Convergence Zones - Where the action is
When geologists use the term
"convergence zone", they are discussing the region where two tectonic
plates are colliding, with one plate sliding beneath the other. The result
is geological turbulence: fault zones that produce earthquakes, and generated
heat that gives rise to explosive volcanoes. When meteorologists use the
term "convergence zone", they are describing a phenomenon in the
atmosphere which works in an analogous fashion. Near the equator, warm air
rises and colder air moves in beneath it. As the warm air rises, it forms huge
bands of clouds and thunderstorms over the ocean, an area called the
Intertropical Convergence Zone, or ITCZ, which is an obvious feature of global
weather satellite imagery [Figure 1 (high resolution-2.5 MB)]. The
clouds indicate the formation of Hadley cells (Figure 2) in the
atmosphere.
When oceanographers use the term "convergence zone", they too
are speaking of an area of converging forces. In this case, the forces in
opposition are strong ocean currents. The result is that oceanic convergence
zones are usually marked by sharp demarcations in temperature and water mass
characteristics, and outbreaks of high biological productivity.
The featured image at the top of this page is a SeaWiFS image obtained on
February 5, 1999, offshore of the coast of Argentina in South America. The
most notable features in this image is a long and narrow band of high
productivity, stretching for hundreds of kilometers from near Buenos Aires and
the Rio de la Plata estuary, toward Patagonia. This band of high
productivity marks the convergence zone between two current systems — the
warm, coast-hugging, southward-flowing Brazil Current and the cold,
northward-flowing Malvinas/Falkland Current.
The interaction of the Brazil Current and the Malvinas Current is quite
interesting. The Malvinas current is actually an offshoot of the Antarctic
Circumpolar Current, a branch that veers northward along the South American
continental shelf. The boundary between the cold Malvinas Current water and
warmer inshore water parallels the coast until about the latitude of Buenos
Aires, where the Malvinas encounters the Brazil Current. This interaction
creates a very complicated fluid dynamics problem: the flow of the Malvinas
Current is turned (retroflected) into the South Atlantic Ocean, while the warm
Brazil Current waters are pushed toward the coast. The exact location of this
boundary varies with the seasons, as seen in sea surface temperature imagery
(Figure 3).
The result of this
water motion, which causes the bright band of productivity seen in the SeaWiFS
image, can be visualized as two rotating cylinders of water parallel to each
other (Figure 4). [Remember that the actual situation is much more
complicated, as it involves the mixing of different water mass types,
interaction with bottom topography, and is also influenced by surface winds.]
At the convergence zone, where the two cylinders meet, the "inside"
circulation of each cylinder is moving downward (a water movement called
downwelling). In the cylinder on the cold side (the Malvinas Current side) of
the convergence zone, the "outside" circulation is upward (called
upwelling, of course), bringing nutrients to the surface. When the
phytoplankton utilize the nutrients and sunshine at the surface, they grow
rapidly, leading to high phytoplankton concentrations. On the warm side (the
Brazil Current side), the upward circulation near the coast only brings warm,
low nutrient water to the surface, so the surface water on this side of the
convergence zone has low phytoplankton concentrations. The large difference
in the abundance of phytoplankton chlorophyll is clearly visible in the SeaWiFS
ocean color image.
Now for a bit more about the physical oceanography of this situation (this
is still very basic). The areas on the outside circulation of the cylinders,
where upwelling occurs, are zones of "negative convergence", more
commonly called divergence. Because mass is conserved, convergence in
one area requires an equal amount of divergence somewhere else to balance
it. So, to summarize: convergence usually means downwelling, divergence
usually means upwelling, and upwelling frequently (but not always) is
associated with enhanced biological productivity.
Convergence zones
are of interest to oceanographers for several reasons. Because they
frequently mark the boundaries of currents, their position can be used to model
the interaction of different oceanic current systems. Obviously, biological
oceanographers study them because they are areas of high biological activity,
from microscopic phytoplankton to large fish like marlin or bluefin tuna. Even
loggerhead turtles have been shown to frequent the area of convergence zones in
the Pacific Ocean. (Backscatter magazine, November 1999, page 29-30).
A variety of remote sensing technologies can be used to view convergence
zones, due to the abrupt changes in water mass characterisitics that define
them. One of the most interesting applications that can view convergence
zones is synthetic aperture radar (SAR). SAR is extremely sensitive to very
small variations in the surface of the ocean, so it can actually be used to
view the wakes of ships. Convergence zones show up clearly due to several
factors. One factor is that a height difference of a few centimeters actually
marks the boundary of the warm water and the cold water, so that the zone can
actually be marked by waves and surface turbulence. Another factor is that
chemical substances called surfactants (either natural or from human sources,
like oil) concentrate in these zones, and they can actually affect the small
waves that form at the surface of the ocean from the wind. Thus, convergence
zones show up as smooth areas, or slicks, due to the reduced wave action caused
by the presence of surfactants. Figure 5 shows a SAR image obtained from an
airplane (using the Jet Propulsion Laboratory’s AirSAR instrument) of an
area off the coast of California, where the dark areas are smooth slicks, and
the bright areas are areas of increased wave action. Note near the center of
the image where the bright and dark areas are roughly parallel, delineating the
convergence zone.
One of the most dramatic examples of a convergence zone was called the
"Line in the Sea". It occurred in the Pacific Ocean in 1992, and was
photographed from the Space Shuttle [Figure 6 (high-resolution-375 K) (alternate view-350 K)].
Researchers in the Joint Global Ocean Flux Study (JGOFS) Equatorial Pacific
program also happened to be in this area at the same time, and documented the
sharp changes that occurred at this particular convergence zone (Yoder et al.,
1993).
Convergence zones
need not be as dramatic as the "Line in the Sea" or the boundary of
the Brazil Current and the Malvinas Current. Observation of a smooth lake
surface during a rainstorm will also show the development of smooth linear
areas parallel to rougher areas. These features are due to the slight
differences in temperature between rain water and lake water cause the
formation of upwelling and downwelling areas. Surfactants converge here too,
dampening the wave action and causing the smooth areas on the surface. These
features are commonly called "windrows", but the technical term for
them is Langmuir cells, after Irving Langmuir, who first described the
circulation that caused them.
References
James A. Yoder, Steven G. Ackleson, Richard T. Barber, Pierre Flament and
William M. Balch. 1994: A Line in the Sea. Nature 371: 689-692
Associated URLs
ACKNOWLEDGMENT
The authors wish to thank Dr. Dennis Kirwan of the
University of Delaware for assistance with the text of this article. We
also thank Dr. James Yoder (University of Rhode Island) who provided Space
Shuttle photographs of the "Line in the Sea". We also thank the authors
of the Web pages which provided associated information and imagery.
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