1. SO442 – Lesson 8
Monsoon circulations

2. 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

3. 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

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

5. 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”

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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)

13. North American MonsoonL
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

14. 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

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