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.