Joe LeMonnier 
Jet Stream, 
Winter Airflow Around Tibet Plateau 
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In wintertime, cold air settles over the 
Tibet plateau, causing the formation 
of a low-pressure system with a 
cyclonic, or counterclockwise, airflow 
in the upper atmosphere at altitudes of 
about 30,000 feet. The effect is 
especially pronounced when there is 
prolonged and heavy snow cover on the 
plateau. South of the Himalayas, a 
west- to east-flowing jet stream is 
established and dry winter weather 
prevails over India and Southeast Asia. 
temperature and pressure differences 
between Tibet and India are reversed 
from what they were in winter. In ac- 
cordance with the above-stated law, an 
anticyclonic curvature is introduced 
into the prevailing jet stream, causing it 
to flow from east to west over the south- 
ern slopes of the Himalayas. This 
change in flow takes place in spring, 
and the jet stream reverts to its winter 
pattern in fall, the two reversals coin- 
ciding with the start of summer and 
winter monsoon seasons, respectively, 
in India. 
Anomalies in ocean temperatures 
cause airflow phenomena similar to 
those seen on land and can also lead to 
the same kinds of feedback mechanisms 
in weather patterns. A number of scien- 
tists have noted that large areas of 
warmer than normal sea-surface tem- 
peratures in the North Pacific and 
North Atlantic tend to coincide either 
with persistent or recurrent high-pres- 
sure patterns in the atmosphere over 
these regions. On the other hand, large 
cold-water anomalies in the North Pa- 
cific and North Atlantic have been 
found to be associated with low-pres- 
sure systems in the atmosphere. 
Low-pressure regions are associated 
with stormy and cloudy weather, 
whereas high-pressure regions usually 
have fair and relatively calm weather. 
In stormy weather, the ocean is stirred 
up by high waves, and cooler water 
from below rises to the surface, thus 
perpetuating below-normal sea-surface 
temperatures that, in turn, attract more 
stormy wegther. A big low-pressure re- 
gion over the North Pacific during Jan- 
uary 1977, for instance, coincided with 
a large cold-water anomaly. The ten- 
dency for prevalent high-pressure pat- 
terns to develop over warm water 
diminishes the turbulent mixing of 
ocean water. Cold water, which is 
heavier than warm water, rises only 
when forced to by the wind. Thus high- 
pressure patterns keep the sea-surface 
layers relatively warm. Sunshine under 
relatively cloudless skies also contrib- 
utes its share to keeping the ocean 
warmer than normal. 
The thermal effects just described are 
usually of much smaller amplitudes 
than the deflection of airflow around 
Tibet and over the Rocky Mountains. 
Nevertheless, the jet stream wave pat- 
terns that are mainly set up by the 
Rocky Mountains and the Tibet pla- 
teau do seem to be affected by sea-sur- 
face temperatures. For example, in the 
eastern United States the cold winters 
of 1976-77 and 1977-78 occurred 
when the surface waters of the central 
North Pacific were abnormally cold 
but the surface waters off the west coast 
of the United States were warmer than 
usual. With this temperature distribu- 
tion, the central North Pacific was 
characterized by stormy, low-pressure 
weather and a stronger than normal 
ridge — an extended high-pressure sys- 
tem — settled over the west coast re- 
gions of the United States and Canada, 
including the Rocky Mountains. The 
eastern side of this strong, persistent 
ridge served as an effective conveyor 
belt, ferrying cold arctic air into the 
central and eastern United States. 
The 1980 summer heat wave in the 
south-central United States was 
marked by a persistent anticyclone in 
the same area. Meteorologists suspect 
that this phenomenon was triggered by 
the Rocky Mountains (which the jet 
stream tends to cross with an anticy- 
clonic curvature). The anticyclone’s 
persistence was most likely caused by 
the combined effects of an unchanging 
pattern of large-scale airflow waves be- 
tween Tibet and the Rockies, surface 
temperatures over the Pacific, and feed- 
back from the heat wave itself. The dry 
and hot soil of the south-central United 
States acted on the atmosphere as a 
giant furnace, heating up the lower tro- 
posphere and thereby extending the de- 
flecting effects of the Rockies on the 
tropospheric airflow patterns farther to 
the east and southeast than is normally 
the case. 
Snow cover can also bring about 
feedback that can cause certain weather 
patterns to become either persistent or 
repetitive, resulting in large seasonal 
anomalies. An example of such a feed- 
back effect is the extended low-pressure 
region, or trough, that often settles over 
the central United States downwind of 
the Rocky Mountains. Such troughs 
create ideal conditions for the develop- 
ment of winter blizzards over eastern 
Colorado. These storms ordinarily 
move in a northeasterly direction, blan- 
keting the country with several inches 
of snow. The snow blanket reflects the 
sun’s radiation back to space, keeping 
the underlying soil from warming up. 
As a consequence, cold air moving 
down from Canada remains cold for ex- 
tended periods and can cause repeated 
blizzards. Thus an extensive snow blan- 
ket reinforces the trough originally pro- 
duced by the barrier effect of the Rocky 
Mountains. These combined effects 
contribute to lengthy periods of below- 
normal temperatures. 
Snow cover in Tibet also comes into 
play. Normally, Tibetan winters are 
dry, resembling winters in the central 
Rocky Mountains. The average annual 
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