1. REGIONAL MONSOON SYSTEMS

2. Outline Definition of Monsoon The Monsoon Makers Annual Monsoon Cycle
Variability of the monsoons
Regional Monsoon Systems
The Indian Monsoon
The East Asia Monsoon
The Australian Monsoon/ East Asia Winter Monsoon
The African Monsoon
The American Monsoons

3. 1) Indian Monsoon
The Indian monsoon is made up of a number of components:
The monsoon trough over northern India;
The Mascarene anti-cyclonic system;
The low level cross-equatorial jet;
The Tibetan high pressure system;
The tropical easterly jet;
Monsoon cloudiness
Rainfall

4. The Indian Monsoon: surface features
Monsoon trough:
Formed over northern India;
Northern Hemisphere Summer as part of the global ITCZ;
Associated with surface low pressure;
Mascarene high:
Situated over the south-east Indian Ocean (30o S, 50o E);
Generates a large outflow of air;
The air moves north over the equator where it becomes a south-westerly flow known as the low level cross-equatorial jet;

5. The Indian Monsoon
Fig. 2 from Krishnamurti and Bhalme (1976) Mean Sea Level Pressure for July

6. The Indian Monsoon and surface circulation
Fig 5 from Krishnamurti and Bhalme (1976)
Low level cross-equatorial jet:
Maximum intensity from June to August;
The jet splits in two branches at around 10o N, 60o E at this time;
Arrive over central west and southern coasts of India;
Important for rainfall amounts over western India.
Monthly progression of the low-level cross-equatorial jet

7. Other important characteristics of the Indian Monsoon:
The Tibetan high:
Upper level anticyclone;
Located over northern India above the surface monsoon trough; (low level convergence matched by upper level divergence)
Well established during summer;
Moves in a south-southeast direction following the zone of maximum surface heating and low pressure;
The outflow of air from the southern flank of the Tibetan high gives rise to the Tropical easterly jet.
Tropical easterly jet lasts from June to September

8. Northern Hemisphere
Upper level surface
High pressure
upper levels
Low pressure
upper levels
Upward vertical movement transports air molecules to upper levels.
More molecules imply in the increase of the weight of the column of the air above a given upper level surface
Low pressure
High pressure
Low level Divergence
Low level Convergence

9. Reasons for the easterly jet:
During summer, the Tibetan Plateau, which is located in subtropical regions, intensify the ascent increasing air pressure in high levels
Temperature Gradient
High level High Pressure + -
High level Easterly Jet
High Pressure
Ascending air
Hot surface
Low pressure system:

10. High level winds and the Indian Monsoon
Figs. 6 from Krishnamurti and Bhalme (1976) Monthly mean 200 hPa wind field
JAN
APRIL
JULY
OCT
Example of wind
(85 knots)
50 + 3x10 + 5
Tibetan High
Tropical Easterly

11. Convective processes and cloud in the Indian Monsoon
Fig. 1 from Krishnamurti and Bhalme (1976)
Cloud cover:
Important component;
Manifestation of moist convective processes;
Varies in both space and time.

12. The Seasonal cycle of precipitation, low and high level circulation for the Asian-Australian Monsoons
Arrows represent low-level winds (at 850mb)
Streamlines are tangent to the direction of the wind in 200mb (they indicate the formation and maintenance of the Tibetan High
Colors indicate OLR (remember low values of OLR are related to convective activity in tropical regions).

13. The Indian Monsoon Variability
Inter-annual variability (year to year) ENSO:
The ENSO phenomenon is clearly associated with the Indian Monsoon
El Niño  Weaker Monsoon
During the summer monsoon season: active and break phases
Active phases of the Indian Monsoon:
Trough intensifies  Cloud development  Mascarene high intensifies  Tibetan high intensifies  tropical easterly jet intensifies;
Break phases of the Indian Monsoon:
Opposite characteristics are observed

14. Regional Monsoon Systems
1) The Indian Monsoon
2) The East Asia Monsoon
3) The Australian Monsoon/ East Asia Winter Monsoon
4) The African Monsoon
5) The American Monsoon

