The subtropical extent of NPIW is objectively defined by the Turner angle (in dotted line) ( Tu = tan −1(α∂θ/∂ z − β∂ S/∂ z, α∂θ/∂ z + β∂ S/∂ z), where α is the thermal expansion coefficient and β is the haline contraction coefficient). First, salinity (in white line) at the core and two neighbor surfaces is examined ( Figure 2). These surfaces encompass the NPIW layer from about 300 to 800 m with a distance of about 100 m between a pair of surfaces. Best-framed neutral density surfaces are mapped to follow the core (σ N = 26.9) and boundaries (σ N = 26.5 and σ N = 27.4) of NPIW with an additional surface (σ N = 27.2) chosen between the core and lower boundary. To identify such a pathway, an updated hydrography including the recently completed World Ocean Circulation Experiment (WOCE) sections is used in a source water-mass mixing scheme. The entrance path of NPIW therefore has to lie in the eastern North Pacific but west of the CC. Unlike the South Pacific where the Peru Current, an eastern boundary current, contains the salinity minimum of Antarctic Intermediate Water (AAIW), the California Current (CC) is not part of NPIW as its low salinity increases monotonically with depth. The entrance path of NPIW into the subtropical gyre is intricate. Identification of the NPIW Pathway on Neutral Density Surfaces Dynamically, the shortcut is inconsistent with a gyre distribution of NPIW because NPIW sources would be most likely carried by the first recirculation branch of the Kuroshio Extension (KE) into the subtropical gyre center where the salinity maximum and oxygen minimum are located. You questioned this shortcut because immediately south of the MWR in the western subtropical gyre, a required isopycnal salinity minimum is not found but occupied instead by a salinity maximum, whereas a salinity minimum lies far in the opposite side of the North Pacific Ocean in the northeastern subtropical gyre. Various studies have attempted to resolve the issue how the source waters cross the SATFZ and form NPIW in the subtropical gyre, but generally favor a shortcut of NPIW directly from the western subarctic region to the western subtropical gyre. The pathway of NPIW source water into the subtropical gyre transits a broad SATFZ of about 800 km in width between the Subarctic Front (SAF) and Kuroshio Extension Front (KEF) where currents dominantly flow eastward. The thick dashed line (in red) marks transformation of OIW and GAIW by cabbeling to the subtropical NPIW along the SATFZ (see You for details). The exporting transport of 2.7 ± 0.2 Sv is a basin-wide mean (see You et al.
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