Monsoons (Global Warming)

A monsoon is any wind that reverses direction seasonally. These occur in many parts of the globe, but the most famous is the Indian monsoon wind. The name monsoon is an adoption of the Arabic word mausim, which means season. For centuries, Arab sailors took the seasonal mausim as a wind that would carry them to India and beyond. The term was originally applied to the Indian monsoon. However, as the phenomenon came to be studied more thoroughly, the term monsoon has been applied to other annual weather cycles in both tropical and subtropical regions. It has also been applied to evidence of weather patterns in the geologic past, such as a monsoon system over the prehistoric super-continent of Pangaea, and to some extreme continental climate patterns.

Monsoons can occur in summer months or in winter months. Those that occur in summer months usually blow as a prevailing wind from a westerly direction. The westerly monsoons usually bring great amounts of rain as the rising air comes from the ocean and brings moisture that falls as rain. In the winter months, monsoon winds usually blow in an easterly direction. Because easterly winds usually blow from the land, they generally bring dry weather. They may also bring drought as the easterlies diverge or shift about. The intensity of monsoons varies annually. Some years they are stronger than in others. The strength of the annual monsoons is affected by the Earth’s total climate system. As the amount of energy available varies, the system has different outputs, such as the strength of the monsoons, which results in variable amounts of annual rainfall.


INDIAN MONSOON

The Indian monsoon is a wind that blows seasonally over India and the land areas surrounding India. The winds blow in different directions at different times of the year. The monsoon winds blow from the southeast from April to October. From November to March they blow from the northeast. The monsoon brings rains when it blows from the southwest across the warm waters of the Indian Ocean. Hundreds of millions of people depend upon the rains it brings. When it blows from the northeast, dry air prevails to create the dry season. Monsoons also occur in northern Australia, the northern and eastern coasts of Asia, some of the Pacific Coast of North America, South America, and Sub-Saharan Africa. In addition, the Indian Ocean has monsoon areas that include parts of the east coast of the African continent.

Differences in the temperatures on land and sea cause the monsoon winds. The temperature differences occur because the heat transfers in the ocean happen in a different manner than the heat transfers on land. In the ocean, sunlight heats the water to a depth of about 64 ft. (50 m.). Winds and wave action churns the ocean waters and as it heats and cools with annual seasonal changes energize or slow wind action. The heating and cooling of the land and waters of the Indian Ocean cause the temperature differences. Sunlight striking the surface of the Earth heats it to much higher temperatures than it does ocean water. As the heated air rises, a large area of low pressure caused by the rising air is formed. The relatively cooler and denser air that blows across the Indian Ocean flows in to fill the low-pressure areas with its heavier air. At the same time, the relatively cooler air is laden with moisture, but is heated over the land so that the moisture rises is cooled at high elevations, condenses, and falls as rain over the land. During the winter, the land is cooler than the Indian Ocean, so as the air over it rises to form low-pressure areas the denser land airflow from the land into the low pressure over the waters of the Indian Ocean.

SOUTHWEST MONSOON

The southwesterly monsoon is a prevailing wind that blows during the wet seasons of April to October. Very heavy rains are brought to India and to countries surrounding the Indian Ocean. These include Pakistan, Bangladesh, Burma, Thailand, the Arabian Peninsula (Oman, Saudi Arabia, the United Arab Republic, and Yemen) and neighboring African countries that border the Indian Ocean and the Arabian Sea. The rains are very heavy and bring the majority of the rainfall to the region each year. There are places in India that receive over 200 in. of rain per year (over 500 cm.). The warm moisture in the southwesterly monsoon falls as rain over the foothills of the Himalaya Mountains. The southwest Indian monsoon is the source of water for most agriculture in the region. If the monsoon fails, or is not strong, there will be drought with the starvation of millions.

The Tibetan Plateau affects the Indian monsoon. The vast Himalayan Mountain system has reached great heights because the India plate is elevating it. Theorists argue that the land’s formation and the development of the Indian monsoon began about 8 million years ago. However, other scientists have hypothesized that an older monsoon existed that also was tied to the Tibetan plateau.

From June through September, the southwestern monsoon wind prevails in India. In the Great Indian Thar Desert, and the areas to the north and south of it, the Indian subcontinent develops very hot temperatures during the long days of the summer months. The hot air causes lower pressure to prevail in the area. Moisture laden air flows into the Indian subcontinent from the Indian Ocean. When the warm moist air reaches the Himalayas, storm clouds form, because the great height of the Tibetan plateau blocks the moist air. The warm air rises to heights where it cools enough that rain falls in varying amounts. When the oceanic southwest monsoon reaches the Indian subcontinent, it splits into two parts. The eastern part becomes the Bay of Bengal branch; the western part becomes the Arabian Sea Branch of the Southwest Monsoon.

