Interannual variability of spring rainfall over South China in association with the North Pacific Oscillation and North Atlantic Oscillation as revealed by reanalysis data and CMIP6 simulations

The interannual variability of South China spring rainfall (SCSR) is examined in relation to the North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO) based on longer-term reanalysis datasets and simulation outputs from Coupled Model Intercomparison Project Phase 6 (CMIP6) for the period 1901-2014. The interannual fluctuations of SCSR exhibit a significantly positive (negative) correlation with those of the spring NPO (NAO), thus the above-normal and below-normal SCSR tend to occur during the positive NPO (NPO+) and negative NPO (NPO-) phases, whereas the negative NAO (NAO-) and positive NAO (NAO+) phases favor the above-normal and below-normal SCSR, respectively. However, the variations in spring NPO and NAO are independent of each other, suggesting that anomalous SCSR depends on circulation anomalies induced jointly by various combinations of the NPO-NAO phases. Composite analyses show that the synergetic impacts on SCSR of the out-of-phase NPO+/NAO- combination are through the enhanced dipole circulation patterns, with both the subtropical negative and mid-latitude positive sea level pressure anomalies intensifying as compared to those for the individual NPO+ phase in the lower troposphere. The intensified low-level moisture convergence associated with the southwestward-extending subtropical anticyclone is in conjunction with the upper-level divergence over South China due to the northward-extending westerly-jet, resulting in more SCSR. Inversely, another out-of-phase NPO-/NAO+ combination favors less SCSR. In contrast, the preference for normal SCSR appears in the in-phase NPO+/NAO+ and NPO-/NAO- combinations, because the circulation anomalies in the NPO+ (NPO-) phase tend to be partly offset by those in the NAO+ (NAO-) phase. Importantly, the above respectively and jointly influencing mechanisms of the spring NPO and NAO on SCSR are validated by historical simulations from some CMIP6 models.