The origin of the North Pacific Intermediate Water

Abstract

Possible source regions of the North Pacific Intermediate Water (NPIW) were investigated with synoptic conductivity‐temperature‐depth surveys. In the Okhotsk Sea Kuril Basin a pycnostad (referred to as the Okhotsk Sea Mode Water (OSMW)) was found in the density range of 26.6–27 σθ. The pycnostad is not seen in the Western Subarctic Gyre (WSAG); thus it is an indicative water peculiar to the Okhotsk Sea. The maximum thickness is at 26.7–26.9 σθ (centered at 26.8 σθ) which coincides with the NPIW density range in the North Pacific Subtropical Gyre. In the Oyashio area east of the southern Kuril Islands and Hokkaido, waters that have intermediate temperature, salinity, and potential vorticity between the OSMW and the WSAG water were distributed, suggesting a following transport route of the NPIW source water. The thick (thus low‐Q; Q is potential vorticity) and low‐salinity water mass outflowing from the Okhotsk Sea mixes with the relatively warm, saline, and high‐Q WSAG water to form the Oyashio water. A coastal part of the Oyashio water flows southwestward along the east coast of the southern Kuril Islands and Hokkaido, retaining the pycnostad (Q minimum). A large part of the low‐Q Oyashio water flows further southward and merges with the Kuroshio Extension and then flows eastward, forming new NPIW. In this process a salinity minimum develops and the Q minimum disappears because of the mixing with a saline Kuroshio water (old NPIW) without a pycnostad in the NPIW density range. These results suggest that the origin of NPIW is the water mass in the Okhotsk Sea (OSMW) and the density of the water mass primarily determines the NPIW density centered at around 26.8 σθ.

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

“…Figure 13b indicates the NPIW pressure, ~ 400 dbars at the outer rim and 700 dbars in the center of the gyre. NPIW salinity is largely the same as Talley The low potential vorticity contour is a useful indicator for identifying the ventilation source of NPIW [Talley, 1988;Yasuda, 1997]. Yasuda [1997] found that OSMW is associated with low salinity and low potential vorticity.…”

“…NPIW salinity is largely the same as Talley The low potential vorticity contour is a useful indicator for identifying the ventilation source of NPIW [Talley, 1988;Yasuda, 1997]. Yasuda [1997] found that OSMW is associated with low salinity and low potential vorticity. Talley [1993] noted the unitbrm distribution of potential vorticity in the subtropical gyre on isopycnal surface c•0=26.8 and suggested that it has implications for NPIW ventilation.…”

“…Altogether, these properties lead to consistent distributions. Several previous authors such as Talley [1988], and Yasuda [1997], among others, have used potential vorticity to trace NPIW. Therefore we regard potential vorticity as a useful tracer in our study.…”

“…Very recently, Yasuda confirmed the existence of a pycnostad in the Kuril Basin of the Okhotsk Sea, which he refers to as Okhotsk Sea Mode Water (OSMW). OSMW has a density range of c•0=26.6-27.0, the same as NPIW in the subtropical gyre, and is estimated to provide •-1 Sv (1 Sv=106 m 3 s-l) of transport for NPIW renewal in the subtropics [Yasuda, 1997]. An updated review of the Okhotsk Sea study can be found in the work of Preller and Hogan [1998].…”

Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 1. A pitfall of general circulation model studies

“…Yasuda et al [1996] also showed that the salinity minimum can be created by isopycnal mixing of the Kuroshio/Oyashio waters with different structures of salinity and velocity and that vertical mixing shifts the newly formed salinity minimum toward depth for asymmetric salinity structure. Moreover, Yasuda [1997] concluded that NPIW is formed by the Oyashio transport maximum of 26.8rr 0 attributed to the Oyashio potential vorticity (f P-109/ O z) minimum from the mode water in the Sea of Okhotsk.…”