Research reveals new insights into how Earth鈥檚 orbit influences seasonal cycles
Researchers have long been aware of the varying distance between Earth and the sun because of Earth鈥檚 elliptical orbit, but new research published in coauthored by a 糖心vlog 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 , 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 鈥 in particular a large pool of locally cool surface waters known as the 鈥淧acific cold tongue鈥 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鈥檚 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.
鈥淲e found that the variation in the Earth-Sun distance also drives seasonal changes in climate,鈥 Atwood said. 鈥淭his new finding raises questions about the links in the a complex and dynamic system in the equatorial Pacific.鈥
The distance between the Earth and the sun varies during the year because of the planet鈥檚 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鈥檚 distance from the sun can greatly affect the annual cycle of the cold tongue. This is distinct from the effect of Earth鈥檚 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 Ni帽o-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鈥檚 lead researcher John Chiang, UC Berkeley professor of geography.
鈥淭he curious thing is that the annual cycle from the distance effect is slightly longer than that for tilt 鈥 around 25 minutes, currently 鈥 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,鈥 Chiang said.
Chiang noted that the distance effect is already incorporated into climate models 鈥 though its effect on the equatorial Pacific was not recognized until now 鈥 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鈥檚 orbital precession is known to have affected the timing of the ice ages, for example.
The distance effect 鈥 and its 22,000-year variation 鈥 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.
鈥淲e learn in science classes as early as grade school that the seasons are caused by the tilt of Earth鈥檚 axis,鈥 added study coauthor Anthony Broccoli of Rutgers University. 鈥淭his 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 鈥榙istance effect鈥 may be a more important effect on climate than had been recognized previously.鈥
Atwood said the team鈥檚 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?
鈥淥ur findings call for a re-examination of ancient climate records in the tropical Pacific,鈥 Atwood said. 鈥淧revious research has largely focused on El Ni帽o, but we鈥檝e 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.鈥
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.