Abstract/Summary

Climate variability in the Pacific has an important influence on climate around the globe. In the period from 1981 to 2012, there was an observed large-scale cooling in the Pacific. This cooling projected onto the negative phase of the Pacific Decadal Oscillation (PDO) and contributed to a slowdown in the rate of near-surface temperature warming. However, this cooling pattern is not simulated well by the majority of coupled climate models and its cause is uncertain. We use large multi-model ensembles from the 6th Climate Model Intercomparison Project, and an ensemble of simulations with HadGEM3-GC3.1-LL that is specifically designed to sample the range of uncertainty in historical anthropogenic aerosol forcing, to revisit the role of external forcings. We show that anthropogenic aerosols can drive an atmospheric circulation response via an increase in North Pacific sea-level pressure and contribute to a negative PDO during this period in several global climate models. In HadGEM3, this increase in North Pacific sea-level pressure is associated with an anomalous Rossby Wave train across the North Pacific which is also seen in observations. Our results provide further evidence that anthropogenic aerosols may have contributed to the observed cooling in the Pacific in this period. However, the simulated cooling in response to aerosol forcing is substantially weaker than the warming induced by greenhouse gases, resulting in simulations that are warming faster than observations, and further highlighting the need to understand whether models correctly simulate atmospheric circulation responses.