The Equatorial Westerly Theory has been propounded by Flohn. He states that the equatorial westerly is the south-west monsoon that is originates because of inter-tropical convergence.
The Equatorial Westerly
According to him, the thermal effect as the main reason for the origin of the monsoon. He suggested that the tropical monsoon of tropical Asia is simply a modification of the planetary winds of the tropics. He thinks of the thermal low of northern India and the accompanying monsoon as simply an unusually great northward displacement of the Northern Inter-Tropical Convergence Zone (NITCZ). The seasonal shift of the ITCZ has given the concept of Northern Inter- Tropical Convergence Zone (NITCZ) in summer (July) and Southern Inter-Tropical Convergence Zone (SITCZ) in winter (Jan.). The fact that the NITCZ is drawn to about 30° latitude may be associated with the unusually high temperature over north India.
According to this interpretation the main westerly current of the monsoon is simply the expanded equatorial westerlies which lie embedded in the great mass of tropical easterlies or the trade winds. NITCZ is the zone of clouds and heavy rainfall.
In summer, the inter-tropical convergence (ITC) happens in the north of the equator because of the northern movement (sometimes up to the Shiwalik foothills) of the thermal equator. The equatorial westerly now begins blowing towards the low pressure created on the Indian peninsula. It is the south-west monsoon. The Inter Tropical Convergence Zone (ITCZ) is known as the monsoon trough. In winter, the area of low pressure changes into that of high pressure because of winter solstice and the north-east trade winds once again become active.
Interpretation of South-west Monsoon by the Equatorial Westerly Theory
This theory was based on the thermal effect, it was also criticised on the basis on which was done of the thermal theory.