Persistent multidecadal power of the Indian Summer Monsoon

Abstract

The instrumental record of Indian Summer Monsoon (ISM) precipitation displays two complete manifestations of an inferred multidecadal cycle. Hitherto, no precipitation-sensitive proxy record from the Indian subcontinent has had the necessary resolution and length to adequately assess whether this observed feature is an inherent aspect of the ISM system on longer timescales. Here we present compelling evidence for persistence of this cycle using a millennial length (AD 600–1550) and sub-annually resolved speleothem oxygen isotope record (δ 18O) from Dandak Cave in east-central India. The record displays a high degree of correlation with a speleothem-based Asian monsoon reconstruction from Wanxiang Cave in north-central China on annual to decadal timescales showing the regional significance of these findings. The observed period in our monsoon reconstruction is similar to that associated with the Gleissberg solar cycle and multidecadal sea surface temperature variability in the north Atlantic (AMO), both of which are often cited as the prominent pacemakers of ISM variability on multidecadal timescales. We document transient coherence between ISM precipitation amount and solar variability that persists exclusively in the century prior to and during the Medieval Climate Anomaly (nominally, AD 950–1300). The non-stationary nature of the SFV–monsoon relationship presented here may be evidence of the time-varying influence of tropical ocean–atmosphere dynamics on the solar–monsoon link; however, it is not possible to show with significance that this period of coherence is anything more than an artifact of two timeseries with similar spectra. We therefore, are inclined to interpret our record as evidence of a minimal role of solar variability in driving persistent multidecadal variability of the ISM. Multidecadal SST variability in the North Atlantic remains as the likely alternative driver for persistence of this cycle. Regardless of the causative mechanism(s), the amplitude and regional signature of the observed cycle in ISM precipitation highlight its societal importance with respect to forecasting ISM precipitation on multidecadal timescales.