Greenhouse Warming Weakens the Seasonal Cycle of the Eastern Boundary Upwelling

The response of upwelling in the eastern boundary upwelling systems (EBUSs) to anthropogenic climate change has attracted much scientific attention due to its core role in nourishing marine ecosystems. A decade‐old hypothesis suggests that greenhouse warming may intensify upwelling‐favorable winds and subsequently the upwelling in EBUSs, but the impact of greenhouse warming on the seasonal cycle of upwelling remains unknown. Using the recent generation of global climate simulations under a high carbon emission scenario, we show a universal weakening of the upwelling seasonal cycle in the major EBUSs. This is mainly ascribed to the projected weakened seasonal cycle of the upwelling‐favorable winds. In addition, long‐term changes in geostrophic transport exert a nonnegligible contribution to the changes in the upwelling seasonal cycle. Our study suggests that the upwelling in the EBUSs is likely to have a more complicated response to greenhouse warming than previously thought. Plain Language Summary The eastern boundary upwelling systems (EBUSs), along the eastern boundaries of the Pacific and Atlantic basins, contain the most diverse marine ecosystems and provide competitive commercial industries. Thus, the potential changes of EBUSs to anthropogenic climate change are of concern for the associated ecosystem and coastal communities. Greenhouse warming threatens the ecosystems by changing the mean‐state eastern boundary upwelling and its seasonal cycle, with the latter being limitedly discussed in previous studies. Using a series of state‐of‐the‐art climate simulations, we show that the seasonal cycle of the upwelling in the major EBUSs will be weakened under a high carbon emission scenario. This attenuated seasonal cycle of upwelling is majorly caused by the weakened seasonal cycle of the equatorward alongshore winds. Moreover, we further investigate the role of geostrophic transport in upwelling seasonality's changes under global warming. We find that the cross‐shore geostrophic transport also plays a nonnegligible role in the response of upwelling seasonality to greenhouse warming, counteracting the wind‐driven changes in the Chile Current System but enhancing upwelling changes in other EBUSs. Key Points The seasonal cycle of upwelling in the major eastern boundary upwelling systems is weakened under greenhouse warming The weakened eastern boundary upwelling seasonal cycle is mainly attributed to the projected alongshore wind changes Geostrophic transport