EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Exploring the impact of aerosol radiative forcing uncertainty on shifts in ITCZ position and tropical rainfall in the near-term future

Amy Peace1, Ben Booth2, Ken Carslaw1, Leighton Regayre1, Lindsay Lee1, David Sexton2, and John Rostron2
Amy Peace et al.
  • 1University of Leeds, Institute of Climate and Atmospheric Science, School of Earth and Environment, United Kingdom
  • 2Met Office Hadley Centre, Exeter, United Kingdom

Anthropogenic aerosol emissions over the industrial period have caused a negative but highly uncertain radiative forcing. This negative radiative forcing has had a cooling effect mainly over the northern hemisphere, affecting the atmospheric interhemispheric energy balance. Consequently aerosols have been linked to observed dynamical responses over the industrial period that depend on the atmospheric interhemispheric energy balance, such as changes in the position of the Intertropical Convergence Zone (ITCZ) and resultant tropical precipitation shifts. However, over the course of the 21st century anthropogenic aerosol emissions are predicted to decline. The reduction in anthropogenic aerosol emissions will cause a positive radiative forcing relative to present day, creating a warming effect in the northern hemisphere. Hence, if the strength of aerosol radiative forcing modulates the magnitude of shifts in the ITCZ, then the large uncertainty in aerosol radiative forcing will limit our understanding of how tropical precipitation will shift in the near-term future.

We use a perturbed parameter ensemble (PPE) of a global coupled climate model to investigate the link between aerosol radiative forcing and ITCZ and tropical rainfall shifts in the near-term future. The PPE consists of 20 simulations of the UK Met Office’s GC3.05 model with parameters perturbed from a range of model schemes. The ensemble was designed to sample uncertainties in future changes, and as a result spans a range of aerosol radiative forcings.

The PPE reveals both northward and southwards shifts in the ITCZ position across the ensemble in the latter half of the 20th century and first half of the 21st century, as well as changes in width and intensity of the ITCZ. We find a correlation between the shift in the ITCZ position and the magnitude of aerosol radiative forcing and AOD trends. However, the correlations in our ensemble are not as strong as those cited in previous studies that use multi-model ensembles. The potential causes of this difference are investigated. We also compare our model output to aerosol, cloud and radiation observations in attempt to identify the most plausible future aerosol-driven climate responses.

How to cite: Peace, A., Booth, B., Carslaw, K., Regayre, L., Lee, L., Sexton, D., and Rostron, J.: Exploring the impact of aerosol radiative forcing uncertainty on shifts in ITCZ position and tropical rainfall in the near-term future , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10202,, 2020

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-10202, Valerio Lembo, 06 May 2020

    I would like to thank the authors for the interesting results that have been shown here.

    The issue of relating aerosol forcing asymmetries with the ITCZ meridional displacement has been a challenging topic, especially when it comes to investigating the historical simulations with a multi-model ensemble approach. The zonal mean Hadley cell has been typically related to energy budget and implied meridional energy transports using different approaches, also involving precipitation maxima over the Tropics. An interesting review of the theoretical background can be found here:

    Schneider, T., Bischoff, T. & Haug, G. Migrations and dynamics of the intertropical convergence zone. Nature 513, 45–53 (2014).

    We tried to dig into the topic with Martin Wild at ETH, using sensitivity experiments of ECHAM simulations for the historical period:

    Lembo, V., Folini, D., Wild, M. et al. Energy budgets and transports: global evolution and spatial patterns during the twentieth century as estimated in two AMIP-like experiments. Clim Dyn 48, 1793–1812 (2017).

    And looking into CMIP5 datasets, isolating the role of the GHG forcing:

    Lembo, V., Folini, D., Wild, M. et al. Inter-hemispheric differences in energy budgets and cross-equatorial transport anomalies during the 20th century. Clim Dyn 53, 115–135 (2019).

    I would be glad to discuss these results with you and share my thoughts on the topic!



    • AC1: Reply to CC1, Amy Peace, 08 May 2020

      Hi Valerio,

      Thanks for your comment.

      I have had a read through your papers, and you have some nice results!

      I would be interested to discuss more on the effect of aerosol forcing and cross-equatorial energy transport in the 20th century, and near-term future implications.