21st Century Upwelling and Air-Sea CO2 Flux Trends in the EBUS in CMIP6 MPI-ESM Realisations

The Eastern Boundary Upwelling Systems (EBUS) in the subtropical Atlantic and Pacific Oceans are regions where wind-induced coastal upwelling results in cold, nutrient-rich surface waters, leading to high productivity. Changes in these regions are of significant interest due to their importance to fisheries, economies, biological productivity, diversity, and the CO₂ cycle. Here, we examine future trends in upwelling and surface CO₂ fluxes across the four EBUS, simulated with different versions of the Earth System model MPI-ESM driven by different carbon emissions scenarios. Our objectives are to test the hypothesis of a more substantial intensification of upwelling in the EBUS regions located polewards and to investigate the impact of upwelling changes on CO₂ surface fluxes.
Using several realisations and high and low-resolution simulations enables us to analyse the internal climate variability and the effect of horizontal resolution on upwelling trends. Our study shows that upwelling does not intensify in the poleward subregions of all four EBUS but instead decreases in all the equatorward subregions. In these simulations, upwelling intensifies in the poleward subregions of the Humboldt and Canary upwelling systems, whereas it decreases in all subregions of the Benguela and California upwelling systems. The model resolution is not relevant for the directions of simulated change in upwelling. The poleward expansion of the Hadley Cell and, thus, the poleward displacement of the subtropical highs drive the change. This high-pressure cell moves offshore in the South Atlantic, which might lead to the negative trends in South Benguela. However, the realism of this westward shift might be questionable, as Earth System models struggle to simulate the South Atlantic high at its observed position. The decrease‚ in California upwelling may be due to the offshore shift of the subtropical high over the North Pacific or the summertime contraction of the Hadley Cell over the North Pacific.
The CO₂ flux from the atmosphere into the ocean shows a general increase in the oceanic CO₂ sink under the high-emission scenario, but a decrease under the low-emission scenario. These changes are not consistent with trends in upwelling but rather with atmospheric CO₂ concentrations. An exception is the North Canary subregion, which remains a CO₂ source in all scenarios, even though upwelling intensifies there.