monsoons

Tropical M. D. Eastin

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• 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


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


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


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


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)


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


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)


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


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


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


(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)


(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


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


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.

http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl

monsoons

Documents

monsoon circulation

monsoon heatingtropicalm

major monsoon systems

strong onshore flow

southwest precipitation

ducted flow

terrain elevated heat

sensible heat fluxes