Satellite Sees Double Zones of Converging Tropical Winds Around The World
NASA’s Quick Scatterometer 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 observing ocean
winds, Quick Scatterometer, also known as 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 is the region that circles Earth near the
equator, where the trade winds of both the Northern and Southern hemispheres come
together. North of the equator, strong sunshine and warm water of the equator heats the
air in the region, 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, have confirmed that there is an Intertropical
Convergence Zone north of the equator and a parallel one south of the equator.
Variation in the location of the Intertropical Convergence Zone 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 region can result in severe
droughts or flooding in nearby areas.
“The double Intertropical Convergence Zone is usually only identified in the
Pacific and Atlantic oceans on a limited and seasonal basis,” said Dr. Timothy Liu, of
NASA’s Jet Propulsion Laboratory, Pasadena, Calif., lead researcher on the project. In
the eastern Pacific Ocean, the southern zone is usually seen in springtime. In the
western Atlantic Ocean, the southern zone was recently clearly identified only in the
summertime. However, Quikscat’s wind data has seen the southern Intertropical
Convergence Zone in all seasons across the entire Atlantic Ocean and the eastern
Pacific. “Quikscat’s wind data confirms there is a double Intertropical Convergence
Zone, and that it exists all year long,” Liu said.
This is a major find, as the existence, location, and seasonality of the double
Intertropical Convergence Zone had remained controversial since 1969.
For most of the time, the southern Intertropical Convergence Zone is weaker
than the northern one, which is why it has been so hard to detect before. The southern
Intertropical Convergence Zone is weaker because it blows over cooler water that
comes up from the lower depths of the ocean. 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 Intertropical Convergence Zone because the air gets
squeezed together.
What is unique about the southern Intertropical Convergence Zone is that there
is not a north and south wind as with the northern Intertropical Convergence Zone. The
southern Intertropical Convergence Zone consists of only 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. JPL
manages the Quikscat mission for NASA.
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 Tuesday, July 9. 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 JPL and funded by the Physical Oceanography,
the Ocean Vector Wind, and the Tropical Rainfall Measuring Mission programs of
NASA. JPL is a division of the California Institute of Technology, Pasadena.
For more information and images, see
http://www.jpl.nasa.gov/images/earth/atlantic_ocean/index.html .
For more information on Quikscat, go to
http://winds.jpl.nasa.gov/missions/quikscat/quikindex.html .
Twice-daily maps of Quikscat winds over global oceans can be viewed and
downloaded in near-real-time at http://airsea-www.jpl.nasa.gov/seaflux .
