Quantifying the role of aerosol forcing for historical Pacific multi-decadal variability

Abstract:

Studies have suggested that anthropogenic aerosols have influenced multi-decadal variability in the Pacific over the historical period, but the topic remains a subject of debate. Climate models differ widely in their representation of aerosol processes and resulting forcing, as well as in their representation of Pacific multi-decadal variability. Such model diversity renders a robust quantification of the role aerosol forcing for Pacific variability difficult. However, understanding what drives multidecadal variability in the Pacific is imperative for improving near-term climate projections and therefore merits further investigation.

Here, we show how aerosol forcing interacts with other external forcings and internal climate variability to generate simulated multi-decadal Pacific variability since the 1850s in a large ensemble of climate model simulations with HadGEM3-GC3.1, where anthropogenic aerosol emissions are scaled by a range of factors. Our results show a forced response in the Pacific Decadal Oscillation to anthropogenic aerosols in this model, particularly since the 1980s. This period coincides with a period of rapid increase in Asian aerosol emissions, and we discuss the mechanisms by which anthropogenic aerosol emissions affect the Pacific in HadGEM3-GC3.1. We assess the robustness of this response by comparing results across the models partaking in the Sixth Phase of the Coupled Model Intercomparison Project (CMIP6). Finally, we discuss the implication of these results for understanding real world Pacific variability, particularly in the context of understanding the discrepancy between observed and simulated temperature trends since the 1980s seen in many CMIP6 models.