How a South Asian monsoon occurs (Southwest and Northeast)?

      

    Monsoon is one of the seasonal winds that bring a huge amount of rain over the tropical regions during a particular season. The monsoon wind comes under the classification of global air circulation which blew over the earth, due to the temperature difference in it. Global air circulation carries the main responsibility to satisfy the temperature over the earth.

To know more about the global air circulation with a clear representation, please visit the below article,

https://sciencetopic03.blogspot.com/2021/12/what-is-global-air-circulation-and-how.html

The word monsoon comes from the Arabic word “mausim” which means seasons. Monsoon is normally a sea breeze, happening on an immense scale. A sea breeze is the flow of wind from sea to land. During the daytime, when the land gets hotter than the ocean, a pressure difference will be created. This pressure difference causes the movement of air from the high-pressure region to low pressure region. This movement of air is called as wind. The monsoon winds happen over in many regions of the world like South Asia, Africa, Australia, and the Pacific coast of Central America. Among all these, the most noticeable monsoon that brings a dramatic change is the South Asian monsoon (Indian Monsoon) which majorly affects the climate of India. In this article let’s see about the monsoon wind patterns and how does it occur over India.

Creation of Wind Flow

As India is an agricultural country, most of the agricultural parts of India depend upon the Monsoon rain. A slight delay in the rain could greatly affect the Indian economy. The monsoon season occurring in the Indian subcontinent is majorly classified into two types, namely southwest monsoon (summer monsoon) and northeast monsoon (winter monsoon). The Indian monsoon season starts with the southwest monsoon and ends with the northeast monsoon. The southwest monsoon brings rain majorly to the western coast of India, while the northeast monsoon brings rain to the eastern coast of India. The power of the summer monsoon is higher than the winter monsoon. This is because, as the summer monsoon blew over the northern parts of India, the Himalayas prevent the cool air from reaching South India as it retreats. The summer monsoon blew over India, over the month of June to August and the winter monsoon blew over the month of September to February. These monsoonal winds occur at a huge pattern due to the huge temperature contrast between the Indian plate and the Indian Ocean. Now let’s see the principle of occurrence.

Difference between the Southwest and Northeast Monsoon

During the month of March, April, May, the northern hemisphere starts to face the sun more than the southern hemisphere. This causes the Indian plate to get hotter than the water body surrounding it. By getting hot, the air over the surface of the land gets rises up and creates a low pressure over the area. Similarly, air over the ocean will be comparatively cooler than the land, which makes it to sink and creates a high pressure. This high pressure is named as Mascarene high. As a result of both the pressure, a pressure difference will be created and the movement of airflow between the two pressure zones takes place. This flow of wind from the sea to land is called the southwest monsoon.

Southwest Monsoon

As the southwest monsoon winds start to enter the Indian plate, it enters the Indian subcontinent through the southwest direction thereby initiating the monsoon season. The reason why it enters in the southwest direction is due to the shifting of the Intertropical Convergence Zone (ITCZ) which we will see later in this post. The monsoon winds enter the Indian subcontinent in three main different branches. One collides with the Western Ghats, which causes the air to rise up and makes condensed to form clouds and rain. The first place to experience rainfall in India is Kerala in the Western Ghats region. The second branch flows over the Bay of Bengal and collides with the Eastern Himalayan Ranges. And the third one proceeds to the Thar Desert. Each brings rainfall to the respective areas. Among all these places, the place of Mumbai on the western coast and Mawsynram in the Northeastern part experience severe rainfall during this season. Particularly, the place Mawsynram receives the highest rainfall of 1872cm of all over the world.

