COMPONENTS OF THE SOUTH – WEST MONSOON SYSTEM

HEAT LOW:

A ‘Heat low’ develops over the sub- continent in the pre monsoon months and it is located over the central parts of Pakistan in July. (The low is centered over east Madhya Pradesh in April, Punjab in May and Pakistan in June and July. There is little retreat to east and south in August and September). This heat low exists only in the lowest 1.5km and is over lain by a well marked ridge extending into upper troposphere, which is a part of sub-tropical high pressure belt. There is frictional coverage in the heat low and consequent weak ascent of air, while subsidence is likely in the ridge above.

The intensity of the ‘heat low’ has been correlated with monsoon activity. Below normal pressure in the heat low region and above normal pressure in the peninsula are regarded as favoring monsoon activity over India. Rummage (1971) has shown that surface pressure at Jacobabad in the ‘heat low’ is inversely related to the monsoon rains over a strip of the sub-continent between 18°N and 27°N.

MONSOON TROUGH

From the seasonal low over Pakistan a trough extends south-eastwards into Gangetic West Bengal. At surface level the trough line runs from Ganganagar to Calcutta through Allahabad. This trough is seen in the upper levels up to about 500mb (6km), the trough line shifting southwards with height. At 4km it runs from Bombay to Sambalpur. The temperature to the north of the trough is about 2°c higher than that to the south, which explains the tilting of trough southwards with height. The monsoon trough is regarded as the equatorial trough of the northern summer in the Indian longitudes.

The position of the trough line varies from day-to-day and in a way it controls the monsoon rainfall over India. When the trough line runs along the foot hills of Himalayas we get the ‘break-monsoon’ conditions. Swing of the trough to the central parts of India is with monsoon depressions.

Along the mean position of the monsoon trough, rainfall is a minimum. Mean rainfall is more to the south, perhaps due to the heavy rainfall in the left forward sector of monsoon depressions, which travel west-north-west-wards, a little to the south of the mean position of the monsoon trough. The rainfall increases to the north of the mean monsoon trough towards the foot of the Himalayas.

Normally some rain occurs near the surface trough on account of the convergence in it. The latent instability of the airmass enhances rainfall in trough, when it is over Bihar and Uttar Pradesh, even without a low. The trough rapidly shifts north or south, which can be even 5 degrees latitude per day.

MASCARENE HIGH

This is the high pressure area at sea level south of the equator in the Indian Ocean. Its name comes from the Mascarene Islands east of Madagascar. The normal position of this anti-cyclone is located near 30°S 50°E.However it has longitudinal and latitudinal variance also .The Position and intensity of this high are found to be very important in relation to performance of monsoon.

LOW LEVEL JET

Existence of strong winds of 50kt at 1km A.S.L. near Scotra in S.W. Arabian Sea was reported by Bunker (1965). Joseph and Roman (1966) showed that westerly low level Jet streams occur over peninsular India on many days during monsoon months with core at about 850mb and core speeds of the order of 40 to 60 knots. Large wind shears in the horizontal and vertical are found associated with this jet stream.

Find later (1966,1967) established the existence of a southerly low level jet crossing the equator over the east areas of Kenya during the months April to October. The jet winds often reached speeds of 50 to 100 knots. Jet Maximum was around 850mb. In 1969 Find later showed that these jet streams were part of major low level on air current in the monsoon flowing from the vicinity of Mauritius, through Kenya, Somalia and across Arabian Sea to the West coast of Peninsular India and further eastwards. This current reaches its maximum development in July. It has been calculated that this current circulating in the western periphery of the monsoon regime, could account for about half of the total cross-equatorial transport of air in the lower troposphere in July.

It is found that there is a strong association between the strengthening of the cross equatorial low level jet and the monsoon rainfall of Peninsular India, the latter lagging behind the former by a few days.

With further studies on the L.L.J., Find later (1974) suggested that the jet downstream from the SomaliCoast, splits into two branches, the northern branch intersecting the west coast of India near 17°N, while the southern branch passes through the southern tip of India. MONEX-1979 has not been able to fully support this view.

TIBETAN HIGH

This is a large anticyclone that is known to have its largest amplitude near 200 mb during the south-west monsoon season. A sequence of its formation and movement is as follows. This anticyclone first forms in April just to the north of Borneo. In May it is located over southern Indochina. By June it is found over the northern part of Burmaand thereafter it moves over the Tibetan Highlands. This anticyclone starts moving south-south-eastwards in September and it is found over Malaysia in November. Thereafter it loses its identity. During the monsoon season this anti cyclone is called ‘Tibetan High’.

The Tibetan High some-times shift much to the west of its usual position. In such a situation monsoon may extend further west-ward into Pakistan and in extreme cases into north Iran.

Radiation balance over the Himalayan ranges and Tibet should be important for the dynamics of ‘Tibetan High’ heating by deep moist convection (by cumulonimbus) should also be equally, if not more, important.

TROPICAL EASTERLY JET (TEJ)

One of the most fascinating features of the monsoon is the upper tropospheric tropical easterly jet at 150-100 mb. This jet has wind of 80-100 kts., with the strongest winds just to the west of the southern tip of India, over the Arabian Sea. The jet forms in June and is present upto September. The observational aspects of this jet have been studied by koteswaram (1958).

TEJ runs from east coast of Vietnam to the west coast of Africa, through the southern portions of peninsular India. Over Africa its location is near 10°N. Normally the jet is in an accelerating stage from south China Sea to south India and decelerates thereafter. Positions and speed of the jet fluctuates from day-to-day.

Mokashi (1970) studied the TEJ using data of peninsular RW stations and Colombo and GanIsland. The core of the jet is between Colombo and Trivandrum in June near 14.5 km. between Trivandrum and Madras in July at 14.5-15.0km., near Trivandrum in August at 14.5 km and near Colombo in September at 14.2 km.