Formation and spreading of subtropical mode water in the North Pacific

Abstract

A time‐averaged description of the formation and spreading of North Pacific subtropical mode water (NPSTMW) is presented. The formation of NPSTMW is studied using TRANSPAC expendable bathythermograph data. The data are transformed into a stream coordinate system and averaged. The coordinate system uses the position of the Kuroshio front as its origin. Maps are presented of the temperature structure and mixed layer distribution in winter relative to the Kuroshio front. The formation of NPSTMW occurs in two separate stages, cooling and thickening. As water flows at the surface in the Kuroshio in winter, the water is cooled by the intense ocean‐atmosphere heat flux taking place in the area. When water leaves the Kuroshio to the south, the thermocline underneath becomes deeper, convection is able to penetrate deep into the water column, and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature. The area south of the Kuroshio, where relatively homogeneous waters are found, is the source region for NPSTMW. At the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become NPSTMW. The Levitus (1982) data are used to study the way in which NPSTMW is spread away from its source region by the subtropical circulation. The distribution of potential vorticity (PV) on four NPSTMW isopycnals is studied, with low PV used as a tracer for NPSTMW. The PV reveals a pattern of “differential spreading,” whereby NPSTMW formed in the western part of the source region is advected to the west after its formation. NPSTMW formed farther east is advected more to the south or east after formation. Differential spreading allows NPSTMW to be found in a large part of the northwestern subtropical gyre, well away from its source region. The distribution of NPSTMW described by Masuzawa (1969), with NPSTMW getting colder from west to east, is explained as a combination of the distribution of NPSTMW at its source and the way it is spread after it is formed.

Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

“…This suggests that part of the STMW formed in the upstream KE recirculation gyre can be expected to be advected downstream. Indeed, STMW with similar properties has been observed as far downstream as the date line (e.g., Bingham 1992;Suga et al 1997;Oka and Suga 2003). The synoptic KESS survey in May 2004 also revealed that STMW was more intense and larger in thickness near the center of the recirculation gyre.…”

“…12d that the low-PV signal of the mixed layer is in contact with the underlying STMW from mid-February to early April, it is important to emphasize that these two low-PV waters have different poten- tial densities. 1 Previous studies by Bingham (1992), Huang and Qiu (1994), and Oka and Suga (2003) have noted that STMW formed in the KE recirculation gyre tended to spread south-and southwestward through the process of lateral induction. With the observed T-S properties slightly colder and fresher than those shown in Fig.…”

“…Characterized by a layer of minimum potential vorticity (PV), STMW can be detected over a wide geographic domain from 130°E to the date line and from the Kuroshio-KE to the subtropical front. STMW has been extensively studied (e.g., Suga et al 1989;Bingham 1992;Suga and Hanawa 1995;Yasuda and Hanawa 1997), and an updated and comprehensive review of STMW may be found in Hanawa and Talley (2001).…”

Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

“…In particular, in the western North Pacific between 30-45 • N, mixed layer depths can often exceed 200 m (Ohno et al, 2009;Oka et al, 2007 (Bingham, 1992;Hanawa and Suga, 1995), known to be enriched in Al (Measures et al, 2005), outcrops or forms south of the Kuroshio Extension (generally south of 36 • N, Fig. 1) (Chen, 2008;Oka et al, 2007).…”

Turbulent heat fluxes during an intense cold-air outbreak over the Kuroshio Extension Region: results from a high-resolution coupled atmosphere–ocean model

“…During the northern winter, when cold outbreaks of continental air masses spread eastward over the warm water south of the Kuroshio, large heat losses to the atmosphere result in episodes of oceanic convection, forming NPSTMW (Masuzawa 1969;Suga et al 1989;Bingham 1992;Suga and Hanawa 1995;Hanawa and Suga 1995;Oka and Suga 2003). According to these studies, the water mass forms in a mixed layer deeper than 200 m, and its core layer temperature and salinity range is 16°C to 18.2°C and 34.75 to 34.85 psu, respectively.…”