Interhemispheric Asymmetry in Air‐Sea CO ₂ flux Trends Across Eastern Boundary Current Systems Under a High Warming Scenario

Abstract

Eastern boundary current systems (EBCSs) transport cold, carbon‐rich polar waters toward the equator. Wind‐driven coastal upwelling within EBCSs fuels intense biological activity, modulating upper ocean pH, middepth oxygen levels, and carbon cycling. While past studies have primarily examined upwelling dynamics and productivity responses to climate change, the future of carbon uptake in EBCSs remains poorly understood. Using results from multiple Earth system models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we show that during 2015–2100 the Southern Hemisphere EBCSs are projected to experience significantly greater positive trends in air‐sea CO ₂ flux (F _CO2 ), implying stronger uptake or reduced outgassing of CO ₂ , compared to their Northern Hemisphere counterparts. Such asymmetry arises from weakening of EBCSs owing to greater poleward shifts of the subtropical gyres over the Southern Hemisphere compared to the Northern Hemisphere. These changes enhance the poleward advection of warm tropical waters while suppressing the supply of carbon‐rich waters from high latitudes by the EBCSs, thereby increasing (reducing) upper ocean CO ₂ uptake (outgassing) in the EBCSs despite complex upwelling trends. Across the CMIP6 models, there exists a consistent link between the magnitude of poleward shifts of the subtropical gyres and the magnitude of positive F _CO2 trends in the EBCSs. Our findings reveal a robust interhemispheric contrast in carbon uptake, offering new insights into how large‐scale circulation shifts influence both global and regional carbon budgets.