Researchers at the University of California, Berkeley have used computer simulations to investigate the effect of the changing distance between Earth and the sun on annual El Niño weather cycles in the eastern equatorial Pacific Ocean.
This factor influences the cold tongue of surface waters stretching westward along the equator from the coast of South America, which impacts the El Niño-Southern Oscillation (ENSO), which in turn affects weather in California, much of North America, and often globally.
According to the research, the Earth-sun distance slowly varies over the course of the year because Earth’s orbit is slightly elliptical. Currently, at its closest approach – perihelion – Earth is about three million miles closer to the sun than at its farthest point, aphelion. As a result, sunlight is about 7% more intense at perihelion than at aphelion.
The research demonstrates that the slight yearly change in our distance from the sun can have a large effect on 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.
John Chiang, professor of geography at UC Berkeley, said, “Because the period of the annual cycle arising from the tilt and distance effects are slightly different, their combined effects vary over time. 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 noted that the distance effect is already incorporated into climate models – through its effect on the equatorial Pacific was not recognized until this research – and his 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. The 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.
Alyssa Atwood, a former UC Berkeley postdoctoral fellow who is now an assistant professor at Florida State University in Tallahassee, said, “Theory tells us that the seasonal cycle of the cold tongue plays a key role in the development and termination of ENSO events. Because of this, many of ENSO’s key characteristics are synced to the seasonal cycle. For example, ENSO events tend to peak during Northern Hemisphere winters, and they don’t typically persist beyond northern or boreal spring months, which scientists refer to as the ‘spring predictability barrier’. Because of these linkages, it is reasonable to expect that the distance effect could also have a major impact on ENSO – something that should be examined in future studies.”
Chiang said, “Very little attention has been paid to the cold tongue seasonal cycle because most people think it’s solved. There’s nothing interesting there. What this research shows is that it’s not solved. There’s still a mystery there. Our result also begs the question of whether other regions on Earth may also have a significant distance effect contribution to their seasonal cycle.”
Anthony Broccoli of Rutgers University, said, “We learn in science classes as early as grade school that the seasons are caused by the tilt of Earth’s axis. 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.”
Chiang, Atwood, Broccoli and their colleagues reported their findings in the journal Nature.
“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,” Broccoli said. “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 – the point at which the earth is closest to the sun – has an important influence on climate in the tropical Pacific.”
The key distinction is that changes in the sun’s distance from Earth don’t affect the Northern and Southern hemispheres differently, which is what gives rise to the seasonal effect due to Earth’s axial tilt. Instead, they warm the eastern “continental hemisphere” dominated by the North and South American and African and Eurasian landmasses, more than it warms the Western Hemisphere – what Chiang calls the marine hemisphere because it is dominated by the Pacific Ocean.
As Earth gets closer to the sun in its elliptical orbit, the continent-dominated hemisphere heats up more than the ocean-dominated or ‘marine’ hemisphere, generating trade winds that affect the Pacific cold tongue and likely the El Niño/La Niña cycle that determines whether California gets rain or drought.
“The traditional way of thinking about monsoons is that the Northern Hemisphere warms up relative to the Southern Hemisphere, generating winds onto land that bring monsoon rains,” Chiang said. “But here, we’re actually talking about east-west, not north-south, temperature differences that cause the winds. The distance effect is operating through the same mechanism as the seasonal monsoon rains, but the wind changes are coming from this east-west monsoon.”
The winds generated by this differential heating of the marine and continental hemispheres reportedly alter the yearly variation of the easterly trades in the western equatorial Pacific, and thereby the cold tongue.
“When Earth is closest to the sun, these winds are strong. In the offseason, when the sun is at its furthest, these winds become weak,” Chiang said. “Those wind changes are then propagated to the eastern Pacific through the thermocline, and basically it drives an annual cycle of the cold tongue, as a result.”
The research indicated that the distance effect on the cold tongue is about one-third the strength of the tilt effect, and they enhance one another, leading to a strong annual cycle of the cold tongue. About 6,000 years ago, they canceled one another, yielding a muted annual cycle of the cold tongue. In the past, when Earth’s orbit was more elliptical, the distance effect on the cold tongue would have been larger and could have led to a more complete cancellation when out of phase. Though Chiang and his colleagues did not examine the effect of such a cancellation, this is projected to have had a worldwide effect on weather patterns. Chiang emphasized that the distance effect on climate, while clear in climate model simulations, would not be evident from observations because it cannot be readily distinguished from the tilt effect.
“This study is purely model based. So, it is a prediction,” he said. “But this behavior is reproduced by a number of different models, at least four. And what we did in this study is to explain why this happens. And in the process, we’ve discovered another annual cycle of the cold tongue that’s driven by Earth’s eccentricity.”
Atwood noted that, unlike the robust changes to the cold tongue seasonal cycle, changes to ENSO tend to be model-dependent. She said, “While ENSO remains a challenge for climate models, we can look beyond climate model simulations to the paleoclimate record to investigate the connection between changes in the annual cycle of the cold tongue and ENSO in the past. To date, paleoclimate records from the tropical Pacific have largely been interpreted in terms of past changes in ENSO, but our study underscores the need to separate changes in the cold tongue annual cycle from changes in ENSO.”