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Climate: Scientists study link between tropical belt expansion and multi-decadal oceanic circulation patterns

Long-term Pacific Ocean cycles could be driving SW drought

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What’s the role of natural climate variability in Southwestern droughts?

Staff Report

FRISCO — If you’re ready to blame drought in the southwestern U.S. on global warming, it might be time to rethink that conclusion.

According to scientists with the University of California, Riverside, dry conditions in the region may be linked with an expansion of the Earth’s tropical belt during the past few decades. And that expansion is likely driven by multi-decadal sea surface temperature variability in the Pacific Ocean, according to the new study. Other explanations for this widening have been proposed, including radiative forcing due to greenhouse gas increase and stratospheric ozone depletion.

Now, a team of climatologists, led by researchers at the University of California, Riverside, posits that 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 long-lived El Niño-like pattern of Pacific climate variability that works like a switch every 30 years or so between two different circulation patterns in the North Pacific Ocean. It also includes, the researchers say, anthropogenic pollutants, which act to modify the PDO.

“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 Robert J. 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’s team has found that the recent tropical widening is largely driven by the PDO.

“Although this widening 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 the climate, including shifts in large-scale atmospheric circulation, like storm tracks, and major climate zones. For example, in Southern California, tropical widening may be associated with less precipitation.

Of particular concern are the semi-arid regions poleward of the subtropical dry belts, including the Mediterranean, the southwestern United States and northern Mexico, southern Australia, southern Africa, and parts of South America. A poleward expansion of the tropics is likely to bring even drier conditions to these heavily populated regions, but may bring increased moisture to other areas.

Widening of the tropics would also probably be associated with poleward movement of major extratropical climate zones due to changes in the position of jet streams, storm tracks, mean position of high and low pressure systems, and associated precipitation regimes. 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.

Allen’s research team also showed that prior to the recent (since ~1980 onwards) tropical widening, the tropical belt actually contracted for several decades, consistent with the reversal of the PDO during this earlier time period.

“The reversal of the PDO, in turn, may be related to the global increase in anthropogenic pollutant emissions prior to the ~ early 1980s,” Allen said.

Allen’s team analyzed IPCC AR5 (5th Assessment Report) climate models, several observational and reanalysis data sets, and conducted their own climate model experiments to quantify tropical widening, and to isolate the main cause.

“When we analyzed IPCC climate model experiments driven with the time-evolution of observed sea surface temperatures, we found much larger rates of tropical widening, in better agreement to the observed rate–particularly in the Northern Hemisphere,” Allen said. “This immediately pointed to the importance of sea surface temperatures, and also suggested that models are capable of reproducing the observed rate of tropical widening, that is, they were not ‘deficient’ in some way.”

“In this case, we found tropical widening — particularly in the Northern Hemisphere — is completely eliminated,” Allen said. “This is true for both types of models–those driven with observed sea surface temperatures, and the coupled climate models that simulate evolution of both the atmosphere and ocean and are thus not expected to yield the real-world evolution of the PDO.

“If we stratify the rate of tropical widening in the coupled models by their respective PDO evolution,” Allen said, “we find a statistically significant relationship: coupled models that simulate a larger PDO trend have larger tropical widening, and vice versa. Thus, even coupled models can simulate the observed rate of tropical widening, but only if they simulate the real-world evolution of the PDO.”

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