The Bay of Bengal branch of the Southwest Monsoon flows over the Bay of Bengal and northward toward Calcutta and then meets the eastern area of the Himalayan Mountains, where it drops huge quantities of rain over Northeast India, northern Burma, and Bangladesh. Cherrapunji, located on the slopes of the Himalayas in Shillong, is one of the wettest spots on Earth. The remaining portion of the Bengal Branch of the Southwest Monsoon then flows westward over the Grand Trunk Road in the Indo-Gangetic Plain. Great quantities of rain pour down during the height of the seasonal monsoon. The Arabian Sea Branch of the Southwest Monsoon bounces off of the Western Ghats of the Indian subcontinent. The monsoon then moves northward, producing rain along the west coast of the Indian subcontinent. The Ghats are steps, or hills that are shorter steps to the Decca plateau. The eastern Ghats are located on the Bay of Bengal side of the Indian subcontinent. They receive considerably less rain than do the western Ghats, which are on the Arabian Sea side of the Indian subcontinent.

Monsoons that occur in summer months usually blow as a prevailing wind from a westerly direction. These westerly monsoons usually bring great amounts of rain, as the rising air comes from the ocean and brings moisture that falls as rain.

Monsoons that occur in summer months usually blow as a prevailing wind from a westerly direction. These westerly monsoons usually bring great amounts of rain, as the rising air comes from the ocean and brings moisture that falls as rain.

Without the monsoon, India would be much more arid. About 80 percent of the rainfall in India is associated with the monsoon. The huge agricultural population in India is totally dependent upon the monsoon rains for growing rice and other crops. In the 1990s, a few days delay affected the future crops. On the other hand, the Bengal Bay Branch of the

Indian monsoon has regularly brought floods. The monsoon is also a great benefit to Indian city-dwellers because it brings relief from the heat and rains that wash away debris from the cities. The first day of June is considered the normal date for the beginning of the Southwest Monsoon. However, in September, the northern part of the Indian subcontinent begins to cool rapidly. The denser air now blocks the flow of the Southwest Monsoon and, instead, reverses in the direction of the retreating monsoon (Northeast Monsoon) that flows from the Indian subcontinent into the Indian Ocean.

NORTHEAST MONSOON

This retreating monsoon also brings rain to areas that received less during the time of the advancing monsoon. The cooler and drier air flowing from the high Himalayas across the Bay of Bengal bring rain to areas of the eastern part of the Indian peninsula. The Northeast Monsoon brings less rain than falls during the Southwest Monsoon, but it bring vital moisture to areas that were missed earlier in the summer.

In southern Asia, the northeastern monsoon appears in December and lasts until early March. In Central Asia, wintertime temperatures are cold so that a dome of high-pressure air develops. As the jet stream passes over the area, it splits into the polar jet and the subtropical jet stream. The subtropical flow passes over southern Asia, creating clear skies over India. At the same time, warm water from the Philippines eastward develops into a low pressure system. The system spreads over Southeast Asia, Indonesia, and northern Australia. The winds then flow in a monsoon trough into the swamp wet areas of northern Australia and into the interior of the continent, bringing much-needed summertime rains.

SUB-SAHARAN MONSOON

The African monsoon in sub-Saharan Africa is different from the monsoon rains that water the vast Seren-geti Plains or the areas of East Africa that adjoin the Indian Ocean. The sub-Saharan monsoon is caused by seasonal shifts in the Intertropical Convergence Zone. The Sahara Desert reaches great temperatures in the summertime and creates hot air low-pressure centers. As they move off of the African continent from June to December, they cross the Atlantic Ocean and have the potential to develop into hurricanes. However, in the winter, the dry northeastern trade winds and the extreme harmattan winds shift and bring dry weather to the regions to the south. In the summer, they act to bring moisture to the savannah regions and to the Sahel as far east as the Sudan.

AMERICAN MONSOONS

The South American Monsoon is a key factor in Brazil’s weather patterns. Seasonal winds from the Atlantic bring summertime rains that feed the Amazon basin. They also frequently bring flooding to Rio de Janeiro.

The North American monsoon begins in late June and ends in September. It blows from the Pacific Ocean onto Mexico is a relatively weaker version of the Indian monsoon. It is an invasion of humid air that brings thunderstorms. The North American monsoon is sometime called the Summer Monsoon, Mexican Monsoon, Southwest Monsoon, Desert Monsoon, or the Arizona Monsoon. It begins in the south off of the coast of Mexico. It soon affects areas such as the region of Guadalajara. As the Mexican Monsoon spreads, it affects the Sierra Madre Occidental, Arizona, New Mexico, Utah, Colorado, West Texas, and California. In California, it brings moisture-laden winds to the Peninsular Ranges and the Transverse Ranges in Southern California.

As the Desert Monsoon, it brings up to 70 percent of the rain that falls in the Mohave and Sonoran Deserts. Compared to the Sahara, these deserts appear wet. In the upper elevations, the rains also bring moisture that mitigates the summer temperatures with its natural drying effects. Oddly, while reducing drought conditions, the monsoon rains can increase wintertime plant growth, which, in turn, provides fuel for wildfires. The thunderstorms over the Grand Canyon or in the high mountains can create hazards from flash floods. Or, lightening can strike to begin fires or to injure or kill the unwary.

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