Sun's Cycle Alters Earth's Climate

Weather patterns across the globe are partly affected byconnections between the 11-year solar cycle of activity, Earth?s stratosphereand the tropical Pacific Ocean, a new study finds.

The study could help scientists get an edge on eventuallypredicting the intensity of certain climate phenomena, such asthe Indian monsoon and tropical Pacific rainfall, years in advance.

The sun is the ultimate source of all the energy on Earth;its rays heat the planet and drive the churning motions of its atmosphere.

The amount of energy the sun puts out varies over an11-year cycle (this cycle also governs the appearanceof sunspots on the sun's surface as well as radiation storms that can knockout satellites), but that cycle changes the total amount of energy reachingEarth by only about 0.1 percent. A conundrum for meteorologists was explaining whetherand how such a small variation could drive major changes in weather patterns onEarth.

Earth-space connection

An international team of scientists led by the National Center for Atmospheric Research (NCAR) used more than a century of weatherobservations and three powerful computer models to tackle this question.

The answer, the new study finds, has to do with the Sun'simpact on two seemingly unrelated regions: water in the tropical Pacific Ocean and air in the stratosphere, the layer of the atmosphere that runs fromaround 6 miles (10 km) above Earth's surface to about 31 miles (50 km).

The study found that chemicals in the stratosphere and seasurface temperatures in the Pacific Ocean respond during solar maximum in a waythat amplifies the sun's influence on some aspects of air movement. This canintensify windsand rainfall, change sea surface temperatures and cloud cover over certaintropical and subtropical regions, and ultimately influence global weather.

"The sun, the stratosphere, and the oceans areconnected in ways that can influence events such as winter rainfall in North America," said lead author of the study, Gerald Meehl of NCAR. "Understandingthe role of the solar cycle can provide added insight as scientists work towardpredicting regional weather patterns for the next couple of decades."

The findings are detailed in the Aug. 28 issue of thejournal Science.

How it happens

The changes occur like this: The slight increase in solarenergy during the peak production of sunspots is absorbed by stratosphericozone, warming the air in the stratosphere over the tropics, where sunlightis most intense. The additional energy also stimulates the production ofadditional ozone there that absorbs even more solar energy.

Since the stratosphere warms unevenly, with the mostpronounced warming occurring nearer the equator, stratospheric winds are alteredand, through a chain of interconnected processes, end up strengthening tropicalprecipitation.

At the same time, the increased sunlight at solar maximum? a peak of sunspot and solar storm activity we're currently headed toward? causes a slight warming of ocean surface waters across the subtropicalPacific, where sun-blocking clouds are normally scarce. That small amount ofextra heat leads to more evaporation, putting additional water vapor into theatmosphere. The moisture is carried by trade winds to the normally rainy areasof the western tropical Pacific, fueling heavier rains and reinforcing theeffects of the stratospheric mechanism.

These two processes reinforce each other and intensify theeffect.

These stratospheric and ocean responses during solar maximumkeep the equatorial eastern Pacific even cooler and drier than usual, producingconditions similar to a La Nina event. However, the cooling of about 1-2degrees Fahrenheit is focused farther east than in a typical La Nina (theopposite sistereffect of the warm-water El Nino), is only about half as strong, and isassociated with different wind patterns in the stratosphere.

The solar cycle does not have as great an effect on Earth'sclimate as the El Nino cycle.

But the Indian monsoon, Pacific sea surface temperatures andprecipitation, and other regional climate patterns are largely driven by risingand sinking air in Earth's tropics and subtropics. The new study could helpscientists use solar-cycle predictions to estimate how that circulation, andthe regional climate patterns related to it, might vary over the next decade ortwo.

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