Rossby wave breaking (RWB) variability over several regions of the Pacific basin is shown to be the mechanism by which the atmosphere forces the Pacific Decadal Oscillation (PDO), accounting for the majority of PDO interannual variability for any month of the year. Composite analyses based on hundreds of RWB cases show that anticyclonic (cyclonic) RWB is associated with a warm, moist (cool, dry) column that extends down to a surface anticyclonic (cyclonic) circulation, and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that are strong enough to generate the SST anomalies associated with the PDO. In regions where RWB variability affects the PDO, the relative frequency of RWB is related to El Ni\~no-Southern Oscillation (ENSO) and the two leading patterns of internal atmospheric variability: the Northern Annular Mode and the Pacific-North American Pattern. These results explain the free troposphere-to-surface segment of the atmospheric bridge concept wherein El Ni\~no anomalies emerge in summer and modify circulation patterns that act over several months to force sea surface temperature anomalies in the extratropical Pacific during late winter or early spring. Within the atmospheric bridge cycle, RWB variability during July-August and December-January accounts for more than 80$\%$ of the January PDOI.