The Hadley circulation has large climate impacts at low latitudes by transferring heat and moisture between the tropics and subtropics. Climate projections show a robust weakening of the Northern Hemisphere Hadley circulation by the end of the twenty-first century. Over the past several decades, however, atmospheric reanalyses indicate a strengthening of the Hadley circulation. Here we show that the strengthening of the circulation in the Northern Hemisphere is not seen in climate models; instead, these models simulate a weakening of the circulation in the past 40 years. Using observations and a large ensemble of model simulations we elucidate this discrepancy between climate models and reanalyses, and show that it does not stem from internal climate variability or biases in climate models, but appears related to artefacts in the representation of latent heating in the reanalyses. Our results highlight the role of anthropogenic emissions in the recent slowdown of the atmospheric circulation, which is projected to continue in coming decades, and question the reliability of reanalyses for estimating trends in the Hadley circulation.
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The data used in the manuscript are publicly available for CMIP5 data (https://esgf-node.llnl.gov/projects/cmip5/), LE (http://www.cesm.ucar.edu/), ERA-I (https://www.ecmwf.int), NCEP2 (https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html), JRA55, MERRA-2 and CFSR2 (https://rda.ucar.edu/ and https://esgf.nccs.nasa.gov/projects/create-ip/) and GPCP (https://www.esrl.noaa.gov/psd/data/gridded/data.gpcp.html).
The code for calculating the KE equation is available upon request from firstname.lastname@example.org.
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Supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, which is administered by UCAR’s Cooperative Programs for the Advancement of Earth System Science (CPAESS). L.M.P. is grateful for the continued support of the US Natural Science Foundation.
The authors declare no competing interests.
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Chemke, R., Polvani, L.M. Opposite tropical circulation trends in climate models and in reanalyses. Nat. Geosci. 12, 528–532 (2019). https://doi.org/10.1038/s41561-019-0383-x
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