Decadal-scale reduction in net primary production in the western subarctic North Pacific: impact of lateral transport of dissolved iron from the Sea of Okhotsk

Abstract The subarctic North Pacific (SNP) is a high-nutrient low-chlorophyll region in which decoupling of dissolved iron (dFe) and macronutrient is an essential control of primary production. In this study, we evaluate the influence of decadal-scale climate change on the net primary production (NPP) in the SNP by performing a hindcast experiment for 1979–2016 using an ice–ocean coupled model with a simple biogeochemical model with iron cycle. Simulation results show a significant NPP decrease in the subtropical–subarctic gyre boundary (SGB) region since the 1990s; the trend is −48 mgC m −2 d −1 /37 years with a magnitude that is 14.3% of the climatological mean NPP. The NPP decrease in the SGB is prominent during the spring, indicating weakening of the spring bloom. Diagnostic analysis of simulation data reveals that the NPP decrease in the SGB can be explained by the decrease in both dFe and light availability. Sensitivity experiments indicate that wind-driven circulation change mainly explains both the reduction in dFe and the light availability through the northward expansion of oligotrophic subtropical water, but the thermohaline change in the Sea of Okhotsk also has a non-negligible effect on the dFe reduction. Results from our numerical model simulations suggest the importance of lateral and vertical advection of dFe on the decadal-scale changes of NPP in the SNP.