How does the North Pacific Meridional Mode affect the Indian Ocean Dipole?

This study reveals a strong relationship between the North Pacific Meridional Mode (PMM) in boreal spring and the Indian Ocean Dipole (IOD) in following autumn. A positive spring PMM tends to be followed by a positive IOD and vice versa. The mechanism for the influence of the PMM on the IOD is then investigated. Positive spring PMM-related cyclonic and SST warming anomalies over the subtropical North Pacific propagate southward to the equatorial central Pacific in the following summer via wind-evaporation-SST feedback. SST warming and enhanced atmospheric heating in summer in the equatorial central Pacific induce an anomalous Walker circulation with ascending anomalies over the tropical central Pacific and descending anomalies over the Maritime Continent. The descending anomalies over the Maritime Continent result in southeasterly wind anomalies off the west coast of Sumatra, which lead to cold SST anomalies in the southeastern tropical Indian Ocean via modulating surface heat flux and upwelling of cold water. The associated increase in zonal SST gradient in the tropical Indian Ocean leads to low-level easterly wind anomalies and contributes to warm SST anomalies in the western tropical Indian Ocean via oceanic dynamic process. The warm and cold SST anomalies in the tropical western and southeastern Indian Ocean further develop to an IOD event in the following autumn via a positive air-sea interaction. The above process for the PMM influence on IOD can be simulated in the long historical simulations of coupled climate models. This study suggests that the spring PMM is a potential precursor for the occurrence of IOD event in the following autumn.