Northeast Monsoon

      After traveling over the northern states, the winds collide with the Himalayan regions which stop the further progression of the winds. Now, most of the states will get cooler and pressure over them increases. At this time (September to November), the sun starts moving down to the equator, thus shifting the low-pressure zone over the ocean. This causes the partial reversal of wind from those new high-pressure zones to the low-pressure zones. Thus the pre-monsoon ends and the retreating monsoon starts. During this retreating season, the eastern coast receives the majority of rainfall. Whereas, during the pre-monsoon season, the western coast receives the majority of rainfall. The southwest monsoon carries more moisture as it from sea to land. But during retreating monsoon, it will be so dry. So only a part of these winds, that blew over the Bay of Bengal picks up the moisture and gives rain to Andhra and Tamil Nadu. These winds blew from northeast to southwest direction. During the months of December to February, the retreating peaks, where the sun moves from the equator to the Tropic of Capricorn. This creates a complete high pressure over the land and low pressure over the ocean. This causes the full reversal of the air, from the high-pressure region to the low-pressure region (land to sea). The movement of air during this period of monsoon is called the Northeast monsoon. This is similar to the southwest monsoon but brings air from land to sea. A detailed diagram of the movement of the sun with respect to the different monsoon seasons is shown below.

Different monsoon seasons

 As the monsoon winds bring rain only when it collides with the mountains, it is termed as orographic rain. The word orographic is related to mountains. To know more about the orographic process, please visit the following article,

https://sciencetopic03.blogspot.com/2021/12/difference-between-snow-sleet-freezing.html

Orographic Rain

    

Tropical Cyclones

    One of the interesting features of the northeast monsoon is, it leads to the formation of many cyclones in the Bay of Bengal. The Bay of Bengal is considered as the hotbed for tropical cyclones. Also, these tropical cyclones happen only during the northeast monsoon and not during the southwest monsoon. This is because during the northeast monsoon the Bay of Bengal region starts facing the sun more, and creates the heat around the ocean. This picks up low-pressure zones (warm ocean water is required) over the oceans. But during the southwest monsoon, there will not be required heat for the cyclones to be created due to the inadequate moisture. For a cyclone to occur there must be enough heat and enough moisture surrounding it. If any of them is made cut the cyclone disappears. That’s why cyclones are disappeared when reaching land. Also, we can observe, that the Bay of Bengal experiences more cyclones than the Arabian Sea. This is because the Bay of Bengal is warmer than the Arabian ocean due to its fresh water and saltwater contrast and its non-mixing property. The fresh water in the Bay of Bengal gets continuously warmed up and stays afloat and creates a hotbed for tropical cyclones.

Before the monsoon takes place over the Indian subcontinent, a fair few atmospheric changes will also be taken place, which strengthens the monsoon winds, thereby marking the remarkable period for the people of India.

The first atmospheric change that takes place over the northern hemisphere is the shifting of the ITCZ (Intertropical Convergence Zone). This is the zone where the trade winds from both the northern and the southern hemispheres get converged. The ITCZ is exactly situated in the equator as the Sun’s concentration is more at the equator. Now as the sun shifts north during summer, the ITCZ also shifts north as part of the global air circulation. Due to the shifting, the trade winds that are meant to converge at the ITCZ will need to travel further north to converge at the ITCZ. This extra movement makes it cross the equator, which changes the direction of the trade winds from west to east. This change in direction is due to the Coriolis force.

Shifting of Intertropical Convergence Zone (ITCZ)

Coriolis Effect

        The Coriolis Effect happens due to the spinning of the earth. As the earth spins, the equator experiences a much higher speed compared to the poles. So the wind present on the equator moves much faster than the wind present on the poles. If the air from the equator moves towards the poles, it is always propelled ahead (led); because the areas around the equator are moving much slower than the equator. Similarly, when the air from the poles moves to the equator, it is propelled backward (lagged); because the area around the poles is moving much faster than the pole regions. That’s why the trade winds change their direction when crossing the equator. And this principle carries the main responsibility for the monsoon winds entering in the southwest direction to the Indian plate.

Deflection of trade winds due to Coriolis effect

The next atmospheric change that happens is due to the jet stream. Jet streams are a kind of fast-moving wind that flows over the tropopause. There are generally two kinds of permanent jet streams namely Subtropical Jet stream and Polar jet stream. As the Subtropical Jet stream flows over the subtropical region, it is also made to move over the northern part and gets altered as the Tropical Easterly Jet. This happens due to the extreme heat in the Tibetan plateau.

