Met.Monograph on SW Monsoon - (IMD), Pune

CHAPTER 2CLIMATIC PATTERNS2.1 In this chapter the mean distribution of important climatological elements during themonsoon is described, tracing their evolution from the preceding season and decay at the end.2.2 Sea–level Pressure Distribution2.2.1 Development of a low due to increased heating over India starts in March itself, withslightly higher pressures over the Arabian Sea and the Bay of Bengal. By April, the global landlows have begun establishing themselves along about 10 o N in north Africa and about the Tropicof Cancer in the Indian region and Burma. Peninsular India south of 20° N has a tapering shapeand is narrow south of 15 o N. The latter portion comes under considerable maritime influence.This makes the heating over land more prominent to the north of 20 o N and hence the axis of thelow is at a more northerly latitude over India (Fig. 2.l). There is a weak cell over upper Sind andanother in Bihar and east Uttar Pradesh. Over the Peninsula, a trough forms with axis alonglongitude 78° E. A ridge runs from Arabia into the west Arabian Sea where a clockwise windcirculation is found around 14 o N, 60 o E. A similar circulation is also present over the Bay ofBengal around 14 o N, 90 o E. The sub–tropical high in the south Indian Ocean is along 30 o S.2.2.2 By May, the summer continental low pressure areas completely dominate north Africaand Asia. Its main centre over India is near 30 o N, 75 o E with an extension as a trough upto Orissa.The heat low is still more marked in June with the main centre over Pakistan. In these months,there are subsidiary centres of low pressure in Africa and other parts of Asia. By May, the troughover the Peninsula has shifted to 79° E along the Madras Coast.2.2.3 Monsoon activity is maximum in July when the low pressure area ex tending fromnorth Africa to northeast Siberia is most intense. Its main centre is over north Baluchistan andneighbourhood (Fig. 2.2). A trough lies over north India with axis from Sriganganagar to the HeadBay, which is referred to as the 'monsoon trough'. Pressure gradient is strong south of this trough.The Indian Ocean 'High' has strengthened and is centred at about 30° S, 60° E. Pressurecontinuously decreases over the Indian Ocean northwards of this high pressure belt. Weak ridgesare present in the Arabian Sea off the west coast of India and in the Bay off Tennaserim coast andover Burma. The weak trough of the premonsoon months in the eastern Peninsula now lies just offthe east coast of the south Peninsular India; it persists through the monsoon months and is morepronounced in September.2.2.4 In August, the intensity of the Afro–Asian low is decreasing. By September, pressuresrise north of 40° N over Asia and the Afro–Asian low is oriented east–west. The pressure gradientsouth of monsoon trough which is maximum in July and slightly less in June and August,decreases to half in September. The ridge off the west coast is displaced in September to about65° E.2.2.5 By October, the trough over northern India shifts to the Bay of Bengal, with the troughline along 13° N and the pressure field is flat over the country (Fig. 2.3). The low pressure beltruns from Africa to the west Pacific between the equator and 20° N with Centres over Africa, theBay of Bengal 'and the west Pacific. The Asian 'High' is establishing along 50° N and is centred atabout 90° E.2.2.6 To summarize, the chief features of the surface pressure distribution in the monsoonseason are the heat low over Pakistan, the monsoon trough thence to Head Bay and the strongpressure gradient to the south.