A monsoon is traditionally a seasonal reversing
wind accompanied by corresponding changes in precipitation but is now used to describe
seasonal changes in atmospheric circulation and precipitation associated with
annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ)
between its limits to the north and south of the equator. Usually, the term
monsoon is used to refer to the rainy phase of a seasonally changing pattern,
although technically there is also a dry phase. The term is also sometimes used
to describe locally heavy but short-term rains.
The major monsoon systems of the world consist of the
West African, Asian—Australian, the North American, and South American
monsoons.
The term was first used in English in British India
and neighbouring countries to refer to the big seasonal winds blowing from the
Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the
area.
History :
Asian monsoon –
Strengthening of the Asian monsoon has been linked to
the uplift of the Tibetan Plateau after the collision of the Indian
subcontinent and Asia around 50 million years ago. Because of studies of
records from the Arabian Sea and that of the wind-blown dust in the Loess
Plateau of China, many geologists believe the monsoon first became strong
around 8 million years ago. More recently, studies of plant fossils in China
and new long-duration sediment records from the South China Sea led to a timing
of the monsoon beginning 15—20 million years ago and linked to early Tibetan
uplift. Testing of this hypothesis awaits deep ocean sampling by the Integrated
Ocean Drilling Program. The monsoon has varied significantly in strength since
this time, largely linked to global climate change, especially the cycle of the
Pleistocene ice ages. A study of Asian monsoonal climate cycles from 123,200 to
121 ,210 years BP, during the Eemian interglacial, suggests that they had an
average duration of around 64 years, with the minimum duration being around 50
years and the maximum approximately 80 years, similar to today.
A study of marine plankton suggested that the South
Asian Monsoon (SAM) strengthened around 5 million years ago. Then, during ice
periods, the sea level fell and the Indonesian Seaway closed. When this
happened, cold waters in the Pacific were impeded from flowing into the Indian
Ocean. It is believed that the resulting increase in sea surface temperatures
in the Indian Ocean increased the intensity of monsoons. In 2018, a study of
the SAM's variability over the past million years found that precipitation
resulting from the monsoon was significantly reduced during glacial periods
compared to interglacial periods like the present day. The Indian Summer
Monsoon (ISM) underwent several intensifications during the warming following
the Last Glacial Maximum, specifically during the time intervals corresponding
to 16,100—14,600 BP, 13,600—13,000 BP, and 12,400—10,400 BP as indicated by
vegetation changes in the Tibetan Plateau displaying increases in humidity
brought by an intensifying ISM. Though the ISM was relatively weak for much of
the Late Holocene, significant glacial accumulation in the Himalayas still
occurred due to cold temperatures brought by westerlies from the west.
During the Middle Miocene, the July ITCZ, the zone of
rainfall maximum, migrated northwards, increasing precipitation over southern
China during the East Asian Summer Monsoon (EASM) while making Indochina drier.
During the Late Miocene Global Cooling (LMCG), from 7.9 to 5.8 million years
ago, the East Asian Winter Monsoon (EAWM) became stronger as the subarctic
front shifted southwards. An abrupt intensification of the EAWM occurred 5.5
million years ago. The EAWM was still significantly weaker relative to today
between 4.3 and 3.8 million years ago but abruptly became more intense around 3.8
million years ago as crustal stretching widened the Tsushima Strait and enabled
greater inflow of the warm Tsushima Current into the Sea of Japan. Circa 3.0
million years ago, the EAWM became more stable, having previously been more variable
and inconsistent, in addition to being enhanced further amidst a period of
global cooling and sea level fall. The EASM was weaker during cold intervals of
glacial periods such as the Last Glacial Maximum (LGM) and stronger during interglacials
and warm intervals of glacial periods. Another EAWM intensification event
occurred 2.6 million years ago, followed by yet another one around 1.0 million
years ago. During Dansgaard—Oeschger events, the EASM grew in strength, but it
has been suggested to have decreased in strength during Heinrich events. The
EASM expanded its influence deeper into the interior of Asia as sea levels rose
following the LGM; it also underwent a period of intensification during the
Middle Holocene, around 6,000 years ago, due to orbital forcing made more
intense by the fact that the Sahara at the time was much more vegetated and
emitted less dust. This Middle Holocene interval of maximum EASM was associated
with an expansion of temperate deciduous forest steppe and temperate mixed
forest steppe in northern China. By around 5,000 to 4,500 BP, the East Asian
monsoon's strength began to wane, weakening from that point until the present
day. A particularly notable weakening took place —3,000 BP. The location of the
EASM shifted multiple times over the course of the Holocene: first, it moved
southward between 12,000 and 8,000 BP, followed by an expansion to the north
between approximately 8,000 and 4,000 BP, and most recently retreated southward
once more between 4,000 and 0 BP.
