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The role of Himalayan Massif-Tibetan Plateau and the mid-tropospheric sub-tropical ridge over north India during the advance phase of the southwest monsoon

The advance of monsoon over the Indian sub-continent is not a continuous process. It advances in a phased manner. It has been observed from large sample of the data that the monsoon current often stagnates outside northwest India. Gujarat, northwest Madhya Pradesh and west Uttar Pradesh for a long period resulting in delaying its advance considerably over these areas. The cause of such prolonged stagnation can be identified to the mechanic Himalayas - Tibetan plateau on the mid-tropospheric westerly flow in absence of any synoptic scale forcing over north India. During 1976, 1982 and 1991 there was prolonged stagnation of southwest monsoon over northwest India and neighbouring areas and, in 1985, it was over north Konkan during June-July. In those years, the sub-tropical ridge (STR) in the middle troposphere over India was weak or absent during the initial phase of advance of monsoon. In absence of the STR, the westerly trough activity in the mid-tropospheric levels extended to southern latitude disrupting the monsoon flow and bringing prolonged stagnation. The observation was confirmed on a test study conducted during the prolonged stagnation of the monsoon of 1995. On the other hand, in the year 1990, the mid- tropospheric STR became prominent from middle of June over north India and it helped in restricting southward extension of westerly troughs. Consequently the equatorial trough remained organize gradually over north India and caused the gradual advance of SW monsoon over the entire country without considerable.