Improvement of Seasonal Carbon Cycles in HNLC Regions of a Global Ocean Biogeochemical Model Using a Simple Marine Ecosystem Module

Abstract A recent community‐wide evaluation revealed that many state‐of‐the‐art global ocean biogeochemical models displayed seasonal surface flux variations in the subpolar North Pacific that were out‐of‐phase with observation‐based estimates. This study aimed to improve one of those models based on the understanding that the carbon drawdown due to biological production during the warm season dominates the seasonal surface variation there. Specifically, we increased the maximum biological production rate and applied a limitation based on dissolved iron concentrations. This treatment improved the seasonal variation without negatively impacting other High‐Nutrient‐Low‐Chlorophyll regions, even in a simple Nutrient‐Phytoplankton‐Zooplankton‐Detritus‐type marine ecosystem model. The improvement was achieved through enhancing the vertically one‐dimensional processes: carbon drawdown from the surface during the warm season; remineralization below; upwelling by the wind‐driven circulation; and re‐entrainment into the surface mixed layer in winter. We also examined a seasonally concentrated anthropogenic uptake in this region as seen in the simulation. In the contemporary condition, the amplitudes of both thermally‐ and non‐thermally‐driven seasonal variations of increase by comparable factors due to the long‐term increase in the dissolved inorganic carbon concentration, resulting in an amplification of the seasonal variation of total . Simultaneously, the oceanic gradually lags behind the increasing atmospheric concentration, resulting in suppressed outgassing and enhanced absorption. This results in the seasonally concentrated absorption of anthropogenic , demonstrating the importance of properly expressing the seasonal variability of when simulating the circulation and feedback processes of carbon in Earth system models.