Long range forecast of the onset of the monsoon

Abstract

A study of the cross-section charts along longitude 75 deg. E from latitude 8 deg. to 55 deg. N for the 200 mb lever and the monthly mean constant pressure charts for all standard levels from 850 to 100 mb for 1he area, equator to Lat. 55 deg. Nand .Long. 20 deg. To 100 deg. E for the months of April and May for two contrasting years, 1971 when the southwest monsoon advanced 7 to 10 days ahead of the normal date over most parts of the country and 1972, when the advance of the monsoon was delayed by 10 to 15 days, shows that a few upper air parameters of pre-monsoon months at 200 mb are useful for forecasting the onset of .the monsoon over the country outside south Peninsula, and northwest India. These parameters are (i) the nature and magnitude of the horizontar shear of the zonal westerlies near Lat. 30 deg. to 35 deg. N, over the longitudinal belt 60 deg. to 100 deg. E, (ii) the presence or absence of a trough near Long. 85 deg. E over northeast India, (iii) wind .anomaly pattern over Pakistan and neighbourhood and (iv) the departure from normal of upper air temperature over the area Lat. 30. deg. to 40 deg, N and Long, 80 deg. to 100 deg. E. The paper also presents the result~ of -ve1'1ficatlon of the utility of the above, parameters in forecasting the onset of the monsoon for the years, 1975 to 1979 and the anomaly pattern for the period, 1.957 to 1974 as well. It has been found that the parameter~ provide reliable indication regarding the onset of monsoon over Bihar, Orissa and Madhya Pradesh. Some indication can also be drawn for monsoon onset over Gujarat, north Peninsula and east Uttar Pradesh.