Indian Monsoon

The Indian monsoon affects India and its surrounding water bodies. It blows from the north-east in the winter and from the south-west in the summer. During June and July, this process brings heavy rains to the region. The surface easterlies (winds) only reach latitudes near 20° N in February, and even then, they have a strong northern component. They retreat northward quickly, changing the upper-air circulation. This is the time between one monsoon and the next. The high-sun season reaches the Equator in late March and moves north. With it comes atmospheric instability, convection clouds, and rain. Northern India is still dominated by the westerly subtropical jet stream, with north-east north-easterly surface winds.

Reason for Different Types of Indian Monsoon

Peninsular India has a much cooler climate than northern India. Heatwaves are widespread in northern and central India now. North-west India continues to be dry and hot. Generally, thunderstorms/thunder showers in the evening occur in Rajasthan, Punjab, Haryana, and Uttar Pradesh. At least three other states have evening thunder showers. This type of a furious thunderstorm is known as “Andhi” or “North-western Storm.” Except for the Deccan plateau, tropical convergence and equatorial westerlies move north to bring rain on the Indian Peninsula. Kerala and the west coast get rainy. Early June is when the Kerala coast experiences its monsoon season.

Understanding Indian Monsoon 

The Indian Peninsula receives rain due to inter-tropical convergence and equatorial westerlies heading north. Northeast monsoon or winter monsoon dominates peninsular India, and lasts from May to February. Most of India is rainy from June through September. The south-west monsoon contributes to around 75% of India’s annual rainfall. The north-east monsoon brings rain to Tamil Nadu, Andhra Pradesh, Karnataka, and Kerala. October brings blue skies as temperatures rise by mid-October and decrease faster than in south India. Travelling to the north-west of India takes about a month. Now, let’s understand the different types of monsoons in India.

The Indian monsoon consists of four significant seasons, which are:

1. Winter Monsoon
2. Summer Monsoon (Pre-Monsoon Season) 
3. South-West Monsoon Season
4. The North-East Monsoon or Post-Monsoon Season/ Winter Season (January – February)

Summer Monsoon / Pre-monsoon season (March-May)

At this time, a low-pressure area progressively replaces the high-pressure area. Seasonal cyclonic winds acquire strength. During May and June, north-west India and Pakistan are susceptible to high solar radiation. This causes a thermal high, which causes warm air to rise, causing a low-pressure area—the thermally produced low-pressure zone. North-west India has the highest temperatures during this season. Rajasthan’s midday temperatures range from 40°C to 50°C. Peninsular India’s temperature is substantially cooler than northern India’s. Heatwaves are widespread now in north and central India. North-west India continues to be dry and hot. Northwest India is primarily arid and hot, but intense convection can generate thunderstorms in Rajasthan, Punjab, Haryana, and Uttar Pradesh. Evening thunderstorms are likely in Assam, West Bengal, and Bihar. Because they appear to be coming from the northwest, these severe thunderstorms are termed ‘andhis’. The Indian Peninsula receives rain due to inter-tropical convergence and equatorial westerlies heading north. Pre-monsoon showers hit Kerala and the western coast. The Indian monsoon season usually begins in late May or early June along the Kerala coast.

Winter Monsoon (January to February)

Anticyclonic winds characterise the cold weather season. Winter monsoon. It lingers till February. Cold winds dominate north-west India. These are dry, cold, and persistent winds. That means generally clear skies. Temperature distribution in India decreases from south to north. This period is characterised by a cold air mass across northern India, causing chilly waves. Nocturnal temperatures are common. Cold advection from the north causes frosty nights this time of year. Subtropical westerly jet stream moves over Northern India at 12 km. The central core of the jet stream can reach speeds of up to 400 km/h. Westerlies dominate north of 20°N, while north-easterlies dominate the south of this latitude. Rainfall from western disturbances moving across northern India falls on northwest India. Winter rains help rabi crops.

These western disturbances also alter the climate, favouring crop plants. The north-east monsoon, or winter monsoon, dominates peninsular India. India’s south-eastern coast receives rain.

 Other Monsoons

• South-west monsoon (June – September)

A seasonal low-pressure system has formed across north-west India and Pakistan due to the extreme warmth. Across northern India, southeasterly winds have supplanted westerlies. A monsoon trough extends from the seasonal low-pressure area to the head Bay of Rajasthan, Uttar Pradesh, Bihar, and West Bengal.

There is a south-to-north pressure gradient in the Indian Ocean, and the temperature drops from north to south. Winds are southeasterly north of the trough. West to south-west winds prevail south of the monsoon trough, whereas south-east winds dominate south India. The subtropical westerly jet stream alters 12 km north of the Himalayas. Tibetan High, a tropospheric anticyclone, has developed above Tibet. The monsoon persists across India due to semi-permanent synoptic elements. Water is needed for agriculture. Success in farming depends on the monsoon. Duration of the wet season impacts agricultural productivity. June–September is the rainy season in India. In early June, the monsoon crosses the Kerala coast. North-west India travel takes around a month. Storms and squalls mark the Indian monsoon. After the monsoon, the weather becomes calmer and less stormy. Most of India’s yearly rainfall comes from the south-west monsoon. North-west and northern India get their rain from western disturbances. In southern India, the rains occur from north-east monsoon or pre-monsoon thundershowers.

• North-east Monsoon/ Post Monsoon (October – December)

The south-west monsoon is retreating from several parts of India. West Indian low-pressure area is replaced with the anticyclonic high-pressure area. Over central India, dry northwestern or western winds replace most south­easterly winds. In the Southern Hemisphere’s tropical regions, high temperatures cause a weakening of the high-pressure area over the Indian Ocean. Anticyclonic circulation strengthens when the ridgeline crosses central India. Over south India, north-eastern north-easterly winds replace westerly winds. Initially dry over land, these winds become moist as they cross the Bay of Bengal. The northeast monsoon brings rain to Tamil Nadu, Andhra Pradesh, Karnataka, and Kerala. October brings blue skies. As a result, temperatures in north India rise by mid-October and decrease faster than in south India. Consequently, north Indian temperatures are lower than south Indian temperatures. Although the north and northwest of India have dry weather, western disturbances might create a shortage of rainfall.

Conclusion

Indian monsoon blows from the north-east in the winter and from the south-west in the summer. Northern India is dominated by the westerly subtropical jet stream, with north-eastern north-easterly surface winds. Most of the Deccan plateau remains dry, except The Indian Peninsula, which gets rain due to inter-tropical convergence and equatorial westerlies flowing north. Anticyclonic winds characterise cold air mass across northern India. North-west India has the highest temperatures during this season.