Chapter 5: Monsoon Flashcards
The term monsoon means
seasonal reversal of winds
The term monsoon means seasonal reversal of winds, which originated from
the Arabic word, mausem or mawsim meaning season.
Following are the four possible definitions of the monsoon
- The prevailing wind direction changes by at least 1200 between January & July.
- The average frequency of prevailing wind in January & July should exceed by 40%.
- The mean resultant winds in at least one of the months should exceed 3m/sec.
- Fewer than one cyclone anticyclone alteration should occur every two years in either month in 5° Latitude-longitude grids
Following are the four possible definitions of the monsoon ………………………… satisfy these four criteria
The countries lying between 30N°, 25S°, 30E°, 170E°
Halley (1686) & Hadley (1735) suggested that
differential heating was the primary cause of annual cycle of monsoon circulation.
Thus we can precisely say that main driving mechanisms of the monsoon are
- The differential heating of the land and Sea.
- Swirl introduced to the winds by the rotation of the earth.
From the reversal of winds’ hypothesis we can say that generally there are two monsoon circulations
- Summer monsoon
- Winter monsoon
Winter monsoon (Fig-1a) is characterized by the four following
quasi-permanent features
- The Siberian anti-cyclone/High pressure area.
- Trough at equator in the Indonesia/Australian region.
- High pressure-cell over west Pacific and strong westerly sub-tropical Jet (STJ) aloft and
- A zone of heavy precipitation along Malaysia and Indonesia near the equator
On the other hand the following semi-permanent features can characterize summer or
southwest monsoon
- The high-pressure area over the Indian Ocean near 30deg South and 50 deg East.
- Heat low over landmass of Pakistan and Indian sub-continent with its elongated southeastward trough.
- The Tibetan anti-cyclone at 200 hpa.
- The Tropical easterly Jet (TEJ) and
- Northward shifting (about north of 30 deg North) of westerly sub-tropical jet (STJ)
- The Quasi-biennial Oscillation (QBO), being major feature of equatorial stratosphere.
- A Low Level Jet stream (LLJ)
- ITCZ (Inter Tropical Convergence Zone)
- Southern Oscillation Index – SOI (difference of sea level pressure of Tahiti and Darwin.)
- ENSO (El Nino/Southern Oscillation)
Heat Low
With the northward march of sun across the equator in northern hemisphere, the continents surrounding the Arabian sea start receiving tremendous amount of heat- not only in form of sun radiation but also as heat emitted out from the earths surface.
the heat flux from the earth’s surface in the atmosphere is equivalent of
160 watts/m2 for the month of June over arid zones of Pakistan, NW-India, Saudi Arabia and Middle East countries.
stimates that the heat flux from the earth’s surface in the atmosphere is equivalent of 160 watts/m2 for the month of June over arid zones of Pakistan, NW-India, Saudi Arabia and Middle East countries. This is much larger than corresponding value of
15 watts/m2 for month of December
As a consequence of this large input of power, a
trough of low-pressure forms extending from Somalia northwards across Arabia into Pakistan and NW- India.
Towards the ………………… over this region heat low is well established.
end of May/early June
Towards the end of May/early June over this region heat low is well established. It persists over there for
the whole season from June towards the mid of September and a strong South westerly wind spreads over Arabian Sea, Bay of Bengal and Suburbs.
In Pakistan we observe that Sun starts scorching down the southern areas of Pakistan by the end of
March or early April
April and May are the months during which soaring heat absorbed by the southern Pakistan (Sindh and south Balochistan) raises the day temperatures as high as up to plus
40s
The peculiar geographical feature of Asian continental landmass gives rise to
extreme thermal contrast between land in the north & Ocean in the south in both summer and winter, which is very crucial factor in most pronounced circulation in this part of the globe
This heat low is
shallow generally extending up to 850hpa. At 700hpa it is over-lain by well-marked ridge of the sub-tropics
surface pressure inside the heat low is
inversely proportional to the rainfall for the areas in belt 180N to 270N
High-Pressure area over Indian Ocean (IOHP):-
Southern hemispheric circulation of the monsoon region is dominated by anti- cyclonic circulation around a high-pressure region off the coast of Madagascar.
Southern hemispheric circulation of the monsoon region is dominated by anti- cyclonic circulation around a high-pressure region off the coast of Madagascar. More often a high-pressure area is located in
the south Indian Ocean roughly along 500E and 300S during the summer monsoon
More often a high-pressure area is located in the south Indian Ocean roughly along 500E and 300S during the summer monsoon (as shown in Fig-3). This is also known as
Mascarene high
This is also known as Mascarene high and it has got a quite significant role in the
cross- equatorial flow during northern summer.
it has got a quite significant role in the cross- equatorial flow during northern summer. It gives way to
low-level Jet (LLJ) with speed between 40 and 100kts, which is well marked off the African coast.
