[A-CG35] Global Carbon Cycle Observation and Analysis

[ACG35-11] CO₂ sink variability in marginal seas of the western North Pacific

*Yosuke Iida¹, Masao Ishii² (1.Japan Meterological Agency, 2.Meteorological Research Institute)

Keywords:carbon cycle, oceanic CO2 sink, marginal sea

Owing to many efforts of atmosphere and ocean observations and data synthesis projects during the decades, we are now, to some extent, able to reconstruct surface ocean carbonate variables and sea-air CO₂ flux from observation data in combination with some machine learning methods, even in highly variable coastal and marginal seas. We investigated the variability in CO₂ sink/source in marginal seas of the western North Pacific by using observation-based reconstruction of surface ocean carbonate variables. Reconstruction of monthly sea surface pCO₂ and sea-air CO₂ flux fields in marginal seas by 0.25° latitude and longitude were derived from using a moving-window multiple linear regression (MW-MLR) method, in which sea surface temperature and salinity, sea surface dynamic height, and chlorophyll-a concentrations were used as explanatory variables for estimating total alkalinity (TA) and total dissolved inorganic carbon (DIC) concentration. Observation data used in the learning processes of the MW-MLR method were obtained from Global Ocean Data Analysis Project version 2 2022 (GLODAPv2.2022: Lauvset et al. 2022) and Surface Ocean CO₂ Atlas version 2022 (SOCATv2022: Bakker et al. 2016), whose fCO2 data were converted to DIC in combination with estimated TA. Uncertainties in estimating TA and DIC were σ=9.5 μmol/kg and σ=13.6 μmol/kg, respectively, which were calculated from prediction intervals of MLR. The total uncertainty in estimating pCO₂ was σ=39.3 μatm when the carbonate system calculation errors were considered. The results showed that marginal seas around Japan were net CO₂ sinks and the Bering Sea was a net source for the atmosphere during the period 1998-2022 (Fig. 1). While a primary cause of variabilities in CO₂ sink is the pCO₂ variability in all seas, wind speed is also significant, especially in the Sea of Japan (Fig. 2). Variabilities in pCO₂ are dominated by those in sea surface temperature upon the long-term CO₂ trend of increase at most seas and by those in salinity at the East China Sea (Fig. 3), which are highly affected by the large river inflow. Such estimates and analyses of marginal seas’ CO₂ sink can provide a more precise figure of the regional and also global carbon cycles.