There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and theNorthernHemisphere subtropical high have very weak covariations with the abovemetrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 amongmetrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important tomake sure the metric used is appropriate for the specific phenomena and impacts being examined.
Bibliographical noteFunding Information:
We thank Karen Rosenlof for helpful comments and discussions on earlier versions of the manuscript, and Damianos Mantsis for initial discussions regarding OLR metrics. This paper arises from discussions within the International Space Science Institute (ISSI) Tropical Width Diagnostics Intercomparison Project and the U.S. Climate Variability, Predictability and Change (U.S. CLIVAR) Working Group on the Changing Width of the Tropical Belt. The authors thank the entire members of these groups, the ISSI and U.S. CLIVAR offices, and sponsoring agencies (ESA, Swiss Confederation, Swiss Academy of Sciences, University of Bern, NASA, NOAA, NSF, and DOE). All model data used in this paper are freely available through the Earth System Grid Federation (https:// pcmdi9.llnl.gov/projects/esgf-llnl/). We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups for producing and making available their model output. For CMIP, the U.S. Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. DW acknowledges support from NSF AGS-1403676.
© 2018 American Meteorological Society.
- Hadley circulation
- Hydrologic cycle
- Meridional overturning circulation