Indian monsoon-ENSO relationship on interdecadal timescale

Empirical evidence is presented to support a hypothesis that the interdecad al variation of the Indian summer monsoon and that of the tropical SST are parts of a tropical coupled ocean-atmosphere mode. The interdecadal variati on of the Indian monsoon rainfall (IMR) is strongly correlated with the int erdecadal variations of various indices of El Nino-Southern Oscillation (EN SO). It is also shown that the interannual Variances of both IMR and ENSO i ndices vary in phase and follow a common interdecadal variation. However, t he correlation between IMR and eastern Pacific SST or between IMR and South ern Oscillation index (SOI) on the interannual timescale does not follow th e interdecadal oscillation. The spatial patterns of SST and sea level press ure (SLP) associated with the interdecadal variation of IMR are nearly iden tical to those associated with the interdecadal variations of ENSO indices. As has been shown earlier in the case of ENSO, the global patterns associa ted with the interdecadal and interannual variability of the Indian monsoon are quite similar. The physical link through which ENSO is related to decreased monsoon rainfa ll on both interannual and interdecadal timescales has been investigated us ing National Centers for Environmental Prediction-National Center for Atmos pheric Research reanalysis products. The decrease in the Indian monsoon rai nfall associated with the warm phases of ENSO is due to an anomalous region al Hadley circulation with descending motion over the Indian continent and ascending motion near the equator sustained by the ascending phase of the a nomalous Walker circulation in the equatorial Indian Ocean. It is shown tha t, to a large extent, both the regional Hadley circulation anomalies and Wa lker circulation anomalies over the monsoon region associated with the stro ng (weak) phases of the interdecadal oscillation are similar to those assoc iated with the strong (weak) phases of the interannual variability. However , within a particular phase of the interdecadal oscillation, there are seve ral strong and weak phases of the interannual variation. During a warm east ern Pacific phase of the interdecadal variation, the regional Hadley circul ation associated with El Nino reinforces the prevailing anomalous interdeca dal Hadley circulation while that associated with La Nina opposes the preva iling interdecadal Hadley circulation. During the warm phase of the interde cadal oscillation, El Nino events are expected to be strongly related to mo nsoon droughts while La Nina events may not have significant relation: On t he other hand, during the cold eastern Pacific phase of the interdecadal SS T oscillation, La Nina events are more likely to be strongly related to mon soon floods while El Nino events are unlikely to have a significant relatio n with the Indian monsoon. This picture explains the observation that the c orrelations between IMR and ENSO indices on the interannual timescale do no t follow the interdecadal oscillation as neither phase of the interdecadal oscillation favors a stronger (or weaker) correlation between monsoon and E NSO indices.