1 / 16

Monsoons

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

Télécharger la présentation

Monsoons

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Monsoons M. D. Eastin

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

  3. 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 M. D. Eastin

  4. 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 Asian-Australian Monsoon M. D. Eastin

  5. 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 M. D. Eastin

  6. Indian Monsoon Dry Season Wet 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 • South slope of Himalayas DJF Surface winds JJA Surface winds DJF DJF Rainfall JJA Rainfall M. D. Eastin

  7. 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 (helpsinitiate anddrive the monsoon) • The East African Highlands act as barrier to low-level easterly winds (increases the inflow) M. D. Eastin

  8. 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 Heating M. D. Eastin

  9. 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) M. D. Eastin

  10. 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 M. D. Eastin

  11. Indian Monsoon • Interannual Variability (ENSO) • La Nina orNormal 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 M. D. Eastin

  12. African Monsoon Precipitation Rate / Low-level Winds • Annual Variability • Characterized by a N-S shift in precipitation • and an onshore-offshore flow reversal • In DJFoffshore 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 Mountains Warm Warm Cool JJA Hot Mountains Cool M. D. Eastin

  13. (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) M. D. Eastin

  14. (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 M. D. Eastin

  15. 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) M. D. Eastin

  16. 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. M. D. Eastin

More Related