Why are the tropics expanding?

https://doi.org/10.1175/JCLI-D-16-0735.1

The Tropics are Expanding

Over the last several decades, the area of the globe that climate scientists and vacationers alike call “the tropics” has gotten larger. No, the Tropics of Cancer and Capricorn haven’t shifted, those are still located at the latitudes 23.5 N and 23.5 S. Climate scientists define the tropics based on the climate of the region. The tropics are regions with “continually high temperatures and considerable precipitation.” (http://glossary.ametsoc.org/wiki/Tropical_climate) This type of climate is normally restricted to a band across the midsection of the Earth, yet scientists have observed the band inching closer and closer to more traditionally temperate climates. What gives?

 

Who’s to Blame?

The go-to culprit would be anthropogenic, or human-caused, climate change. Climate change is already making other parts of the world both warmer and wetter. In this case, however, climate change is a red herring. Human-caused climate change will certainly increase the proportion of tropical climates in the future, but natural climate variability is to blame for the most recent expansion.

Wait, is this an anti-climate change activist opinion piece? Nope. Our planet is warming and humans are the primary cause. However, just because the average global temperature is increasing at an alarming rate that doesn’t mean natural climate cycles have stopped. Some locations still have cooler-than-average and warmer-than-average winters and summers. This is what we call natural climate variability.

 

Natural Cycles vs. Climate Change

A lot of climate research is about teasing out the trends of human-caused global climate change and the natural changes that can occur on a year-to-year or decade-to-decade cycle. One such study conducted by scientists at Oak Ridge National Laboratory in Oak Ridge, Tennessee was aimed at investigating the cause behind the recent expansion in the tropical boundaries.

They found through a modeling study that an interaction between two multi-year climate cycles, the El Niño-Southern Oscillation and the Pacific Decadal Oscillation, are causing the current widening of the tropics. Interaction between El Niño and the Pacific Decadal Oscillation is a still an area of active research, but analysis shows that the Pacific Decadal Oscillation can have an enhancing or dampening effect on El Niño.

How changing sea surface temperatures affect global weather patterns . In the top image, warmer-than-average sea surface temperatures dominate in the central Pacific which shifts tropical storminess to the east of Asia. This is a classic El Niño pattern. The bottom image shows the opposite. Cooler-than-average sea surface temperatures arrive in the central Pacific which brings a stormier-than-average season to Southeast Asia. This is a La Niña pattern.
Image Credit: (climate.gov)
The Pacific Decadal Oscillation index is a measure of the the strength of the Pacific Decadal Oscillation. Notice that a positive or negative event can occur over several years. Image Credit: University of Washington (http://research.jisao.washington.edu/)

The Future

The tropical climate is expected to recede back from its current expansion once the El Niño-Southern Oscillation and the Pacific Decadal Oscillation move out of phase with each other.

This will impact wet and dry seasons across the globe, and heavily impact the path tropical storms like Hurricane Harvey and Hurricane Maria.

Over the next several decades the effects from our warming globe are likely to surpass the signal from natural climate variability. This means that the tropics will expand beyond its current climatological mean. Then the answer to the question “what is causing the expanding tropics?” will almost certainly be global, human-induced climate change.

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Geneva Gray

Geneva Gray is a graduate student at North Carolina State University in Atmospheric Sciences. She researches climate change and applied statistical methodologies. When she isn’t writing to meet a deadline, she’s knitting something cute for her infant daughter.

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