One sunspot mystery solved, researchers say

Answer: by what scientists call "wave energy." The layers don't necessarily mix. Rather, just as waves travel and transmit energy through water without necessarily moving the water forward or backward, waves travel – and transmit energy – through different layers of the atmosphere. In this case, changes in stratospheric circulation affect the troposphere by reinforcing, through these energetic waves, certain wind patterns. These strengthened winds cause stormier conditions over the western Pacific.

Then there's the bottom-up component. At the equator, heated air rises. Once cooled, that air descends over the subtropics. Where the hot humid air rises and cools, you generally get storms and rainfall; where it sinks, you generally get clear skies and not much precipitation. (For this reason, many of the world's deserts occur at subtropical latitudes.)

During sunspot years, a little more energy hits the sea surface of the Pacific. The subtropics, which naturally have few clouds to reflect the sun's energy back into space, warm more than other areas as a consequence. That increases evaporation and water vapor. The westerly trade winds carry the extra moisture to the western tropical Pacific. More rain falls there. The entire cycle is reinforced.

The end result of these "top down" and "bottom up" processes working together: during solar maximum years, the subtropics are more cloudless than usual, and more sunlight – which is a little stronger – hits the ocean. That warms the ocean more than usual, raising the observed temperature. This increased warmth isn't evenly distributed: The trade winds are stronger than usual, too, making the eastern Pacific cooler, drier, and less stormy than usual. The western Pacific, meanwhile, gets more warm water and more rain. If that sounds familiar, that's because it mimics, very generally, the conditions that prevail during La Niña years.

Not everyone is persuaded that the study satisfactorily explains the sunspot-earth connection. A news article accompanying the Science study airs some scientists' doubts:

The study “is not nearly as conclusive as they would have it,” says Joanna Haigh of Imperial College London, who developed the top-down mechanism. Among additional critiques, she and others say the researchers ran the model too few times to give reliable results. “The atmosphere and oceans are a big coupled system,” she says, “but it’s incredibly complicated.”

For those wondering how the study bears on global warming, Gerald Meehl, lead author on the study, says that it doesn't – at least not directly. (For more on sunspots' possible role in global warming, see Monitor colleague Pete Spotts's article.

Global warming is a long-term trend, Dr. Meehl says in a phone conversation. By contrast, this study attempts to explain the processes behind a periodic occurrence. But, he says, a model finally able to reproduce a complex phenomenon observed in the real world does suggest that our climate models – the same ones we use to predict what will happen to global climate as we ratchet up CO2 concentrations – are improving. And that will, inevitably, have an effect on the climate discussion.

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