Characteristics of Indian Monsoon

Table of Contents

Context: The onset of southwest monsoon over Kerala has been delayed by couple of days this year.

What is Indian Monsoon?

Monsoon is a seasonal reversal of winds that brings moisture-laden winds to the Indian sub-continent, causing rainfall.
It can also be described as seasonal changes in atmospheric circulation and precipitation due to annual latitudinal oscillation of the Inter Tropical Convergence Zone (ITCZ) between north and south of the equator.
Features of Indian monsoon:
- Dry spell and wet spell: Monsoon in India has clearly marked wet phase and dry phase, characterized by weeks where there is no rainfall.
- Unequal distribution: Monsoon rains are unequally distributed with peninsular part of India receiving more rainfall than the plains.
- Topographic influence: Monsoons are influenced by topography. The Western side of the Western Ghats receives heavy rainfall whereas the eastern side is deficient.
- Fixed schedule: Monsoons in India have typically a fixed schedule. It usually begins by June first week and ends by September.

Source of water: The monsoon accounts for nearly 80% of the rainfall in India. It is responsible for replenishing depleted water resources of the country.
Irrigation: Majority of India is dependent on monsoon rainfall for irrigating their crops. This makes it critical for India’s food security.
Economy: India is primarily an agro-economy with fast majority depending on agriculture for their livelihoods. Delay of monsoon can hamper economic growth and can have domino effect.
Sustain flora and fauna: Vast biodiversity in India is supported through rains caused during monsoon season. Monsoons are responsible for maintaining ecological balance.

During summer, the northward shift of ITCZ helps develop a low-pressure area over interior Asia as well as north and north-western India.
- A high-pressure system over the southern Indian Ocean develops at the same time.
The low-pressure system over the Indian landmass attracts the southeast trade winds of the southern hemisphere, which turns right towards the low-pressure areas over the Indian subcontinent as a result of Coriolis force.
These south-westerly blowing winds enter the Indian peninsula as the southwest monsoon. Since they blow over warm oceans, they bring abundant moisture to the subcontinent.
After 100 to 120 days, the low pressure system over north and north-west India weakens due to southern shift of ITCZ. This leads to the retreat of the monsoon winds.

Mascarene high: The presence of this high pressure area near Madagascar helps in dispersal of moisture laden winds towards Indian sub-continent.
Low pressure on Tibetan plateau: During summers, Tibetan plateau gets immensely heated, thereby creating a low pressure area that attracts trade winds.
Northward shift of Inter-tropical convergence zone (ITCZ): The northward shift of ITCZ over north and northwest India creates a low pressure area known as monsoon trough.
Differential cooling of land and sea: Differential cooling generates a low pressure on the landmass of India while surrounding seas experience comparatively high pressure.
Shift of westerly jet stream: This shift to the north of the Himalayas creates the requisite temperature for arrival of monsoonal winds.
Southern Oscillation: El Nino or La Nina conditions either strengthen or weaken the Indian monsoons depending on their occurrence.

Onset and advance of monsoon

The onset of the monsoon over Kerala indicates the start of the four-month (June-September) southwest monsoon season, responsible for providing 70% of India’s annual rainfall.
However, onset does not mean the first rain of the season. First rains start in certain places even before the declaration of onset.
A delay or an early onset of monsoon has no impact on the quality or amount of rainfall, or its regional distribution across the country.
The onset of the monsoon occurs when there is a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.

Certain conditions have to be met before IMD declares onset of monsoon over India:

Rainfall intensity	Declaration of onset takes place if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep, after May 10, record at least 2.5 mm of rain for two consecutive days. These stations are Amini, Minicoy, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kochi, Kottayam, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru. Onset over Kerala is declared on the second day, if specific wind and temperature criteria are also fulfilled.
Wind conditions	Westerlies’ depth must be up to 600 hectopascal in the area that is bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. The zonal wind speed over the area bound by 5-10ºN latitude (Maldives to Kochi) and 70-80ºE longitude (Arabian Sea to Chennai) must be of the order of 15-20 knots at 925 hPa.
Heat	The Outgoing Longwave Radiation (OLR) value, which is combined value of the energy emitted to space by the Earth’s surface, oceans, and atmosphere, should be below 200 watt per sq m in the area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes.

South-west monsoon:
- South-west monsoon in India is divided into two parts—the Arabian Sea branch and Bay of Bengal branch.
- The Arabian Sea branch causes orographic rainfall along the western slopes of Western Ghats from the coast of Kerala to Gujarat.
- The Bay of Bengal branch moves over Bay of Bengal, collecting moisture on the way, and strikes the northeastern part of India.
- The Eastern Himalayas divert the winds towards the Indo-Gangetic Plains.
North-east monsoon:
- As Sun starts moving towards southern hemisphere, change in pressure conditions cause cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards Indian Ocean.
- On its way, cold dry wind picks up some moisture from the Bay of Bengal and pours it over eastern part of the peninsular India. This is called northeast monsoon.

A monsoon break refers to the dry spell when no rain occurs during the Monsoon season.
During this period, the Monsoon trough shifts closer to the Himalayan foothill, leading to sharp decrease in rainfall over most parts of the country.
Reasons for monsoon breaks vary for different regions:
- In northern part of the country, rains decline if rain-bearing clouds are not frequent along the monsoon trough.
- Over western coast, there is decline in rainfall when winds blow parallel to the coast.

Retreating monsoons start occurring by September end. Due to southward movement of the sun, low pressure area starts moving southwards.
The southwest winds become weaker and starts retreating from northern India towards northern Bay of Bengal and southeast India.
By December, the low pressure area completely vanishes from Indian peninsular region. This marks the complete withdrawal of monsoon.
Technical write-off is made when the bank removes an account from the NPA category even as it continues to make efforts to recover the