- The Tibetan Plateau or as the Himalayan Plateau in India,[3][4] is a vast elevated plateau in South Asia, Central Asia[5][6][7][8] and East Asia, It is the world's highest and largest plateau above sea level, with an area of 2,500,000 square kilometres (970,000 sq mi)
- The Tibetan Plateau contains the headwaters of the drainage basins of most of the streams in surrounding regions. Its tens of thousands of glaciers and other geographical and ecological features serve as a "water tower" storing water and maintaining flow. It is sometimes termed the Third Pole because its ice fields contain the largest reserve of fresh water outside the polar regions.
- role of the Tibetan plateau in the Indian Monsoon: Indian monsoon is a complex system characterized by seasonal reversal in wind direction in south and South East Asia. Monsoon system is modified trade winds guided by various landforms such as Himalayas and water systems such as Pacific. Tibetan plateau especially has very major role to play in genesis of monsoon.
- Landforms play a major role in deciding the path of propagation as well as intensity of rainfall, as most of the rainfall in India is orographic in nature. The Himalayas plays a major role in genesis of monsoon as well as monsoon breaks. Tibet Plateau is an enormous block of a high ground act as a formidable barrier as well as a heat source.
- Tibetan Heating creates a low pressure in summer which gets coupled with Massacarene High pressure area and thus helps in initiation of Monsoon.
The Mascarene Islands or Mascarenes or Mascarenhas Archipelago is a group of islands in the Indian Ocean east of Madagascar consisting of the islands belonging to the Republic of Mauritius
- The vast mountain range also acted as a tall barrier, preventing cold, dry air in the northern latitudes from entering the subcontinent and subduing the warm, moisture-laden winds from the oceans that drive the monsoon.
- During southwest monsoon, a thermal anticyclone appears over Tibet, which the resultant formation of dynamic anti-cyclogenesis. On the south side of the anticyclone, the tropical jet stream is from. The plateau accents the northward displacement of the jet stream in the middle of the October. The withdrawal sub-tropical westerly jet stream to the north Himalayas marks the onset of monsoon season in India.
Thermal anticyclones are more typical in winter; they occur over continents and often can persist for a long time.
Anticyclone, any large wind system that rotates about a centre of high atmospheric pressure clockwise in the Northern Hemisphere and counterclockwise in the Southern. Its flow is the reverse of that of a cyclone
The Tropical Easterly Jet (Jet stream) is the meteorological term referring to an upper level easterly wind that starts in late June and continues until early September. ... The strongest development of the jet is at about 15 km above the Earth's surface with wind speeds of up to 40 m/s over the Indian Ocean.
- With the Tibetan heating and ENSO acting independently of each other, the two factors taken together could have predictive( being an indication of the future or of future conditions) value for rainfall in the monsoon’s early and late phases.
The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal.
El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral
ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe. We also focus on ENSO because we can often predict its arrival many seasons in advance of its strongest impacts on weather and climate.
- The tropical easterly jet stream is formed at an eastern longitude of India then moves towards westwards across India and the Arabian Sea to eastern Africa. This upper-level easterly jet stream creates a flow of air on the south side of Tibetan Plateau that reaches down to low levels over northernmost India. During summer, the insolation heating of air above Tibet Plateau weakens the western subtropical jet stream south of the Himalayas with the resultant reversal of pressure gradient and wind flow over northern India.
- Thus the presence of Tibet Highland is very important, even if there is no significant barrier effect on the flow of air.
How does the Tibet Plateau influence the monsoon system of the Indian Subcontinent?
Defintion:
The seasonal reversing wind followed by corresponding changes in precipitation,but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea is called Monsoon.
Indian mansoon:The monsoon system of the Indian subcontinent has appeared as truly massive interruptions and reversal of the normal global atmospheric circulation, which make it different from the rest of the world.
It envisages that monsoon circulation is not only the reason for the outcome of the thermally induced surface low and high-pressure centres, but also the influence of Tibetan plateau and the jet stream on the origin of monsoonal circulation over the Indian subcontinent and its adjoining areas.
How does the Tibet Plateau influence the monsoon system of the Indian Subcontinent?
Tibet Plateau is an enormous block of a high ground act as a formidable barrier as well as a heat source. It accents the northward displacement of the jet stream in the middle of the October. This abrupt onset of the summer monsoon in the beginning of June is promoted by the hydrodynamic effect the Himalayas and not by the thermally induced low-pressure centre over northwest India.
Recent theory envisions that the Tibet plateau is the high level source of heat during summer time. During southwest monsoon, a thermal anticyclone appears over Tibet, which the resultant formation of dynamic anti-cyclogensis. On the south side of the anticyclone, the tropical jet stream is from.
As a result, there is a sensible heat transfer from the elevated surfaces of the Himalayas and Tibet to the atmosphere. Besides this, large amounts of latent heat released by monsoon rains over India are also added to the upper troposphere anticyclone. Thus the presence of Tibet Highland is very important, even if there is no significant barrier effect on the flow of air.
The Tibetan plateau and the Indian monsoon
In a paper published in the journal Nature in 2010, William Boos and Zhiming Kuang of Harvard University in the U.S argued that the Himalayas’ role as a barrier was the crucial factor for the monsoon.
However, Balaji Rajagopalan and Peter Molnar of the University of Colorado at Boulder in the U.S. have taken the view that the Tibetan plateau should not be written off so easily. The heating of the plateau correlated with monsoon rainfall but only during the early and late parts of the season, they observed in a paper published recently in Journal of Geophysical Research: Atmospheres.
As the vast Tibetan plateau, high up in the mountains, warmed during the summer months, it heated the air above, which then rose and created an area of low pressure, explained Dr. Rajagopalan. That belt of low pressure sucked in moisture from the oceans, thus initiating the monsoon.
