Eastern North Pacific Subtropical Mode Water in a general circulation model: Formation mechanism and salinity effects

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 investig ated using an ocean general circulation model that is forced by monthly cli matological temperature, salinity, and wind stress at the sea surface. An e quation based on the ventilated thermocline theory is used to diagnose the initial PV of a water mass right after its subduction. In this equation, th ree factors affect the initial PV: the spacing of density outcrop lines, th e mixed layer depth gradient, and the vertical velocity at the bottom of mi xed layer. Among them the wide spacing between outcrop lines is the most im portant for ESTMW's low PV instead of the deep mixed layer, which is most i mportant 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 densit y gradient in the eastern North Pacific and contributes to the formation of the ESTMW. An additional experiment forced with observed freshwater flux d emonstrates that the southward advection of fresher water from the high lat itude along the eastern boundary is the cause of this low-salinity tongue.