Australian monsoon –
The January ITCZ migrated further south to its present
location during the Middle Miocene, strengthening the summer monsoon of
Australia that had previously been weaker.
Five episodes during the Quaternary at 2.22 Ma (PL-1),
1.83 Ma (PL-2), 0.68 Ma (PL-3), 0.45 Ma (PL-4) and 0.04 Ma (PL-5) were
identified which showed a weakening of the Leeuwin Current (LC). The weakening
of the LC would have an effect on the sea surface temperature (SST) field in
the Indian Ocean, as the Indonesian Throughflow generally warms the Indian
Ocean. Thus these five intervals could probably be those of considerable
lowering of SST in the Indian Ocean and would have influenced Indian monsoon
intensity. During the weak LC, there is the possibility of reduced intensity of
the Indian winter monsoon and strong summer monsoon, because of change in the
Indian Ocean dipole due to reduction in net heat input to the Indian Ocean
through the Indonesian Throughflow. Thus a better understanding of the possible
links between El Nino, Western Pacific Warm Pool, Indonesian Throughflow, wind
pattern off western Australia, and ice volume expansion and contraction can be
obtained by studying the behaviour of the LC during Quaternary at close
stratigraphic intervals.
South American monsoon –
The South American summer monsoon (SASM) is known to
have become weakened during Dansgaard—Oeschger events. The SASM has been
suggested to have been enhanced during Heinrich events.
Process –
Monsoons were once considered as a large-scale sea
breeze caused by higher temperature over land than in the ocean. This is no
longer considered as the cause and the monsoon is now considered a
planetary-scale phenomenon involving the annual migration of the Intertropical
Convergence Zone between its northern and southern limits. The limits of the
ITCZ vary according to the land—sea heating contrast and it is thought that the
northern extent of the monsoon in South Asia is influenced by the high Tibetan Plateau.
These temperature imbalances happen because oceans and land absorb heat in
different ways. Over oceans, the air temperature remains relatively stable for
two reasons: water has a relatively high heat capacity (3.9 to 4.2 Jg-1
K-1), and because both conduction and convection will equilibrate a
hot or cold surface with deeper water (up to 50 metres). In contrast, dirt,
sand, and rocks have lower heat capacities (0.19 to 0.35 jg-1 K-1 ), and they can only
transmit heat into the earth by conduction and not by convection. Therefore,
bodies of water stay at a more even temperature, while land temperatures are
more variable.
During warmer months sunlight heats the surfaces of
both land and oceans, but land temperatures rise more quickly. As the land's
surface becomes warmer, the air above it expands and an area of low pressure
develops. Meanwhile, the ocean remains at a lower temperature than the land,
and the air above it retains a higher pressure. This difference in pressure
causes sea breezes to blow from the ocean to the land, bringing moist air
inland. This moist air rises to a higher altitude over land and then it flows back
toward the ocean (thus completing the cycle). However, when the air rises, and
while it is still over the land, the air cools. This decreases the air's
ability to hold water, and this causes precipitation over the land. This is why
summer monsoons cause so much rain over land.
In the colder months, the cycle is reversed. Then the
land cools faster than the oceans and the air over the land has higher pressure
than air over the ocean. This causes the air over the land to flow to the
ocean. When humid air rises over the ocean, it cools, and this causes
precipitation over the oceans. (The cool air then flows towards the land to
complete the cycle.)
Most summer monsoons have a dominant westerly
component and a strong tendency to ascend and produce copious amounts of rain
(because of the condensation of water vapor in the rising air). The intensity
and duration, however, are not uniform from year to year. Winter monsoons, by
contrast, have a dominant easterly component and a strong tendency to diverge,
subside and cause drought.
Similar rainfall is caused when moist ocean air is lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, and this produces condensation.
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