Tibetan High at
200hpa
Tibetan High at (200hpa):-
During July a ridge at 200hpa at 28°N and east of 80°E is often known as Tibetan High.
During July a ridge at 200hpa at 28°N and east of 80°E is often known as Tibetan High. Various studies show that its center is at about
980E
During July a ridge at 200hpa at 28°N and east of 80°E is often known as Tibetan High. Various studies show that its center is at about 980E and is distinct from the
Pacific high at 140°E
mean position of ridge line over (tiberan high)
India and south Indian Ocean at 200hpa
Tibetan high is said to be caused by
widespread thunderstorms over southeastern Tibet in pre- monsoon months of April & May releasing, thereby considerable amount of latent heat into atmosphere through rainfall.
…………………….. result in the formation of this high. (tibetan high)
Basically the intense heating of Plateau and the radiational balance
The westerly Sub-tropical Jet (STJ) is found in
the north of this high (tibetan high)
Tropical Easterly Jet (TEJ) in
south of tibetan high
tibetan high importance
steering force for the monsoon systems
Tropical Easterly Jet (TEJ)
South of the sub-tropical ridge over Asia, the easterly flow concentrates into a jet stream with its central region passing roughly along 13.5N° at 100 hpa over southern India and Gulf of Aden.
TEJ runs from
east coast of Vietnam to the west coast of Africa where its location is around 10°N
TEJ being in normal position with
above average wind speed in the jet core region leads to a good monsoon
A careful study of the easterly jet suggests that its core is located at
higher altitude than the core of the westerly Jet in extra-tropical altitudes as such the level of maximum wind in the easterly jet is about 13km while the core of westerly jet is about 9km.
A careful study of the easterly jet suggests that its core is located at higher altitude than the core of the westerly Jet in extra-tropical altitudes as such the level of maximum wind in the easterly jet is about 13km while the core of westerly jet is about 9km. Cyclonic sheer is found
south of the jet
TEJ
Cyclonic sheer is found south of the jet and anti-cyclonic sheer is found
to the north of jet
Cyclonic sheer is found south of the jet and anti-cyclonic sheer is found to the north of jet. ……………………………… are divergent
TEJ
Usually east of 70E° the easterlies in jet
Cyclonic sheer is found south of the jet and anti-cyclonic sheer is found to the north of jet. Usually east of 70E° the easterlies in jet are divergent, and …………………………… producing convergence
TEJ
down- stream west of 70°E they tend to decelerate
Cyclonic sheer is found south of the jet and anti-cyclonic sheer is found to the north of jet. Usually east of 70E° the easterlies in jet are divergent, and down- stream west of 70°E they tend to decelerate producing convergence. This vergence distribution at upper level is associated with
development of weather systems in the lower tropospheric levels in these areas.
Sub-Tropical (Westerly) Jet - STJ
Another synoptic feature of onset & good monsoon is the weakening of the upper tropospheric westerly jet over northern Pakistan & adjoining India and the ultimate shift of sub tropical westerly Jet (STJ) to the north of Himalayas.
The northward shifting of STJ gives way to the more
penetration & advancement of monsoon towards higher latitudes.
The Quasi-biennial Oscillation (QBO)
QBO – Quasi biennial oscillation is a major factor of the equatorial Stratosphere.
QBO – Quasi biennial oscillation is a major factor of the equatorial Stratosphere. At levels between
60hpa & 10hpa, alternative spells of easterly & westerly winds are observed with periods of two years or slightly above.
…………………………….. association with QBO.
The analysis of the power spectrum of monsoon rainfall has revealed that rainfall has got some
Low Level Jet (LLJ)
Besides the TEJ there is yet another Jet flowing intermittently over Somali coast, Arabian Sea & peninsular India. This is named as low-level Jet–LLJ
LLJ is taken to represent a
narrow current in the monsoon field in the lower levels of atmosphere with wind speeds above 40-60kts.
brought out the existence of LLJ over
Indian peninsula.
The core of the LLJ is roughly
250-350km wide, 600-900km long & one km deep
Flowing over………………….LLJ causes ………………
Arabian Sea & Peninsula India
convergence ahead
Flowing over Arabian Sea & Peninsula India LLJ causes convergence ahead. Further there world be a generation of
positive relative vorticity in the northern side of the Jet core & negative vorticity in the southern side.
