Decadal variability of simulated phytoplankton and nitrate associated with the Kuroshio Extension instability

Abstract Decadal modulation of the Kuroshio Extension (KE) between stable and unstable states influences phytoplankton and nitrate concentrations in the KE region. Decadal variability of phytoplankton and nitrate concentrations is examined using the outputs of an eddy-resolving coupled physical–biological model. The model captures decadal variations in phytoplankton and nitrate concentrations south and north of the KE jet. During unstable states, south of the KE jet, vertical nitrate movement of the isopycnal surfaces, vertical nitrate mixing, and meridional shifts in the KE jet contribute to a large spring phytoplankton bloom. The winter MLD becomes shallower earlier than during stable states, leading to an earlier supply of high nitrate concentrations into the euphotic layer and a phytoplankton bloom occurring one month earlier. The nutricline also becomes shallower throughout the year, and the shallow nutricline maintains the high subsurface phytoplankton concentrations. During stable states, the winter MLD and nutricline become shallower north of the KE jet, leading to a large and earlier spring phytoplankton bloom. The offline analysis of the nitrate budget over the top 100 m suggests that the horizontal nitrate flux south of the KE jet during unstable states enhances phytoplankton biomass in spring. Conversely, vertical nitrate flux north of the KE jet during stable states enhances phytoplankton biomass in spring. Decadal modulation of the KE induces distinct surface and subsurface distributions of phytoplankton and nitrate on the southern and northern flanks of the KE jet.