Exploring the Impact of Aerosol Radiative Forcing Uncertainty on Tropical Precipitation Shifts in the Near-Term Future

Abstract:

Anthropogenic aerosols emissions have caused a negative but highly uncertain radiative forcing over the industrial era, causing a cooling effect mostly over the northern hemisphere. Hence, anthropogenic aerosols have been linked to historical climate responses, such as changes in the position of the Intertropical Convergence Zone (ITCZ) and resultant tropical precipitation shifts that are governed by the atmospheric interhemispheric energy balance. In the near-term future, anthropogenic aerosol emissions reductions will cause a positive radiative forcing relative to present day, unmasking warming in the northern hemisphere. If the strength of aerosol radiative forcing is able to modulate the magnitude of modelled ITCZ shift historically, then the large uncertainty in aerosol radiative forcing may also limit our understanding of tropical precipitation shifts in the near-term future.

We investigate the link between aerosol radiative forcing uncertainty and ITCZ and tropical precipitation shifts in the latter half of the 20^th century and first half of the 21^st century under scenarios RCP8.5 and RCP2.6. We use a perturbed parameter ensemble (PPE) of a global coupled climate model that was designed to sample the uncertainty in future climate changes. The PPE consists of 15 transient climate simulations of the UK Met Office’s GC3.05 model with 47 parameters perturbed across a range of model schemes, and as a result spans a range of aerosol radiative forcings.

The PPE reveals a southward shift of the ITCZ and tropical precipitation in most ensemble members in the latter half of the 20^th century. Whereas, in the first half of the 21^st century the PPE shows both northward and southward shifts in the ITCZ position and tropical precipitation across the ensemble, with a further northward shift in RCP2.6. We find a correlation between the shift in the ITCZ position and the magnitude of aerosol radiative forcing. However, the correlations in our single-model ensemble are not as strong as those in previous studies that use multi-model ensembles. The potential drivers of these differences, and of the shifts themselves are investigated. Lastly, we compare our model output to aerosol, cloud and radiation observations in attempt to identify the most plausible future climate responses in our ensemble.