Creation of Tropical Easterly Jet Stream

Creation of Tropical Easterly Jet Stream

There is also a formation of the non-permanent jet stream called Somalian Jet. This jet gives an extra push to the monsoon winds. This jet occurs due to the heavy temperature contrast between the Indian Ocean and the African Coast, which creates a huge sea breeze over the coast that is diverted by the Coriolis force to strengthen the Indian Monsoon winds. 

Creation of Somali Jet stream

To know more about the formation of Tropical Easterly Jet, Somali Jet, and the principle of formation Jet streams, please visit the following article,

https://sciencetopic03.blogspot.com/2022/03/jet-streams-types-of-jet-streams-effect.html

So the combination of the ITCZ, Tropical Easterly Jet, Somali Jet, and the differential pressure in the land and sea, converts the monsoon into a sudden outburst in the regions of India.

Even though so many factors support the Indian monsoon, some things affect the monsoon over a certain period. These are called climate drivers which can change a prevailing climate. When coming into the monsoon season, climate change is caused by the effects of ENSO (El Nino Southern Oscillation) and Indian Ocean Dipole (IOD). These effects are carried out by the sea surface temperatures. If the sea surface temperatures vary in the Pacific Ocean or the Indian Ocean, it will have a large impact on the Indian monsoon. So the climate drivers play a vital role in determining a season. Let’s first see about ENSO and its effects on the Indian monsoon.

The word El Nino means “Christ child”, which is named due to its time of occurrence during Christmas. El Nino is a temperature driving force, which occurs at the water surface, rather than between land and sea in the case of monsoon. The affecting period of ENSO will be once in 2-7 years. This effect occurs in the Pacific Ocean. Due to the large surface area of water, effects like El Nino develop over this area. And particularly this effect happens over the south pacific of the equator.

During a normal year, the trade winds converging over the Intertropical Convergence Zone, blow at a strong pace, which pushes the warmer pacific water to the west side, as shown in the figure. So the warmer water begins to evaporate and causes the air to rise and get clouds to form and rain. Meanwhile on the eastern coast of the pacific, cold deep water will emerge from the bottom of the ocean leading to rich mineralized content. This creates a great fishing activity during the time at this place. This process is called coastal upwelling. As the clouds become dry over the western coast, it begins to move to the eastern coast. Now it sinks down and makes the cycle continues. This is the progress during the normal course of a year.

During normal year

But during an El Nino year, due to the weakening of the trade winds, the two ocean currents exchange their place where the warm water surface takes the eastern coast and the cold deep water takes the western coast. Now the pressure difference also exchanges and makes the low pressure over the eastern coast and high pressure over the western coast. During El Nino, the low-pressure condition developed across the Peruvian coast draws up the cold ocean currents from the west side. This decreases the intensity of the Mascarene high pressure developed in the Indian Ocean; thus reducing rainfall to India. And not every ENSO affects the Indian monsoon.

During El Nino year

Indian Ocean Dipole

       Unlike the El Nino which happens in the Pacific Ocean, the Indian Ocean Dipole plays its part in the Indian Ocean. It too concentrates on the sea surface temperatures. As shown in the picture, the IOD has three cycles. During the positive phase, the trade winds will be strong, which pushes the warm currents over the western coast and cold currents over the eastern coast. Thus rain clouds and floods will occur over the western coast. During the negative cycle, the westerlies will be strong and the trade winds will be weak. Thus the warm currents will shift to the eastern coast. Thus the eastern coast will experience rainfall and the western coast experience drought. A Neutral phase will have a calm season over both the coast. A positive cycle is always better for a good Indian monsoon. A positive IOD can even result in a good monsoon even it was an El Nino year. This is how a monsoon season is categorized and delivered by Mother Nature!

Positive Cycle of IOD

Negative Cycle of IOD