Geostrophic flows control future changes of oceanic eastern boundary upwelling

Equatorward alongshore winds over major eastern boundary upwelling systems (EBUSs) drive intense upwelling via Ekman dynamics, surfacing nutrient-rich deep waters and promoting marine primary production and fisheries. It is generally thought, dating back to Bakun's hypothesis, that greenhouse warming should enhance upwelling in EBUSs by intensifying upwelling-favourable winds; yet this has not been tested. Here, using an ensemble of high-resolution climate simulations with improved EBUS representation, we show that long-term upwelling changes in EBUSs differ substantially, under a high-emission scenario, from those inferred by the wind-based upwelling index. Specifically, weakened or unchanged upwelling can coincide with intensified upwelling-favourable winds. These differences are linked to long-term changes of geostrophic flows that dominate upwelling changes in the Canary and Benguela currents and strongly offset wind-driven changes in the California and Humboldt currents. Our results highlight the controlling role of geostrophic flows in upwelling trends in EBUSs under greenhouse warming, which Bakun's hypothesis overlooked. Oceanic eastern boundary currents are regions with strong upwelling, which is expected to intensify with global warming through enhanced winds. Here the authors show that geostrophic flow dominates over wind effects on long-term upwelling changes for the major eastern boundary upwelling systems.

Automated summary

Eastern boundary upwelling systems (EBUSs) are driven by equatorward alongshore winds, which upwell nutrient-rich deep waters, promote marine primary production and fisheries. It was previously believed that greenhouse warming would enhance upwelling in EBUSs by intensifying upwelling-favourable winds, but this has not been tested. This study reveals that long-term upwelling changes in EBUSs differ substantially from those inferred by wind-based upwelling index, with weakened or unchanged upwelling despite intensified winds. These differences are attributed to the controlling role of geostrophic flows, which offset wind-driven changes in certain currents.