UCP Seminar: Tropical Widening: What, When, Where (and Why)

Tropical Widening: What, When, Where (and Why)

US CLIVAR Working Group on the Changing Width of the Tropical Belt

Paul Staten (Indiana University) and Kevin Grise (University of Virginia)

OCT 13th 12:00 - 1:00 PM FL2-1022

The tropics are often thought of as warm, moist regions near the Equator, but the edges of the tropics near 30˚ latitude are characterized by hot, dry regions, which are home to the world’s largest deserts. The boundary of the tropics has been moving poleward in both the Northern and Southern Hemispheres in recent decades, potentially contributing to desertification, habitat loss, and food insecurity as the subtropical dry zones encroach poleward toward more populated midlatitude regions. This important problem is the focus of the US CLIVAR Working Group on the Changing Width of the Tropical Belt. This talk will highlight recent activities and findings of the working group. We will focus on three key activities.

First, there are a myriad of metrics used to measure the observed widening of the tropics, ranging from surface impacts (precipitation) to the atmospheric general circulation (Hadley circulation and jet streams) to atmospheric chemistry and transport. Unfortunately, different metrics yield widely different estimates for how quickly the tropics have been widening. Our working group is working to narrow the range of uncertainty in recent tropical widening estimates by providing guidance on the choice and calculation of these metrics.

Second, it is commonly reported that the computer models used to project climate change impacts cannot capture the magnitude of recent tropical expansion. However, this conclusion is based upon comparison of the observed trends with the “forced” response from global climate models, which only accounts for the climate change signal without the influence of natural climate variations. When natural variability is properly accounted for, the models can much better capture the magnitude of the observed tropical expansion. Our working group is working to better understand the role of decadal variability in sea surface temperatures, particularly those in the Pacific Ocean (Pacific Decadal Oscillation), which appear to be key in explaining the rapid widening of the tropics over the last 30 years.

Finally, as the globe warms, climate models predict that the tropics will continue to widen, but where these impacts will be felt most acutely at regional levels remains poorly understood. Our working group is working to understand how and why different regions respond differently to tropical widening. Preliminary results indicate that, while a range of climate phenomena can widen the tropics, they each produce unique regional fingerprints, suggesting that the spatial distribution of regional impacts will depend on the underlying cause of the tropical widening.


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Announcement Timing: 
October 12, 2017
Calendar Timing: 
Friday, October 13, 2017 - 12:00pm to 1:00pm