Regional Predictors

The progressive development of surface heat low over central parts of Pakistan and adjoining northwest India and the decaying wintertime upper air westerly wind regime in the pre-monsoon season play a very important role in the evolution and performance of Indian summer monsoon. In view of this, several workers have developed predictors for the monsoon rainfall using data on various surface as well as upper-air meteorological parameters from India and adjoining regions. Details on some of the important predictors are presented below:

(i) Pre-monsoon surface pressure and thermal fields over India

Monsoon being the result of land-sea heating contrast involving large-scale seasonal reversal of pressure, temperature and winds, many studies have been carried out to identify useful predictors based on the pressure and thermal fields during antecedent winter and pre-monsoon seasons. Parthasarathy et al. (1992c) developed a predictor parameter, which they called West Central India (WCI) pre-monsoon (MAM) pressure, represented by the mean of sea level pressure (SLP) at six stations (Jodhpur, Ahmedabad, Bombay, Indore, Sagar and Akola) located in the core region of high correlation, which showed a CC of -0.63 (significant at 1% level) with the AISMR during 1951-80. Earlier, Parthasarathy et al., 1990 found that the mean surface temperatures at these stations during MAM season also showed high correlation (0.6) with monsoon rainfall for the period 1951-80.

Mooley and Paolino (1988), using maximum and minimum temperature data for the period 1901-75, have shown that a predictor based on May minimum temperatures over western Indian region has good potential for LRF. Krishna Kumar et al. (1995) identified two predictors based on the minimum temperatures during March over East Peninsular India and during May over west central India.

The operational LRF model (Gowariker et al,, 1991) of the India Meteorological Department also uses three minimum temperature parameters representing northern, central and east coastal areas of India.

(ii) Pre-monsoon 500 hPa Ridge location over India

The mean latitudinal location of the 500 hPa ridge along 75°E in April over India, first identified by Banerjee et al. (1978), is considered to be one of the most important predictors. The mid-tropospheric anticyclone over southern India migrates from 11.5°N in January to its northernmost position of 28.5°N during July. From October, the ridge starts shifting back southward. Mooley et al. (1986) found a CC of 0.71 (significant at 0.1% level) between the April ridge location and AISMR during 1939-84; a more northward location indicates better performance of the monsoon and vice versa. It is conjectured that the northward and southward displacements of this mid-tropospheric anticyclonic circulation are related to the seasonal march of the solar radiation and the associated diabatic heat source. The anomalies in the seasonal evolution of the mid-tropospheric circulation, as measured by the April ridge location, can be taken to be a good precursor of the slowly varying planetary-scale circulation. A delayed northward displacement of the ridge is considered to indicate large-scale anomalous descending motion over the Indian region (Shukla and Mooley, 1987). In a detailed diagnostic study using daily locations of the 500 hPa ridge during the pre-monsoon months of March-May for the period 1967-87, Krishna Kumar et al. (1992) found that the ridge location in March showed a CC of -0.47 with AISMR, while in April it showed a CC of +0.63. They also found that the negative correlation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant with the monsoon rainfall of northern India. The difference between the two ridge locations (April minus March) shows a CC of 0.73 with AISMR. Though the 500 hPa ridge has shown consistently significant relation with monsoon rainfall in recent years, the subjectivity involved in the determination of its location imposes some limitation on its reliability.

(iii) Upper tropospheric winds over India

Monsoon circulation over India involves marked changes in the upper tropospheric wind field. Keeping this in view, many recent studies have shown that the upper air winds during their pre-monsoon transition phase can provide a useful predictor. Verma and Kamte (1980) and Joseph et al. (1981) have identified the association between Indian monsoon rainfall and 200 hPa meridional wind component for the month of May, and indicated its potential for prediction of the seasonal rainfall. Recently, Parthasarathy et al., (1991a) have further investigated the relationship between meridional wind index (arithmetic average of 200 hPa meridional component of wind for May at Bombay, Delhi, Madras, Nagpur and Srinagar) and AISMR by using an extended data set for the period 1964-88 and found a CC of -0.72 (significant at 0.1% level).

Copyright © 1997   D. B. Stephenson and K. Rupa Kumar   Permission is granted to make copies for individual use, but not for redistribution.   Last Updated: January 24, 1998.