Monsoon (/mɒnˈsuːn/) is traditionally defined as 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 the asymmetric heating of land and sea. 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 sometimes incorrectly used for locally heavy but
short-term rains, although these rains meet the dictionary definition of monsoon.
The major monsoon systems of the world consist of the West African and Asia-Australian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal has been debated.
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.
Etymology
Monsoon clouds over Lucknow, Uttar Pradesh, India
The etymology of the word monsoon is not wholly certain. The English monsoon came from Portuguese monção, ultimately from Arabic mawsim (موسم "season"), "perhaps partly via early modern Dutch monson."
History
Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau after the collision of the Indian sub-continent 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 marine plankton suggested that the Indian Monsoon
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.
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 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 through flow 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
through flow. Thus a better understanding of the possible links between
El Niño, 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.
Strength of impact
On May 28, in the dry season
On August 28, in the rainy season,Western Ghats in 2010
The impact of monsoon on the local weather is different from place to
place. In some places there is just a likelihood of having a little
more or less rain. In other places, quasi semi-deserts are turned into
vivid green grasslands where all sorts of plants and crops can flourish.
The Indian Monsoon turns large parts of India from a kind of
semi-desert into green lands. See photos only taken 3 months apart in
the Western Ghats. In places like this it is crucial for farmers to have
the right timing for putting the seeds on the fields, as it is
essential to use all the rain that is available for growing crops.
Process
Monsoons are large-scale sea breezes
which occur when the temperature on land is significantly warmer or
cooler than the temperature of the ocean. 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 J g−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 J g−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 temperature 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.
Global monsoon
Africa (West African and Southeast African)
Southeast African moonsoon clouds, over Mayotte island.
The monsoon of western Sub-Saharan Africa is the result of the seasonal shifts of the Intertropical Convergence Zone and the great seasonal temperature and humidity differences between the Sahara and the equatorial Atlantic Ocean.
It migrates northward from the equatorial Atlantic in February,
reaches western Africa on or near June 22, then moves back to the south
by October. The dry, northeasterly trade winds, and their more extreme form, the harmattan, are interrupted by the northern shift in the ITCZ and resultant southerly, rain-bearing winds during the summer. The semiarid Sahel and Sudan depend upon this pattern for most of their precipitation.
North America
Incoming monsoon clouds over Phoenix, Arizona
The North American monsoon (NAM) occurs from late June
or early July into September, originating over Mexico and spreading into
the southwest United States by mid-July. It affects Mexico along the Sierra Madre Occidental as well as Arizona, New Mexico, Nevada, Utah, Colorado, West Texas and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of Southern California,
but rarely reaches the coastal strip (a wall of desert thunderstorms
only a half-hour's drive away is a common summer sight from the sunny
skies along the coast during the monsoon). The North American monsoon is
known to many as the Summer, Southwest, Mexican or Arizona monsoon. It is also sometimes called the Desert monsoon as a large part of the affected area are the Mojave and Sonoran deserts. However, it is debatable whether the North and South American weather patterns with incomplete wind reversal should be counted as true monsoons.
Asia
The Asian monsoons may be classified into a few sub-systems, such as
the Indian Subcontinental Monsoon which affects the Indian subcontinent
and surrounding regions including Nepal, and the East Asian Monsoon
which affects southern China, Taiwan, Korea and parts of Japan.
South Asian monsoon
Southwest monsoon
Onset dates and prevailing wind currents of the southwest summer monsoons in India
The southwestern summer monsoons occur from July through September. The Thar Desert
and adjoining areas of the northern and central Indian subcontinent
heat up considerably during the hot summers. This causes a low pressure
area over the northern and central Indian subcontinent. To fill this
void, the moisture-laden winds from the Indian Ocean rush into the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas. The Himalayas act like a high wall, blocking the winds from passing into Central Asia, and forcing them to rise. As the clouds rise, their temperature drops, and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm (390 in) of rain annually.
The southwest monsoon is generally expected to begin around the
beginning of June and fade away by the end of September. The
moisture-laden winds on reaching the southernmost point of the Indian Peninsula, due to its topography, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch.
The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, India,
thus making this area the first state in India to receive rain from the
Southwest Monsoon. This branch of the monsoon moves northwards along
the Western Ghats (Konkan and Goa)
with precipitation on coastal areas, west of the Western Ghats. The
eastern areas of the Western Ghats do not receive much rain from this
monsoon as the wind does not cross the Western Ghats.
The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards North-East India and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram, situated on the southern slopes of the Khasi Hills in Meghalaya, India, is one of the wettest places on Earth. After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state,
pouring rain all along its way. June 1 is regarded as the date of onset
of the monsoon in India, as indicated by the arrival of the monsoon in
the southernmost state of Kerala.
The monsoon accounts for nearly 80% of the rainfall in India.
Indian agriculture (which accounts for 25% of the GDP and employs 70%
of the population) is heavily dependent on the rains, for growing crops
especially like cotton, rice, oilseeds
and coarse grains. A delay of a few days in the arrival of the monsoon
can badly affect the economy, as evidenced in the numerous droughts in
India in the 1990s.
