Monsoons

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Monsoons
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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

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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 worlds population

Population Density
American Monsoon
African Monsoon
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
Wet Season
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
• West coast of India
• Bay of Bengal

DJF
JJA
Surface winds
Surface winds
DJF
JJA
DJF
Rainfall
Rainfall
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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
• 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
• Moisture convergence and forced ascent
• over land produces deep convection and
• latent heat release
• Both heat sources continue to drive the

Mean Temperature (200-500mb)
N-S cross-section through Monsoon
Heating
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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)
E-W cross section (A-B)
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Indian Monsoon

• Interannual Variability (ENSO)
• El Nino
• Warmer SSTs combined with a reverse
• Walker circulation increases near-
• equatorial 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

Walker Circulation
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Indian Monsoon

• Interannual Variability (ENSO)
• La Nina or Normal years
• 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

Walker Circulation
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African Monsoon
Precipitation Rate / Low-level Winds

• Annual Variability
• Characterized by a N-S shift in precipitation
• and an onshore-offshore flow reversal
• 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)
• 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

DJF
Mountains
Warm
Warm
Cool
JJA
Hot
Mountains
Cool
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(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
• 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

Date of Precipitation Maximum
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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 Centers
(CDCs) Interactive Plotting and Analysis
Webage ( http//www.cdc.noaa.gov/cgi-bin/PublicDa
ta/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 MonsoonENSO 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.