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WIREs Clim Change
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The expanding tropics: a critical assessment of the observational and modeling studies

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This review provides comprehensive coverage of the tropical expansion literature to date. The primary focus is on the annual‐ and zonal‐mean behavior of the phenomenon. An idealized model that identifies the mean meridional circulation as a hemisphere‐wide structure with significant tropical–extratropical interaction is introduced as background for the understanding of the expansion and the methodologies used for detection. A variety of metrics from different data sources have been used to identify an expansion of the global tropics since 1979 by 1°–3° latitude in each hemisphere, an average trend of approximately 0.5°–1.0° decade−1. The symmetry of this expansion—whether Northern and Southern hemispheres are expanding at the same rate—is unclear. Limitations of observational datasets, including reanalyses, prevent a more precise determination at this time. General circulation models are able to qualitatively reproduce this expansion, but generally underestimate its magnitude. Multiple factors have been identified as potential drivers of the expansion, including increasing greenhouses gases, stratospheric ozone depletion, and anthropogenic aerosols. No single factor by itself appears to explain the full expansion, perhaps a shortcoming of the models or experiment design. It may be that some combination of these forcings is producing the change, but the relative contribution of each forcing to the expansion is currently unknown. The key issues remaining to be resolved are briefly summarized at the end. WIREs Clim Change 2014, 5:89–112. doi: 10.1002/wcc.251

Conflict of interest: The authors have declared no conflicts of interest for this article.

Schematic diagram detailing the components and physical processes of the idealized mean meridional circulation (MMC). Diagram represents the hemispheric annual and zonal‐mean flow; a mirror image about the equator would represent the global circulation. Labels in all capitals represent the main features of the MMC. Bold labels refer to subcomponents of the circulation. Text in italics identify important physical processes. See text for further discussion.
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Summary of result of observational tropical expansion studies, broken down by categories. Pluses (+) represent SH values, crosses (×) NH values of tropical expansion trend. Symbols in the corresponding hemisphere indicates expansion, symbols in the ‘opposite’ hemisphere indicate contraction. Within a particular methodology, the vertical position of each symbol is randomized to improve clarity of individual symbols; no other meaning is implied. Colors of symbols refer to source and/or methodology, with legends for each category in boxes to side of plot, where numbers represent the cited reference.
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Time‐latitude profiles of annually averaged NOAA AVHRR OLR for uncorrected (left) and the equatorial crossing time (ECT)‐corrected data (Ref ) for the period 1979–2008. Vertical dashed lines indicate the changes in the satellite. Contours are every 10 W m−2, with the 250 W m−2 contour colored in red. The trend along this contour is also indicated with a dashed line.
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Time series of the peak intensity (vertically‐averaged maximum Ψ) of the Hadley cell in the eight reanalyses for the Northern (top) and Southern (bottom) Hemispheres. (Reprinted with permission from Ref . Copyright 2013 The American Meteorological Society). Time series from ERA40 (post 2002) and 20CR (post 2008) are partially extrapolated from statistical relations with the other reanalyses Ψ.
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Time series of the position of the subtropical edge of the Hadley cell as defined from the streamfunction in the eight modern reanalyses for the Northern (top) and Southern (bottom) Hemisphere. (Reprinted with permission from Ref . Copyright 2013 The American Meteorological Society). Time series from ERA40 (post 2002) and 20CR (post 2008) are partially extrapolated from statistical relations with the other reanalyses.
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