Examining the respective roles of greenhouse-gas and aerosol forcing for modes of multi-decadal variability

The respective roles of aerosol and greenhouse-gas forcing in modulating the phasing and amplitude of large-scale modes of multi-decadal variability remain poorly understood, despite the attention that has been devoted to trying to separate the influence of forcing from internal variability in modes such as the Atlantic Multidecadal Variability and the Pacific Decadal Oscillation, for instance. However, understanding what drives multidecadal variability in these basins is imperative for improving near-term climate projections.

Here, we show how aerosol and greenhouse-gas forcing interact with internal climate variability to generate indices of multi-decadal variability in the Atlantic, using a large ensemble of historical simulations with HadGEM3-GC3.1 for the period 1850-2014, where anthropogenic aerosol emissions are scaled to sample a wide range in historical aerosol forcing. These results are complemented by early results from new stabilised warming simulations with the same climate model and analysis of future projections from models partaking in the Sixth Phase of the Coupled Model Intercomparison Project (CMIP6).