With the Tibetan heating and ENSO acting independently of each other, the two factors taken together could have predictive value for rainfall in the monsoon’s early and late phases.
Journal:
Despite recent challenges, conventional wisdom has held that heating over the Tibetan Plateau leads to increased Indian summer monsoon rainfall via enhancement of
cross-equatorial circulation aloft, and a concurrent strengthening of both the Somali Jet and westerly winds that bring moisture to southern India.
Somali Jet:
A low-level southwesterly jet over the Arabian Sea in the summer months, off the coast of Somalia. It is the northern branch of a cross-equatorial flow, giving rise to a major supply of moisture in support of the Asian summer monsoon.
Westerly wind:
When air moves in a definite direction, it is called wind. If the winds move from west to east, they are called westerlies. If they move from east to west, they are called easterlies
Conclusion of Journal:We show that such heating, quantified by monthly estimates of moist static energy in the atmosphere just above the surface,
correlates with summer monsoon rainfall, but only in the early (20 May to 15 June) and late (September 1 to 15 October) monsoon season.Correlations during the main monsoon
season (15 June to 31 August) are small and insignificant. The positive correlations with early and late monsoon season,
however, allow for heating over Tibet to modulate as much as
~30% of the total rainfall. Furthermore, we demonstrate that heating over Tibet is independent
of the El Niño Southern Oscillation, so that together they explain a substantial portion of
variability in the early and late season rainfall, providing potential predictability
Given the two different views, we ask the question:
Does heating over the Tibetan Plateau correlate with the
strength of the monsoon as measured in different ways?
Toward that end we sought correlations of moist static
energy (MSE) in the atmosphere just above the surface over
Tibet with different measures of monsoon strength. Moist
static energy is given by
where Cp is the heat capacity at constant pressure, T is
temperature in Kelvin, Lv is latent heat of vaporization, q is
specific humidity, g is gravity, and Z is height. Because Z
does not change with time,
We correlated average MSE of surface air in the
region 30N–36N; 75E–90E, hereafter “Tibet MSE” with
gridded daily rainfall of [Rajeevan et al., 2006] over India
during early, middle, and late periods of the monsoon season
(Figure 2). Correlations between rainfall and MSE were
made for the same period. Rainfall in the early period
(20 May to 15 Jun), exhibits significant positive correlations
exceeding 0.4 over central India and its west coast
(Figure 2a). The correlation weakens markedly to become
insignificant during the peak monsoon season (15 June to
31 August, Figure 2b) before reappearing quite strongly
during the late monsoon period (1 September to 15 October,
Figure 2c). This raises the interesting suggestion that the
heating over the Tibetan Plateau correlates with the Indian
monsoon only during the early and late periods of the
monsoon season
5. Conclusions
[25] Recently, Boos and Kuang [2010] argued that the
principal role that Tibet plays in affecting the Indian monsoon
is to block cool dry air from northwest of Tibet. Although they
did not deny a warming over Tibet a role in the monsoon, their
work implies that such warming has a modest influence. We
found an insignificant correlation between Tibetan heating,
as quantified by moist static energy of surface air, and rainfall
in the main monsoon period (15 June to 31 August)
(Figure 2b). We show, however, that heating over the Tibetan
plateau does correlate with, and therefore may modulate Indian
monsoon rainfall in the early (20 May to 15 June) and late
(1 September to October 15) monsoon seasons, which together
contribute about a third of the seasonal rainfall. Simple regressions
suggest that variations in heating over Tibet might
account for as much as 20% of seasonal total rainfall.
Furthermore, we demonstrate that heating over Tibet is largely
independent of ENSO, so that together they can explain a
substantial portion of variability in the early and late season
rainfall, and therefore provide potential predictability at crucial
times of crop management—sowing and harvesting, respectively.
These links between heating over Tibet and ENSO with
rainfall over India may explain the wet conditions over India
during early Holocene time and provide a quantitative link
between a rise of Tibet and a stronger Somali Jet. They might
also provide some insights into mechanisms that could play a
role in the monsoon variability under a warmer climate that
the current crop of dynamical models need to capture better.
The Holocene Epoch began 12,000 to 11,500 years ago at the close of the Paleolithic Ice Age and continues through today.
MY conclusion:
- The Himalayas and the Tibetan Plateau have a profound influence on the monsoon. It is now believed that the Asian monsoon circulation basically results from the differential heating between the Eurasian land mass and adjacent oceans and is influenced by the thermal and dynamic effects of the Tibetan Plateau.
- Tibet Plateau plays a crucial role in initiating the Monsoon circulation over the Indian subcontinent. The summer time heating of the Plateau of Tibet was the most important factor in the causation and maintenance of the Monsoonal circulation. The Plateau of Tibet affects the atmosphere in two ways
- Acting as a physical barrier and
- Acting as a high level heat source.
In the mid October, when the Plateau becomes very cold, proves to be the most important factor in causing the advance of the westerly jet south of the Himalayas by bifurcating it into two parts.
The summer time heating of the Tibetan Plateau makes it a high level heat source. This heat engine produces a thermal anticyclone over this region during summer in the middle part of troposphere. Winds coming out of this anticyclone in a clockwise movement over plateau give birth to easterly jet streams which flow from East to West on the southern side of this upper air anticyclone.
These upper air easterlies descend into the permanent high pressure area formed over the southern Indian Ocean and intensifies the high pressure area already present there. It is from this high pressure cell that the onshore winds start blowing towards the thermally induced low pressure area, developed in the northern part of the Indian subcontinent. After crossing the equator these surface winds change direction and become South-West monsoon.
Chandan Jhinkwan
University Of hyderabad
msc earth and ocean science(Meterological field)
