Sun teams up with atmosphere, ocean to ramp up Pacific storms
Sunset over the Pacific at Arica, Chile. Image courtesy of the University Corporation for Atmospheric Research (UCAR) in Boulder, CO.
When it comes to teasing out the factors affecting Earth’s climate, the Sun is a compelling character. A rare breed of enthusiasts got pretty vocal about the Sun in the past year, when the great orb stayed quiet about a year longer than expected between the end of Solar Cycle 23 and the beginning of Solar Cycle 24, which has now begun — maybe. More about that in a minute.
The “enthusiasts” I mean are the ones who suspect that natural factors (e.g., not human-caused carbon emissions) are major players in any warming or cooling trends on Earth. They point to the Maunder Minimum in the 17th Century, when a lull in solar activity (as measured by sunspots) was linked with cooling in parts of the globe, especially Europe. Some members of the “global warming is a hoax” camp even latched onto the idea that if we were to succeed in reducing our carbon emissions, we could actually sabotage a sort of blanket that would help insulate us should the Sun stay mum. Sounds to me like a cop out — a cheap way to avoid cleaning up our act. Chances are we won’t get to find out if they’re right anyway, because the Sun appears to be ramping up toward another maximum around 2013.
Regardless, the Sun’s power to influence Earth’s climate is highlighted anew in a Science study this week.
The new study is suggesting a sun-climate connection that’s disproportionately large given the small overall variation in the Sun’s output (up to a tenth of one percent) during different parts of its cycle. And Earth’s response isn’t all about temperature, the authors report. Their models reveal a three-way interaction between the Sun, the atmosphere and the ocean, such that a peak in solar activity results in more precipitation over the western tropical Pacific, and cooler, drier conditions over the equatorial eastern Pacific.
Earth and the Sun, as viewed by the Space Shuttle Discovery. Credit: NASA
The connection between peaks in solar energy and cooler water in the equatorial Pacific was first discovered by study co-author Harry Van Loon of the National Center for Atmospheric Research in Boulder, Colorado and Colorado Research Associates.
Long-term records show precipitation increases up to 10 or 15 percent that coincide with solar maxima, said lead author Gerald Meehl, also of NCAR.
Meehl and his colleagues have now described the underpinnings of the connection.
First, they confirmed that the slight increase in solar energy during the peak production of sunspots is absorbed by stratospheric ozone.
The energy warms the air in the stratosphere over the tropics, where sunlight is most intense, while also triggering the production of ozone there that absorbs even more solar energy. 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 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, producing additional water vapor. In turn, 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.
The top-down influence of the stratosphere and the bottom-up influence of the ocean work together to intensify this loop and strengthen the trade winds. As more sunshine hits drier areas, these changes reinforce each other, leading to less clouds in the subtropics, allowing even more sunlight to reach the surface, and producing a positive feedback loop that further magnifies the climate response.
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. The cooling of about 1-2 degrees F is focused farther east than in a typical La Nina, is only about half as strong, and is associated with different wind patterns in the stratosphere.
Solar maximum could potentially enhance a true La Nina event or dampen a true El Nino event. The La Nina of 1988-89 occurred near the peak of solar maximum and yielded an unusually mild and dry winter in the southwestern United States, for example.
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 authors say their new work 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.
That is, if the Sun behaves itself.
The Sun normally goes through 11-year activity cycles, measured most commonly by the number of sunspots. Solar cycle 23 bottomed out in 2007 and we should have seen an uptick in sunspots — heralding Solar Cycle 24 — last year. But the Sun didn’t start stirring again until this spring. (I wrote about that for National Geographic News, here.)
Doug Biesecker, a physicist at NOAA’s Space Weather Prediction Center, also in Boulder, said the Sun seems to have started Solar Cycle 24, but he’s not willing to guarantee it yet.
“We’re clearly seeing an uptick in the long-term smoothed number [of sunspots] in the early part of 2009,” he said. Biesecker tries to look at the overall trend rather than individual monthly numbers, but the past couple of months have been a bit puzzling. “July was one of the bigger months of late,” he said, “but we haven’t had a single sunspot in all of August. “The odds are in favor that we’re already in the new cycle,” he said. “I would argue it will be the end of the year before we know definitively.”
Sources: University Corporation for Atmospheric Research (UCAR), Science, and a brief interview with Biesecker. See a previous blog post on solar behavior here.
