Seasonal evolution and aging of the subtropical mode water in the North Pacific

Abstract We examined the aging and fate of subtropical mode water (STMW) in the North Pacific using Argo float profiles of dissolved oxygen obtained since 2008. A majority of STMW moved nearly on isopycnal surfaces, but faster increase in potential vorticity in water parcels with potential density anomaly (σ θ ) < 25.3 kg m − 3 and apparent oxygen utilization (AOU) < 30 µmol kg − 1 indicated that the upper STMW was apt to be rapidly entrained into the seasonal pycnocline within one year. A histogram of AOU in April showed an increase of about 20 µmol kg − 1 during the first year (5.5 × 10 − 2 µmol kg − 1 day − 1 ). The AOU rate of increase was relatively large during the first several months in newly formed STMW and then declined. The estimated aging curve of AOU in STMW can be interpreted as the aging process of a cluster including some moderate oxygenation and suggested that AOU reached 50 µmol kg − 1 in a few years. Most of the STMW older than one year stagnated in the range AOU = 40–60 µmol kg − 1 and σ θ = 25.4–25.6 kg m − 3 before gradually dissipating. In the old STMW with AOU > 40 µmol kg − 1 , AOU increased with potential density. It was inferred that this relationship resulted from diapycnal mixing and long-term accumulation of a small amount of dense, aged STMW. The oxygen rate of change was estimated to be more than approximately − 5.0 × 10 − 2 µmol kg − 1 day − 1 above a depth of 200 m and smaller in the deeper layer.