There are a number of clear examples in the instrumental period wherepositive El Ni?o–Southern Oscillation (ENSO) events were coincident with a severely weakened Indian summermonsoon (ISM). ENSO's influence on ISM precipitation has therefore remainedthe centerpiece of various predictive schemes of ISM rainfall for over acentury. The teleconnection between ISM precipitation and ENSO has undergonea protracted weakening since the late 1980s, suggesting the strength ofENSO's influence on ISM precipitation may vary on multidecadal timescales.The recent weakening has occurred despite the fact that the ENSO system hasexperienced variance levels during the latter part of the 20th century thatare as high as any period in the past millennium. The recent change in theENSO–ISM coupling has prompted questions as to whether this shift representsa natural mode of climate variability or a fundamental change in ENSO and/orISM dynamics due to anthropogenic warming or aerosol impacts on the ISM. Herewe place the 20th century ENSO–ISM relationship in a millennial context byassessing the phase relationship between the two systems across the timespectrum using a a series of high-resolution reconstructions of ENSO and theISM from tree rings, speleothems and corals. The results from all the proxiessuggest that in the high-frequency domain (5–15 yr), warm (cool) seasurface temperatures in the eastern tropical Pacific lead to a weakened(strengthened) monsoon. This finding is consistent with the observedrelationship between the two systems during the instrumental period. However,in the multidecadal domain (30–90 yr) the phasing between thesystems is reversed such that periods of strong monsoons were, in general,coincident with periods of enhanced ENSO variability. This result iscounterintuitive to the expectation that enhanced ENSO variance favors anasymmetric increase in the frequency of El Ni?o events and therefore aweakened monsoon system. The finding implies that the prominent multidecadalvariability that characterizes the last 1000 yr of the ISM is notlikely attributable to multidecadal shifts in ENSO. If there is a continuedtrend towards enhanced ENSO variance in the coming decades, the resultspresented here do not suggest this will force a reduction in ISMprecipitation.
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