Simulated decadal variations of surface and subsurface phytoplankton in the upstream Kuroshio Extension region

Abstract

Using outputs from an ecosystem model embedded in an eddy-resolving ocean general circulation model that can realistically simulate decadal modulations of the Kuroshio Extension (KE) between stable and unstable states, decadal variations of phytoplankton concentration in the upstream KE region are investigated. During stable states of the KE, surface phytoplankton concentrations are anomalously suppressed to the south of the KE front, while those to the north are anomalously enhanced. Although the surface phytoplankton concentration anomalies are prominent only during winter to spring, significant subsurface anomalies centered around 60 m depth persist even in summer and autumn. Anomalies persist throughout the year in phytoplankton biomass integrated over the upper 100 m despite the strong surface anomalies during the spring bloom season. An analysis of nitrogen concentration anomalies suggests that the vertical movement of the isopycnal surfaces, vertical mixing of nutrients, and meridional shifts in the KE jet contribute to the anomalous phytoplankton biomass.