Western North Pacific anticyclone change associated with the El Nino-Indian Ocean Dipole coupling

摘要

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 Nino. Regardless of the data sets obtained from observations and long-term (1,300years) model simulations, it is found that the WNPAC is strongly developed with larger circulation intensities when El Nino co-occurs with a positive phase of IOD (i.e., El Nino-IOD coupling), whereas it is weakly developed when El Nino solely occurs (i.e., El Nino-IOD decoupling). Sensitivities computed by ratios of the WNPAC intensities to the El Nino intensities from the three cases of El Nino-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 Nino-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 Nino-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.