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

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 degrees-3 degrees latitude in each hemisphere, an average trend of approximately 0.5 degrees-1.0 degrees decade(-1). The symmetry of this expansionwhether Northern and Southern hemispheres are expanding at the same rateis 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. (C) 2013 John Wiley & Sons, Ltd.