Difference Between Advancing and Retreating Monsoons
The southwest monsoon gets its name from the south-westerly winds that blow from the south-western region and is responsible for rainfall throughout the subcontinent. While having maximum intensity during the summer season, they gradually lose their strength and retreat in the upcoming seasons, during which parts of the country get rain in the off-season while passing over the regions.
Advancing and retreating Monsoons are two different phases of the monsoon that impact the country's rainfall pattern, along with its agriculture and overall ecosystem balance.
Let's learn about advancing and retreating monsoons to better understand the difference between them.
The "advancing monsoon" is the name for the southwest monsoon winds that blow over the warm oceans and bear moisture, bringing rainfall to the Indian subcontinent. Before delving deeper into their intensity and impact, it is essential to have a brief idea of how they are formed.
There are four chief seasons—spring, summer, fall, and winter—that follow each other and have their temperature and weather patterns repeated yearly. Summer is the hottest of the four temperature seasons. It occurs from June to August and is accompanied by more extended hours of sunlight and hot, dusty dry winds that flow over the Indo-Gangetic plain regions.
The heating of the land during the hot summer seasons heats the air that rises, creating a low-pressure area on the land and a high-pressure area in the Indian Ocean with the moving of the high-pressure air into the low-pressure regions.
The low-pressure land system attracts southeast trade winds from the Southern Hemisphere to fill the vacuum. These winds turn right after crossing the Equator due to Coriolis force (a force caused by the rotation of the earth that deflects the winds) in the direction of the Indian subcontinent and enter the Indian peninsula as the southwest monsoon or advancing monsoons.
On entering India, the advancing monsoons are divided into the Arabian Sea and Bay of Bengal branches. The Arabian Sea branch, also referred to as the Southwest Monsoons, moves along the Arabian Sea and, within a short interval of entering the Indian Peninsula, arrives in the north-western and North-Eastern regions in early June.
The Bay of Bengal branch, or Southeast Monsoon, as it is referred to because of its eastern location, moves along the Bay of Bengal and arrives in the north-eastern regions during the first week of June. While Kerala is one of the first cities to receive rain due to the southwest advancing monsoons, areas such as Mumbai and parts of Punjab receive rain soon after. Because of its extreme north location, rains arrive in Delhi ten days later than rains in Mumbai.
The southeast monsoons hit Assam within the first week of their arrival and were then distributed over the other regions before being deflected towards the west over the Gangetic Plains. By mid-June, the advancing monsoon arrives in places like Kutch and the central part of India, while western Uttar Pradesh, Haryana, and the whole of Punjab experience it in the first week of July. During this time, the Arabian Sea and the Bay of Bengal branches merge over the Gangetic Plain's north western regions. By mid-July, Himachal Pradesh and the rest of the country will experience the effects of advancing monsoon rains.
These monsoons last for the next three months, with breaks between them, until the end of September, after which they weaken. During the months of October and November, the southwest monsoon winds become weaker and begin to retreat from northern and central India. The returning pattern is referred to as a "retreating monsoon," which differs from advancing monsoons in parameters of humidity, intensity, and climate change.
The temperature in the northern part of India begins to decline in October because of the sun's rays moving south of the Equator and the advent of the winter season. This weakens the low-pressure system and weakens the southwest winds, resulting in the receding of the advancing monsoons that have already started to wane.
The retreating monsoons are often confused with the north-east monsoons because of their occurrence at similar times. But they are two different things. Receding Monsoons or retreating monsoons occur from October to January and are usually a clear period, while north-eastern monsoons occur in late December and are centred in the north-eastern regions.
While the disappearance of clouds makes the climate relatively dry, the soil retains its wetness during this time. The withdrawal period of retreating monsoons is gradual, unlike that of advancing monsoons, which have a quicker onset period. This period is also associated with a sudden increase in temperature because of receding clouds and is referred to as "October Heat."
Rainfall during this period is unevenly distributed because of the irregular intensity of the wind and the impact of low pressure that is greater near the water bodies and reduced in the central regions. While blowing from the northeast to the Bay of Bengal, it picks up moisture from the ocean and releases it in the southern- peninsular areas.
Retreating monsoons are the transition period from hot, rainy weather to cold winter. As the monsoon starts retreating towards the south, the pressure gradient becomes low, giving rise to cyclonic conditions. Several cyclones emerge during this time and cause immense damage in parts of Eastern India, especially Odisha, Andhra Pradesh and West Bengal. The retreating monsoon brings rainfall in varying amounts to different places across India. Some areas receive heavy rainfall, while others witness scanty rain. While parts of Gujarat, Maharashtra, Western Madhya Pradesh, Andhra Pradesh, Karnataka, Eastern Rajasthan, Punjab, Haryana, and parts of Uttar Pradesh receive little or no rain, Western Rajasthan, Kutch, and the Ladakh region of Jammu and Kashmir receive little or no rain.
During this time, the slopes of the Western Ghats, a large area of the northern plain, and the coastal belts of Odisha, Andhra Pradesh, and Tamil Nadu receive a lot of rain. The north-eastern regions of India, like Assam, Meghalaya, Mizoram, and Nagaland, receive very heavy rain.The southwest monsoons withdraw from the Coromandel coast in mid-December. In Punjab, the southwest monsoons withdraw in the second week of September. Tamil Nadu gets the most rainfall out of the retreating monsoons, partly because its location does not enable much rain from the advancing monsoons and partly because of the tendency of the retreating monsoons to pick up moisture from the Bay of Bengal before reaching Cape Comorin, resulting in heavy rainfall in the southernmost regions.