Weather Bee: How did monsoon restart after a three-week pause?
Cyclonic circulations that developed on both coasts this week have strengthened the monsoon winds and driven away the dry air from the west.
After staying stationary for around three weeks, the Southwest monsoon finally started advancing beyond the peninsular and north-eastern regions on June 16. This long pause is one reason why the monsoon was almost on schedule for eastern Bihar and northern Maharashtra although it arrived on the Kerala coast eight days before schedule. What explains the long pause and the progress thereafter? The short and technical answer is that cyclonic circulations that developed on both coasts this week have strengthened the monsoon winds and driven away the dry air from the west. Here is how that happened explained in a simpler way.
Monsoon is commonly understood as the season of rainfall. Even by that definition – which is correct but can’t help us in finding the precise day the season begins or ends – the season seems to have made progress this week. As the maps below show, most regions beyond peninsular India were very dry in the week ending June 15. This changed in the two days ending June 17, when rain increased in most parts of India, but particularly over Gujarat and the northern parts of the east coast.
To be sure, as of June 17, the limit of the monsoon technically passed only through Gujarat, Madhya Pradesh, Chhattisgarh, Jharkhand, and Bihar. In other words, while regions north of these states might have been rainy on June 16 and June 17, the monsoon had not arrived there. How do we know this? As this column explained earlier, monsoon is the rainy season for most of India because of several atmospheric patterns. These atmospheric patterns had developed only up to the regions described above until June 17.
The understanding of the monsoon as the presence of particular atmospheric conditions can also help in understanding why it stalled for around three weeks. One reason, as shown in the map below, is that dry winds were blowing in from the west over large parts of north-western India and even parts of central India. This prevented the further development of the strong westerly (blowing from the west) winds even over Maharashtra, a region where these types of winds generally prevail under active monsoon conditions.
To be sure, the forces that guide the movement of the wind are pressure differences and the earth’s rotation. The combination of these two forces is such that wind generally moves in an anticlockwise direction around regions of low pressure in the northern hemisphere. As a map of wind and pressure from last week shows, the pressure gradients at the time were not helpful for drawing the westerly wind over peninsular India further northward. This has changed now. There are two regions of low pressure -- one around Gujarat and another around southern west Bengal – that have created anticlockwise wind movement. (Meteorologists call these winds cyclonic circulations because the movement of the wind is like a cyclone, although the wind speed is much slower than in a cyclone.) These regions of low pressure have pulled the moist, westerly monsoon winds further northward this week.
It is the changes in pressure and winds described above that have made the week so far rainy across large parts of India and jump-started the monsoon’s progress again. However, since the monsoon winds had only progressed up to the bottom half of the country, regions further north are rainy because of factors other than the monsoon winds.
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