Subtle connections between the solar cycle, the stratosphere and the tropical Pacific Ocean generate regular weather patterns that could help predict the intensity of climate phenomena years in advance.

More than a century of weather observations and three computer models helped scientists led by the National Center for Atmospheric Research (NCAR) to arrive at an answer to the riddle of how such a small variation in the energy that reaches Earth across the 11 year solar cycle – just 0.1 percent – can drive major changes in weather patterns on our home planet.

The answer came from two unlikely places – chemicals in the stratosphere and sea surface temperatures in the Pacific Ocean, both of which respond to the solar maximum in a way that amplifies the Sun's influence on some aspects of air movement. This in turn can intensify winds and rainfall, change sea surface temperatures and cloud cover over certain tropical and subtropical regions, and ultimately have a knock on affect on global weather patterns.

"The Sun, the stratosphere, and the oceans are connected in ways that can influence events such as winter rainfall in North America," says NCAR scientist Gerald Meehl and lead author of the research presented this week in the journal Science. "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."

One line of evidence came from the tiny increase in solar energy during the peak production of sunspots that is absorbed by ozone in the stratosphere, which warms the air over the tropics where sunlight is most intense. This stimulates the production of more ozone that absorbs even more solar energy. Since the stratosphere warms unevenly, with the most pronounced warming occurring at lower latitudes, stratospheric winds are altered such that tropical precipitation is strengthened.

At the same time, the increased sunlight at solar maximum causes a slight warming of ocean surface waters across the subtropical Pacific, where Sun-blocking clouds are normally scarce. The small amount of extra heat leads to more evaporation, producing additional water vapor. This moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, fueling heavier rains and reinforcing the atmosphere-ocean feedback loop.

The effect is noticed during solar maximum when the equatorial eastern Pacific is even cooler and drier than usual. This is a similar characteristic of the La Nina event, but the cooling is focused farther east and is about half as strong, as well as being associated with different wind patterns in the stratosphere. Earth’s response to the solar cycle continues for a year or two following peak sunspot activity, and then evolves into a pattern similar to El Nino as slow-moving currents replace the cool water over the eastern tropical Pacific with warmer water.

There is even evidence to suggest that solar maximum could enhance a La Nina event or dampen an El Nino event if their timings coincide. For example, a particularly strong La Nina occurred in 1988-89, near the peak of solar maximum. Other regular climatic events, such as the Indian monsoons, are largely driven by the rising and sinking of air around the tropics, so 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.

"With the help of increased computing power and improved models, as well as observational discoveries, we are uncovering more of how the mechanisms combine to connect solar variability to our weather and climate," says Meehl.

The team conducted three simulations that provided overlapping views of the climate system. The first model analysed the relation between sea surface temperatures and the lower atmosphere, which produced a small cooling in the equatorial region of the Pacific during solar maximum. The second model focused on the influence of ozone and found that tropical precipitation was increased, but on a much smaller scale than the observed patterns. The third model contained ocean-atmosphere interactions combined with the presence of ozone and produced a response in the tropical Pacific during peak solar years that was close to actual observations.

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