New climate simulations show the annual change in the planet's distance from the star also affects a seasonal cycle that impacts weather in North America and globally
Weather and climate modelers understand pretty well how seasonal winds and ocean currents affect El Niño patterns in the eastern equatorial Pacific Ocean, impacting weather throughout the United States and worldwide.
But new computer simulations show that the effect of the changing distance between Earth and the sun has gone unrecognized.
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. This slight variation in the Earth-Sun distance drives unexpected seasonal changes in climate, researchers say.
The research published in Nature and led by Rutgers University-New Brunswick, the University of California, Berkeley, Florida State University and other institutions demonstrates the yearly change in our distance from the sun, albeit slight, can have a sharp effect on the annual cycle of the "cold tongue" of surface waters stretching westward along the equator from the coast of South America. This is distinct from the effect of Earth's axial tilt during the seasons, which is understood to cause the annual cycle of the cold tongue.
The cold tongue influences the El Niño-Southern Oscillation (ENSO), which impacts weather in California, in much of North America and often globally.
"We learn in science classes as early as grade school that the seasons are caused by the tilt of Earth's axis," said Anthony J. Broccoli, co-director of the Rutgers Climate Institute and a coauthor of the study. "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."
The main driver of alterations in global weather is seasonal change. Earth's equator is tilted relative to its orbit around the sun, so the Northern and Southern hemispheres are illuminated differently. When the sun shines directly overhead in the north, it's warmer in the north and colder in the south, and vice versa.
The researchers said these yearly changes have major effects on the Pacific equatorial trade winds, which blow from southeast to northwest across the south and equatorial Pacific and push surface waters westward, causing upwelling of cold water along the equator that creates a tongue of cold surface water that stretches from Ecuador through the Pacific - nearly one-quarter the circumference of the planet.
The yearly hemispheric changes in seasonal temperatures alter the strength of the trade winds, causing a yearly cycle in the temperature of the cold tongue. This, in turn, has a major influence on ENSO, which typically peaks during Northern Hemisphere winter.
The occurrence of El Niño - or its opposite, La Niña - helps determines whether California and the West Coast will have a wet or dry winter and whether the Midwest and parts of Asia will have rain or drought.
"In studying past climates, much effort has been dedicated to trying to understand if variability in the tropical Pacific Ocean - that is, the El Niño/La Niña cycle - has changed in the past," said Broccoli, who is a Distinguished Professor in the Rutgers School of Environmental and Biological Sciences. "We chose to focus instead on the yearly cycle of ocean temperatures in the eastern Pacific cold tongue. Our study found that the timing of perihelion - that is, the point at which the earth is closest to the sun - has an important influence on climate in the tropical Pacific."
In 2015, Broccoli, along with then-graduate student Michael Erb, employed a computer climate model to show that the distance changes caused by Earth's elliptical orbit sharply altered the cold tongue yearly cycle.
The researchers noted the distance effect already is incorporated into climate models - however, its effect on the equatorial Pacific wasn't recognized until now - and the findings won't alter weather predictions or climate projections. But the 22,000-year cycle may have had long-term historical effects. Earth's orbital precession-the change in orientation of the Earth's rotational axis-is known to have affected the timing of the ice ages, for example.
The distance effect - and its 22,000-year variation - may affect other weather systems on Earth. ENSO, which originates in the equatorial Pacific, likely is affected because its workings are closely tied to the seasonal cycle of the cold tongue.
"Our field is focused on El Niño, and we thought that the seasonal cycle was solved," said John Chiang, lead researcher of the study and UC Berkeley professor of geography. "But then we realized that the result by Erb and Broccoli challenged this assumption. What this research shows is that it's not solved. There's still a mystery there. Our result also begs the question whether other regions on Earth may also have a significant distance effect contribution to their seasonal cycle."
Other coauthors of the study include Alyssa Atwood of Florida State University, Daniel Vimont of the University of Wisconsin in Madison; Paul Nicknish, a graduate student at the Massachusetts Institute of Technology; William Roberts of Northumbria University in the United Kingdom; and Clay Tabor of the University of Connecticut.
