Subtropical Mode Water in the 137°E Section

The subtropics mode water (STMW) in the North Pacific Ocean has been investigated based on the data of long-term observations along the 137°E meridian, which have been performed by the Japan Meteorological Agency since 1967 for winter and since 1972 for summer. STMW cores were identified as vertical potential vorticity minima, and examined by the use of apparent oxygen utilization as an indicator of the age of STMW. The main results can be summarized as follows. 1) A major post of the STMW appearing in the summer (or the winter) sections is the water formed in the immediately previous winter, its age being half a year (or one year). 2) Within half a year after its formation the STMW can be advected to the 137°E section only as far south as about 26°N and as far as about 23°N within one year. 3) Typical potential temperature in summer was higher than in winter, with salinity higher and potential density lower. 4) Less STMW was observed during the period of the typical large meander of the Kuroshio in the later 1970s. 5) The salinity of STMW was relatively low before 1981; it increased considerably in the summer of 1981 and has since showed a slowly decreasing trend.