Subtle connections between the 11-year-solar cycle, the
stratosphere and the tropical Pacific Ocean work in sync to
generate periodic weather patterns that affect much of the globe,
according to research results appearing this week in the journal
Science.
The findings will help scientists get an edge on predicting the
intensity of certain climate phenomena, such as the Indian monsoon
and tropical Pacific rainfall, years in advance.
"It's been long known that weather patterns are well-correlated
to very small variations in total solar energy reaching our planet
during 11-year solar cycles," says Jay Fein, program director in
the National Science Foundation (NSF)'s Division of Atmospheric
Sciences, which funded the research. "What's been an equally long
mystery, however, is how they are physically connected. This
remarkable study is beginning to unravel that mystery."
An international team of authors led by the National Center for
Atmospheric Research (NCAR) in Boulder, Colo., used more than a
century of weather observations and three powerful computer models
to tackle one of the more difficult questions in meteorology: if
the total energy that reaches Earth from the Sun varies by only 0.1
percent across the approximately 11-year solar cycle, how can it
drive major changes in weather patterns on Earth?
The answer, according to the study, has to do with the Sun's
impact on two seemingly unrelated regions.
Chemicals in the stratosphere and sea surface temperatures in
the Pacific Ocean respond during solar maximum in a way that
amplifies the Sun's influence on some aspects of air movement.
This can intensify winds and rainfall, change sea surface
temperatures and cloud cover over certain tropical and subtropical
regions, and ultimately influence global weather.
"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, the lead author of the
paper. "Understanding the role of the solar cycle can provide added
insight as scientists work over the next decade or two toward
predicting regional weather patterns."
The results builds on recent papers by Meehl and colleagues
exploring the link between the peaks in the solar cycle and events
on Earth that resemble aspects of La Niña events, but are
distinct from those larger patterns associated with changes in
pressure and known as the Southern Oscillation.
The connection between peaks in solar energy and cooler water in
the equatorial Pacific was first discovered by Harry Van Loon of
NCAR and Colorado Research Associates, a co-author of the
paper.
The contribution by Meehl and his colleagues is to document that
two mechanisms that had been previously theorized in fact work
together to amplify the response in the tropical Pacific.
The team first confirmed a theory 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 the sunlight is most intense, while also stimulating the
production of additional ozone there that absorbs even more solar
energy.
Since the stratosphere warms unevenly, with the most pronounced
warming occurring at lower latitudes, stratospheric winds are
altered and, through a chain of interconnected processes, end up
strengthening tropical storms and precipitation.
At the same time, the increased sunlight at solar maximum causes
a slight warming of ocean surface waters, especially 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 intensifies the climate response.
These stratospheric and ocean responses during solar maximum
keep the eastern Pacific even cooler and drier than usual,
producing conditions similar to a La Niña event.
However, the cooling of about 1-2 degrees Fahrenheit is focused
further east than in a typical La Niña, is only about half
as strong, and is associated with different wind patterns in the
stratosphere.
Earth's response to the solar cycle continues over the year or
two following peak sunspot activity. The La Niña-like
pattern triggered by the solar maximum tends to evolve into a
pattern similar to El Niño, as slow-moving currents replace
the cool water over the eastern tropical Pacific with warmer
water.
Again, the ocean response is only about half as strong as with
El Niño, and the lagged warmth is not as consistent as the
cold event-like pattern that occurs during peaks in the solar
cycle.
Solar maximum could potentially enhance a true La Niña
event or dampen a true El Niño event. The La Niña of
1988-89 occurred near the peak of solar maximum.
That La Niña became unusually strong and was associated
with significant changes in weather patterns, such as an unusually
mild and dry winter in the southwestern United States.
The Indian monsoon, Pacific precipitation and sea surface
temperatures, and other regional climate patterns are largely
driven by rising and sinking air in Earth's tropics and
subtropics.
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.
To tease out the elusive mechanisms that connect the Sun and
Earth, the study team needed three computer models that provided
overlapping views of the climate system.
One model, which analyzed the interactions between sea surface
temperatures and lower atmosphere, produced a small cooling in the
equatorial Pacific during solar maximum years.
The second model, which simulated the stratospheric ozone
response mechanism, produced some increases of tropical
precipitation but on a much smaller scale than the observed
patterns.
The third model contained ocean-atmosphere interactions as well
as the role of ozone. It showed, for the first time, that the two
combined to produce a response in the tropical Pacific during peak
solar years that was close to actual observations.
"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," Meehl says.
SOURCE