Tropical Forcing of the Pacific Decadal Oscillation Tied to Central Pacific ENSO

Abstract The Pacific decadal oscillation (PDO) is the leading empirical mode of sea surface temperature (SST) variability in the North Pacific, exhibiting a horseshoe-shaped pattern. The PDO can be driven by tropical Pacific El Niño–Southern Oscillation (ENSO), associated with its eastern Pacific (EP) type as traditionally thought. However, recent studies suggested that the PDO can also be induced by the central Pacific (CP)–type ENSO. Which ENSO flavor dominates the tropical forcing of the PDO remains unraveled. To robustly address this question, we use multiple observational SST datasets, a tropical Pacific pacemaker experiment, and multimodel historical simulations from phase 6 of the Coupled Model Intercomparison Project for the period 1900–2014. We show that the PDO is tied to CP ENSO forcing. Specifically, relative to EP ENSO–excited Aleutian low (AL) anomaly, CP ENSO–triggered AL shifts southeastward and displays a northwest–southeast tilted structure. As a result, it drives not only cold SST anomalies in the western and central North Pacific, similar to those forced by EP ENSO, but also prominent warm SST anomalies off the west coast of North America and in the subtropical northeastern Pacific, thus contributing more to the PDO. Our study highlights the key role of CP ENSO in forcing the PDO, advancing the current theoretical framework of PDO mechanisms. Significance Statement Traditionally, the PDO can be a response of the canonical (i.e., EP) ENSO forcing. This notion is, however, challenged by recent studies that pointed out the role of CP ENSO in forcing the PDO. Yet, the relative forcing strength between EP and CP ENSOs has not been quantified. Here, we robustly show that the PDO is dominantly driven by CP ENSO. This result advances the current PDO mechanisms on the tropical Pacific forcing.