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

Abstract

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