retrogression of long waves. These rules are discussed
in the following text.
PROGRESSION OF LONG WAVES.—
Progression (eastward movement) of long waves is
usually found in association with relatively short wave
lengths and well defined major troughs and ridges. At
the surface, there are usually only one or two prominent
cyclones associated with each major trough aloft.
Beneath the forward portion of each major ridge there
is usually a well developed surface anticyclone moving
toward the east or southeast. The 24-hour height
changes at upper levels usually have a one-to-one
association with major troughs and ridges; that is,
motion of maximum height fall and rise areas associated
with major trough and ridge motion. The tracks of the
height change centers depend on the movement and
changes in intensity of the long waves.
STATIONARY LONG WAVE PATTERNS.—
Once established, stationary long wave patterns usually
persist for a number of days. The upper airflow
associated with the long wave pattern constitutes a
steering pattern for the smaller scale disturbances (short
waves). These short waves, with their associated height
change patterns and weak surface systems, move along
in the flow of the large scale, long wave pattern. Short
wave troughs deepen as they move through the troughs
of the long waves, and fill as they move through the
ridges of the long waves. The same changes in intensity
occur in sea level troughs or pressure centers that are
associated with minor troughs aloft. Partly as a result
of the presence of these smaller scale systems, the
troughs and ridges of the stationary long waves are often
spread out and hard to locate exactly.
RETROGRESSION OF LONG WAVES.— A
continuous retrogression of long wave troughs is a rare
event. The usual type of retrogression takes place in a
discontinuous manner; a major trough weakens,
accelerates eastward, and becomes a minor trough,
while a major wave trough forms to the west of the
former position of the old long wave trough. New major
troughs are generally formed by the deepening of minor
troughs into deep, cold troughs.
Retrogression is seldom a localized phenomenon,
but appears to occur as a series of retrogressions in
several long waves. Retrogression generally begins in
a quasi-stationary long wave train when the stationary
wavelength shows a significant decrease. This can
happen as a result of a decrease in zonal wind speed, or
of a southward shift in the zonal westerlies. Some
characteristics of retrogression are as follows:
l Trajectories of 24-hour height change patterns at
500-hPa deviate from the band of maximum wind.
l New centers appear, or existing ones rapidly
increase in intensity.
. Rapid intensification of surface cyclones occurs
to the west of existing major trough positions.
Location Of The Jet Stream
The AG2 TRAMAN, volume 1, discusses the
migration of the jet stream both northward and
southward. Some general considerations can be made
concerning this migration and the movement of waves
in the troposphere:
. In a northward migrating jet stream, a west wind
maximum emerges from the tropics and gradually
moves through the lower midlatitudes. Another
maximum, initially located in the upper midlatitudes,
advances toward the Arctic Circle while weakening.
Open progressive wave patterns with pronounced
amplitude and a decrease in the number of waves due to
cutoff centers exist. The jet is well organized and
troughs extend into low latitudes.
. As the jet progresses northward, the amplitude of
the long waves decrease and the cutoff lows south of the
westerlies dissipate. By the time the jet reaches the
midlatitudes, a classical high zonal index (AG2
TRAMAN, volume 1) situation exists. Too, we have
weak, long waves of large wavelength and small
amplitude, slowly progressive or stationary. Few
extensions of troughs into the low latitudes are present,
and in this situation, the jet stream is weak and
disorganized.
. As the jet proceeds farther northward, there will
often be a sharp break of high zonal index with rapidly
increasing wave amplitudes aloft. Long waves
retrograde. As the jet reaches the upper midlatitudes and
into the sub-Arctic region, it is still the dominant feature,
while a new jet of the westerlies gradually begins to
form in the subtropical regions. Long waves now begin
to increase in number, and there is a reappearance of
troughs in the tropics. The cycle then begins again.
With a southward migrating jet, the processes are
reversed from that of the northward moving jet. It
should be noted that shortwaves are associated with the
jet maximum and move with about the same speed as
these jet maximums.
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