15. 2) East Asia Monsoon
The East Asian can be divided into seven major components:
The Australian high;
The low-level cross equatorial jet at about 110o E
East Asia Summer Monsoon trough/ Zones of Convergence ;
Tropical Easterly Jet (Upper level return flow);
The Western Pacific High
The Mei-Yu Front Zone
Mid Latitude Disturbance

16. Regional Monsoon Systems 2) East Asia Monsoon
East Asian Monsoon also originates in air flows from:
The Indian Ocean (Indian monsoon airflow);
Western Pacific high (south-east flow from its western flank).
The low-level cross equatorial jet:
Carries with it large amounts of moisture (Inter-hemispheric mass exchange);
Stronger when the pressure difference between the Australian high and the East Asian trough is enhanced;
Over the Malaysian Peninsula the low-level jet meets the westerly Indian monsoonal flow and produces large amounts of rainfall.
Monsoon trough:
Stretches from western Pacific across Indo-China;
Associated with zones of convergence, cloudiness and rainfall.

17. 2) East Asia Monsoon
(1) The low-level cross equatorial jet at about 110o E;
(2) Southwesterlies
(3) East Asia Summer Monsoon trough/ Zones of Convergence
(4) The Western Pacific High
(5) The Mei-Yu Front Zone
(6) Mid Latitude Disturbances
(7) Tibetan High
(8) Tropical Easterly Jet (Upper level Return Flow); 7
Fig 18b from Zhou and Lau (1998)
Shading  topography.
Dashed lines  easterlies

18. 2) Components of the East Asia Monsoon: Test your memory!
1) East Asia Monsoon Trough or convergence zones
High level High
2) Mei-Yu Front
3) Tibetan High
4) Western Pacific High:
Low level
5) High level easterly jet
6) Low-level southerly
7) Low-level jet
8) Midlatitude disturbances

19. Regional Monsoon Systems 2) East Asia Monsoon Variability
ENSO:
ENSO onset in spring or summer:
El Niño (La Niña) decreased (increased) rainfall over Indo-China, South China, Philippines region.
El Niño  rainfall below normal over eastern China.

20. Regional Monsoon Systems
1) The Indian Monsoon
2) The East Asia Monsoon
3) The Australian Monsoon/ East Asia Winter Monsoon
4) The African Monsoon
5) The American Monsoon

21. 3) Australian Monsoon / East Asia Winter Monsoon
Asian Summer monsoon Jun-Sep
Asian Winter monsoon Dec-Mar
Winds (arrows) and convergence zones (dots). Prevailing winds at 700mb (3000m) and 500mb (5800m) levels
Note the changes in low and high level winds from summer to winter.
Note how the intertropical convergence zone moves to the Southern Hemisphere and affects North Australia – The Australian Monsoon.

22. 3) Australian Monsoon / East Asia Winter Monsoon
South Hemisphere Extension of the Larger Asia Winter Monsoon System. Three major features of circulation:
Monsoon trough over South China Sea;
Surge of cold air within the northerly monsoon current from the northern Asia anticyclone system;
Reversal of upper level winds from easterly to westerly over southern China.

23. Regional Monsoon Systems 3) Australian Monsoon / East Asia Winter Monsoon

24. 3) Australian Monsoon Variability
Interannual timescale: ENSO
El Niño  weaker Australian monsoon.
Intraseasonal time-scales (active and break phases) (modulated by the Madden-Julian Oscillation.

25. Regional Monsoon Systems
1) The Indian Monsoon
2) The East Asia Monsoon
3) The Australian Monsoon/ East Asia Winter Monsoon
4) The African Monsoon
5) The American Monsoon

26. Regional Monsoon Systems 4) African Monsoon
Differ from India and East Asia Systems in:
Magnitude;
Thickness of flow;
Geographical coverage;
High latitude air masses are not involved.

27. Regional Monsoon Systems 4) African Monsoon
West African Monsoon:
Surface pressure trough;
The Inter-Tropical Convergence Zone (ITCZ);
South Atlantic (St. Helena) and North African (Saharan) subtropical high pressure systems.
East African Monsoon:
Arabian and Mascarene high pressure systems;
East African low level jet;
West African mid-tropospheric jet stream;
Tropical easterly jet stream;
Subtropical jet streams.

