Presenter: Lydia Keppler (Max-Planck-Institute for Meteorology)

Description:
Both physical and biogeochemical processes affect the amount of dissolved inorganic carbon (DIC) in the water column, potentially leading to large variations, especially in the upper ocean. In turn, variations in upper ocean DIC modulate the air-sea flux and thus affect the overall carbon sink. However, to date the interannual variability of the upper ocean DIC is largely unknown at a global scale. Here, we investigate this variability in the upper 1500 m, extending the temporal resolution of the Mapped Observation-Based Oceanic Dissolved Inorganic Carbon monthly climatology (MOBO-DIC_clim; Keppler et al., 2020) to resolve monthly fields of the global ocean from January 2004 through January 2020 (MOBO-DIC_2004-2020). Results reveal the largest interannual variability in the thermocline of the Western Equatorial Pacific, where the temporal standard deviation of DIC exceeds 17 mumol kg^-1, with a mean of 10 mumol kg^-1. This variability is strongly tied to ENSO, and specifically to the flattening of the west-east gradient of the thermocline during El Nino periods. The interannual variability in the North Atlantic is largest in the western subtropics, with standard deviations up to 7 mumol kg^-1. In the Southern Ocean Subduction Zone, the interannual variability is comparatively weaker in magnitude, with mean standard deviations of 2 mumol kg^-1 at 150 m, i.e., the depth level with the most variability. Most of the remaining ocean displays similarly low levels of interannual variability. Our study reveals that interannual variations in DIC are dominated by processes connecting the surface ocean to the interior and only moderately connected to the suggested vacillations in the air-sea CO₂ flux.