Role of Changes in Mean Temperatures versus Temperature Gradients in the Recent Widening of the Hadley Circulation

Abstract

The Hadley circulation (HC) has widened in recent decades, and it widens as the climate warms in simulations. But the mechanisms responsible for the widening remain unclear, and the widening in simulations is generally smaller than observed. To identify mechanisms responsible for the HC widening and for model–observation discrepancies, this study analyzes how interannual variations of tropical-mean temperatures and meridional temperature gradients influence the HC width. Changes in mean temperatures are part of any global warming signal, whereas changes in temperature gradients are primarily associated with ENSO. Within this study, 6 reanalysis datasets, 22 Atmospheric Modeling Intercomparison Project (AMIP) simulations, and 11 historical simulations from phase 5 of the Climate Modeling Intercomparison Project (CMIP5) are analyzed, covering the years 1979–2012. It is found that the HC widens as mean temperatures increase or as temperature gradients weaken in most reanalyses and climate models. On average, climate models exhibit a smaller sensitivity of HC width to changes in mean temperatures and temperature gradients than do reanalyses. However, the sensitivities differ substantially among reanalyses, rendering the HC response to mean temperatures in climate models not statistically different from that in reanalyses. While global-mean temperatures did not increase substantially between 1997 and 2012, the HC continued to widen in most reanalyses. The analysis here suggests that the HC widening from 1979 to 1997 is primarily the result of global warming, whereas the widening of the HC from 1997 to 2012 is associated with increased midlatitude temperatures and hence reduced temperature gradients during this period.

Additional Information

© 2014 American Meteorological Society. Manuscript received 21 February 2014, in final form 17 June 2014. This research was supported by the U.S. National Science Foundation (Grant AGS-1049201) and Israeli Science Foundation Grant 1537/12. All calculations presented here were performed using the Geophysical Observation Analysis Tool (GOAT), a freely available MATLAB-based tool for retrieval, analysis, and visualization of geophysical data (http://www.goat-geo.org).

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