10 Feb 2022
10 Feb 2022
Status: this preprint is currently under review for the journal BG.

High-resolution modelling of long-term trends in the oxygen and carbon cycles of the Benguela upwelling system

Katharina Dorothea Six and Uwe Mikolajewicz Katharina Dorothea Six and Uwe Mikolajewicz
  • Max Planck Institute for Meteorology, Hamburg, Germany

Abstract. We investigate driving forces of the biogeochemistry of the Benguela upwelling system (BUS) and their temporal changes over the 20th century. For this purpose, we developed a global ocean-only model in a stretched grid configuration, which resolves meso-scale circulation structures in the area of interest. The biogeochemical module of this model is extended by a more comprehensive nitrogen cycle to account for the specific nitrogen loss processes common in eastern upwelling systems. The model is forced by 110 years of atmospheric reanalysis data. To assess the impact of meso-scale circulation structures on local biogeochemical processes we compare our results to a set-up with a coarser horizontal resolution, comparable to the ocean component of Earth system models used for anthropogenic climate projections. In the higher spatial resolution we find enhanced intermediate depth ventilation (200–1000 m) with concurrent reduced loss of bioavailable nitrogen and a high shelfbound biological production, in line with observations. Moreover, only in the high resolution setup do multi-decadal trends of deoxygenation match observation-based estimates. Our study supports the view that the presence of meso-scale circulation structures exerts a major influence on biogeochemical patterns, especially on mid-depth oxygen concentrations. Furthermore, we show for the first time that by including this high spatio-temporal variability of the circulation, the regional anthropogenic carbon uptake of the BUS over the 20th century is lower than in the coarse resolution model. This indicates that, at least for some regions, the pathway of changes in the marine biogeochemistry as projected by state-of-the-art coarse resolution Earth system models is associated with high uncertainty.

Katharina Dorothea Six and Uwe Mikolajewicz

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-27', Anonymous Referee #1, 05 May 2022
  • RC2: 'Comment on bg-2022-27', Anonymous Referee #2, 15 May 2022

Katharina Dorothea Six and Uwe Mikolajewicz

Katharina Dorothea Six and Uwe Mikolajewicz


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Short summary
We developed a global ocean biogeochemical model with a zoom on the Benguela upwelling system. We show that the high spatial resolution is necessary to capture long-term trends of oxygen of the recent past. The regional anthropogenic carbon uptake over the last century is lower than compared to a coarser resolution ocean model as used in Earth system models. This suggests that, at least for some regions, the changes projected by these Earth system models are associated with high uncertainty.