FY 2025

High-frequency correlations between winds and pCO2 change the California Coastal Upwelling System from a CO2 sink to a source

Song, R., T. DeVries, R. Li, A.J. Sutton, U. Send, and H.C. Frazão

Geophys. Res. Lett., 52(14), e2025GL115470, doi: 10.1029/2025GL115470, View open access article at AGU/Wiley (external link) (2025)

Net sea-air CO₂ flux can be calculated from observations of seawater and atmosphere partial pressure of CO₂ (pCO₂) and estimates of the gas transfer velocity. Typically, these quantities are calculated at a monthly resolution, which misses potentially important high-frequency temporal variability. Here, we calculated sea-air CO₂ flux at a 3-hourly resolution using a 10-year mooring data set (2011–2020) from the central California coastal upwelling region. We identified a significant flux of CO₂ from the ocean to the atmosphere due to a positive correlation between seawater pCO₂ and wind speed at timescales of hours to days, particularly during the late spring and early summer upwelling season. Accounting for this variability changes the region from a net sink to a net source of CO2 to the atmosphere. These findings imply that CO₂ fluxes computed from monthly-resolution data may miss important shorter-term variability that contributes to a net outgassing of CO₂ from the ocean.

Plain Language Summary The exchange of carbon dioxide (CO₂) between the ocean and atmosphere plays a crucial role in regulating Earth's climate. In this study, we examined how accurately this exchange is captured along the California coast, where seasonal wind-driven upwelling brings CO₂-rich waters to the surface. We compared CO₂ flux estimates using data collected every 3 hours for 10 years with those derived from monthly averages. We found that monthly averages often miss important short-term variability, particularly during upwelling seasons. These seasonal changes cause the region to emit more CO₂ into the atmosphere than previously thought, challenging the perception that the coastal California Current is a CO₂ sink. High-frequency calculations revealed that winds and ocean conditions are closely linked, driving this variability in CO₂ exchange. Our findings highlight the need for continuous and high-resolution measurements to better quantify sea-air CO₂ exchange.