Oceanography | Vol. 38, No. 3
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eastward to exit the ECS. Most of the data from these cruises are
publicly accessible, having been previously published or available
through datasets. Each cruise recorded measurements of nutrients
and dissolved oxygen (DO)—parameters influenced by biological
activity as well as by temperature and salinity fluctuations.
Apparent oxygen utilization (AOU) is calculated by subtract-
ing the measured DO from its saturation value at the in situ tem-
perature and salinity (AOU = [O2]theoretically saturated – [O2]measured;
C.T.A. Chen, 1981). AOU serves as a crucial indicator of the net
production or consumption of DO. Unlike DO, AOU is unaf-
fected by temperature and salinity variations, making it a more
robust indicator of biological respiration and oxygen consump-
tion in seawater.
Further, the PN line data from 2000 to 2020 were selected for
this study, with long-term observations conducted semi-annually
to quarterly during this period by the Japan Meteorological
Agency. Additional longitudinal data were obtained from a
repeated-measurement study southeast of Taiwan along 21.75°N,
with sampling conducted in November 1990, June 1991, May 1994,
May 1995, October 1995, September 1996, July 2004, May 2006,
June 2011, and August 2015.
Collection of these shipboard data, although covering a large
area, occurred in different years. For data synthesis across simi-
lar sampling periods, hydrographic data from the same study
areas were collated during September and October 2000.
Shipboard CTD data extending from the northeastern coast of the
Philippines to northeastern Taiwan were sourced from the Ocean
Data Bank. Concurrently, model temperature and salinity data
for September 2000 were obtained from the HYbrid Coordinate
Ocean Model (HYCOM). To estimate nutrient concentrations,
empirical formulas derived from repeated measurements along
21.45°N during 10 cruises were applied, utilizing temperature
data in conjunction with the specific water mass characteristics of
SCS-like (South China Sea-like) and WPS-like (West Philippine
Sea-like) waters (Figure S1 in the online supplementary materi-
als). This categorization is based on whether the maximum salin-
ity (S-max) is lower or greater than 34.75, which approximates the
median of the S-max values for typical WPS (S ≈ 34.94) and typi-
cal SCS (S ≈ 34.53) around σθ = 24.5–25 (Figure S2A).
RESULTS
HYDROGRAPHIC FEATURES EAST OF LUZON ISLAND
Figure 2 focuses on the origin of the Kuroshio Current, where the
North Pacific Equatorial Current (NPEC) encounters the NPIW
near the Philippines. The cross-sectional (14°N, INDOPAC)
parameters encompass temperature (T), salinity (S), AOU, nitrate
(NO3), phosphate (PO4), and silicate (Si(OH)4). While temperature
shows a decreasing trend with depth, AOU, nitrate, phosphate, and
silicate all exhibit an increasing trend with depth (Figure 2). The
primary thermocline is observed at depths ranging from 200 m to
500 m, coinciding with the nutricline. Salinity, on the other hand,
behaves differently, increasing initially with depth until reaching a
maximum at around 200 m, referred to as Kuroshio Tropical Water
(KTW). Below this point, salinity decreases with depth until it
reaches a minimum, namely the KIW at approximately 400 m, and
then increases again (Tsuchiya, 1968; Qu et al., 2000).
For the profile made east of Luzon Island (Figure 2), an essen-
tial observation is that parameter contours remain relatively flat
below 400 m east of 126°E within the NPIW domain. However,
above 400 m and west of 126°E, the contours tilt upward by as
much as 100 m. For example, the 1 µM contour for NO3 is around
160 m deep east of 126°E (Figure 2), where it is too deep for phyto-
plankton to utilize sunlight for photosynthesis. Near the Philippine
coast, this contour reaches depths of about 100 m, closer to the
euphotic zone, where some light penetration can support biologi-
cal production. This phenomenon is primarily driven by the force
of the pressure gradient and the Coriolis effect (Qu et al., 1998;
Schaeffer et al., 2013; F. Liao et al., 2022). It is noteworthy that
AOU is negative above 100 m, reflecting net primary productivity
FIGURE 1. (A) The map shows the main pathways of the Kuroshio Current
and its branches, indicated by colored arrows. Different symbols represent
various sampling stations and research lines: KEEP-MASS, TPS-24, ORI-179,
ORI-462, and INDOPAC (triangles); CTD data collected in September 2000
(crosses). Shaded areas indicate HYCOM model data from September 2000.
Red, orange, and purple indicate regions mostly influenced by the West
Philippine Sea (WPS), while blue denotes areas mainly influenced by the
South China Sea (SCS). (B,C) Repeated sampling tracks at the PN Line and
21.75°N, respectively. The different area numbers correspond to Table S1.