2011 - Cooperative Institute for Research in Environmental Sciences ...

Peter Molnar
Tibet and the Asian Monsoon System
I devoted a part of my research effort in FY11 to synthesizing
evidence that pertains to the growth of the Tibetan Plateau
and to the Asian monsoon, in modern and geologic time.
The extreme breadth and height of the Tibetan Plateau
are commonly assigned key roles in the Asian monsoon
system, and hence most imagine that the geologic history
of the Asian monsoon is closely related to the growth of
Tibet. According to this view, heating of the air immediately
above Tibet induces ascent and cross-equatorial circulation
that comprises the upper branch of the monsoon circulation.
A new view is emerging, however, in which the role of
Tibet is little more than a barrier to the flow of cool, dry air
from northern Eurasia, and heating over the plateau plays a
minor role, at least in the South Asian (or Indian) monsoon.
Together with William Boos of Harvard University and
David Battisti, I reviewed not only this new view, but also
the geologic history of Tibet and the Asian monsoon (Molnar
et al., 2010).
One of the breakthroughs in geology in the past 20 years
has been the development of methods for determining
paleoelevations. When applied to Tibet, virtually all studies
show elevations comparable to present-day elevations, with
the one exception in northern Tibet (Figure 1). If a part of
Tibet rose recently, since about 10 million years ago, when
some evidence suggesting a strengthening of the monsoon
occurred, that part must be northern Tibet.
The view that heating of Tibet plays a key role in the
strength of the monsoon loses some credibility when upper
atmospheric temperatures are plotted (Figure 2); the hottest
upper troposphere is not over Tibet, but to its south over
northern India. Moreover, current theories hold that the
edge of the monsoon circulation should lie over the region
of highest specific entropy, which also lies not over Tibet, but
over northern India (Figure 2). It seems that the Himalaya,
the southern edge of Tibet, plays a key role by blocking
cool, dry air from farther north, air that would reduce the
specific entropy of air over India if it could interact with the
Figure 1: Topographic map of Tibet showing estimates of paleoaltimetry.
40 CIRES Annual Report 2011
hot, moist air formed over the Indian subcontinent. That
blockage, not the heating of Tibet, allows the South Asian
monsoon to become very strong. If so, the growth of Tibet is
unlikely to have played a key role in the development of the
South Asian monsoon.
This review presents a summary of the recent thinking on
both the growth of Tibet and its significance for the geologic
history of the Asian monsoon.
Figure 2: (Left) Upper-tropospheric temperature (250 hPa) over Asia in
July; the maximum overlies northern India and Pakistan, not the Tibetan
Plateau. (Right) The moist entropy in July on a terrain-following model
level within 50 hPa (about 500 m) of the surface. All quantities are means
for 1979–2002 from the ERA-40 reanalysis data set.