Impact of cross-equatorial flow on intra-seasonal variability of Indian summer monsoon rainfall

Abstract

850-hPa zonal wind gradient anomaly between the region-1 [Equation-25°N; 30°E-75≡E] and region-2 [Equation-1-°S; 30°E-75°E] is used to represent the cross-equatorial flow. Long-term daily mean of 1st June to 30th September, for the period 1951-2000, of gradient time series is correlated with Indian summer monsoon daily mean rainfall, for the same period. The correlation coefficient is 0.92, which is statistically significant at 1% level. In order to understand the impact of 850-hPa zonal wind gradient on daily rainfall activity over India during monsoon season, the composite analysis of daily rainfall activity over India during extreme positive and negative 850-hPa zonal wind gradient anomaly is performed. This analysis reveals that when 850-hPa zonal wind gradient anomaly in last 20 days of May is extremely positive then almost all days in June show above-normal rainfall activity and vice versa. The correlation coefficients between 20-day mean of rainfall departure and previous 20-day mean of 850-hPa zonal wind gradient anomaly, for the period 1951-2003, are computed. This correlation analysis suggests that statistically significant (at 5% level) relationship is seen only up to the 30th June rainfall and from this point onward the relationship becomes insignificant. The result also indicates that 850-hPa zonal wind gradient anomaly, in last 20-days of May, is important for daily rainfall activity over India during onset phase of monsoon but, when the monsoon season sets in then it loses its significance. Copyright 2008 by the American Geophysical Union.