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Subject: G5) What determines the movement of tropical
cyclones?
Contributed by Chris Landsea
Tropical cyclones - to a first approximation - can be thought of as
being steered by the surrounding environmental flow throughout
the depth of the troposphere (from the surface to about 12 km or
8 mi). Dr. Neil Frank, former director of the U.S. National
Hurricane Center, used the analogy that the movement of
hurricanes is like a leaf being steered by the currents in the
stream, except that for a hurricane the stream has no
set boundaries.
In the tropical latitudes (typically equatorward of
20°-25°N or S), tropical cyclones usually move toward
the west with a slight poleward component. This is because
there exists an axis of high pressure called the subtropical
ridge that extends east-west poleward of the storm. On the
equatorward side of the subtropical ridge, general easterly winds
prevail. However, if the subtropical ridge is weak - often
times due to a trough in the jet stream - the tropical cyclone
may turn poleward and then recurve back toward the east. On the
poleward side of the subtropical ridge, westerly winds prevail
thus steering the tropical cyclone back to the east. These
westerly winds are the same ones that typically bring
extratropical cyclones with their cold and warm fronts
from west to east.
Many times it is difficult to tell whether a trough will allow the
tropical cyclone to recurve back out to sea (for those folks on the
eastern edges of continents) or whether the tropical cyclone will
continue straight ahead and make landfall.
For more non-technical information on the movement of tropical
cyclones, see Pielke and Pielke's "Hurricanes:
Their Nature and Impacts on Society". For a more detailed,
technical summary on the controls on tropical cyclone motion,
see Elsberry's chapter in "Global Perspectives
on Tropical Cyclones".
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