Tropical Revelations: Unearthing the Impacts of Hydrological Sensitivity on Global Rainfall

Georgia Tech researcher Jie He set out to predict how rainfall will change as Earth’s atmosphere continues to heat up. In the process,  he made some unexpected discoveries that might explain how greenhouse gas emissions will impact tropical oceans, affecting climate on a global scale.

“This is not a story with just one punch line,” said He, assistant professor in Georgia Tech’s School of Earth and Atmospheric Sciences, whose most recent work appeared in the journal Nature Climate Change. “I didn’t really expect to find anything this interesting—there were a few surprises.”

He is principal investigator of the Climate Modeling and Dynamics Group, which combines expertise in physics, mathematics, and computer science to study climate change. The team’s latest study, a collaboration with Mississippi State University and Princeton University, examines hydrological sensitivity in the planet’s three tropical basins: the central portions of both the Pacific and Atlantic oceans and most of the Indian Ocean, an equatorial belt girding the Earth between the Tropic of Cancer (north) and Tropic of Capricorn (south).

Hydrological sensitivity (HS) refers to the precipitation change per degree of surface warming. Hydrological sensitivity is a key metric researchers use in evaluating or predicting how rainfall will respond to future climate change. Positive HS indicates a wetter climate, while negative HS indicates a drier climate.

“The projection of hydrological sensitivity and future precipitation has been widely investigated, but most studies look at global averages — nobody had yet looked closely at each individual basin,” He said. “And the real impact on global climate change will come from the regional scale.”

In other words, what happens in tropical waters has far-reaching effects.

Long Reach of the Tropics

He wanted to specifically examine the tropical basins because they already have a well-known influence on remote locations: El Niños and La Niñas. These weather patterns that shift every couple of years are examples of tropical oceanic precipitation changes that have a global impact.

“These precipitation changes create heating and cooling in the atmosphere that set off atmospheric waves affecting remote climates across the globe,” He said. During El Niño winters, for example, the southeastern U.S. typically gets more precipitation than usual.

But El Niños and La Niñas are naturally occurring, whereas the tropical precipitation changes He identified are projected as outcomes of human-induced global warming — a simulation, part of a climate model.

Climate models are an essential tool for He and other researchers, who use them to simulate possible future scenarios. These are computer programs that rely on complex math equations to project the atmospheric interactions of energy and matter likely to occur across the planet.

What surprised He was the substantial difference in HS between tropical basins. Essentially, in He’s model the Pacific tropical basin has an HS more than twice as large as the Indian basin, with the Atlantic basin projected as a negative value.

“It was surprising because these differences can’t be explained by the mainstream theories on tropical precipitation changes,” He said. “In other words, none of the theories we knew would have predicted it.”

Modeling the Sensitive Future

The effects of such diverging hydrological sensitivity would be widespread, according to He. For example, his experiments suggest that the continental U.S. will get wetter, and the Amazon will become drier.

“If these model projections are true, these effects will materialize as the climate continues to warm,” said He, who can’t predict exactly how long it will be before these effects can be detected in actual observations of our three-dimensional world.

That’s because they only have reliable observations of oceanic tropical precipitation since 1979. Precipitation changes over decades are strongly affected by internal climate variability — that is, climate change that isn’t caused by humans. When human-induced precipitation changes are significantly greater than internal climate variability, we should be able to detect the wide-ranging effects of diverging hydrological sensitivity.

But the challenges of continuing climate change do not allow the luxury of waiting until every aspect of climate projection becomes a reality, He noted, adding, “We are relying on climate projections to some extent to guide our adaptation and mitigation plans. Therefore, it is important to study and understand the climate projections.”

Based on the scenario projected by climate models used in He’s research, the effects of El Niños and La Niñas on remote climates will become stronger.

“What we can imply is that this strengthening would be partly due to the diverging HS among tropical basins,” He concluded.

While the future effects of HS on El Niños and La Niñas weren’t discussed in this study, He believes it would make a very interesting research subject going forward.