28. Regional Monsoon Systems 4) African Monsoon Variability
ENSO:
El Niño (La Niña) Subsidence (ascent) over west Africa and thus below (above) average convection.

29. Regional Monsoon Systems
1) The Indian Monsoon
2) The East Asia Monsoon
3) The Australian Monsoon/ East Asia Winter Monsoon
4) The African Monsoon
5) The American Monsoon

30. Regional Monsoon Systems 5) American Monsoon

31. 5) American Monsoon: seasonal variation in convective activity (indicated by low values of OLR) and sea surface temperature (climatology)
Sea surface temperature (oC, colors) and outgoing long wave radiation (Wm-2, contours) for December-February (left panel) and June-August (right panel). (Courtesy V. Kousky.)

32. 5) What are the main features of the American Monsoon that are similar to the Asian Monsoon?
Schematic illustration of the North and South American monsoon systems (left and right panels, respectively). Shading indicates precipitation and dashed lines indicate convergence zones. Small arrows show low-level (900 hPa) winds, and thick arrows represent low-level jets. An "H" shows a subtropical surface high center, and an "A" indicates the monsoon high level anticyclone. (Adapted from V. Kousky.)

33. Schematic vertical (longitude–pressure) cross section through the North America Monsoon System at 27.5°N.
Schematic vertical cross section across South America displaying the major large-scale
elements affecting the South America Monsoon System(from CLIVAR Web site online at

34. Anomaly = Observed – Mean
The recognition of the SA Monsoon is relatively recent. This is because winds do not change 120o from summer to winter. However, if we remove the annual mean, then the anomalies clearly change.
Remember:
Anomaly = Observed – Mean
Westerly anomalies
Easterly anomalies
Zhou e Lau (1998)

35. The South America Monsoon System: wind anomaly and precipitation
(Oct)

36. Although it is not observed a reversal of the low level wind between summer and winter over South America, there is a reversal in the wind anomalies in comparison with the annual average.

37. South America Monsoon System
Fig 18a from Zhou and Lau (1998)
Shading  topography.
Dashed lines  easterlies
(1) The low-level cross equatorial jet
(2) Northwesterlies
(3) The Chaco Low (monsoon trough)
(4) The South Atlantic High
(5) The South Atlantic Covergence Zone (SACZ)
(6) Mid Latitude Disturbances
(7) Bolivian High
(8) Upperlevel Return Flow.

38. South America Summer Precipitation
Low-Level Cross Equatorial Jet
Energy Released in the Form of Latent Heat
Bolivian High
Tropical Easterly Jet
Upper Level Jet
Northwesterlies
SACZ
Chaco Low

39. Outline Definition of Monsoon The Monsoon Makers Annual Monsoon Cycle
Variability of the monsoons
Regional Monsoon Systems
The Indian Monsoon
The East Asia Monsoon
The Australian Monsoon/ East Asia Winter Monsoon
The African Monsoon
Easter North Pacific Monsoon/American Monsoon?
Summary

40. SUMMARY The Monsoon Makers: Seasonal heating; Moisture Processes;
The Earth’s Rotation.
Common features in monsoon systems:
Cloudiness and heavy rainfall [moisture processes];
Cross-equator low level jet [seasonal heating + moisture processes];
Low level circulation influenced by Subtropical anticyclones [moisture processes + Earth’s rotation];
Monsoon trough (low level convergence) and associated convergence zones [moisture processes + Earth’s rotation];
Upper level high pressure (upper level divergence) and associated upper level return flow [moisture processes + Earth’s rotation];
Mid-latitude disturbances (NOT in African monsoon).

41. SUMMARY
Although three main monsoon systems were traditionally recognized (African, Asian and Australian), recent studies have shown that a monsoon regime exists over tropical America.
American

42. SUMMARY
Internal dynamic and boundary forcing  variations in the onset, duration, intensity, and cessation of the monsoons.
ENSO:
El Niño (La Niña) decreased (increased) rainfall over Indo-China, South China, Philippines region as well as west Africa region.
El Niño  rainfall below normal over Indian, eastern China and Australian monsoons.