Weather patterns across the globe are partly affected by connections between the 11-year solar cycle of activity, Earth's stratosphere and the tropical Pacific Ocean, a new study finds.
The study could help scientists get an edge on eventually predicting the intensity of certain climate phenomena, such as the 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 an 11-year cycle (this cycle also governs the appearance of sunspots on the sun's surface as well as radiation storms that can knock out satellites), but that cycle changes the total amount of energy reaching Earth by only about 0.1 percent. A conundrum for meteorologists was explaining whether and how such a small variation could drive major changes in weather patterns on Earth.
An international team of scientists led by the National Center for Atmospheric Research (NCAR) used more than a century of weather observations and three powerful computer models to tackle this question.
The answer, the new study finds, has to do with the Sun's impact on two seemingly unrelated regions: water in the tropical Pacific Ocean and air in the stratosphere, the layer of the atmosphere that runs from around 6 miles (10 km) above Earth's surface to about 31 miles (50 km).
The study found that chemicals in the stratosphere and sea surface temperatures in the Pacific Ocean respond during solar maximum in a way that amplifies the sun's influence on some aspects of air movement. This can intensify winds and rainfall, change sea surface temperatures and cloud cover over certain tropical and subtropical regions, and ultimately influence global weather.
"The sun, the stratosphere, and the oceans are connected in ways that can influence events such as winter rainfall in North America," said lead author of the study, Gerald Meehl of NCAR. "Understanding the role of the solar cycle can provide added insight as scientists work toward predicting regional weather patterns for the next couple of decades."
The findings are detailed in the Aug. 28 issue of the journal Science.
How it happens
The changes occur like this: The slight increase in solar energy during the peak production of sunspots is absorbed by stratospheric ozone, warming the air in the stratosphere over the tropics, where sunlight is most intense. The additional energy also stimulates the production of additional ozone there that absorbs even more solar energy.
Since the stratosphere warms unevenly, with the most pronounced warming occurring nearer the equator, stratospheric winds are altered and, through a chain of interconnected processes, end up strengthening tropical precipitation.
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 subtropical Pacific, where sun-blocking clouds are normally scarce. That small amount of extra heat leads to more evaporation, putting additional water vapor into the atmosphere. The moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, fueling heavier rains and reinforcing the effects of the stratospheric mechanism.
These two processes reinforce each other and intensify the effect.
These stratospheric and ocean responses during solar maximum keep the equatorial eastern Pacific even cooler and drier than usual, producing conditions similar to a La Nina event. However, the cooling of about 1-2 degrees Fahrenheit is focused farther east than in a typical La Nina (the opposite sister effect of the warm-water El Nino), is only about half as strong, and is associated with different wind patterns in the stratosphere.
The solar cycle does not have as great an effect on Earth's climate as the El Nino cycle.
But the Indian monsoon, Pacific sea surface temperatures and precipitation, and other regional climate patterns are largely driven by rising and sinking air in Earth's tropics and subtropics. The new study could help scientists use solar-cycle predictions to estimate how that circulation, and the regional climate patterns related to it, might vary over the next decade or two.