The Eastern North Pacific Subtropical Mode Water (ESTMW) is a water mass of low potential vorticity (PV) and appears as a weak pycnostad or thermostad. Distinct from other subtropical mode waters, it forms in the absence of a deep winter mixed layer. The formation mechanism of this ESTMW is investigated using an ocean general circulation model that is forced by monthly climatological temperature, salinity, and wind stress at the sea surface. An equation based on the ventilated thermocline theory is used to diagnose the initial PV of a water mass right after its subduction. In this equation, three factors affect the initial PV: the spacing of density outcrop lines, the mixed layer depth gradient, and the vertical velocity at the bottom of mixed layer. Among them the wide spacing between outcrop lines is the most important for ESTMW's low PV instead of the deep mixed layer, which is most important for classical mode waters. It is found that weak gradients in both sea surface temperature and salinity in the direction of mixed layer flow are important for the low PV formation. A low-salinity tongue that extends southeastward off North America is responsible for the small surface density gradient in the eastern North Pacific and contributes to the formation of the ESTMW. An additional experiment forced with observed freshwater flux demonstrates that the southward advection of fresher water from the high latitude along the eastern boundary is the cause of this low-salinity tongue. ¿ 2001 American Geophysical Union