A Lagrangian study of surface pCO2 dynamics in the eastern equatorial Pacific Ocean

[1] We characterized pCO₂ variability on hourly to weekly timescales during the 14‐day GasEx‐2001 Lagrangian drifter experiment in the eastern equatorial Pacific Ocean. Underway pCO₂ was recorded at 5 m depth as the ship closely followed a drogued drifter. Dissolved O₂ (DO) was measured at 10 and 15 m depths using in situ sensors deployed on the drogue. Diel pCO₂ and DO variability is evaluated using a simple model that accounts for air‐sea exchange, vertical mixing, heating, and net community metabolism. Mixed‐layer depths and local (vertical) entrainment are estimated with the Price Weller Pinkel (PWP) mixed‐layer model. The mean observed pCO₂ was 472.0 ± 1.8 μatm with a diel increase of 2–6 μatm on most days, near coincident in time with the diel peak in temperature. The biogeochemical model reveals that heating was the primary source of diel pCO₂ variability, but net community production and depletion of CO₂ in the shallow warm layer due to air‐sea gas exchange reduced the heating‐driven peak by ∼1–4 and 1–2 μatm each day, respectively. The same model parameterizations also accurately predict the diel DO amplitude. In the model, atmospheric exchange depletes total CO₂ and DO in the surface layer, and the depleted water is mixed with the isolated underlying water during nocturnal convection. The 10‐ and 15‐m DO time series corroborate these predicted dynamics. Over the 14‐day study, net heating offset the expected ∼14 μatm decrease due to air‐sea CO₂ exchange and net community production, resulting in a nearly constant mean pCO₂. Consequently, net heating acts to sustain high air‐sea CO₂ fluxes in the upwelled equatorial Pacific water as the water advects westward in the South Equatorial Current.