Changes of the Pacific Decadal Oscillation (PDO) under global warming are investigated by using the fifth phase of the Coupled Model Intercomparison Project (CMIP5) outputs and a theoretical midlatitude air-sea coupled model. In a warming climate, the decadal variability of PDO is found to be significantly suppressed, with the amplitude reduced and the decadal cycle shifted toward higher-frequency band.  We used the theoretical model, put forward by Goodman and Marshall (GM model) to underpin the potential mechanisms. The GM model exhibits a growing coupled mode that resembles the simulated PDO.

  It is found that the suppression of the PDO appear to be associated with the acceleration of Rossby wave, due to the enhanced oceanic stratification under global warming. This shortens the PDO period and reduces PDO amplitude by limiting the growth time of the coupled mode. The GM model also suggests that the increase of growth rate due to strengthening of oceanic stratification tends to magnify the PDO amplitude, counteracting the Rossby wave effect. This growth rate influence, however, plays a secondary role.