Seasonal behaviour of NCEP-NCAR longwave cloud radiative forcing and its relationship with all-India summer monsoon rainfall

Using National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) longwave cloud radiative forcing (LWCRF) reanalysis at the top of the atmosphere (TOA) for the period 1949-2006, the seasonal behaviour of the LWCRF and its relationship with the all-India summer monsoon rainfall (AISMR) during the winter (December-January-February, DJF), pre-monsoon (March-April-May, MAM) and summer monsoon (June- July-August-September, JJAS) seasons has been examined. The LWCRF over the Bay of Bengal region (15-20° N and 87.5-92.5° E) during the pre-monsoon season (MAM) is found to be significantly related to AISMR. The correlation coefficient (CC) between AISMR and LWCRF is 0.419, significant at the 1% level. The composite anomalies for excess minus deficient rainfall years of the LWCRF during MAM over the same region strongly support a significant relationship between LWCRF and AISMR. Thus, the LWCRF over the Bay of Bengal region in the pre-monsoon season appears to be a good indicator of the forthcoming monsoon rainfall.