Monsoons

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