September 2025 | Oceanography
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in the euphotic zone. The AOU reaches 25–50 µmol kg–1 at the
depth where the nitrate concentration is approximately 1 µM. This
suggests that significant organic matter decomposition, a pro-
cess that consumes oxygen, occurs at a depth of around 160 m
east of 126°E (Figure 2).
Temperature, salinity, and AOU contours all show an upward
trend near the Philippine coast (west of 126°E, Figure 2), accom-
panied by a shoaling of nutriclines, including nitrate, phosphate,
and silicate, toward the coast. This increase in nutrients is not asso-
ciated with SCS waters, as little SCS water is present in the study
area east of the Philippines (Lien et al., 2015).
UPWELLING AND NUTRIENT DYNAMICS
SOUTHEAST OF TAIWAN
The situation changes significantly off southeast Taiwan. In the
NPIW domain (east of 126°E in Figure 2), contours remain rel-
atively flat below 400 m. However, to the southeast of Taiwan
(Figure 3, 21.75°N), waters shallower than 600 m start to tilt up
toward the west. The southern tip of a ridge that lies between the
Philippines and Taiwan is located at approximately 121°E, with the
SCS to its west and the WPS to its east. The temperature, AOU, and
nutrient contours all indicate a westward upwelling feature that
occurs most prominently west of 121°E (Figure 3). This upwell-
ing is so intense that the 1 µM nitrate contour, which was previ-
ously at a depth of 100 m west of 126°E (Figure 2), emerges at a
depth of 50 m at 121°E (Figure 3). This phenomenon, including
the influence of the SCS water, is beyond the scope of this study but
undoubtedly results in higher nutrient concentrations compared
to the Kuroshio (K.-C. Yang et al., 2015).
It is important to note that Kuroshio waters are significantly
modified by water exchange involving SCS water in the LS
(C.T.A. Chen and Huang, 1996; Matsuno et al., 2009; Yuan et al.,
2014; Nakamura, 2020). Figures 2 and 3 illustrate that in the
Kuroshio and the WPS regions, the S-max reaches values near 35.
However, along 21.45°N latitude, waters with a salinity of 34.8 do
not appear to penetrate the SCS west of 121°E. Extensive upwell-
ing and vertical mixing in the SCS weaken the salinity maximum,
and decrease temperature, but elevate nutrient concentrations in
the subsurface layer (Figure S3; Chao et al., 1996; Lu et al., 2020).
These characteristics gradually attenuate along the Kuroshio’s
northward path.
In September 2000, CTD salinity cross sections from stations
northeast of Luzon Island to the northern LS indicate that the
S-max is around 34.8 east of 120.7°E (Figure S4A–C). This obser-
vation is consistent with the climatological salinity distribution in
the LS. In the sigma-theta range of 24.6 to 24.9 (corresponding to
a water depth of approximately 150–250 m), waters with S = 34.8
are found east of 120.5°E in the northern LS (Y. Chen et al., 2016).
In other words, the S-max is reduced due to mixing with lower
S-max SCS seawater west of 120.7°E (Figure S4D). The S-max in
the Kuroshio east of Taiwan decreases from >34.8 offshore to ~34.6
as it nears the coast (Figure S4E–G; Mensah et al., 2014).
FIGURE 2. Zonal cross sections of temperature, salinity, potential density anomaly (σθ), apparent oxygen utilization (AOU), nitrate, phosphate, and silicate east
of the Philippines based on INDOPAC, Leg II, data. Color shading and black contours represent the values of individual parameters.