Research reveals new insights into how Earth’s orbit influences seasonal cycles

Alyssa Atwood is an assistant professor with the Department of Earth, Ocean and Atmospheric Science.

Researchers have long been aware of the varying distance between Earth and the sun because of Earth’s elliptical orbit, but new research published in Nature coauthored by a Florida State University atmospheric scientist is shedding light for the first time on how this phenomenon impacts annual temperature cycles in the tropics.

Alyssa Atwood, an assistant professor with the Department of Earth, Ocean and Atmospheric Science, worked with researchers from the University of California, Berkeley to analyze how the distance between the Earth and the sun influences weather cycles in the eastern equatorial Pacific Ocean — in particular a large pool of locally cool surface waters known as the “Pacific cold tongue” that stretches westward along the equator from the coast of South America to the middle of the Pacific.

Researchers found there are two yearly cycles that affect ocean temperatures in the Pacific cold tongue. The first, which has long been recognized by scientists, is associated with the tilt of Earth’s axis tilt relative to its orbit around the sun. This tilt is why seasons are reversed in the Northern and Southern hemispheres.

But the second cycle was a new discovery for scientists.

“We found that the variation in the Earth-Sun distance also drives seasonal changes in climate,” Atwood said. “This new finding raises questions about the links in the a complex and dynamic system in the equatorial Pacific.”

The distance between the Earth and the sun varies during the year because of the planet’s slightly elliptical orbit. At its closest approach, Earth is about 3 million miles closer to the sun than at its farthest point. As a result, sunlight is about 7 percent more intense when Earth is closest to the sun.

The research, led by the University of California, Berkeley, shows that the slight yearly variation in Earth’s distance from the sun can greatly affect the annual cycle of the cold tongue. This is distinct from the effect of Earth’s axial tilt on the seasons, which is currently understood to cause the annual cycle of the cold tongue.

The findings are pivotal given that changes in tropical Pacific climate can have global impacts. The cold tongue annual cycle also influences the El Niño-Southern Oscillation (ENSO), which impacts weather across much of North America, including Florida and, often, worldwide.

Because the period of the annual cycle arising from the tilt and distance effects are slightly different, their combined effects vary over time, according to the study’s lead researcher John Chiang, UC Berkeley professor of geography.

“The curious thing is that the annual cycle from the distance effect is slightly longer than that for tilt — around 25 minutes, currently — so over a span of about 11,000 years, the two annual cycles go from being in phase to out of phase, and the net seasonality undergoes a remarkable change, as a result,” Chiang said.

Chiang noted that the distance effect is already incorporated into climate models — though its effect on the equatorial Pacific was not recognized until now — and their findings will not alter weather predictions or climate projections. But the 22,000-year phase cycle may have had long-term, historical effects. Earth’s orbital precession is known to have affected the timing of the ice ages, for example.

The distance effect — and its 22,000-year variation — also may affect other weather systems on Earth. And ENSO, which also originates in the equatorial Pacific, is likely affected because its workings are closely tied to the seasonal cycle of the cold tongue.

“We learn in science classes as early as grade school that the seasons are caused by the tilt of Earth’s axis,” added study coauthor Anthony Broccoli of Rutgers University. “This is certainly true and has been well understood for centuries. Although the effect of the Earth-sun distance has also been recognized, our study indicates that this ‘distance effect’ may be a more important effect on climate than had been recognized previously.”

Atwood said the team’s discovery may only be one piece of the puzzle. Given the capacity of the tropical Pacific to modulate global climate, it begs the question: to what extent does eccentricity contribute to the annual cycle in other regions through its impact on the Pacific cold tongue?

“Our findings call for a re-examination of ancient climate records in the tropical Pacific,” Atwood said. “Previous research has largely focused on El Niño, but we’ve shown that this other mechanism plays an important role. In fact, this cycle may help us understand the mechanism behind long-term changes in ENSO.”

Researchers from the University of Wisconsin-Madison, Northumbria University in the U.K., and the University of Connecticut also collaborated on the study.

This research was supported by grants from the National Science Foundation, the Ministry of Science and Technology, Taiwan, and the National Center for Atmosphere Research Advanced Study Program.

Robert Sanders of the University of California, Berkeley, contributed to this story.