Monsoon of South Asia

The Monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.
The unique geographical features of the Indian subcontinent, along with associated atmospheric, oceanic, and geographical factors, influence the behavior of the monsoon. Because of its effect on agriculture, on flora and fauna, and on the climates of nations such as Bangladesh, Bhutan, India, Nepal, Pakistan, and Sri Lanka – among other economic, social, and environmental effects – the monsoon is one of the most anticipated, tracked, and studied weather phenomena in the region. It has a significant effect on the overall well-being of residents and has even been dubbed the "real finance minister of India".

Definition

The word monsoon, derived from the Arabic word موسم meaning "season", colloquially refers to a season of greatly intensified precipitation that occurs in some coastal regions in the tropics and subtropics. Scientifically however, while generally defined as a system of winds characterized by a seasonal reversal of direction, there are several more detailed definitions used by various meteorological and climatological sources. Some examples:

The American Meteorological Society calls it a name for seasonal winds, first applied to the winds blowing over the Arabian Sea from the northeast for six months and from the southwest for six months. The term has since been extended to similar winds in other parts of the world.
The Intergovernmental Panel on Climate Change describes a monsoon as a tropical and subtropical seasonal reversal in both surface winds and associated precipitation, caused by differential heating between a continental-scale land mass and the adjacent ocean.
The India Meteorological Department defines it as the seasonal reversal of the direction of winds along the shores of the Indian Ocean, especially in the Arabian Sea, which blow from the southwest for half of the year and from the northeast for the other half.
Colin Stokes Ramage, in Monsoon Meteorology, defines the monsoon as a seasonal reversing wind accompanied by corresponding changes in precipitation.

Thus, similar seasonal precipitation patterns in other parts of the world are not always true monsoons.

Background

The first people to observe the combined pattern of the monsoons' branches over different regions of South Asia were sailors in the Arabian Sea who traveled between Africa, India, and Southeast Asia.
The monsoon can be categorized into two branches based on their spread over the subcontinent:

Arabian Sea branch
Bay of Bengal branch

Alternatively, it can be categorized into two segments based on the direction of rain-bearing winds:

Southwest monsoon
Northeast monsoon

Based on the time of year that these winds bring rain to India, the monsoon can also be categorized into two periods:

Summer monsoon
Winter monsoon

The complexity of the monsoon of South Asia is not completely understood, making it difficult to accurately predict the quantity, timing, and geographic distribution of the accompanying precipitation. These are the most monitored components of the monsoon, and they determine the water availability in India for any given year.

Changes of the Monsoon

Monsoons typically occur in tropical areas. One area that monsoons impact greatly is India. In India monsoons create an entire season in which the winds reverse completely.
The rainfall is a result of the convergence of wind flow from the Bay of Bengal and reverse winds from the South China Sea.
The onset of the monsoon occurs over the Bay of Bengal in May, arriving at the Indian Peninsula by June, and then the winds move towards the South China Sea.

Effect of geographical relief features

Although the southwest and northeast monsoon winds are seasonally reversible, they do cause precipitation on their own.
Two factors are essential for rain formation:

Additionally, one of the causes of rain must happen. In the case of the monsoon, the cause is primarily orographic, due to the presence of highlands in the path of the winds. Orographic barriers force wind to rise. Precipitation then occurs on the windward side of the highlands because of adiabatic cooling and condensation of the moist rising air.
The unique geographic relief features of the Indian subcontinent come into play in allowing all of the above factors to occur simultaneously. The relevant features in explaining the monsoon mechanism are as follows:

The presence of abundant water bodies around the subcontinent: the Arabian Sea, Bay of Bengal, and Indian Ocean. These help moisture accumulate in the winds during the hot season.
The presence of abundant highlands like the Western Ghats and the Himalayas right across the path of the southwest monsoon winds. These are the main cause of the substantial orographic precipitation throughout the subcontinent.
# The Western Ghats are the first highlands of India that the southwest monsoon winds encounter. The Western Ghats rise abruptly from the Western Coastal Plains of the subcontinent, making effective orographic barriers for the monsoon winds.
# The Himalayas play more than the role of orographic barriers for the monsoon. They also help confine it to the subcontinent. Without them, the southwest monsoon winds would blow right over the Indian subcontinent into Tibet, Afghanistan, and Russia without causing any rain.
# For the northeast monsoon, the highlands of the Eastern Ghats play the role of orographic barrier.
Features of monsoon rains

There are some unique features of the rains that the monsoon brings to the Indian subcontinent.

