Evaluating Uncertainty in Aerosol Forcing of Tropical Precipitation Shifts
Anthropogenic aerosol emissions have been linked to driving climate responses such as shifts in the location of tropical rainfall. In the CMIP5 multi-model ensemble, there is a strong correlation between the strength of the inter-hemispheric aerosol forcing and the latitudinal shift in tropical precipitation shifts from 1950 to 1985. Researchers from the University of Leeds, Met Office Hadley Centre, Lawrence Livermore National Laboratory, and the University of Exeter analyzed a perturbed physics ensemble (PPE) performed with the HadGEM3-GC3.05 climate model designed to sample model uncertainty in climate projections. The PPE spans a comparable range of aerosol forcing to CMIP5 and is used to investigate the influence of aerosol radiative forcing on tropical precipitation shifts. While tropical precipitation shifts respond to the inter-hemispheric temperature contrast, they do not strongly respond to the strength of historical inter-hemispheric aerosol radiative forcing in the PPE, contradicting the results from CMIP5. In contrast, members with a large hemispheric difference in historical aerosol radiative forcing show a further northward tropical precipitation shift in response to future aerosol reductions.
Aerosols remain one of the most uncertain aspects of climate modeling, limiting our ability to predict future climate change. This paper innovatively questions the role of anthropogenic aerosol radiative forcing in modulating tropical precipitation shifts over the historical and future periods using a PPE. However, the results depend on the analyses of a single model and use a sample size too small to account for the effects of internal variability. A broader analysis involving the CMIP6 multi-model ensemble and other single-model PPEs that span a range of aerosol radiative forcing and climate sensitivity values would help deepen our understanding of the relationship between precipitation response to warming and future aerosol forcing across multiple emission scenarios.
An observed southward shift in tropical rainfall over land between 1950 and 1985, followed by a weaker recovery post-1985, has been attributed to anthropogenic aerosol radiative forcing and cooling of the Northern Hemisphere relative to the Southern Hemisphere. We might therefore expect models that have a strong historic hemispheric contrast in aerosol forcing to simulate a further northward tropical rainfall shift in the near-term future when anthropogenic aerosol emission reductions will predominantly warm the Northern Hemisphere. We investigate this paradigm using a perturbed parameter ensemble (PPE) of transient coupled ocean–atmosphere climate simulations that span a range of aerosol radiative forcing comparable to multi-model studies. In the 20th century, in our single-model ensemble, we find no relationship between the magnitude of pre-industrial to 1975 inter-hemispheric anthropogenic aerosol radiative forcing and tropical precipitation shifts. Instead, tropical precipitation shifts are associated with major volcanic eruptions and are strongly affected by internal variability. However, we do find a relationship between the magnitude of pre-industrial to 2005 inter-hemispheric anthropogenic aerosol radiative forcing and future tropical precipitation shifts over 2006 to 2060 under scenario RCP8.5. Our results suggest that projections of tropical precipitation shifts will be improved by reducing aerosol radiative forcing uncertainty, but predictive gains may be offset by temporary shifts in tropical precipitation caused by future major volcanic eruptions.