SATELLITE
SEES DOUBLE ZONES OF CONVERGING TROPICAL WINDS AROUND THE
WORLD
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![QuikSCAT's Look at Surface Winds Coming Together Over 4 Months](https://webarchive.library.unt.edu/web/20080920162322im_/http://www.gsfc.nasa.gov/gsfc/earth/pictures/20020711itczworks/itczwindst.gif)
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NASA's
QuikSCAT satellite has confirmed a 30-year old largely unproven
theory that there are two areas near the equator where the
winds converge year after year and drive ocean circulation
south of the equator. By analyzing winds, QuikSCAT has found
a year-round southern and northern Intertropical Convergence
Zone. This find is important to climate modelers and weather
forecasters because it provides more detail on how the oceans
and atmosphere interact near the equator.
The
Intertropical Convergence Zone (ITCZ) is the region that circles
the Earth near the equator, where the trade winds of both
the Northern and Southern Hemispheres come together. North
of the equator, strong sun and warm water of the equator heats
the air in the ITCZ, drawing air in from north and south and
causing the air to rise. As the air rises it cools, releasing
the accumulated moisture in an almost perpetual series of
thunderstorms. Satellite data, however, has confirmed that
there is an ITCZ north of the equator and a parallel ITCZ
south of the equator.
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![The Northern and Southern ITCZs](https://webarchive.library.unt.edu/web/20080920162322im_/http://www.gsfc.nasa.gov/gsfc/earth/pictures/20020711itczworks/equatormap1t.gif)
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Variation
in the location of the ITCZ is important to people around
the world because it affects the north-south atmospheric circulation,
which redistributes energy. It drastically affects rainfall
in many equatorial nations, resulting in the wet and dry seasons
of the tropics rather than the cold and warm seasons of higher
latitudes. Longer term changes in the ITCZ can result in severe
droughts or flooding in nearby areas.
"The
double ITCZ is usually only identified in the Pacific and
Atlantic Oceans on a limited and seasonal basis," said
Timothy Liu, of NASA's Jet Propulsion Laboratory and California
Institute of Technology, Pasadena, Calif., and lead researcher
on the project. In the eastern Pacific Ocean, the southern
ITCZ is usually seen springtime. In the western Atlantic Ocean,
the southern ITCZ was recently clearly identified only in
the summertime.
However,
QuikSCAT's wind data has seen the southern ITCZ in all seasons
across the entire Atlantic Ocean and the eastern Pacific.
"QuikSCAT's wind data confirms there is a double ITCZ,
and that they exist all year long," Liu said.
This
is a major find for the science community, as the existence,
location, and seasonality of the double ITCZ had remained
controversial since 1969.
For
most of the time, the southern ITCZ is weaker than the northern
one, which is why it has been so hard to detect before. The
southern ITCZ is weaker because it blows over cooler water
that comes up from the lower depths of the ocean, called upwelling.
Over cooler water, the air doesn't rise as easily as warm
air, and the winds from higher altitudes are mixed less than
they would with warmer water and air. Cooler water therefore
causes weaker surface winds. Therefore, the surface winds
from the south slow down as they approach the equator, and
this causes the southern ITCZ because the air gets squeezed
together.
What
is unique about the southern ITCZ is that there is not a north
and south wind as with the northern ITCZ. The southern ITCZ
only consists of a southern wind coming into the equator.
"The
convergence of winds in the southern hemisphere creates a
counter-clockwise rotation which affects ocean circulation,"
Liu said. "For the first time, we can give a better explanation
of the ocean movements driven by winds."
The
SeaWinds instrument on QuikSCAT is a specialized microwave
radar that measures both the speed and direction of winds
near the ocean surface. Launched June 19, 1999, from California's
Vandenberg Air Force Base, the spacecraft operates in a Sun-synchronous,
near-polar orbit, circling Earth every 100 minutes, taking
approximately 400,000 measurements over 90 percent of Earth's
surface each day.
These
findings are being presented at the 2002 Western Pacific Geophysics
Meeting, Wellington Convention Centre, Wellington, New Zealand,
in a session titled "Southern Intertropical Convergence
Zones," on Friday, July 9th at 2:15 p.m., Session A22A-05,
Location: MF-LionHV2. The results will be published in a paper
entitled, "Double Intertropical Convergence Zones - a
New Look Using a Scatterometer," in an upcoming issue
of Geophysical Research Letters.
This
research was performed at the Jet Propulsion Laboratory, California
Institute of Technology and funded by the Physical Oceanography,
the Ocean Vector Wind, and the TRMM Programs of NASA.
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