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 influences 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 and comparably third simulations in the equatorial and southeast 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 in particular, the Atlantic summer bloom is 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 sea-air 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 sea-air CO2 interannual variability associated with the Atlantic Niño and Niña events. We stress that physical process and its improvement are responsible for modeling the marine biogeochemical process as the first simulations, where only climatological surface ocean dynamics are corrected, provides the better improvements of marine biogeochemical processes.
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