|Title||Future aerosol reductions and widening of the northern tropical belt|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Allen RJ, Ajoku O.|
|Journal||Journal of Geophysical Research-Atmospheres|
|Type of Article||Article|
|Keywords||atmosphere; circulation; climate-change; expansion; shift; width|
Observations show that the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Although part of this signal is related to natural climate variability, studies have identified an externally forced contribution primarily associated with greenhouse gases (GHGs) and stratospheric ozone loss. Here we show that the increase in aerosols over the twentieth century has led to contraction of the northern tropical belt, thereby offsetting part of the widening associated with the increase in GHGs. Over the 21st century, however, when aerosol emissions are projected to decrease, the effects of aerosols and GHGs reinforce one another, both contributing to widening of the northern tropical belt. Models that have larger aerosol forcing, by including aerosol indirect effects on cloud albedo and lifetime, yield significantly larger Northern Hemisphere (NH) tropical widening than models with direct aerosol effects only. More targeted simulations show that future reductions in aerosols can drive NH tropical widening as large as greenhouse gases, and idealized simulations show the importance of NH midlatitude aerosol forcing. Mechanistically, the 21st century reduction in aerosols peaks near 40 degrees N, which results in a corresponding maximum increase in surface solar radiation, NH midlatitude tropospheric warming amplification, and a poleward shift in the latitude of maximum baroclinicity, implying a corresponding shift in atmospheric circulation. If models with aerosol indirect effects better represent the real world, then future aerosol changes are likely to be an important-if not dominant-driver of NH tropical belt widening.
Despite the above uncertainties, we have shown the importance of anthropogenic aerosols—particularly their indirect effects on clouds—in driving northern hemisphere tropical expansion in CMIP5 models across all four RCPs, in targeted CAM3/5 simulations using RCP4.5, and in more idealized aerosol experiments with CAM5. The response is consistent with a relatively simple mechanism, related to NH midlatitude warming amplification. Although geostrophic adjustment does not explicitly allow cause and effect to be separated, the maximum decrease in aerosol load—near 40°N—generally corresponds to the location of maximum warming. This shifts the latitude of the maximum climatological meridional temperature gradient poleward, which implies a corresponding shift in the jet. If models with larger aerosol forcing (i.e., those that include aerosol indirect effects) better represent the real world, then future aerosol changes are likely to be an important—if not dominant—driver of NH tropical belt widening.