Oceanography | Vol. 38, No. 3
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DISCUSSION
WATER MASS DYNAMICS IN LUZON STRAIT
The water mass exchange in the LS is well known for its sandwich-
like pattern. In the upper layer (water depth shallower than
500 m), there is a complex two-way exchange, but the net flux is
westward. In the middle layer (water depth between 500 m and
1,500 m), the net flux is eastward, while in the deep layer (water
depth deeper than 1,500 m), the net flux is westward. Though
the major source of SCS seawater is WPS seawater, the upwell-
ing and intense vertical mixing cause the salinity and temperature
maxima to decrease while the salinity and temperature minima
increase in the SCS (C.-T.A. Chen et al., 2001, 2006; Gan et al.,
2006; Y. Liu et al., 2000; Tian et al., 2006).
Figure S1 presents temperature/salinity plots that indicate dif-
ferent water masses, which are simplified into two types—SCS-like
and WPS-like—based on the salinity maximum in this study area.
The corresponding locations are marked as red triangles and pink
Xs, and blue triangles and light blue Xs in Figure 1. The value of
34.75 represents the S-max from an equal mixture of WPS and SCS
portions. Although mixing in the subsurface water may differ from
that in the middle layer, this study focuses only on the upper layer.
The WPS-like water occupies the regions east of Luzon Island, the
LS, and the eastern side of the Kuroshio Current. It is worth not-
ing that a WPS-like station at 119°E (Figure 1), which was expected
to exhibit an SCS-like pattern, may result from incomplete mix-
ing. The SCS-like water is found in the western Luzon Islands, the
northwest LS, and off the east coast of Taiwan, to the left of the
Kuroshio, between approximately 21.75°N and 25°N, and even
extends to the ECS region (Figure 1). The influence of fresher ECS
water reduces the salinity in the region north of 25°N, where the
lowest salinity value, around 28, is found in the surface layer (not
shown in Figure S1A). Some data indicate vertical mixing phe-
nomena off the northeast coast of Taiwan that also involve shelf
water and further reduce the salinity to below the typical SCS
water pattern (Hsueh et al., 1992). As the WPS water mixes with
SCS water along the isopycnal, the mixed water exhibits lower tem-
perature and salinity values than the original WPS water. This mix-
ing also suggests an increase in nutrient concentration, as the SCS
has higher nutrient values than the WPS at the same depth in the
upper layer (Figure S2).
NUTRIENT DISTRIBUTION AND
LATITUDINAL TRENDS
It’s important to note that various oceanographic factors, such as
stratification, vertical mixing, eddy-driven transport, typhoons,
winter cooling, horizontal advection, and geostrophic currents
interacting with terrain effects, play significant roles in controlling
the behavior and distribution of nutrients in the regions studied
(Andres et al., 2015; Uchiyama et al., 2017; Jan et al., 2019; Y. Zhang
et al., 2020). These physical conditions and processes influence the
transport and availability of nutrients in the marine environment,
contributing to the observed patterns in nutrient concentrations.
FIGURE 6. Zonal cross sections of temperature, salinity, σθ, AOU, nitrate, phosphate, and silicate northern of the ECS based on the KEEP-MASS data. Color
shading and black contours represent the values of individual parameters.