Understanding the drivers of the air-sea CO2 flux seasonal variability in the upwelling systems off Peru and Baja California

The open ocean plays a critical role in mitigating climate change by absorbing approximately 25% of annual anthropogenic carbon dioxide (CO₂). In contrast, Eastern Boundary Upwelling Systems (EBUSs) are net sources of CO₂, primarily due to the high concentrations of dissolved inorganic carbon (DIC) from upwelled waters. However, the carbon dynamics in EBUSs exhibit significant variability, both temporally and spatially, with differences between systems. This study focuses on two Pacific Ocean EBUSs with distinct physical characteristics: the upwelling systems off Peru and off Baja California, where the relative contribution of  Ekman transport and pumping, and geostrophic compensation to upwelling differ. Based on seasonal simulations of a regional biogeochemical model configured for the two regions, we characterise the seasonal variability of CO₂ fugacity (FCO₂) in these systems, and identify the processes driving this variability through a Taylor expansion of the flux formulation. Our results show that FCO₂ is highly dynamic and exhibits notable spatial variability. The processes influencing FCO₂ seasonality differ between subregions. Off Peru, the primary drivers of FCO₂ seasonal variability are: the oceanic partial pressure of CO₂ (pCO₂), primarily influenced by changes in DIC, and alongshore winds (Ekman transport). Similarly, off Baja California, changes in pCO₂ are the dominant contributor to the FCO₂ seasonality, with DIC and sea surface temperature (SST) also playing significant roles. This comparative analysis deepens our understanding of how large-scale climate processes shape FCO₂ dynamics, offering valuable insights for interpreting future changes in CO₂ fluxes within EBUSs.