Western North Pacific anticyclone change associated with the El Niño–Indian Ocean Dipole coupling

It is known that the lower-tropospheric anomalous anticyclone over the western North Pacific (WNPAC) is strongly influenced by the sea surface temperature (SST) variations in the tropical Pacific and Indian Oceans. In this study, we re-examine a role of the dominant inter-annual SST variability in the Indian Ocean basin (represented by the Indian Ocean Dipole [IOD]) in the formation and development of the WNPAC during El Niño. Regardless of the data sets obtained from observations and long-term (1,300 years) model simulations, it is found that the WNPAC is strongly developed with larger circulation intensities when El Niño co-occurs with a positive phase of IOD (i.e., El Niño–IOD coupling), whereas it is weakly developed when El Niño solely occurs (i.e., El Niño–IOD decoupling). Sensitivities computed by ratios of the WNPAC intensities to the El Niño intensities from the three cases of El Niño–IOD coupling strengths further capture the aforementioned changes, supporting the idea that the IOD has an active role in modulating the WNPAC intensity during its development phase in a co-operation with the known El Niño-related processes that drive the WNPAC. Additional analyses suggest that the pronounced atmospheric convection anomalies occurring over the western Indian Ocean during the WNPAC development season, which are only manifested in the El Niño–IOD coupled cases, may play an essential role for the WNPAC intensity modulation by concurrently enhancing the subsidence anomalies over the western Pacific including the Maritime Continent through a change in the tropical Walker circulation.