The Earth Research Institute (ERI) and the Department of Geography CLIMATE DYNAMICS COLLOQUIUM: Tropical Width Changes Through the 20th and 21st Centuries



02/09/2016 - 3:30am


Since ~1979, observations show the tropical belt has widened by 0.25 degrees to 3.0 degrees latitude decade-1, a phenomenon associated with poleward migration of subtropical dry zones and mid-latitude storm tracks. Global climate models also simulate tropical belt widening over this time period, but less than observed. Although studies have linked the widening to anthropogenic activities, including greenhouse gases and stratospheric ozone loss, the reasons for this discrepancy, and the mechanisms driving the expansion, remain uncertain. Here, I analyze multidecadal variability in tropical belt width using the CMIP5 climate model runs, and find that simulated rates of tropical expansion over the past 30 years, particularly in the Northern Hemisphere, are in better agreement with observations than previous models. Models driven by observed sea surface temperatures over this interval yield the largest rate of tropical expansion. NH tropical expansion is linked to the leading pattern of sea surface temperature variability in the North Pacific, the Pacific Decadal Oscillation (PDO). Both models and observations also show that the northern boundary of the tropical belt contracted from 1950 to 1979, coincident with the reversal of the PDO trend. In both time periods, anthropogenic aerosols act to modify the PDO and contribute to the width of the tropical belt. In the 21st century, when aerosol emissions are projected to decrease to near preindustrial levels, the effects of aerosols and greenhouse gases 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 NH tropical widening than models with direct aerosol effects only. More targeted simulations show that future reductions in aerosols can drive widening of the northern tropical belt as large as greenhouse gases, and idealized simulations show the importance of NH mid-latitude aerosol forcing. The 21st century reduction in aerosols peaks near 40N, 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 reductions are likely to be an important—if not dominant—driver of widening the northern tropical belt.


Brief Bio: Dr. Allen received a B.S. in atmospheric science, and an M.S. in Applied Climatology, from Cornell University. He received a PhD in Atmosphere, Ocean and Climate Dynamics from Yale University in 2009. Dr. Allen has completed two postdocs, including UC Irvine (Department of Earth System Science), and Scripps Institution of Oceanography. Since 2011, he has been an Assistant Professor of Climatology at UC Riverside. Dr. Allen’s research use climate models and observations to improve our understanding of the climate system. This includes natural variability and the processes involved, as well as how climate is changing, what is driving that change, and how to adapt to and mitigate such changes. His main research interests include climate variability, land-atmosphere interaction, atmospheric aerosols/short-lived pollutants, and large-scale climate dynamics. He has published over 20 peer-reviewed articles in prestigious journals, including Nature, Nature Geoscience and Nature Climate Change, and has received funding from NASA, NSF and private industry.