A possible mechanism for the weakening of El Ni�o-monsoon relationship during the recent decade

Abstract

Observational data are used to explore the relationship between surface air temperature anomaly gradients and Indian summer monsoon rainfall (ISMR). The meridional temperature anomaly gradient across Eurasia during January directed towards equator (pole) is a very good precursor of subsequent excess (deficient) Indian summer monsoon rainfall (ISMR). This gradient directed towards equator (pole) indicates below (above) normal blocking activity over Eurasia, which leads to less (more) than normal southward penetration of dry and cold mid latitude westerlies over the Indian monsoon region, which ultimately strengthens (weakens) the normal monsoon circulation. These findings suggest a mechanism for the weakening of relationship between El Nino and ISMR. Though there is a strong fundamental association between El Nino (warm ENSO) and deficient Indian summer monsoon rainfall (ISMR), this relationship was weak during the period 1921–1940 and the recent decade (1991–1998). During the El Nino years of 1921–1940 and 1901–1998, the meridional temperature anomaly gradient across Eurasia (Eurasian forcing) during January was directed towards equator. On the other hand, during the El Nino years of 1901–1920 and 1941–1990 this gradient was directed towards pole. Thus during 1921–1940 and 1991–1998, the adverse impact of El Nino on Indian monsoon was reduced by the favorable Eurasian forcing resulting in the weak association between El Nino and ISMR. This finding disagrees with the hypothesis of winter warming over the Eurasian continent as the reason for the observed weakening of this relationship during recent decade.