Further there world be a generation of positive relative vorticity in the northern side of the Jet core & negative vorticity in the southern side. Thus the northern sector ahead of an approaching LLJ core could be a
good favoured zone for good rainfall
Okola & Asnani (1982) having carried out the analysis detected relation between monsoon rainfall over India &
intensity of prominent anticyclone southeast of Madagascar in Indian Ocean called Mascarene High. Hence most probably LLJ is triggered by the outflow from this anti-cyclone.
ITCZ-Inter Tropical Convergence Zone
It is the narrow zone where air masses, from northern and southern hemispheres moving towards the equator, converge producing shear cyclonic vorticity.
Inter Tropical Discontinuity
Over western Africa the ITCZ is often recalled as “Inter Tropical Discontinuity” (ITD)
Over western Africa the ITCZ is often recalled as “Inter Tropical Discontinuity” (ITD) because the weather that is associated with
the ITCZ over east Africa is different from what is observed over western coast of Africa.
The changing position ITCZ is singularly responsible for the
seasonal variations of rain over east Africa.
During the northern hemispheric summer, the ITCZ moves to
the north of Equator
During the northern hemispheric summer, the ITCZ moves to the north of Equator. Probably it merges with an
extended trough in the lower troposphere but there is another opinion as well that it looses its identity.
The ITCZ migrates seasonally as for as
15°N or more of equator in some places
The ITCZ migrates seasonally as for as 15°N or more of equator in some places. Its fluctuation is maximum over
the Indian region during monsoon season
According to them there are two cloud zones between
70°E and 90°E one is the continental in the monsoon zone north of 15°N and the other being the Oceanic over the equatorial regions.
According to them there are two cloud zones between 70°E and 90°E one is the continental in the monsoon zone north of 15°N and the other being the Oceanic over the equatorial regions. Active spell of one is generally associated with
the weak spells of the other and vice-versa.
According to Gadgil, during the onset phase of the Southwest monsoon ITCZ shifts northwards leading to the
establishment of the monsoon zone.
According to Gadgil, during the onset phase of the Southwest monsoon ITCZ shifts northwards leading to the establishment of the monsoon zone. It fluctuates with
monsoon zone for a period of about four weeks.
Southern Oscillation Index (SOI)
Yet another equally important phenomenon having notable effect on summer (southwest) monsoon is Southern Oscillation.
The intensity of (SOI) is represented by the
the pressure difference between Tahiti in French Polynesia and Darwin in northern Australia.
Discovered by Sir Gilbert walker Southern Oscillation postulates
a See-Saw weather pattern between Pacific Ocean and Indian Ocean extending from Africa to Australia
“a See-Saw weather pattern between Pacific Ocean and Indian Ocean extending from Africa to Australia”. He discovered that when pressure tended to be high over
Pacific Ocean it tended to be low over Indian Ocean.
SOI
With pressures being …………..related to rainfall
inversely
With pressures being inversely related to rainfall, the prevailing low pressures over Indian Ocean in the winter months is an indication of
approach of good monsoon in summer.
SOI has a period of varying from
3 to 7 years and consequently pressure departures and their trends are considered to be better predictors than absolute values of pressure.
The El Nino event in the preceding winter is suggestive of a
strong Walker circulation and weak monsoon.
EL Nino/Southern Oscillation (ENSO)
Ocean currents, being significant for Navigation, also have an important effect on climate. In the eastern Pacific a strong cold Peruvian current flows equatorward along the coast of Peru and Ecuador. This causes the upwelling and as a result a cold current starts flowing westward. During this state a high pressure prevails in eastern Pacific near Peru coast and low pressure in western Pacific over north Australia.
Ocean currents, being significant for Navigation, also have an important effect on climate. In the eastern Pacific a strong cold Peruvian current flows equatorward along the coast of Peru and Ecuador. This causes the upwelling and as a result a cold current starts flowing westward. During this state a high pressure prevails in eastern Pacific near Peru coast and low pressure in western Pacific over north Australia. But
near the end of each year a weak warm counter current flows southward along coast of Peru and Ecuador, replacing the cold Peruvian current. This is known as El Nino
Major El Nino events, like the one in 1982 and 1983, are intimately related to the
large-scale atmospheric circulation. With the occurrence of El Nino the barometric pressure drops over large portion of the southern Pacific, whereas in western Pacific near Indonesia and north Australia the pressure rises.
With the occurrence of El Nino the barometric pressure drops over large portion of the southern Pacific, whereas in western Pacific near Indonesia and north Australia the pressure rises.
This reverse trend of pressure distribution in Pacific weakens the Southern Oscillation and hence the cross equatorial flow is dampened which results in weak monsoon.
