The observed widening of the tropics during the late 20th century and its mechanisms

Several lines of research indicate that the Tropics have been widening by a few degree latitudes over the past decades and that this widening may continue into the future. Associated with this widening are important shifts in precipitation and other climate elements, raising the question of what are the exact reasons behind the expansion. There is evidence that the increase in tropospheric static stability associated with global warming is driving this shift, but there may be other still unknown factors. Here, we investigate the relative roles of various stratospheric and tropospheric processes for the tropical expansion and conduct attribution experiments with stratosphere and non-stratosphere resolving versions of the latest GFDL general circulation model. The model is forced with different combinations of sea surface temperatures, carbon dioxide, halogen, and ozone concentrations. The impact of each of these factors and of stratospheric model resolution is isolated by comparing different pairs of simulations that differ by only one of these factors. The results highlight that both, tropospheric and stratospheric climate change, play a prominent role for the recent observed widening of the Tropics. We discuss the significance of these results for the future evolution of the tropical width and its realistic prediction.