Synergistic effect of El Niño and negative phase of Arctic Oscillation on winter precipitation over southern China

We demonstrate that the negative phase of the Arctic Oscillation (AO⁻) and El Niño exert a significant synergistic effect on the enhancement of winter precipitation over southern China (SCWP). The enhancement of SCWP under the co-occurrence of AO⁻ and El Niño arises from both the linear superposition of individual effects and nonlinear interaction between the AO⁻ and El Niño. Physical mechanisms reveal that El Niño favors the negative phase of North Atlantic Oscillation-like circulation, enhancing North Atlantic Rossby wave sources during the concurrent AO⁻. Under the combined influence of AO⁻ and El Niño, Rossby waves initiated from central equatorial Pacific and subtropical North Atlantic synergistically intensified from North Atlantic to Eurasia in three pathways, facilitating the strengthening of a low-pressure anomaly over southern China and amplifies the tropical northwestern Pacific (TNWP) high. The intensified TNWP high is accompanied by stronger descending motion and anticyclonic circulation over the region, which increases southwesterly moisture transport toward southern China, and enhances ascending motion over southern China, leads to increased SCWP. By examining the synergistic effects of AO⁻ and El Niño on SCWP, this study advances our understanding of SCWP variability and its underlying mechanisms.