During winter, the intense cooling of the huge landmass of Asia leads to the formation of a high pressure system over the continent. The centre of the unusually strong anticyclone lies in the vicinity of Lake Baikal in eastern Siberia. The average pressure at its centre is about 1035 mb.
Another secondary anticyclone is found centered in the vicinity of Peshawar in the northern part of Indian subcontinent where the maximum recorded pressure is 1040 mb. In winter, the adjacent oceans are warmer than the extremely cold land mass of the continent.
The whole of North Pacific is covered by ‘Aleutian Low’ with 1002 mb pressure at its centre. Because of the presence of this semi-permanent depression the subtropical ‘Highs’ are displaced to the coast of North America.
There is another low pressure system developed in the equatorial zone of Indian Ocean. Since it is summer in Australia, there also is a low pressure centre.
Thus, during the winter season Asia is dominated by high pressure centers from which there is an outflow of air that spreads towards the south and southeast over Korea, China and Japan.
These continental polar air masses are cold and dry. The pressure gradient which is from land to sea sometimes becomes very steep which results in high velocity winds.
The continental air blowing out from central Asia towards oceanic low pressure centers is termed ‘the winter monsoon’ or ‘the dry monsoon’.
In north and central China the direction of dry and cold winds during the winter months is from northwest and north respectively. Winds blowing out from the central Asian deserts are mostly in the form of dust storms.
However, when these winds have a maritime trajectory, they are moisture-laden and have some potentiality for precipitation. The western coastal region of Japan receives moderate amount of precipitation in the form of snowfall or rainfall.
The air streams from the extremely cold continent later on unite with the north-east trade winds. Now, the trades start blowing from the north-easterly direction towards the south-west.
Because of high mountains of the Himalayas and the Tibet plateau, the cold and dry air from Siberian anticyclone cannot penetrate into the Indian sub-continent.
The winter monsoon over the sub-continent is less vigourous than its counterpart in China. As a matter of fact, the effect of relief is clearly discernible on both the winter as well as the summer monsoons in the Indian sub-continent.
In summer, the temperature and pressure conditions in the Asian continent are reversed. In this season the huge land mass of Asia heats quickly and a strong low-pressure centre develops over there. This development is reinforced by the Intertropical Convergence which reaches its maximum pole-ward migration to a position over Southern Asia.
The thermally-induced low attracts warm and moist air from the oceans. It may be pointed out that the tropical maritime air masses which are unstable and full of moisture cannot by themselves yield precipitation. Precipitation results only when the moisture-laden winds are forced to rise by a landform barrier or any atmospheric turbulence.
Upper air observations have proved that the abrupt onshore movement of hot and humid tropical air in July is caused by changes in the air circulation aloft. The tropical unstable air, when forced to rise by any landform barrier, brings about heavy precipitation. The foothills of the Himalayas, the Western Ghats of India and the Annamese Highlands of Vietnam are the typical examples.
To sum up, the vigour and the development of an ideal monsoonal circulation in Asia may be attributed to two most potent factors, viz. vast size of the continent of Asia, and the east-west high mountains.
These factors are largely responsible for bringing about the winter and summer temperature extremes, which in turn cause seasonal reversal of winds. Depicts the Asiatic monsoon circulation in winter.
Circulation near the earth’s surface has been shown by solid arrows, while circulation in the upper air is shown by dashed arrows. It shows the Asiatic monsoon circulation during summer. Low level circulation is shown by solid wind direction arrows, and upper winds are shown by dashed arrows.