Monsoons
Tropical
M. D. Eastin
Outline
What is a Monsoon?
Societal Impacts of Monsoons
Indian Summer Monsoon (the Big One)
Other Monsoons
Tropical
M. D. Eastin
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
Tropical
M. D. Eastin
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
M. D. Eastin
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
Tropical
M. D. Eastin
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
Tropical
M. D. Eastin
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)
Tropical
M. D. Eastin
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
Tropical
M. D. Eastin
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)
M. D. Eastin
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
Tropical
M. D. Eastin
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
Tropical
M. D. Eastin
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)
Tropical
M. D. Eastin
(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
M. D. Eastin
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)
Tropical
M. D. Eastin
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