Monsoon Mechanism

The word monsoon derived from the Arabic word mausim means seasonal winds. In this system, the direction of the winds reverses seasonally. The first thing we note is that Monsoon is typically considered a phenomenon of tropical south Asia, but it is also experienced over parts of North America and Africa.

Mechanism of Monsoon: Traditional View

Traditionally, monsoon has been considered a result of the differential heating of land and sea.

• In summer, southern Asia develops a low pressure while the pressure over the sea is relatively higher. As a result the air starts flowing towards land from the Indian oceans. The winds coming from ocean carry moisture and thus cause rainfall in summer reason. This is known as the southwest monsoon or summer monsoon.
• In winter, the pressure over land is higher than over the sea and consequently the air starts flowing from land to sea. The air coming from land being dry, these winds do not cause rainfall.

The above explanation is known as the thermal theory of monsoon. This theory explains monsoon as a  regional phenomenon but fails to  explain the total amount of energy / processes involved in the global monsoon circulation.

Mechanism of Monsoon: Modern View

The modern meteorologists seek explanation for the phenomenon of monsoon on the basis of seasonal shift in the position of the global belts of pressure and winds. This is also known as Dynamic Theory.

According to the dynamic theory, monsoons are a result of the shift of the inter-tropical convergence zone (ITCZ) under the influence of the vertical sun. Though the average position of the ITCZ is taken as the equator, it keeps shifting vertical sun towards with the migration of the vertical sun towards the tropics during the summer of the respective hemisphere.

• During summer in the northern hemisphere in the months of May and June, the sun shines vertically over the tropic of cancer. Due to the northward shift of the zone of maximum heating and low pressure at this time the ITCZ also shifts northwards and approaches, the tropic of cancer. The ITCZ being the zone of the lowest pressure in the tropical region is the destination of the trade winds blowing from both the hemispheres.
• With ITCZ situated close to the tropic of cancer the northeast trade winds are confined to an area extending to its north while the southeast trade winds blowing from the southern hemisphere have to cross the equator to reach this area of low pressure. However  as the winds  blowing  from the southern  hemisphere cross the equator  their direction is altered due to Coriolis effect, i.e. they are direction is their right  and thus it give rise to the formation  of a belt of equatorial  westerlies  in the months of many of June northeast and they are called the southwest monsoon.
• As the ITCZ again moves southwards at the end of the summer of the northern hemisphere the areas north of the equator which experienced  the equatorial  westerlies during the summer season come under the influence of the northeast trade winds. These northeasterly winds are called the northeast  The onset of winter season the ITCZ shifts south of the equator and reaches as far south at this time.  in this season the northeast trades blowing towards the ITCZ have to cross the equator  towards  south and as a result they get deflected giving rise to the equatorial westerlies in the southern hemisphere. these westerlies blow form the northwest to the southwest, replacing the trade winds of the southern hemisphere between the ITCZ and the equator. They form the summer monsoon of the southern hemisphere.

We can say that due to the seasonal shift of the wind belts under the influence of the north-south migration of the vertical sun the areas situated in the tropical  zone in the both the hemisphere come under the influence  of the trade winds during the respective winter and the equatorial westerlies during the respective  summer season. The direction of the winds is thus reversed seasonally and it makes up the monsoon system of these regions.

Please note that though, dynamic theory provides a much better explanation of the system of monsoon as a global phenomenon, it does not negate the influence of differential heating of land and sea.

Differential heating still plays an important  role in making monsoon much stronger in certain  of the south-west monsoon factor that explains  the extension  of the southwest monsoon even to the north of the tropic of cancer in northern India.