Seasonal variability in surface water pCO2 and air-sea CO2 fluxes in the Northwestern Pacific Ocean

Abstract

The Northwestern Pacific Ocean is one of the most important carbon sink regions globally. However, spatial variability and seasonal amplitude of surface water CO₂ partial pressure (pCO₂) and air-sea CO₂ fluxes remain unresolved. Surface seawater pCO₂ and auxiliary parameters were investigated in the Northwestern Pacific (10–33°N, 120–158°E) during spring, summer and winter in 2019 and 2020. The air-sea CO₂ fluxes exhibited pronounced seasonal variability, acting as a CO₂ sink of 5.0 ± 4.1 mmol m⁻² d⁻¹ in winter and a CO₂ source of 1.7 ± 1.6 mmol m⁻² d⁻¹ in summer. In summer, the CO₂ source increased with latitude, with the 10–14°N sub-region near equilibrium with the atmosphere (0.6 ± 0.6 mmol m⁻² d⁻¹) and the 27–33°N sub-region displaying the strongest source (3.6 ± 2.1 mmol m⁻² d⁻¹). The air-sea CO₂ flux is primarily driven by variability in surface water pCO₂. During summer, surface water pCO₂ increases with latitude (408.4 ± 5.1, 418.2 ± 9.0 and 455.5 ± 12.4 μatm in 10–14°N, 14–27°N and 27–33°N, respectively), showing a “strange” pattern inverse with sea surface temperature. Temperature normalized pCO₂ (NpCO₂) also increases with latitude. In winter, surface water pCO₂ generally decreases with latitude (379.4 ± 3.8, 372.9 ± 9.1 and 354.5 ± 3.6 μatm in 10–14°N, 14–27°N and 27–33°N, respectively), but NpCO₂ increased with latitude (356.7 ± 7.7, 387.2 ± 13.1 and 434.4 ± 4.4 μatm in 10–14°N, 14–27°N and 27–33°N, respectively). In addition to the dominating temperature effect, different sub-regions have their own unique processes that affect the pCO₂ behavior which in turn influences the air-sea CO₂ fluxes. In the western zone (west of 130°E) of the 10-14°N sub-region, precipitation reduces pCO₂ by 12.4 ± 5.2 μatm in summer and 14.8 ± 4.4 μatm in winter. In the 14-27°N sub-region, the relatively high NpCO₂ is primarily driven by evaporation, with elevated salinity increasing surface water NpCO₂ by 11.1 ± 12.7 μatm in summer and 7.7 ± 15.3 μatm in winter. The 27–33°N sub-region is located in the Subtropical Mode Water and atmospheric CO₂ intrusion (increasing pCO₂ by 44.2 μatm) also have important contributions to the high NpCO₂. Under the context of global warming, the regional changes, such as variations in evaporation and precipitation, have the potential to significantly alter global ocean CO₂ sink/source patterns and weaken the surface ocean’s CO₂ sequestration ability.