Analysis of Argo temperature and salinity profiles (gridded at 0.5° × 0.5° resolution for 2005–12) shows a strong North Pacific Subtropical Mode Water (NPSTMW) volume and density decrease during 2006–09. In this time period, upper-ocean temperature, stratification, and potential vorticity (PV) all increased within the region in and around the NPSTMW low-PV pool, contributing to the NPSTMW volume decrease in two ways: (i) the volume of water satisfying the low-PV constraint that is part of the “mode water” definition decreased, and (ii) some water that was initially in the NPSTMW density range σθ = 25.0–25.5 kg m−3 was transformed into lighter water. Both changes in density and in PV in the NPSTMW region were a manifestation of basinwide changes. A positive PV anomaly started to propagate westward from the central Pacific in 2005, followed by a negative density anomaly in 2007, which caused a dramatic NPSTMW volume and density decrease.;
A Walin estimate of surface formation in the NPSTMW density range accounted better (although not entirely) for the interannual variability of the volume of water in the NPSTMW density range without imposing the PV < 2 × 10−10 m−1 s−1 constraint than did the same estimate with the PV constraint imposed. This underlines the importance of the PV constraint in identifying the mode water. The mode water evolution cannot be fully described from a density budget alone; rather, the PV budget must be considered simultaneously.