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

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 pCO ₂ 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 pCO ₂. 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.