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Sunday, January 13, 2013

"Solar Variability and Terrestrial Climate"

Although discounting its impact on global climate, this article reports on research showing a link between regional climate and even minor differences in solar output. From Phys.Org:
One of the participants, Greg Kopp of the Laboratory for Atmospheric and Space Physics at the University of Colorado, pointed out that while the variations in luminosity over the 11-year solar cycle amount to only a tenth of a percent of the sun's total output, such a small fraction is still important. "Even typical short term variations of 0.1% in incident irradiance exceed all other energy sources (such as natural radioactivity in Earth's core) combined," he says. Of particular importance is the sun's extreme ultraviolet (EUV) radiation, which peaks during the years around solar maximum. Within the relatively narrow band of EUV wavelengths, the sun's output varies not by a minuscule 0.1%, but by whopping factors of 10 or more. This can strongly affect the chemistry and thermal structure of the upper atmosphere.

Several researchers discussed how changes in the upper atmosphere can trickle down to Earth's surface. There are many "top-down" pathways for the sun's influence. For instance, Charles Jackman of the Goddard Space Flight Center described how nitrogen oxides (NOx) created by solar energetic particles and cosmic rays in the stratosphere could reduce ozone levels by a few percent. Because ozone absorbs UV radiation, less ozone means that more UV rays from the sun would reach Earth's surface.
Isaac Held of NOAA took this one step further. He described how loss of ozone in the stratosphere could alter the dynamics of the atmosphere below it. "The cooling of the polar stratosphere associated with loss of ozone increases the horizontal temperature gradient near the tropopause," he explains. "This alters the flux of angular momentum by mid-latitude eddies. [Angular momentum is important because] the angular momentum budget of the troposphere controls the surface westerlies." In other words, solar activity felt in the upper atmosphere can, through a complicated series of influences, push surface storm tracks off course.
... Gerald Meehl of the National Center for Atmospheric Research (NCAR) presented persuasive evidence that solar variability is leaving an imprint on climate, especially in the Pacific. According to the report, when researchers look at sea surface temperature data during sunspot peak years, the tropical Pacific shows a pronounced La Nina-like pattern, with a cooling of almost 1o C in the equatorial eastern Pacific. In addition, "there are signs of enhanced precipitation in the Pacific ITCZ (Inter-Tropical Convergence Zone ) and SPCZ (South Pacific Convergence Zone) as well as above-normal sea-level pressure in the mid-latitude North and South Pacific," correlated with peaks in the sunspot cycle.
The solar cycle signals are so strong in the Pacific, that Meehl and colleagues have begun to wonder if something in the Pacific climate system is acting to amplify them. "One of the mysteries regarding Earth's climate system ... is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific." Using supercomputer models of climate, they show that not only "top-down" but also "bottom-up" mechanisms involving atmosphere-ocean interactions are required to amplify solar forcing at the surface of the Pacific.


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