But this is not the case always as the study advanced by PK.DAS explains that by examining the history of Indian rainfall from 1875 to 1985 shows about
43 years of deficient monsoon rain but only 19 of the 43 deficient years were El Nino years.
But this is not the case always as the study advanced by PK.DAS explains that by examining the history of Indian rainfall from 1875 to 1985 shows about 43 years of deficient monsoon rain but only 19 of the 43 deficient years were El Nino years. This indicates that
there are other factors as well, than EL Nino, which cause a deficiency in monsoon rain.
This indicates that there are other factors as well, than EL Nino, which cause a deficiency in monsoon rain. In the period from 1875 to 1985, there were only 6 so years that El Nino happened but
rainfall wasn’t deficient
which cause a deficiency in monsoon rain. In the period from 1875 to 1985, there were only 6 so years that El Nino happened but rainfall wasn’t deficient. It hence proves that
EL Nino has got same association with poor or indifferent monsoon.
With the end of El Nino event
the pressure difference between these two regions swings back in opposite direction. i.e. low pressure over northern Australia in west Pacific and high in eastern Pacific (fig-8b). This See-Saw pattern of atmospheric pressure between eastern and western Pacific is called Southern Oscillation. It is an inseparable part of the El Nino warming that occurs over central and eastern Pacific every 3 to 7 years. Therefore this phenomenon is often called El Nino/Southern Oscillation or ENSO
In summary the effects of El Nino on world climate are
widespread and variable, as is said “There is no place on earth where weather is indifferent to air and Ocean currents in the tropical Pacific
Southern Oscillation and El Nino events associated with it are now believed to have a significant influence on
he state of weather and climate almost everywhere
Walker cell
feed back from Oceans
Dr.J.Bjerknes from Norway, explaining the Walker cell as a feed back from Oceans, suggests that main drive for the Walker cell is the
difference in temperatures of the Sea Surface (SST) between Indonesia (warm) and eastern Pacific Ocean (cold).
Walker cell is the difference in temperatures of the Sea Surface (SST) between Indonesia (warm) and eastern Pacific Ocean (cold). Cold costal waters off the Peru and Ecuador coasts are generated by
the upwelling i.e. replacement of surface waters by colder water from greater depths. The warm SSTs at Indonesian sector induce the greater cloudiness which, in turn, cuts off the input from solar radiation, thereby reducing the drive behind Walker circulation
SSTs at Indonesian sector induce the greater cloudiness which, in turn, cuts off the input from solar radiation, thereby reducing the drive behind Walker circulation (P.K.DAS). This weakening of the drive behind the Walker circulation causes the
rise in SSTs at the eastern Pacific because of the lesser upwelling and resultantly the difference in western and eastern Pacific SST’s gets narrowed.
Summer and winter monsoon of Asia being principal monsoon of the world appear to be dominated by the
circulations that are either aligned in a north–south or east- west direction.
Summer and winter monsoon of Asia being principal monsoon of the world appear to be dominated by the circulations that are either aligned in a north–south or east- west direction. The rising branch of each circulation is located near
source of heat while descending limb occurs over a heat sink. These are known as Hadley and Walker cells.
For the Asian summer/Southwest Monsoon Tibetan Plateau acts as an
elevated heat source. The ascending air above the source gradually spreads southwards to join a descending limb over north India Ocean near the Mascarene high.
For the Asian summer/Southwest Monsoon Tibetan Plateau acts as an elevated heat source. The ascending air above the source gradually spreads southwards to join a descending limb over north India Ocean near the Mascarene high. The southwesterly winds at the surface form the
return current to complete the Hadley cells
For the Asian summer/Southwest Monsoon Tibetan Plateau acts as an elevated heat source. The ascending air above the source gradually spreads southwards to join a descending limb over north India Ocean near the Mascarene high. The southwesterly winds at the surface form the return current to complete the Hadley cells. In the addition there is an
east-west Walker cell appears to influence the summer monsoon.
In the addition there is an east-west Walker cell appears to influence the summer monsoon. The ascending branch of Walker cell is located over ………………. the decending branch ………….
Indonesia where heat source is generated due to convection and heavy precipitation. The descending limb of the Walker cell is located over the semi-arid region of Pakistan, Northwest India and Middle East.
The performance of Asian summer monsoon is often determined by
the relative importance of Hadley and Walker cells.
The performance of Asian summer monsoon is often determined by the relative importance of Hadley and Walker cells. Good monsoon appear to be associated with
the more intense Hadley cell and relatively weak Walker cell,
poor monsoon occur when
Walker cell is strong and Hadley cell is weak
