Sample essay on the East Asian Monsoon

The East Asian monsoon signifies the seasonal wind reversal over China, Korea, Japan and Taiwan. These countries receive cold air from the Siberian anticyclone. Be­cause of the intensity of high pressure system over central Asia, the cold, dry and stable winds, which generally blow from a westerly direction, are more constant than the summertime winds.

That is why winter monsoons over China and Japan are stronger than summer monsoons. In January the velocity of the cold air-streams is many times higher than that of the surface air-flow in July.

However, the prevailing wind direction is disturbed by local cyclones and anticyclones. Moreover, a perusal of the daily synoptic charts makes it clear that the Siberian High is not of permanent nature, but is only more durable than the thermal lows of Central Europe and Russia.

The outward blowing winds from the Siberian anticyclone draw towards them, families of depressions. These winter depressions follow different routes which ultimately converge on Japan. The cold winter monsoon blows in strong and short bursts, but the dominant current prevails. Generally these winds are dry and stable.

But when such winds are forced to rise by some physical barriers, they yield a little precipitation mostly as snowfall. Along the west coast of Japan, the winter months receive a little higher amount of precipitation than other regions.

In summer, the heating of the continent causes pressure to become generally low. Now there is a shallow depression over the continental interior and a semi-permanent thermally induced low over south-east China.

On the contrary, the sub-tropical high over the Western Pacific is displaced towards north with the mean position of its centre at 40°N, 150°W. At the same time, a semi-permanent high is formed over the Sea of Okhotsk.

According to Trewartha, the summer circulation pattern of the East Asian region consists of three basic air currents. North of about 40″N the deep zonal westerlies dominate. In the upper air, there is a widely fluctuating jet stream at about 40 to 45°N.

The jet stream is characterized by a highly meandering course. To the south and east of the zonal westerlies there is found a current of southwesterly winds which is referred to as the southwest monsoon. It originates in the equatorial region of the Indian Ocean.

According to some meteorologists, the south-west monsoon winds are simply the pole-ward extension of equatorial westerlies which cover a large part of South Asia during the summer months. These air-streams from the warm tropical oceans supply abundant moisture to the East Asian atmosphere.

Contrary to the widely held belief that most of the summer rains in this region are derived from the moisture supplied by the Pacific Ocean, it is in fact the Indian Ocean which is the primary source of supply of moisture for most of the summertime precipitation in East Asian regions.

However, it is true that some Pacific moisture is also drawn towards the continents by westward moving cyclones. Besides, on occasions the sub-tropical southeasterly winds also carry some moisture over the continent from the Pacific Ocean.

The third basic current is the tropical easterlies which flow towards East Asia from the south-east, south, and even south-west. This current picks up moisture from the Pacific Ocean.

However, since these tropical winds are relatively more stable than the southwesterlies, they are not so conducive to summer rainfall in this region. But eastern subtropical China and Southern Japan are exceptions, because these regions receive most of their summer rains from air-streams originating in the Pacific.

Atmospheric disturbances associated with the polar front account for much of the heavy summer rainfall in North China and Manchuria. However, the rainfall of eastern Asia according to certain Chinese climatologists is not of monsoonal origin.

The warm and moist air blowing from the Pacific Ocean also does not contribute substantially to the precipitation in this region. Thus, there are actually two important sources of annual precipitation in eastern Asia: (a) southwesterly winds originating in the equatorial region, and (b) extensive travelling disturbances which develop along the polar front that lies between the zonal westerlies to the north and the equatorial southwesterlies farther south.

As pointed out earlier, the winter monsoon of Eastern Asia originates in the Siberian anticyclone. In its source region in the Baikal area the northwest winter monsoon is extremely cold, dry and stable.

It has not high potentiality for precipitation. The land or sea trajectory of the winter monsoon, and the configuration of the land over which it has travelled determine the temperature of north westerly winds.

During winter China, Japan, and Korea come under the influence of northwesterly cold and dry winds with little or moderate to scanty precipitation. However, the polar front separating the Siberian cold air-currents from the warm and humid trade winds breaks the monotony by giving birth to numerous weather disturbances.

It may be mentioned that the winter monsoon current is shallow and is overlain by the zonal westerlies as well as the jet streams which steer cyclonic storms along their routes. A noteworthy feature of winter-time atmospheric disturbances of eastern Asia is that they are rather smaller in size, less intense, and that their movement is slow.

These characteristics develop because of the upper convergence zone. On the contrary, cyclone development over north-eastern China is more pronounced. Most of these atmospheric disturbances, it may be pointed out, originate as secondary cyclones from the primary disturbances travelling from the west over western Siberia. Even these disturbances fail to give much precipitation.

The reason appears to be the dominance of dry and stable air in lower layers of the atmosphere over this region.

The East China Sea, the ocean area south and east of Honshu, and the Japan Sea are the principal areas of cyclogenesis.

From the foregoing discussion it is evident that the source region of winter monsoon of East Asia is the thermally induced shallow anticyclone of northeast Asia. Over a larger part of Eastern I Asia there is dominance of the polar continental air mass.

But when the cold, dry, and stable winds advance towards the coastal regions or to the south, they are considerably modified by their ascent and descent.

Whenever the winter monsoon winds are more vigorous and steady, the number of distur­bances is less, and the weather is extremely cold and dry.

However, in the coastal regions and towards the south, where these dry and cold polar winds come into contact with the warm and moist maritime air masses, a large number of cyclones originate along the front yielding heavy precipitation. South China and Japan receive heavy precipitation in winter from these distur­bances.

To summarize, the summer monsoon of eastern Asia is weaker than the winter monsoon. The summer monsoon makes the weather of China and Japan hot and humid. Both the temperature and relative humidity of these countries during the summer season are very high.

The southwesterly or southeasterly winds have a high potential for heavy precipitation because of the fact that while blowing over the tropical warm oceans they pick up a large quantity of evaporated moisture.

The winter monsoon, on the other hand, is generally cold, dry, and stable. It has no potential for heavy precipitation. But, wherever the out blowing continental air-streams have their trajectory over the relatively warmer seas, they do pick up some moisture from the water surface and yield a little precipitation on the wind-ward slopes of the elevated coastal highlands or mountain barriers.