SO442 – Lesson 8 Monsoon circulations

What are monsoon circulations? The word “monsoon” comes from Arabic to mean Reversal of winds Globally, there are several areas that are known for monsoon circulations: Indian-Asian-Australian monsoon American monsoon African monsoon All three monsoon circulations are: Near the equator (between 30S and 30N) Related to seasonal shifts in winds (summer to winter) Augmented by significant topography Ethiopian and Great Rift systems of Africa Himalayas and Maritime Continent of Asia/Australia Andes of South America Sierra Madre and Rockies of North America

Asian monsoon The Himalayan Mountains are in South Asia Called the Indian subcontinent India, Pakistan, Bangladesh, Sri Lanka, Maldives South Asia is the world’s second most populous region Behind East Asia (China) The population is growing, raising concerns about food production keeping pace Since achieving in 1947, India and Pakistan have been embroiled in conflict; both countries have nuclear weapons This region is one of the world’s poorest

Creation of the Indian subcontinent and Himalaya Mountains The mountains are directly related to the Asian monsoon Continental Drift

The Asian monsoon “To know India and her people, one has to know the monsoon” (Globalization & Diversity: Rowntree, Lewis, Price, Wyckoff) To the people of India, the monsoons are a source of life General onshore movement in summer General offshore flow in winter Very distinctive seasonal precipitation regime – Two monsoons – wet one & dry one Note: monsoon does not mean “deluge of rain” or “heavy rain”

Monsoon Principles The lower-troposphere wind is a horizontal movement of air from a high pressure area to a low pressure area In Asia, the land surfaces heat up and cool off more quickly and to a greater degree than the nearby water bodies (Indian Ocean, Pacific Ocean, and adjacent seas) During the warmer months, low pressure develops over land while high pressure over the adjacent water bodies This leads to onshore transport of very humid air. Convergence of that air leads to very heavy rainfalls (the wet monsoon) During the cooler months, high pressure develops over land while low pressure over the adjacent land areas This leads to offshore transport of very dry air. Divergence of that air leads to subsidence and clear/stable conditions (the dry monsoon) This results in the shifting of the prevailing winds -- MONSOONS Air circulation during the wet monsoon of Asia

Another view of the South Asian Monsoon: the wet season (summer) Hot air rising (Low Pressure) Air rises,cools, clouds form Rain! Moist air drawn north Hot ground Deccan Plateau Tibetan Plateau Central Asia Himalayas Indian Ocean

Another view of the South Asian Monsoon: the dry season (winter) Air chilled and sinks ( High Pressure) Dry air spills over Himalayas Dry Air= no clouds= NO RAIN Very Cold Ground Deccan Plateau Tibetan Plateau Central Asia Himalayas Indian Ocean

Seasonal shift in the prevailing wind direction Seasons NOT defined by temperature patterns, but by precipitation patterns Note the seasonal temperature variability at the tropical location is only 6F However, the precipitation ranges from < 3 cm in January to > 70 cm in June Nearly 3 meters of precipitation (almost the height of our classroom!) falls per year, mostly in about 6 months! Seasonal shift in the prevailing wind direction Dry monsoon Wet monsoon

Vulnerable populations The monsoon rains are needed (they allow for rice farming) But, when the rains come too fast, or too soon, they can lead to disaster Bangladesh is a country vulnerable to meteorological disasters Not only tropical cyclones, but also flooding from a more active monsoon In all three areas (highlands-red, lowlands-blue, and the peninsula-purple), flooding risks are high! But, particularly so in the blue area Rice is dependent on the monsoons

The monsoons of the Americas The North and South American monsoons share some similar features with the Asian monsoon: Shift in wind direction caused by seasonal differences in solar radiation Nearby mountain system leading to elevated heat source (summer) or elevated cold source (winter) High (cold) and low (warm) pressure systems causing an acceleration of low-level “jets” of wind near the mountains Transport of warm, humid air from nearby oceans onto the land Resulting in very strong seasonality in precipitation patterns http://iri.columbia.edu/~awr/papers/WMO.American.Monsoons.RH.pdf

South American Monsoon The South American monsoon occurs in Brazil, Bolivia, Paraguay, Uruguay, and Argentina To the east of the Andes mountains In the South American monsoon, warm and humid air flows from the Atlantic and Amazon river basins toward the Andes (the elevated heat source) Depending on the location and intensity of the “South Atlantic Convergence zone” (SACZ), that leads to: Heavy rainfall in Uruguay, Argentina, Brazil, and Paraguay (left panel), or Heavy rainfall in northwest Argentina, Bolivia, and Paraguay (right panel)

North American Monsoon L Winter: stratiform precipitation driven by trough systems that enter the continent from the North Pacific ocean. Summer: convective precipitation driven by circulation around the ridge and heat-low (L) over the deserts of SW U.S. and NW Mexico https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1417.pdf

North American Monsoon The North American monsoon evolves from May through July (peaks in July, typically), then back through August and September Rainy season in NW Mexico and SW U.S. occurs in July- Sept https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1417.pdf

More on the North American Monsoon in Mexico Monsoon can occur in one month, two months, or three months ------> Monsoon can be weak, moderate, or strong <------