Fig 2: Formation of ITCZ
This concept was propounded by H. Flohn of German Weather Bureau in 1951. As per him, monsoon system of tropical Asia is a consequence of the seasonal changes in the planetary wind system. These seasonal changes are the result of the seasonal swing of temperature and pressure belts in this region due to changes in overhead position of sun. These planetary winds of tropics are known as trade winds.
During the month of March and September, sun is overhead the equatorial area in tropics. This leads to intense heating which creates a belt of low pressure region. This low pressure belt attracts the north-east trade winds from northern hemisphere and south-east trade winds form southern hemisphere. Convergence of these two trade winds in this belt leads to ascending to air which creates a low pressure situation. This low pressure belt this is known as Inter-tropical Convergence Zone (ITCZ).
With the change in the apparent position of sun towards tropic of cancer, ITCZ changes its position. In July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough. This monsoon trough creates low pressure area over north and northwest India. Due to this shift of ITCZ, the south-east trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes. These trade winds change their direction due to the Coriolis force and start blowing from southwest to northeast. Therefore, it is known as ‘southwest monsoon’.
In winter, due to apparent movement of sun toward tropic of Capricorn, the ITCZ moves southward, and so the reversal of wind direction takes place in Indian sun-continent. Now, the wind blows from northeast to southwest. Therefore, winter monsoon is known ‘northeast monsoon’.
