Oceanic mesoscale eddies enhance the Pacific Decadal Oscillation and its predictability.

The Pacific Decadal Oscillation (PDO) profoundly influences marine ecosystems, fisheries, and global hydroclimate. Although traditionally interpreted as basin-scale oceanic responses to atmospheric stochastic forcing, whether its dynamics involve active ocean feedbacks remains unresolved. Using an unprecedented multicentury eddy-resolving global climate simulation, we find that mesoscale eddy-driven atmospheric anomalies in the Kuroshio Extension (KE) region are critical to PDO evolution. During the PDO cold phase, the northward-shifted meandering KE generates mesoscale sea surface temperature (SST) anomalies that intensify lateral diabatic heating gradient, driving deep updrafts that cool the mid-troposphere and weaken its northern baroclinicity. This suppresses transient eddy momentum flux and facilitates a basin-scale low-pressure anomaly, initiating transition to the warm phase. Concurrently, mesoscale eddy-induced vertical heat transport sustains SST anomalies, providing additional PDO memory. These coupled processes substantially enhance the PDO's predictability. Our findings highlight the previously underappreciated upscale effects of oceanic eddies, offering important insight into decadal climate variability.