''"Bursting"''

Bursting of monsoon refers to the sudden change in weather conditions in India, characterized by an abrupt rise in the mean daily rainfall. Similarly, the burst of the northeast monsoon refers to an abrupt increase in the mean daily rainfall over the affected regions.

Rain variability (''"vagaries"'')

One of the most commonly used words to describe the erratic nature of the monsoon is "vagaries", used in newspapers, magazines, books, web portals to insurance plans, and India's budget discussions.
In some years, it rains too much, causing floods in parts of India; in others, it rains too little or not at all, causing droughts. In some years, the rain quantity is sufficient but its timing arbitrary. Sometimes, despite average annual rainfall, the daily distribution or geographic distribution of the rain is substantially skewed. In the recent past, rainfall variability in short time periods were attributed to desert dust over the Arabian Sea and Western Asia.

Ideal and normal monsoon rains

Normally, the southwest monsoon can be expected to "burst" onto the western coast of India at the beginning of June and to cover the entire country by mid-July. Its withdrawal from India typically starts at the beginning of September and finishes by the beginning of October.
The northeast monsoon usually "bursts" around 20 October and lasts for about 50 days before withdrawing.
However, a rainy monsoon is not necessarily a normal monsoon – that is, one that performs close to statistical averages calculated over a long period. A normal monsoon is generally accepted to be one involving close to the average quantity of precipitation over all the geographical locations under its influence and over the entire expected time period. Additionally, the arrival date and the departure date of both the southwest and northeast monsoon should be close to the mean dates. The exact criteria for a normal monsoon are defined by the India Meteorological Department with calculations for the mean and standard deviation of each of these variables.

File:Moonsoon taragonj 1.jpg|thumb|Monsoon clouds over Taraganj, Rangpur, Bangladesh

Theories for mechanism of monsoon

Theories of the mechanism of the monsoon primarily try to explain the reasons for the seasonal reversal of winds and the timing of their reversal.

Traditional theory

Because of differences in the specific heat capacity of land and water, continents heat up faster than seas. Consequently, the air above coastal lands heats up faster than the air above seas. These create areas of low air pressure above coastal lands compared with pressure over the seas, causing winds to flow from the seas onto the neighboring lands. This is known as sea breeze.

Process of monsoon creation

Also known as the thermal theory or the differential heating of sea and land theory, the traditional theory portrays the monsoon as a large-scale sea breeze. It states that during the hot subtropical summers, the massive landmass of the Indian Peninsula heats up at a different rate than the surrounding seas, resulting in a pressure gradient from south to north. This causes the flow of moisture-laden winds from sea to land. On reaching land, these winds rise because of the geographical relief, cooling adiabatically and leading to orographic rains. This is the southwest monsoon.
The reverse happens during the winter, when the land is colder than the sea, establishing a pressure gradient from land to sea. This causes the winds to blow over the Indian subcontinent toward the Indian Ocean in a northeasterly direction, causing the northeast monsoon. Because the southwest monsoon flows from sea to land, it carries more moisture, and therefore causes more rain, than the northeast monsoon. Only part of the northeast monsoon passing over the Bay of Bengal picks up moisture, causing rain in Andhra Pradesh and Tamil Nadu during the winter months.
However, many meteorologists argue that the monsoon is not a local phenomenon as explained by the traditional theory, but a general weather phenomenon along the entire tropical zone of Earth. This criticism does not deny the role of differential heating of sea and land in generating monsoon winds, but casts it as one of several factors rather than the only one.