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

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  • REGIONAL MONSOON SYSTEMS
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  • 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
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  • 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 cloudiness 7.Rainfall
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  • 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 (30 o S, 50 o E); Generates a large outflow of air; low level cross-equatorial jet The air moves north over the equator where it becomes a south- westerly flow known as the low level cross-equatorial jet;
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  • The Indian Monsoon Fig. 2 from Krishnamurti and Bhalme (1976) Mean Sea Level Pressure for July
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  • 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 Low level cross-equatorial jet: Maximum intensity from June to August; The jet splits in two branches at around 10 o N, 60 o E at this time; Arrive over central west and southern coasts of India; Important for rainfall amounts over western India.
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  • Other important characteristics of the Indian Monsoon: The Tibetan high The Tibetan high: Upper level anticyclone 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; Tropical easterly jet The outflow of air from the southern flank of the Tibetan high gives rise to the Tropical easterly jet. Tropical easterly jet Tropical easterly jet lasts from June to September
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  • 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
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  • Reasons for the easterly jet: 9 During summer, the Tibetan Plateau, which is located in subtropical regions, intensify the ascent increasing air pressure in high levels High level High Pressure Hot surface Low pressure system: Ascending air High Pressure High level Easterly Jet Temperature Gradient +-+-
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  • JAN APRIL JULY OCT Tropical Easterly High level winds and the Indian Monsoon 200 hPa Figs. 6 from Krishnamurti and Bhalme (1976) Monthly mean 200 hPa wind field Tibetan High Example of wind (85 knots) 50 + 3x10 + 5
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  • Convective processes and cloud in the Indian Monsoon Fig. 1 from Krishnamurti and Bhalme (1976) Cloud cover Cloud cover: Important component; Manifestation of moist convective processes; Varies in both space and time.
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  • 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).
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  • The Indian Monsoon Variability Inter-annual variability (year to year) ENSO: The ENSO phenomenon is clearly associated with the Indian Monsoon El Nio 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
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  • 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
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  • 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 110 o E 3.East Asia Summer Monsoon trough/ Zones of Convergence ; 4.Tropical Easterly Jet (Upper level return flow); 5.The Western Pacific High 6.The Mei-Yu Front Zone 7.Mid Latitude Disturbance
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  • 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); Australian high East Asian trough 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.
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  • 2) East Asia Monsoon Shading topography. Dashed lines easterlies (1) The low-level cross equatorial jet at about 110 o 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)
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  • 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
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  • Regional Monsoon Systems 2) East Asia Monsoon Variability ENSO: ENSO onset in spring or summer: El Nio (La Nia) decreased (increased) rainfall over Indo-China, South China, Philippines region. El Nio rainfall below normal over eastern China.
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  • 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
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  • 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. Asian Summer monsoon Jun-Sep Asian Winter monsoon Dec-Mar 3) Australian Monsoon / East Asia Winter Monsoon
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  • 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.
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  • Regional Monsoon Systems 3) Australian Monsoon / East Asia Winter Monsoon
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  • 3) Australian Monsoon Variability Interannual timescale: ENSO El Nio weaker Australian monsoon. Intraseasonal time-scales (active and break phases) (modulated by the Madden-Julian Oscillation.
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  • 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
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  • 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.
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  • Regional Monsoon Systems 4) African Monsoon West 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.
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  • Regional Monsoon Systems 4) African Monsoon Variability ENSO: El Nio (La Nia) Subsidence (ascent) over west Africa and thus below (above) average convection.
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  • 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
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  • Regional Monsoon Systems 5) American Monsoon
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  • 5) American Monsoon: seasonal variation in convective activity (indicated by low values of OLR) and

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