The monsoon is widely welcomed and appreciated by city-dwellers
as well, for it provides relief from the climax of summer heat in June. However, the roads take a battering every year. Often houses and streets are waterlogged and slums
are flooded despite drainage systems. A lack of city infrastructure
coupled with changing climate patterns causes severe economic loss
including damage to property and loss of lives, as evidenced in the 2005 flooding in Mumbai that brought the city to a standstill. Bangladesh and certain regions of India like Assam and West Bengal, also frequently experience heavy floods during this season. Recently, areas in India that used to receive scanty rainfall throughout the year, like the Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season.
The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang. It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains
falls during the three summer months, when the region is under the
monsoon influence; about 70% of that is directly of "cyclonic" (i.e.,
monsoon-driven) origin (as opposed to "local convection").
Northeast monsoon
Monsoon clouds in Kolkata
Around September, with the sun fast retreating south, the northern
land mass of the Indian subcontinent begins to cool off rapidly. With
this air pressure begins to build over northern India, the Indian Ocean
and its surrounding atmosphere still holds its heat. This causes cold
wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.
While travelling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. Cities like Chennai,
which get less rain from the Southwest Monsoon, receive rain from this
Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon. In Southern Asia, the northeastern monsoons take place from October to December when the surface high-pressure system is strongest. The jet stream
in this region splits into the southern subtropical jet and the polar
jet. The subtropical flow directs northeasterly winds to blow across
southern Asia, creating dry air streams which produce clear skies over India. Meanwhile, a low pressure system known as a monsoon trough develops over South-East Asia and Australasia and winds are directed toward Australia.
East Asian Monsoon
The East Asian monsoon affects large parts of Indo-China, Philippines, China, Taiwan, Korea and Japan.
It is characterised by a warm, rainy summer monsoon and a cold, dry
winter monsoon. The rain occurs in a concentrated belt that stretches
east-west except in East China where it is tilted east-northeast over
Korea and Japan. The seasonal rain is known as Meiyu in China, Jangma in Korea, and Bai-u in Japan, with the latter two resembling frontal rain.
The onset of the summer monsoon is marked by a period of
premonsoonal rain over South China and Taiwan in early May. From May
through August, the summer monsoon shifts through a series of dry and
rainy phases as the rain belt moves northward, beginning over Indochina and the South China Sea (May), to the Yangtze River Basin and Japan (June) and finally to North China and Korea (July). When the monsoon ends in August, the rain belt moves back to South China.
Australia
Monsoonal squall nears Darwin, Northern Territory, Australia
Also known as the Indo-Australian Monsoon. The rainy season
occurs from September to February and it is a major source of energy for
the Hadley circulation during boreal winter. The Maritime Continent Monsoon and the Australian Monsoon may be considered to be the same system, the Indo-Australian Monsoon.
It is associated with the development of the Siberian High and the movement of the heating maxima from the Northern Hemisphere to the Southern Hemisphere. North-easterly winds flow down Southeast Asia, are turned north-westerly/westerly by Borneo topography towards Australia.
This forms a cyclonic circulation vortex over Borneo, which together
with descending cold surges of winter air from higher latitudes, cause
significant weather phenomena in the region. Examples are the formation
of a rare low-latitude tropical storm in 2001, Tropical Storm Vamei, and the devastating flood of Jakarta in 2007.
The onset of the monsoon over the Maritime Continent tends to follow the heating maxima down Vietnam and the Malay Peninsula (September), to Sumatra, Borneo and the Philippines (October), to Java, Sulawesi (November), Irian Jaya and Northern Australia
(December, January). However, the monsoon is not a simple response to
heating but a more complex interaction of topography, wind and sea, as
demonstrated by its abrupt rather than gradual withdrawal from the
region. The Australian monsoon (the "Wet") occurs in the southern summer
when the monsoon trough develops over Northern Australia. Over
three-quarters of annual rainfall in Northern Australia falls during
this time.
Europe
The European Monsoon (more commonly known as the return of the westerlies) is the result of a resurgence of westerly winds from the Atlantic, where they become loaded with wind and rain.
These westerly winds are a common phenomenon during the European
winter, but they ease as spring approaches in late March and through
April and May. The winds pick up again in June, which is why this
phenomenon is also referred to as "the return of the westerlies".
The rain usually arrives in two waves, at the beginning of June
and again in mid- to late June. The European monsoon is not a monsoon in
the traditional sense in that it doesn't meet all the requirements to
be classified as such. Instead the return of the westerlies is more
regarded as a conveyor belt that delivers a series of low pressure
centres to Western Europe
where they create unsettled weather. These storms generally feature
significantly lower than average temperatures, fierce rain or hail,
thunder and strong winds.
The return of the westerlies affects Europe's Northern Atlantic coastline, more precisely Ireland, Great Britain, the Benelux countries, Western Germany, Northern France and parts of Scandinavia.
