Recent Tropical Expansion: Natural Variability or Forced Response?

Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979, but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This study reflects the efforts of the US CLIVAR Working Group on the Changing Width of the Tropical Belt to reassess contradictory claims in the past literature about the magnitude and causes of the recent tropical widening and to synthesize key conclusions on the topic. Overall, we find that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses.

The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern and Southern Hemispheres, but models suggest that the factors driving the expansion differ between the hemispheres. In the Southern Hemisphere, increasing greenhouse gases and stratospheric ozone depletion contributed to tropical expansion over the late 20^th century, and if greenhouse gases continue increasing, the Southern Hemisphere tropical edge is projected to shift further poleward over the 21^st century, even as stratospheric ozone concentrations recover. In the Northern Hemisphere, the contribution of increasing greenhouse gases to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the 21^st century. To explain comparable recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere-ocean variability, including the Pacific Decadal Oscillation, has contributed to tropical expansion in both hemispheres, but particularly in the Northern Hemisphere. However, even if observed sea surface temperatures are prescribed, modeled tropical expansion rates still vary widely due to internal atmospheric variability.