What is a Monsoon?

Report
Monsoons
Tropical
M. D. Eastin
Outline
What is a Monsoon?
Societal Impacts of Monsoons
Indian Summer Monsoon (the Big One)
Other Monsoons
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What is a Monsoon?
Definition and Background
• A monsoon is a wind circulation that reverse course on seasonal time scales
• Associated are sharp seasonal contrasts in precipitation
• The primary cause of monsoons are strong thermal contrasts between the land and sea
[Edmond Halley (of comet fame) was first of recognize this forcing]
Three major monsoon systems:
• Asian-Australian (of which the Indian monsoon is the dominant component)**
• African (influences easterly wave development during the summer)
• American (of which the Southwest U.S. monsoon is a part)
Common Characteristics:
• Heavy summer rains and very dry winters
• Seasonal wind reversals
• Large cross-equatorial moisture flux from the winter hemisphere
• Strongly influenced by terrain → elevated heat sources and ducted flow
** We will look at the Indian Monsoon in greatest detail
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Monsoon Impacts
Large Societal Impact on Global Scale
• Monsoonal regions cover roughly ½ of the Tropics (or ¼ of the global surface area) and
plays host to ~65% of the world’s population
Population Density
American
Monsoon
African
Monsoon
Tropical
Asian-Australian
Monsoon
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Monsoon Impacts
Large Societal Impact on Global Scale
• Most agriculture and the economies of these regions are intimately tied to the monsoons
• Interannual (and climatic) variability of monsoon “onset” and intensity can be catastrophic
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Indian Monsoon
Dry Season
Annual Variability
• The dry season (Dec-Feb) is
characterized by offshore flow
toward the southwest
• Deep convection is located in
southern Indian Ocean
• Precipitation over the continent
is very minimal
• The wet season (Jun-Aug) is
characterized by strong onshore
flow from the southwest
• Precipitation is often intense and
frequent
• Three distinct rainfall maxima
Wet Season
DJF
Surface
winds
JJA
Surface
winds
DJF
Rainfall
JJA
Rainfall
West coast of India
Bay of Bengal
South slope of Himalayas
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Indian Monsoon
Impact of Topography
• Very important during the wet season (less so during the dry season)
• The Tibetan Plateau acts as an elevated heat source (helps initiate and drive the monsoon)
• The East African Highlands act as barrier to low-level easterly winds (increases the inflow)
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Indian Monsoon
Impact of Tibetan Plateau
Mean Temperature (200-500mb)
• Solar heating of the Himalayas is quickly
converted to mid-level atmospheric
heating via sensible heat fluxes
• Mid-level heating increases the thickness
between pressure surfaces
• Sets-up a strong pressure gradient at
upper-levels and strong offshore flow
• Lowers surface pressure over land and
induces onshore low-level flow that gains
moisture from the ocean via surface fluxes
Heating
• Moisture convergence and forced ascent
over land produces deep convection and
latent heat release
N-S cross-section through Monsoon
• Both heat sources continue to drive the
monsoon circulation
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Indian Monsoon
Impact of East African Highlands
• Low-level easterlies are blocked by the terrain and diverted northward (Somalia Jet)
• Increases the low-level inflow beyond that driven by the heating over land
• Arguably, without the Tibetan and East-African Highlands, southeast Asia would be a
desert like North Africa
Low Level Flow (z = 1 km)
Tropical
E-W cross section (A-B)
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Indian Monsoon
Interannual Variability (ENSO)
El Nino
Walker Circulation
• Warmer SSTs combined with a reverse
Walker circulation increases nearequatorial convection over the west
Indian Ocean and Africa (i.e. more air
ascends than is diverted northward)
• Less low-level onshore monsoonal flow
occurs results in less convection and
latent heat release → weaker monsoon
• Poleward outflow from the enhanced
equatorial convection also induces
subsidence over the continent, further
suppressing convection
• Severe droughts and famine often occur
in India during strong El Nino Events
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Indian Monsoon
Interannual Variability (ENSO)
La Nina or Normal years
Walker Circulation
• Warm SSTs and enhanced convection
over the equatorial west Pacific drives
a strong “normal” Walker Circulation
and enhanced subsidence over the
west Indian Ocean
• Increased subsidence enhances the
normal monsoon circulations and
increases total monsoon precipitation
• Flooding often occurs across India
during strong La Nina events
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African Monsoon
Precipitation Rate / Low-level Winds
Annual Variability
DJF
• In DJF offshore northeasterly flow dominates
sub-Saharan west Africa, confining the
precipitation to a narrow coastal band
• Onshore south-westerly flow dominates
southern Africa with deep convection
located west of the East African Highlands
(which acts like an elevated heat source)
Tropical
Cool
JJA
Hot
Cool
Mountains
• In JJA onshore southwesterly flow dominates
sub-Saharan west Africa with deep convection
extending northward to ~15ºN
• The very warm Sahara acts like an elevated
(but shallow) heat source, driving the
west Africa monsoon circulation
• Offshore south easterly flow dominates
southern Africa with very little precipitation
Warm
Mountains
Warm
• Characterized by a N-S shift in precipitation
and an onshore-offshore flow reversal
M. D. Eastin
(North) American Monsoon
Annual Variability
• Characterized by a reversal of the low-level flow along the Mexican west coast from
offshore (during the winter dry season, DJF) to onshore (the summer wet season, JJA)
• Monsoon circulation during the wet season is driven by the thermal contrast between
relatively cold ocean and the relatively warm Mexican mountains (an elevated heat source)
JJA Precipitation Rate (mm/day)
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(North) American Monsoon
Intra-seasonal Variability
Date of Precipitation Maximum
• The northward migration of the incoming
solar radiation maximum combined with
the roughly N-S orientation of the
mountain range results in a northward
migration of the elevated heat source
• As a result, the region of deep convection
tends to migrate northward in response
Tropical
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Monsoons
Summary:
• Definition (3 primary monsoons, common characteristics)
• Global Societal impacts
• Indian Monsoon
• Seasonal Variability
• Effects of Topography
• Variability due to ENSO
• African Monsoon (seasonal variability)
• North American Monsoon (seasonal variability)
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References
Adams, D. K., and A. C. Comrie, 1997: The North American Monsoon. Bull Amer. Meteor. Soc., 78, 2197-2213.
Cadet, D., and G. Reverdin, 1981: The monsoon over the Indian Ocean during summer 1975. Part I: Mean fields.
Mon. Wea. Rev., 109, 148-158.
Cadet, D., and G. Reverdin, 1983: The monsoon over the Indian Ocean during summer 1975. Part II: Break and active
monsoons. Mon. Wea. Rev., 111, 95-108.
Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage
( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl )
Fennesey, M. J., and Coauthors, 1994: The simulated Indian monsoon: A GCM sensitivity study, J. Climate, 7, 33-43.
Fu, C., and J. O. Fletcher, 1985: The relationship between Tibet-tropical ocean thermal contrast and interannual
variability of Indian monsoon rainfall , J. Appl. Meteor., 24, 841-847.
Krishnamurthy, V., and B. N. Goswami, 2000: Indian Monsoon–ENSO relationship on interdecadal timescale,
J. Climate, 13, 579-595.
Mooley, D. A., and B. Parthasarathy, 1983: Variability of the Indian summer monsoon and tropical circulation features,
Mon. Wea. Rev., 111, 967-987.
Tropical
M. D. Eastin

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