Decreasing Wintertime Mixed?Layer Depth in the Northwestern North Pacific Subtropical Gyre

The deep winter mixed layer (ML), ?240 m, in the northwestern North Pacific subtropical gyre exchanges heat and absorbs carbon dioxide from atmosphere, receives nutrients through Kuroshio. ML water spreads broadly subsurface ocean over gyre. Compiling historical temperature measurements, we show that depth (MLD) has decreased by ?6% last six decades. Observations, atmospheric reanalyzes, simulation outputs a one-dimensional turbulent closure model, climate projections reveal strengthening of upper-ocean stratification due to surface warming under global is responsible for MLD decrease. Climate models project further decrease attributable marked with warming, up 27%–40% 2100 highest emission scenario. A monitoring system should be established so important changes marine ecosystems can foreseen.