monsoons
DESCRIPTION
Monsoons. Outline. What is a Monsoon? Societal Impacts of Monsoons Indian Summer Monsoon (the Big One) Other Monsoons. What is a Monsoon?. Definition and Background A monsoon is a wind circulation that reverse course on seasonal time scales - PowerPoint PPT PresentationTRANSCRIPT
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 ImpactsLarge 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
AmericanMonsoon
AfricanMonsoon
Asian-AustralianMonsoon
Tropical M. D. Eastin
Monsoon ImpactsLarge 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
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 South slope of Himalayas
DJF
DJF
JJA
JJADJF
Wet SeasonDry Season
Surfacewinds
Surfacewinds
Rainfall Rainfall
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
• 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 monsoon circulation
Mean Temperature (200-500mb)
N-S cross-section through Monsoon
He
atin
g
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) E-W cross section (A-B)
Tropical M. D. Eastin
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
• Severe droughts and famine often occur
in India during strong El Nino Events
Walker Circulation
Tropical M. D. Eastin
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
Tropical M. D. Eastin
African Monsoon
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• Offshore south easterly flow dominates southern Africa with very little precipitation
DJF
JJA
Precipitation Rate / Low-level Winds
Hot
Cool
Mo
un
tain
s
Cool
Warm
Wa
rmM
ou
nta
ins
Tropical 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
• 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
Tropical M. D. Eastin
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)
Monsoons
Tropical M. D. Eastin
ReferencesAdams, 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 activemonsoons. 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.