REGIONAL MONSOON SYSTEMS. Outline Definition of Monsoon The Monsoon Makers Annual Monsoon Cycle Variability of the monsoons Regional Monsoon Systems The.

REGIONAL MONSOON SYSTEMS

Outline• Definition of Monsoon

The Monsoon MakersAnnual Monsoon CycleVariability of the monsoons

Regional Monsoon SystemsThe Indian MonsoonThe East Asia MonsoonThe Australian Monsoon/ East Asia Winter MonsoonThe African MonsoonThe American Monsoons

1) Indian Monsoon

The Indian monsoon is made up of a number of components:

1. The monsoon trough over northern India;2. The Mascarene anti-cyclonic system;3. The low level cross-equatorial jet;4. The Tibetan high pressure system;5. The tropical easterly jet;6. Monsoon cloudiness7. Rainfall

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;

The Indian Monsoon

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

The Indian Monsoon and surface circulation

Monthly progression of the low-level cross-equatorial jet

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.

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

Low pressure

Low level Convergence

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

High pressure upper levels

High pressure

Low pressure upper levels

Upper level surface

Low level Divergence

Northern Hemisphere

Reasons for the easterly jet:

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During summer, the Tibetan Plateau, which is located in subtropical regions, intensify the ascent increasing air pressure in high levelsHigh level

High Pressure

Hot surfaceLow pressure system:

Ascending air

High Pressure

High level Easterly Jet

Temperature Gradient

+

-

JAN APRIL

JULY OCT

Tropical Easterly

High level winds and the Indian MonsoonFigs. 6 from Krishnamurti and Bhalme (1976) Monthly mean 200 hPa wind field

Tibetan High

Example of wind(85 knots)

50 + 3x10 + 5

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.

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).

The Indian MonsoonVariability

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 phasesActive 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

Regional Monsoon Systems

1) The Indian Monsoon

2) The East Asia Monsoon

3) The Australian Monsoon/ East Asia Winter Monsoon4) The African Monsoon

5) The American Monsoon

2) East Asia Monsoon

The East Asian can be divided into seven major components:

1. The Australian high;2. The low-level cross equatorial jet at about 110o E3. East Asia Summer Monsoon trough/ Zones of Convergence ;4. Tropical Easterly Jet (Upper level return flow);5. The Western Pacific High6. The Mei-Yu Front Zone7. Mid Latitude Disturbance

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.

2) East Asia Monsoon

Shading topography. Dashed lines easterlies

(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);

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Fig 18b from Zhou and Lau (1998)

1) East Asia Monsoon Trough or convergence zones

2) Mei-Yu Front

2) Components of the East Asia Monsoon: Test your memory!

High level High

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

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.

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

Winds (arrows) and convergence zones (dots). Prevailing winds at 700mb (3000m) and 500mb (5800m) levelsNote 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.

Asian Summer monsoon Jun-Sep Asian Winter monsoon Dec-Mar

3) Australian Monsoon / East Asia Winter Monsoon

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.

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

3) Australian MonsoonVariability

Interannual timescale: ENSOEl Niño weaker Australian monsoon.

Intraseasonal time-scales (active and break phases) (modulated by the Madden-Julian Oscillation.

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

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.

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:The Inter-Tropical Convergence

Zone (ITCZ);Arabian and Mascarene high

pressure systems;East African low level jet;West African mid-tropospheric

jet stream;Tropical easterly jet stream;Subtropical jet streams.

Regional Monsoon Systems 4) African Monsoon

Variability

ENSO:El Niño (La Niña) Subsidence (ascent) over west Africa and thus below (above) average convection.

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

Regional Monsoon Systems 5) American Monsoon

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.)http://144.16.68.129/faculty/bng/IWM-III-BNG_overview.pdf

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.)http://144.16.68.129/faculty/bng/IWM-III-BNG_overview.pdf

5) What are the main features of the American Monsoon that are similar to the Asian Monsoon?

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-scaleelements affecting the South America Monsoon System(from CLIVAR Web site onlineat http://www.clivar.org).

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

Zhou e Lau (1998)

Westerly anomalies

Easterly anomalies

The South America Monsoon System: wind anomaly and precipitation

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(Oct)

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.

South America Monsoon System

(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.

Shading topography. Dashed lines easterlies

Fig 18a from Zhou and Lau (1998)

South America Summer Precipitation

Bolivian High

Chaco Low

Upper Level JetNorthwesterlies

Low-Level Cross Equatorial Jet

Tropical Easterly Jet

SACZ

Energy Released in the Form of Latent Heat

Outline• Definition of Monsoon

The Monsoon MakersAnnual Monsoon CycleVariability of the monsoons

Regional Monsoon SystemsThe Indian MonsoonThe East Asia MonsoonThe Australian Monsoon/ East Asia Winter MonsoonThe African MonsoonEaster North Pacific Monsoon/American Monsoon?

Summary

SUMMARYThe 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).

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

American

SUMMARYInternal 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.