Few sunspots, more hurricanes, FSU researchers warn
Study indicates link between sun and hurricanes
Florida Today • June 1, 2010
Busy hurricane season expected
The calmest sun in a century may rustle up more hurricanes, as the season officially begins today.
Research by Robert Hodges and Jim Elsner of Florida State University found the probability of three or more hurricanes hitting the United States goes up drastically during low points of the 11-year sunspot cycle, such as we're in now.
Our star is just beginning to eke out of the lowest period for sunspots in a century.
Years with few sunspots and above-normal ocean temperatures spawn a less stable atmosphere and, consequently, more hurricanes, according to the researchers.
Years with more sunspots and above-normal ocean temperatures yield a more stable atmosphere and thus fewer hurricanes.
"The effect is actually amplified under certain conditions," said James Elsner, a geography professor at Florida State University. Hodges is his graduate student. "With fewer sunspots, there's less energy at the top of the atmosphere," Elsner explained, therefore the atmosphere above the hurricane is cooler.
When that happens, the differential creates more atmospheric instability and stronger storms, energizing what might otherwise remain tropical storms into hurricanes.
Chances of three or more hurricanes hitting the U.S. increase from 20 to 40 percent in years when sunspots are in the lowest 25 percent, compared with years when they're in the highest 25 percent, the researchers found.
There's only a 25 percent chance of at least one hurricane hitting the United States in peak sunspot years. The chance spikes to 64 percent in the lowest sunspot years.
The scientists studied the frequency of hurricanes and sunspots from 1851 to 2008, adjusting for other hurricane-influencing factors such as El Nino and changes in sea-surface temperature. They recently presented their findings in a poster at the American Meteorological Society's conference in Arizona. Their work also was published this month in the scientific journal Geophysical Research Letters.
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UV light's effectsFor hurricanes to form, the atmosphere must cool fast enough, at the right heights, to make it unstable enough for storm clouds to form. This thunderstorm activity enables heat stored in the ocean to be unleashed, developing into tropical cyclones. As the "heat-engine theory" of hurricanes goes, storm strength decreases when the layer near the hurricane's top warms.
Although sunspots are cooler, darker blotches on the sun, more solar flares form near the increased magnetic activity within sunspots.
"The spots are kind of an indication of the amount of energy," Elsner explained. "The more sunspots, the more active the sun."
Sunspots ebb and flow on 11-year cycles. There can be anywhere from 200 to 300 sunspots a day at the cycle's peak or as few as one to three dozen at the cycle's low, as has been happening in recent months.
Solar radiation also varies as the spots pass across the sun's surface while the star rotates on its axis about once a month.
The sun's yearly average radiance during its 11-year cycle only changes about one-tenth of one percent, according to NASA's Earth Observatory.
But the warming in the ozone layer can be much more profound, because ozone absorbs ultraviolet radiation. Between the high and low of the sunspot cycle, radiation can vary more than 10 percent in parts of the ultraviolet range, Elsner has found.
When there are more sunspots and therefore ultraviolet radiation, the warmer ozone layer heats the atmosphere below.
Their latest paper shows evidence that increased UV light from solar activity can influence a hurricane's power even on a daily basis.
Sunspots no factorThe researchers say their finding could help improve hurricane intensity forecasting.
But because of the novelty of the research, the National Hurricane Center has yet to factor in sunspots in its forecast. Nor do hurricane seasonal forecasters such as Phil Klotzbach and William Gray's team at Colorado State University.
"Thus far we've not used sunspot information at all," said Chris Landsea, science and operations officer at NOAA's National Hurricane Center in Miami. "If it does prove to be a robust signal, we'll certainly consider including it in the future. But Id' need to see more evidence."
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UV light's effects
For hurricanes to form, the atmosphere must cool fast enough, at the right heights, to make it unstable enough for storm clouds to form. This thunderstorm activity enables heat stored in the ocean to be unleashed, developing into tropical cyclones. As the "heat-engine theory" of hurricanes goes, storm strength decreases when the layer near the hurricane's top warms.
Although sunspots are cooler, darker blotches on the sun, more solar flares form near the increased magnetic activity within sunspots.
"The spots are kind of an indication of the amount of energy," Elsner explained. "The more sunspots, the more active the sun."
Sunspots ebb and flow on 11-year cycles. There can be anywhere from 200 to 300 sunspots a day at the cycle's peak or as few as one to three dozen at the cycle's low, as has been happening in recent months.
Solar radiation also varies as the spots pass across the sun's surface while the star rotates on its axis about once a month.
The sun's yearly average radiance during its 11-year cycle only changes about one-tenth of one percent, according to NASA's Earth Observatory.
But the warming in the ozone layer can be much more profound, because ozone absorbs ultraviolet radiation. Between the high and low of the sunspot cycle, radiation can vary more than 10 percent in parts of the ultraviolet range, Elsner has found.
When there are more sunspots and therefore ultraviolet radiation, the warmer ozone layer heats the atmosphere below.
Their latest paper shows evidence that increased UV light from solar activity can influence a hurricane's power even on a daily basis.
Sunspots no factor
The researchers say their finding could help improve hurricane intensity forecasting.
But because of the novelty of the research, the National Hurricane Center has yet to factor in sunspots in its forecast. Nor do hurricane seasonal forecasters such as Phil Klotzbach and William Gray's team at Colorado State University.
"Thus far we've not used sunspot information at all," said Chris Landsea, science and operations officer at NOAA's National Hurricane Center in Miami. "If it does prove to be a robust signal, we'll certainly consider including it in the future. But Id' need to see more evidence."
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