Researchers Offer an Explanation for Expansion of Tropical Belts

A team of researchers led by a former Scripps Institution of Oceanography, UC San Diego postdoctoral researcher offered a new explanation for why Earth’s tropical belt, bounded by the tropics of Cancer and Capricorn, has progressively expanded since the late 1970s.

Robert Allen, now an assistant professor at the University of California, Riverside, led a NASA-funded study that found the recent widening of the tropical belt is primarily caused by multi-decadal sea surface temperature variability in the Pacific Ocean.

This variability includes the Pacific Decadal Oscillation (PDO), a pattern of Pacific climate variability that switches roughly every 30 years between two different circulation patterns in the North Pacific Ocean.  The researchers say pollutants produced by human activities are another influence that modifies the PDO. The result is an expansion of the range of subtropical deserts that stay dry year-round and the pushing of storm tracks closer to the poles.

“It’s great to see research that uses NASA data and models to unravel the workings of the climate system and help to decipher the differences between internal dynamics and possible influences of human activity, in this case aerosols,” said Jared Entin, program manager of the NASA Energy and Water cycle Study, which funded the research. “The possibility to lead to any improvement in prediction of California precipitation is icing on the cake, because it would help confront a well-known societal challenge.”

Several explanations for the tropical belt widening have been proposed in recent years, such as radiative forcing due to greenhouse gas increase and stratospheric ozone depletion.

“Prior analyses have found that climate models underestimate the observed rate of tropical widening, leading to questions on possible model deficiencies, possible errors in the observations, and lack of confidence in future projections,” said Allen, an assistant professor of climatology in UC Riverside’s Department of Earth Sciences, who led the study. “Furthermore, there has been no clear explanation for what is driving the widening.”

Allen and colleagues, including Scripps atmospheric science researcher Joel Norris, found that the PDO is a more significant variable than previously realized.

"Although the PDO is considered a ‘natural’ mode of climate variability, implying tropical widening is primarily driven by internal dynamics of the climate system, we also show that anthropogenic pollutants have driven trends in the PDO," Allen said.  “Thus, tropical widening is related to both the PDO and anthropogenic pollutants.”

Tropical widening is associated with several significant changes in climate, including shifts in large-scale atmospheric circulation and storm tracks. In Southern California, tropical widening may be associated with less precipitation.  

Semi-arid regions including the Mediterranean, the southwestern United States and northern Mexico, southern Australia, southern Africa, and parts of South America also face the prospect of greater dryness. An increase in the width of the tropics could increase the area affected by tropical storms (hurricanes), or could change climatological tropical cyclone development regions and tracks.

A reversal of the cycle or a global clean-up of air pollution would also influence the dimensions of the tropical belt, said Norris.

“One thing we found in the study is evidence that there was tropical contraction from the 1950s to 1970s so that would suggest if the PDO shifts back again that there might be tropical contraction,” said Norris.

 The study appears in the journal Nature Geoscience.

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