20 Southwest M<strong>on</strong>so<strong>on</strong>Fig. 2.11(b) Upper winds (missing)
<str<strong>on</strong>g>Met</str<strong>on</strong>g>eorological <str<strong>on</strong>g>M<strong>on</strong>ograph</str<strong>on</strong>g> : Synoptic <str<strong>on</strong>g>Met</str<strong>on</strong>g>eorology No. 1/1976 212.7.3 In May, a trough line runs from Gorakhpur to Kanyakumari at 900 mb A weaker troughalso runs from Multan to Badin. Winds all over the country are from some westerly directi<strong>on</strong>. At850 mb, the <strong>on</strong>ly troughs are <strong>on</strong>e over northeast India al<strong>on</strong>g 87° E and another roughly al<strong>on</strong>g12° N in the Bay and the Peninsula. The latter trough persists upto 500 mb. The sub–tropical ridgefrom Arabia extends into the Indian Peninsula at 850 mb and persists upto 500 mb at about thesame positi<strong>on</strong>. Aloft, this ridge line shifts southwards reaching 14 o N at 200 mb, but is againdisplaced northwards to near 20 o N at 100 mb. Aloft of 200 mb, the ridge over the Peninsula is anextensi<strong>on</strong> from the east rather than the west. The westerlies over northern India are str<strong>on</strong>gestbetween 200 and 150 mb, reaching 50 kt at 200 and 150 mb near 30 o N. Speed decreases aloft.Easterlies over the Peninsula are not over 20 kt.2.7.4 In June, by the end of which the m<strong>on</strong>so<strong>on</strong> is established over practically the whole ofthe country, winds are westerlies at 900 mb. The m<strong>on</strong>so<strong>on</strong> trough over northern India betweenwesterlies to the south and easterlies to the north is established <strong>on</strong>ly towards the end of the m<strong>on</strong>th.A trough is present ,over Pakistan and adjoining Iran which persists upto 800 mb. Between 850 mband 500 mb, a trough line exists over the northeast India between 85 o and 90 o E. The sub tropicalridge from the west begins extending into Rajasthan at 700 mb and reaches. Gujarat at 500 mb.Weak easterlies appear at 300 mb south of the ridge line al<strong>on</strong>g (22 N). The anticycl<strong>on</strong>e from the eastextends to 85° E at 300 mb and 70 o E by 150 mb. 'The ridge line is at 25 o N at 150 mb and near 28 oN at 100 mb. Easterlies well to the south of this ridge line strengthen with height and reach 50 ktin some belt over the Peninsula at 100 mb.2.7.5 In July (Fig. 2.12 a and b) , the m<strong>on</strong>so<strong>on</strong> trough runs from Delhi to Calcutta at 900 mb.Westsouthwesterly to westnorthwesterly winds prevail to the south of it and southeasterly winds t<strong>on</strong>orth. A weak trough is also present over Pakistan and neighbourhood. Westerly winds over thePeninsula increase with height from ground and reach a maximum between 900 and 800 mb. Thislevel is near 900 mb al<strong>on</strong>g the west coast and increases to 800 mb in the eastern Peninsula,Maximum speeds are between 20 and 25 kt. A similar wind maximum near 900 mb has also beenfound in the Arabian Sea south of 20 o N, particularly in the western porti<strong>on</strong>s. The m<strong>on</strong>so<strong>on</strong> troughshifts south with height and is near about 23° N at 700 mb but becomes diffuse above 500 mb overthe Peninsula. The trough over Pakistan is not present at 700 mb; instead the sub–tropical highfrom the west extends into northwest India. Thus the warm surface low over Pakistan andneighbourhood is replaced by the subtropical high at 700 mb. At 500 mb, this ridge, the easterliesover northern India and a trough from east central Arabian Sea to Orissa are the chief features.Winds at this level are weak. .At 300 mb, apart from the western ridge al<strong>on</strong>g 30 o N over northwestIndia and Pakistan, winds are easterly over the whole country. Between 200 mb and 100 mb,another ridge from the east develops to the east of l<strong>on</strong>gitude 75 o E (with ridge line at 30° N).Easterly winds strengthen with height from 200 mb reaching a maximum at 100 mb. Speeds arebetween 60 and 80 kt over the Peninsula between 150 and 100 mb. Upper tropospheric easterliesare seen in Gauhati <strong>on</strong>ly in mid–m<strong>on</strong>so<strong>on</strong>. The strengthening of easterlies with height in UttarPradesh and Bihar is less than in central parts and the Peninsula where thermal gradients arestr<strong>on</strong>ger.2.7.6 While the pattern remains the same in August, winds are slightly weaker between 850and 500 mb. In September, winds are weaker at 900 mb also. The anticycl<strong>on</strong>e from west nowextends at 700 mb upto 20° N, 80° E. A cycl<strong>on</strong>ic circulati<strong>on</strong> also covers the north and adjoiningcentral Bay, instead of the trough near 20 o N in earlier m<strong>on</strong>ths. At 500 mb, the ridge has shifted 3°to 4° southwards and middle–latitude westerlies are spreading upto 27 o N,2.7.7 Joseph and Raman (1966) find that the lower tropospheric westerlies over the Peninsulamay develop into low level jet stream <strong>on</strong> many days in July, with a core at about 1.5 km andspeeds of 40–60 kt in the core. The jet appears between 8 o and 18 o N. The wind shear below thecore is more than above (Fig. 2.13).
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