Recent Tropical Expansion: Natural Variability or Forced Response?

Kevin M. Grise, University of Virginia

 

In recent decades, the poleward edges of Earth’s Hadley circulation have shifted poleward in both the Northern and Southern Hemispheres, acting to push subtropical dry zones further poleward.  Previous studies have disagreed on the causes of these observed changes and the ability of global climate models to capture them.  In this presentation, I review some of the key findings from the US CLIVAR Working Group on the Changing Width of the Tropical Belt, which has recently reassessed contradictory claims in the past literature and synthesized key conclusions on the topic.

The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern and Southern Hemispheres, but global climate models suggest that anthropogenic forcing should drive 3–4 times larger circulation shifts in the Southern Hemisphere (SH).  In the SH, increasing greenhouse gases and stratospheric ozone depletion likely contributed to tropical expansion over the late 20th century, and if greenhouse gases continue increasing, the SH tropical edge is projected to shift further poleward over the 21st century, even as stratospheric ozone concentrations recover.  In the Northern Hemisphere (NH), 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 21st century.  Only during autumn months are circulation trends in the NH expected to be comparable to those in the SH.

To explain comparable observed 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 NH.  However, even if observed sea surface temperatures are prescribed, modeled tropical expansion rates still vary widely due to internal atmospheric variability.  Natural variability can induce large trends over 30–40-year periods, so it is difficult to definitively distinguish forced trends from natural variability over the period since 1979.