The North Pacific Climatology of Winter Mixed Layer and Mode Waters

A climatology of the winter mixed layer in the North Pacific Ocean was constructed using hydrographic data from historical archives and recent observational programs, including the World Ocean Circulation Experiment. The main aim was to provide better knowledge about source areas of upper water masses. The authors have endeavored to preserve water properties near the frontal regions by keeping the smoothing scale as small as possible. The resulting climatology shows considerable differences in the mixed layer depth and its water properties from those derived from the World Ocean Atlas (WOA). Maps of the potential vorticity field of the North Pacific pycnocline are presented using the isopycnally averaged climatology, HydroBase. Three distinct lateral minima of potential vorticity are identified as Subtropical Mode Water (STMW), Central Mode Water (CMW), and Eastern Subtropical Mode Water (ESTMW), in the western, central, and eastern parts of the subtropical gyre, respectively. The HydroBase isopycnal climatology is more consistent with the present mixed layer climatology than with the mixed layer from WOA in the sense that the former represents the formation of all mode waters more adequately. The STMW and ESTMW formation areas are associated with the mixed layer front and the small horizontal gradient of the mixed layer density, respectively, which confirms previously proposed formation mechanisms. That is, the low potential vorticity of STMW and ESTMW results from the large lateral induction and the small cross-isopycnal flow, respectively. The CMW formation area is not primarily associated with the mixed layer front, which contrasts with previous ideas. It is suggested that low potential vorticity of CMW is mainly caused by small cross-isopycnal flow rather than through large lateral induction rate. Additional new features of subtropical pycnocline ventilation revealed by the HydroBase isopycnal climatology are also discussed.