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1

The term monsoon means 

seasonal reversal of winds 

2

The term monsoon means seasonal reversal of winds, which originated from 

the Arabic word, mausem or mawsim meaning season. 

3

Following are the four possible definitions of the monsoon 

  1. The prevailing wind direction changes by at least 1200 between January & July.
  2. The average frequency of prevailing wind in January & July should exceed by 40%.
  3. The mean resultant winds in at least one of the months should exceed 3m/sec.
  4. Fewer than one cyclone anticyclone alteration should occur every two years in either month in 5° Latitude-longitude grids 

4

Following are the four possible definitions of the monsoon  .............................. satisfy these four criteria

The countries lying between 30N°, 25S°, 30E°, 170E° 

5

Halley (1686) & Hadley (1735) suggested that  

differential heating was the primary cause of annual cycle of monsoon circulation. 

6

Thus we can precisely say that main driving mechanisms of the monsoon are 

  1. The differential heating of the land and Sea.
  2. Swirl introduced to the winds by the rotation of the earth. 

7

From the reversal of winds’ hypothesis we can say that generally there are two monsoon circulations 

  1. Summer monsoon
  2. Winter monsoon 

8

Winter monsoon (Fig-1a) is characterized by the four following 

quasi-permanent features

  1. The Siberian anti-cyclone/High pressure area.
  2. Trough at equator in the Indonesia/Australian region.
  3. High pressure-cell over west Pacific and strong westerly sub-tropical Jet (STJ) aloft and
  4. A zone of heavy precipitation along Malaysia and Indonesia near the equator 

9

On the other hand the following semi-permanent features can characterize summer or 

southwest monsoon  

  1. The high-pressure area over the Indian Ocean near 30deg South and 50 deg East.
  2. Heat low over landmass of Pakistan and Indian sub-continent with its elongated southeastward trough.
  3. The Tibetan anti-cyclone at 200 hpa.
  4. The Tropical easterly Jet (TEJ) and
  5. Northward shifting (about north of 30 deg North) of westerly sub-tropical jet (STJ)
  6. The Quasi-biennial Oscillation (QBO), being major feature of equatorial stratosphere.
  7. A Low Level Jet stream (LLJ)
  8. ITCZ (Inter Tropical Convergence Zone)
  9. Southern Oscillation Index – SOI (difference of sea level pressure of Tahiti and Darwin.)
  10. ENSO (El Nino/Southern Oscillation) 

10

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. 

11

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.  

12

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 

13

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.  

14

Towards the ..................... over this region heat low is well established. 

end of May/early June

15

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. 

16

In Pakistan we observe that Sun starts scorching down the southern areas of Pakistan by the end of 

March or early April 

17

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 

18

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 

19

This heat low is 

shallow generally extending up to 850hpa. At 700hpa it is over-lain by well-marked ridge of the sub-tropics 

20

surface pressure inside the heat low is 

inversely proportional to the rainfall for the areas in belt 180N to 270N  

21

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. 

22

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  

23

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 

24

This is also known as Mascarene high and it has got a quite significant role in the 

cross- equatorial flow during northern summer. 

25

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. 

26

Tibetan High at 

200hpa 

27

Tibetan High at (200hpa):- 

During July a ridge at 200hpa at 28°N and east of 80°E is often known as Tibetan High.  

28

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 

29

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 

30

mean position of ridge line over (tiberan high)

India and south Indian Ocean at 200hpa