Distinct Effects of Early Positive Indian Ocean Dipole Events on East Asian Seasonal Rainfall by Modulating the Western North Pacific Anomalous Anticyclone

Abstract As one of the most prominent modes of interannual variability in tropical oceans, the Indian Ocean Dipole (IOD) exerts global climate and socioeconomic impacts. Compared to the canonical positive IOD (pIOD) peaking in boreal autumn, more pIOD events peaking in boreal summer have occured during the recent decades, which is referred to as early pIOD. Rainfall anomalies associated with the early pIOD show increased precipitation over the Maritime Continent and above‐normal precipitation over the Yangtze River region in summer, along with decreased winter precipitation over South China, distinct from the anomalies during canonical pIOD years that are largely modulated by El Niño. These distinctive rainfall responses are primarily modulated by the western North Pacific (WNP) anomalous anticyclone (WNPAC), which slightly strengthens in summer and significantly weakens in winter of the early pIOD years, different from the canonical pIOD. Pacemaker experiments, which prescribe observed sea surface temperatures (SST) anomalies in the Indian Ocean using a state‐of‐the‐art model, further support the influence of early pIOD on the WNPAC, independent of forcing from other oceans. Local air‐sea interaction sustains the warm SST anomalies over the WNP from summer to winter during the early pIOD events, weakening the WNPAC in winter.