Brief notes on the Flohn’s Concept of Monsoon

Flohn published a memoir in 1951 in which he proposed a new explanation of the monsoon. According to him, what is called a monsoon is only the normal seasonal migration of planetary winds following the sun. In fact, it is because of the normal swing of planetary circulation zones that there is a periodical change of wind direction in the lower and the upper levels of the atmosphere.

During the northern summer all the wind belts are displaced towards the north, while during the northern winter they shift towards the south following the apparent movement of the sun in its zenith. The result is that the mean zonal axis of equatorial winds coincides with the 10° N parallel in July and 5° S parallel in January.

The zonal axes of subtropical highs are also slightly displaced. Under these conditions, the regions occupying an intermediate position between two planetary zones are encroached upon by two contrasting annual wind regimes. The monsoon, therefore, represents the succession of these two regimes.

Thus, the so-called winter monsoons of the Indian sub-continent, Indo-China and South China in southern Asia, West Africa, and Northern Australia are nothing but the tropical easterlies, or the northern trade winds.

In summer, the equatorial westerlies which have been displaced northward cover all the regions mentioned above. Similarly, the subtropical countries receive the westerlies of the temperate zone during winter and the subtropical highs during summer.

Flohn has noted that there are two different circulation patterns in the low latitudes according to the continental or oceanic conditions. The continental conditions are exemplified by Central Africa and the Indo-Australian region. The oceanic conditions are represented by the Central Pacific.

Over land, the annual temperature changes are relatively larger because of which the seasonal shifting of temperature and pressure belts amount to many degrees. The oceans present a different picture, for over them only small annual temperature changes are observed.

That is why, over the sea the annual migration of thermal and pressure belts are confined to only a few degrees. According to him, the existence of Asian monsoon is not due to the temperature contrasts between land and sea, but mainly due to the annual migration of thermally produced planetary winds and pressure belts under continental influences.

On the basis of what has been described above Flohn ascribes the origin of monsoon to the thermal response of the tropical continental atmosphere to the annual variation of solar radiation. However, he seems to have ignored the upper atmospheric circulation which makes the Asiatic monsoon a fairly complex system.