In-Phase Pacific Decadal Oscillation and El Niño–Southern Oscillation Enhance the Interannual Variability of Spring Blooms in the Yellow Sea

Although the individual modulations of marine phytoplankton by Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO) have been comprehensively studied, their synergistic effect on the phytoplankton spring bloom (PSB) over the shelf remains unclear. In this study, which was based on satellite data obtained from the Yellow Sea between 2003 and 2020, we quantified the PSB intensity and environmental factor responses to PDO and ENSO, after which we examined the synergistic effect of PDO and ENSO on PSB. The results revealed that the interannual variability of the PSB intensity correlated negatively with the PDO phases (r = -0.48; P < 0.05) and could be enhanced by in-phase ENSO. Further, the mean PSB intensity in the negative-phase PDO (NPDO; 153 mg C/m³) was approximately 28% higher than that in the positive-phase PDO (PPDO; 120 mg C/m³) at the 90% confidence level, whereas the mean PSB intensity during the years of combined NPDO and La Niña (162 mg C/m³) was approximately 53% higher than that during the years combining PPDO and El Niño (106 mg C/m³) at the 95% confidence level. Furthermore, the sea surface temperature (SST) and wind speed accounted for approximately 68% and 22% of the interannual variability of PSB intensity, respectively. Notably, the combination of PPDO with El Niño induced anomalously high SST and reduced the wind speed, thereby reducing the vertical mixing and lowering the availability of nutrients to PSB, and vice versa, for the case of NPDO combination with La Niña. Overall, our quantitative estimation revealed the strengthening effect of the in-phase combination of PDO and ENSO on PSB intensity, which aids the predictions of the change in marine ecosystems and fishery.