Assessing the tropical Atlantic biogeochemical processes in the Norwegian Earth System Model

Abstract. State-of-the-art Earth system models exhibit large biases in their representation of the tropical Atlantic hydrography, with potential large impacts on both climate and ocean biogeochemistry projections. This study investigates how biases in model physics influence marine biogeochemical processes in the tropical Atlantic using the Norwegian Earth System Model (NorESM). We assess four different configurations of NorESM: NorESM1 is taken as benchmark (NorESM1-CTL) that we compare against the simulations with (1) a physical bias correction and against (2 and 3) two configurations of the latest version of NorESM with improved physical and biogeochemical parameterizations with low and intermediate atmospheric resolutions, respectively. With respect to NorESM1-CTL, the annual-mean sea surface temperature (SST) bias is reduced largely in the first simulation and comparably in the third simulation in the equatorial and southeastern Atlantic. In addition, the SST seasonal cycle is improved in all three simulations, resulting in more realistic development of the Atlantic Cold Tongue in terms of location and timing. Corresponding to the cold tongue seasonal cycle, the marine primary production in the equatorial Atlantic is also improved, and the Atlantic summer bloom is particularly well represented during June to September in all three simulations. The more realistic summer bloom can be related to the well-represented shallow thermocline and associated nitrate supply from the subsurface ocean at the Equator. The climatological intense outgassing of air–sea CO2 flux in the western basin is also improved in all three simulations. Improvements in the climatology mean state also lead to better representation of primary production and air–sea CO2 interannual variability associated with the Atlantic Niño and Niña events. We stress that the physical process and its improvement are responsible for modelling the marine biogeochemical process because the first simulations, where only climatological surface ocean dynamics are corrected, provide more improvements in terms of marine biogeochemical processes.