast of Vietnam in the western SCS. This location matches roughly
with the area of high positive Ekman pumping velocity, and especially a
clearly defined lower jet-shape SST (
<
28.5
◦
C) to northeast PRB.
4. Discussion
4.1.
Key factors limiting the growth of phytoplankton
Generally, the availability of nutrients and light radiation are the key factors
limiting the growth of phytoplankton. However, in the SCS light may not
be a key factor because it is located in the tropics, whereas the accessibility
of nutrients may perhaps be the determining factor.
8
,
18
The hydrological
conditions in the study area are complex, varying with space and time, and
the interaction between these factors must have had a considerable influence
on the transport and distribution of nutrients in the study region, ultimately
affecting the growth of phytoplankton. Hence, we place an emphasis on
discussing important hydrological features influencing the distribution of
Chl-
a
concentration in this study.
4.2.
The distribution of Chl-
and oceanic conditions
a
In the southeastern offshore part of Vietnam, where high Chl-
a
concentra-
tion appears (oval-shape in Fig. 2(A)), intense Ekman Pumping upwelling
(Fig. 2(C)) induced by positive SSWS (Fig. 2(B)) eddies will transport cold
water from the sub-surface to surface layer, lowering SST (Fig. 2(D)) and
increasing Chl-
a
concentration (Fig. 2(A)). The concept of Ekman Pump-
ing velocity is seldom introduced in similar previous research. Our results
(Fig. 2(A) and 2(B)) also show that high Chl-
a
concentration matches posi-
tive Ekman Pumping velocity and low SST (Fig. 2(D)) in terms of intensity,
shape and location. The result may mean that the region of high Chl-
a
