On the mechanisms of the suppressed Pacific Decadal Oscillation in a warming climate

In a warming climate, the decadal variability is found to be significantly suppressed, with the amplitude reduced and the decadal cycle shifted toward higher-frequency band. The underlying mechanisms and theoretical explanations are investigated by conducting a series of coupled ocean-atmosphere experiments, studying a theoretical midlatitude air-sea coupled model, and using the outputs from phase 5 of Coupled Model Intercomparison Project (CMIP5).

In global warming, the spatial pattern of the decadal variability preserves while its decadal cycle is significantly suppressed. With the aid of partial-blocking experiment, it is found that the decadal cycle can be attributed to the cross basin timescale of the first-mode baroclinic Rossby waves. The Rossby wave accelerates in response to an enhanced oceanic stratification under greenhouse warming, which shortens the periodicity of decadal variability.

Moreover, the amplitude of decadal variability is found to be reduced in global warming, a result with high inter-model consistency among CMIP5 models. A theoretical model, put forward by Goodman and Marshall (GM model), is used to underpin the potential mechanisms. In this model the decadal variability can be interpreted as a growing coupled mode, propagating westward via Rossby wave during which an initial perturbation keeps growing. The shortened periodicity in global warming reduces the amplitude by limiting the growth time of the coupled mode.