Summer Airflow Around Tibet Plateau 
MONGOLIA 
Stian Mts 
Cool Waters 
Warm Plateau 
PEOPLE'S 
■ REPUBLIC OF 
CHINA 
jet Stream 
SOUTHEAST 
.ASIA': 
INDIA 
precipitation over wide areas of Tibet is 
less than sixteen inches and comes 
down mainly in the form of summer 
showers. Nevertheless, extensive win- 
ter storms occasionally bury most of 
the Tibet plateau under a lasting blan- 
ket of snow, and these snowfalls can 
have a significant effect on precipita- 
tion in India. 
In April the plateau usually starts to 
act as an elevated heat source for the at- 
mosphere, helping to generate a high- 
pressure system in the upper tropo- 
sphere over Tibet. This system becomes 
the dominant feature of the Tibetan 
summer, and its strength, to a large ex- 
tent, controls the Indian summer mon- 
soon. The barrier and thermal effects of 
the Tibet plateau work together to pro- 
duce the rain that hundreds of millions 
of people on the Indian subcontinent 
and in southeastern China rely on to 
sustain their crops. The sequence of 
weather patterns in that part of the 
world tends to go awry, however, when 
winter snows in Tibet are unusually 
deep and consequently resist the melt- 
ing warmth of the sun. 
The dazzling white of the snow sur- 
face reflects the sunlight and slows the 
warming of the soil. Under such condi- 
tions the Tibet plateau becomes a sput- 
tering, ineffective “hot plate” over 
which the atmosphere's monsoon-sus- 
taining anticyclone can build only with 
difficulty. If this Tibetan high-pressure, 
anticyclone system is weak and is estab- 
lished late in the season, the drenching 
rains of the Indian summer monsoon 
will also be late, and the rainy Indian 
summer will be interrupted by pro- 
longed spells of drought with devastat- 
ing effects. Food prices in India and 
Southeast Asia may climb, millions of 
people may go hungry, and the stability 
of governments may be threatened. 
If winter snow cover in Tibet can in- 
fluence the amount of rainfall of the 
subsequent Indian summer monsoon in 
this manner, the obvious question to 
ask is, What causes the variability in Ti- 
betan snow cover? In other words, what 
lies behind this Tibet connection? 
One possible answer relates to the 
amount of solar heat the plateau re- 
flects to space, and that, in turn, de- 
pends to a large extent on the amount of 
snow that falls on the plateau. When 
the plateau is widely blanketed under 
snow, it reflects more of the incoming 
solar energy back to space than when it 
is covered with dry grass and barren 
rock, and this difference in reflectivity 
produces different effects on the atmo- 
sphere. In an ordinary winter, the pla- 
teau receives a minimum amount of 
snow — only a few inches. When the oc- 
casional extensive snow cover does fall, 
the snow, in contrast to the usual bar- 
ren soil, keeps the atmosphere colder, 
promotes more storm activity, and in- 
creases the chance of additional snow- 
fall. Such conditions disturb the usual 
position of weaves in the jet stream flow 
and produce such untoward effects as 
one foot of snow' in Tibet in the winter 
of 1972. But what caused the Tibetan 
snowfall in the first place? 
In studying weather maps from 1960 
to the present, I discovered almost by 
accident that variations in Tibet’s win- 
ter snow cover matched surprisingly 
well with sea-surface temperature 
anomalies over the central North At- 
lantic during the same periods. When 
the North Atlantic was excessively 
As the Tibet plateau heats up during 
spring and summer, it warms the air. 
giving rise to a high-pressure system 
with anticyclonic, or clockwise, 
circulation in the upper troposphere. 
South of the Himalayas, this serves to 
establish an east- to west-flowing jet 
stream, bringing summer monsoon 
rains to India and Southeast Asia. 
warm in the winter, there appeared to 
be little winter snow in Tibet. A warm 
North Atlantic tends to attract a persis- 
tent high-pressure system, which as it 
extends into fall, promotes warm, dry- 
weather in Europe and Asia. (This is 
the kind of weather vintagers love, es- 
pecially in the Bordeaux area, and as 
one might expect, there was also a posi- 
tive correlation between the quality of 
Sauteme wine grown in that region of 
France and warmer than normal tem- 
peratures in the central North Atlan- 
tic.) On the other hand, warm-water 
anomalies in the Atlantic during au- 
tumn and winter promote a trough and 
ridge pattern over Europe and central 
Asia that can send Siberian winter 
storms into Tibet, dumping excessive 
amounts of snow- in that country . 
Having found this correlation, I be- 
gan to search for other possible Tibet 
connections. A link has already been 
demonstrated among the delays, weak- 
nesses, and shifts in the Tibetan sum- 
mer high-pressure system, the Indian 
summer monsoon, and sea-surface tem- 
peratures in the Atlantic. But there 
seems to be a tie-in with the Pacific 
Ocean as well. When there are strong 
summer monsoons in central India and 
a well-developed high-pressure system 
over Tibet, the frequency of typhoons 
in the west Pacific seems to be lessened. 
When there are many typhoons in the 
Pacific — some of them bringing exces- 
sive rains and flooding to eastern Chi- 
na — the Indian summer monsoon 
tends to be dry. 
In the 1920s, when India was still un- 
der British control, British researchers 
pointed out the existence of a “southern 
oscillation” in surface air pressures 
over the land and the sea in the regions 
between the South Pacific and the Indi- 
an Ocean. W’hen the air pressure at 
Djakarta in Java and in the Indian 
Ocean was above normal, for example, 
the Indian monsoon tended to be weak, 
and the sea-surface pressure in the 
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