Paired windward and leeward biogeochemical time series reveal consistent surface ocean CO₂ trends across the Hawaiian Ridge

Sustained time series have provided compelling evidence for progressive acidification of the surface oceans through exchange with the growing atmospheric reservoir of carbon dioxide. However, few long-term programs exist, and extrapolation of results from one site to larger oceanic expanses is hampered by the lack of spatial coverage inherent to Eulerian sampling. Since 1988, the Hawaii Ocean Time-series program has sampled CO₂ system variables nearly monthly at Station ALOHA, a deep ocean site windward and 115 km north of the island of Oahu. Surface measurements have also been made at Station Kahe, a leeward site 12 km from the island and on the opposite side of the Hawaiian Ridge. Despite having different physical settings, the sites exhibit identical rates of surface pCO₂ increase and hydrogen ion accumulation, suggesting that atmospheric forcing dominates over local dynamics in determining the CO₂ trend in the surface waters of the North Pacific subtropical gyre.