Impacts of External Forcing on the Decadal Climate Variability in CMIP5 Simulations*

Decadal climate variability is usually regarded as an internal variability in the climate system. However, using the coupled simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is demonstrated that the external radiative forcing plays an important role in modulating decadal variability of the global mean surface air temperature (SAT). In historical runs, the standard deviations of the global mean SAT exhibit robust increases relative to preindustrial runs, indicating that external forcing acts on decadal variability of the global mean SAT through enhancing amplitude and modulating phase. By comparing model results using different external forcing agents, it is found that the natural forcing agent has the strongest impact on the decadal time scale. Every type of simulation (i.e., the preindustrial, historical, natural forcing, and anthropogenic forcing runs) from almost all the CMIP5 models exhibits a high correlation between the net shortwave (SW) radiative flux at the top of the atmosphere (TOA) and the global mean SAT with a 13-month lag. However, after taking the multimodel ensemble mean for the TOA SW radiative flux and the SAT, respectively, the correlations from the external forcing runs are much higher than those from preindustrial runs. This is because that the decadal SAT anomalies from multiple models cancel each other out in the preindustrial runs without external forcing but generally follow decadal evolution of the external forcing with a 13-month lag. The most significant regional responses to external forcing are found in the tropical Indian and Pacific Oceans, although with different physical mechanisms for the natural and greenhouse gas forcing agents.