The Tibetan High
A dominant feature, which is related to the monsoon’s onset, is the emergence of a zone of high pressure over the plateau of Tibet. The reasons for the formation of the High are not very well understood at present. The average height of the plateau is around 4 km, which corresponds to a pressure of about 600 hPa. This is nearly half the depth of the troposphere. According to the observations of Chinese meteorologists the widespread thunderstorms over the southeastern parts of Tibet in the pre-monsoon months of April and May releases considerable amounts of latent heat into the atmosphere through rainfall. Besides, sensible heat from the plateau, which is observed to be of the order of 145 watts per square metre, much energy is injected into the atmosphere. The plateau thus acts like an elevated ‘heat island’. But, unlike the heat low over Indian sub-continent, thermal convection is stronger over the Tibetan plateau because of its higher elevation. Consequently, there is a heat low over Tibet at elevations around 500 hPa (6km) The ascending air rapidly spreads outwards both to the north and to the south of the plateau, which divergence leads to the formation of an anticyclone over Tibet around 300-200 hPa (9 to 12Km).
Air-Sea Interactions and Temperature Inversions
Let us trace back our steps to the Somali Jet stream and Somali Current. As the strong winds drive away the surface coastal waters towards the east, extremely cold water from the depths of the sea rise upwards to preserve the continuity of mass. This upwelling is brought about by strong low level winds. This generates an atmospheric state where a layer of warm air lies over the colder air beneath. Meteorologists refer to this as a temperature or thermal inversion, because the lapse rate that is usually negative becomes positive when warm air lies over colder air.
In view of the cold sea surface temperatures off the coast of East Africa, there is a pronounced temperature inversion in the atmosphere in this region. The base of the inversion is around 1.0 to 1.5 km above the sea surface. The height of the inversion base increases as we proceed eastwards and becomes gradually less marked. The inversion usually disappears as we move eastwards of 55º E. This temperature inversion inhibits formation of convective activity to the west of 65º E, and usually clear skies are observed on the western sector of the Arabian Sea. But, eastwards of 65º E the inhibiting effect of the inversion wears off and cloud development takes place rapidly. The convective clouds are often accompanied by heavy rain. The inversion layer is thus a measure of the modification of monsoon air as it traverses the Arabian Sea.
As the monsoon wind proceeds over the Arabian Sea it picks up a lot of moisture evapourating from the sea. The constant action of the waves and the bursting of minute air bubbles on the ocean surface causes innumerable salt molecules to be let off into the air. The salt molecules released into the atmosphere form the nucleus for water droplets as water vapour condenses. These salt molecules help water vapour to condense at lower vapour saturation pressure and less cooler temperatures. The Indian Summer monsoon reaches India around early June say before 15th June.
Aircraft observations revealed that the depth of the monsoon current was fairly shallow west of the meridian 65ºE. It was only 1.5 km deep westward of 65ºE. Thereafter, as we proceed eastwards, the depth of the monsoon air rose sharply to about 6 km near the Indian coastline. It is opined that a rapid increase in the depth of the monsoon eastwards of 65ºE was the consequence of an orographic barrier along the west coast of India. On approaching the west coast from westerly or southwesterly direction, the monsoon air was forced to ascend over the Western Ghats. This is a barrier running along the entire coast in a north-south direction. Consequently, the orientation of the barrier forced the moist and comparatively shallow monsoon air to rise abruptly to a height of around 6 km. It is still not clear how far westwards can we extend the influence of this physical barrier.