J. Clim. http://doi.org/qb7 (2013)
The tropics, as defined by climatological patterns of rainfall, evaporation and atmospheric circulation, have expanded polewards over the past 30 years, but the rate and cause of this expansion has remained unclear. Climate model simulations suggest that human-induced changes in the radiative balance of the atmosphere account for just a small fraction of the expansion of the tropical belt.
Xiao-Wei Quan, of the University of Colorado, and colleagues examined the rate at which the subtropical dry zone — which reflects the poleward edges of the tropical belt — expanded between 1979 and 2009, using the NCAR Community Climate model. Taking the average of multiple simulations, in which observed variations in radiative forcing and ocean changes were specified, they show that the rate of subtropical dry zone expansion ranged from 0.1 to 0.2° latitude per decade. The modelled rate of expansion is considerably lower than some previous estimates that rely on reanalysis data suggest. The authors suggest that observational errors make some of these empirically derived expansion rates — which can amount to over 1° latitude per decade — unreliable.
The researchers further show that the growth of the tropical belt over a 30-year period largely results from natural as opposed to human-induced variability in the sensitivity of the atmosphere to ocean warming.