Ecosystem Changes in the Northern South China Sea - Earth Science: New Methods and Studies - page 164

According to Takano et al. (1998) and Liao et al. (2006), there is a cyclonic eddy in
the sea area around Station 4 (near Dongshan Islands) in summer. Whenever a medium
and strong El Nino occurs, the summer monsoon is weak (Zhang et al., 2003; Zhu et al.,
2000), which induces strengthened cyclonic eddy, leading to strong upwelling, result-
ing in low T av and DO av , and high S av , nutrients and Chl-a induced by phytoplankton
growth. During a La Nina event, the opposite occurs. This is due to ENSO/La Nina
events affecting the strength of the summer monsoon related, that is both El Nino and
La Nina make the anomaly of Walker circulation. During El Nino event, the heat con-
vection of the western Pacifi c warm pool moves to the central Pacifi c. In contrast, dur-
ing La Nina event, the heat convection of the western Pacifi c warm pool moves back
to the western Pacifi c. That makes the anomaly of Walk circumfl uence. Namely, when
phase of El Nino (La Nina) fall under the infl uence of the anomal Walker circulation,
subsidence (ascending) air current occur over low latitude and middle latitude of the
east Asia. If this subsidence (ascending) air current fl ow over low latitude occurred in
summer, and joined to west-south monsoon, it will result in weaken (strengthen) of
the summer monsoon (Wang et al., 2001; Zhang et al., 2003; Zhu et al., 2000). The
changes of ecological environment in the sea area around Station 4 in summer respond
to ENSO events, namely, respond to the abnormity of summer monsoon.
Figure 10. The interannual changes of the water column average values of the seawater temperature
(a), (b) salinity, (c) PO 4 -P, (d) SiO 3 -Si, (e) DIN, and (f) DO at the Station 4 of the nSCS in summer
during the observation period.