The simultaneous effect of NAO and SO on the monsoon activity over India

North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are two large‐scale atmospheric oscillations in northern and southern hemisphere respectively. These two oscillations are known to affect Indian summer monsoon (June–September) rainfall (ISMR). It is logical to expect the combined effect of these two oscillations on monsoon rainfall over Indian subcontinent. In the present paper this combined effect of NAO and SO on Indian summer monsoon rainfall is studied. North Atlantic Oscillation (NAO) index in the month of April is showing an inverse and statistically significant relationship with Indian summer monsoon rainfall (ISMR). Southern oscillation (SO) indices from April through December show direct association with ISMR. The present study deals with the simultaneous effect of NAO and SO on monsoon rainfall over India. The effective strength index is newly defined to quantify the simultaneous effect of NAO and SO and its relationship with ISMR is investigated. For this purpose 40 years data (1951–90) has been used.