A new type of mode water in the upper thermocline of the eastern subtropical North Pacific is identified and examined using data from World Ocean Circulation Experiment high-resolution repeat expendable bathythermograph (XBT) section PX37 and archives of historical XBT data. This water mass (labeled Eastern Subtropical Mode Water) is characterized by a subsurface potential vorticity minimum and is located east of Hawaii (Northeast Pacific Basin) in a density range of 24–25.4 σθ. It is a distinct water mass from the classical subtropical mode water (STMW) of the western Pacific. Eastern STMW is formed as a relatively deep late-winter mixed layer, associated with the subtropical/subpolar water mass boundary near 25°–30°N, 135°–140°W, and is capped and subducted into the permanent thermocline. Along a section between San Francisco and Honolulu, Eastern STMW production is seen in every year for which there is adequate data. In this section the volume of Eastern STMW formed each winter and the temperature of the potential vorticity minimum are similar during the periods 1970–1979 and 1991–1997.
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