Interannual variations of outgoing longwave radiation and Indian summer monsoon rainfall

Abstract

The outgoing longwave radiation (OLR) over tropical Asia has been analysed for a relationship with the Indian summer monsoon rainfall (ISMR; June–September) total using 19 years' data. The ISMR is found to be negatively associated with a simultaneous period of OLR variation over west–central India and the south Equatorial Indian Ocean, and positively associated with that of the variation over the east of Madagascar Sea. The OLR of west–central India, particularly, is strongly coupled with the ISMR. The averaged OLR for west–central India (index A) accounts for 92% of the total variance of the ISMR, and hence, index A (with opposite sign) can be considered as a proxy of the ISMR variability. The relationship between index A and the ISMR is found to be stable. The monthly scale variation of these two parameters (i.e. all-India rainfall and OLR averaged for west–central India) also exhibits a significant association during each of the summer monsoon (June–September) months. However, the association appears better in the latter half (August–September), as compared with the first half (June–July), of the season. The ISMR can be estimated with a high degree of accuracy through a linear regression model using index A. Index A, thus, appears useful in updating the ISMR series, which is generally accomplished by averaging the area weighted rainfall of 306 stations distributed over India. Similarly, index A can be computed with a high degree of accuracy through a linear regression model using the ISMR. The relationship can be utilized in filling the OLR data gap for 1978 over west–central India. The study further provides scope for investigating interannual variability of the Indian summer monsoon in relation to climate variability of the global tropics. Copyright © 2000 Royal Meteorological Society