Distinct effects of interdecadal Warm Pool Seesaw and Interdecadal Pacific Oscillation over the tropical Indo-Pacific

Abstract Decadal variability in the tropical Indo-Pacific complicates precipitation projections. Here, we quantify the influence of newly identified Warm Pool Seesaw (WPS) and traditional Interdecadal Pacific Oscillation (IPO) using large ensembles from the Community Earth System Model. Traditional IPO reduces rainfall in convergence zones while enhancing it in the central-eastern Pacific by modifying the Walker Circulation. The newly identified WPS, a dipole SST pattern with warming in the western Pacific while cooling in the southeastern Indian Ocean, also modulates regional precipitation variability. When IPO and WPS occur in phase, their combined effect amplifies precipitation variability, aligning closely with the independent WPS pattern. This highlights the significance of WPS mode for precipitation variability in the Indo-Pacific, although it occurs less frequently than the traditional IPO mode. Conversely, when out of phase, they reinforce the IPO’s impact over the Pacific, while partially offsetting in the southeastern Indian Ocean, thereby increasing precipitation complexity. To better capture decadal precipitation variability, we develop a novel composite index integrating the IPO and WPS indices. This framework provides improved insights into future precipitation predictions.