Changes in the position and width of the tropical belt are societally and ecologically relevant, because they are associated with shifts of the subtropical dry zones. The tropical belt has widened since about 1980, but little is known about its earlier variability. Here we analyse historical surface and upper-level observations, three global reanalysis data sets, and a reconstruction of total column ozone, to show that the northern tropical edge retracted from 1945 to 1980, while the northern Hadley cell shifted southwards in both summer and winter. We present chemistry–climate model simulations that reproduce the retraction and southward shift. We find that retraction of the tropical belt was largely due to cooling sea-surface temperatures north of the Equator and warming south of the Equator, most prominently over the Atlantic. Substantial hydroclimatic anomalies such as European droughts of the 1940s and 1950s and the Sahel drought of the 1970s were associated with this shift of the Hadley cell. Our results suggest that multidecadal changes in the position of the northern Hadley cell are an important component of climate variability.
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The authors wish to acknowledge support by the Swiss National Science Foundation (projects EVALUATE (130407) and FUPSOL-II (147659)), EU FP7 project ERA-CLIM2 (607029) and ERC Grant TITAN (320691). Support for the Twentieth Century Reanalysis Project data set is provided by the US Department of Energy, Office of Science Innovative and Novel Computational Impact on Theory and Experiment (DOE INCITE) programme, and Office of Biological and Environmental Research (BER), and by the National Oceanic and Atmospheric Administration Climate Program Office. We wish to thank all data providers.
The authors declare no competing financial interests.
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Brönnimann, S., Fischer, A., Rozanov, E. et al. Southward shift of the northern tropical belt from 1945 to 1980. Nature Geosci 8, 969–974 (2015) doi:10.1038/ngeo2568
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