Seasonally evolving dominant interannual variability mode of air-sea CO2 flux over the western North Pacific simulated by CESM1-BGC

We applied a season-reliant empirical orthogonal function (S-EOF) analysis based on the results of the Community Earth System Model, version 1-Biogeochemistry, to seasonal mean air-sea CO2 flux over the western North Pacific (WNP) (0A degrees-35A degrees N, 110A degrees E-150A degrees E). The first leading mode accounts for 29% of the total interannual variance, corresponding to the evolution of the El Nio-Southern Oscillation (ENSO) from its developing to decaying phases. During the ENSO developing phase in the summer and fall, the contribution of surface seawater CO2 partial pressure anomalies is greater than that of gas transfer/solubility anomalies, which contribute to increasing oceanic CO2 uptake over the WNP. During the ENSO mature phase in the winter, the anomalous southwesterly northwest of the western North Pacific anticyclone (WNPAC) reduces the surface wind speed in the China marginal sea and thus decreases oceanic CO2 uptake by reducing the gas transfer coefficient. In the subsequent spring, the WNPAC maintains with an eastward shift in position. The anomalous southwesterly warms sea surface temperatures in the China marginal sea by reducing evaporation and thus decreases oceanic CO2 uptake by enhancing surface seawater CO2 partial pressure. This process, rather than the effect of decreasing gas transfer coefficient, dominates CO2 flux anomalies in the spring.