Response of the South Indian runoff-harvesting civilization to northeast monsoon rainfall variability during the last 2000 years: instrumental records and indirect evidence

Compared with the southwest monsoon, the mechanisms and history of northeast monsoon (NEM) variability over South Asia are poorly known. The NEM nevertheless contributes /50% of rainfall to semi-arid southeast India, and has underpinned the success, over the last 2000 years, of a widespread indigenous water resource management system based on rainwater harvesting and storage in man-made lakes. Based on a power spectrum analysis of 60to 100-yr instrumental rainfall records, we show that NEM rainfall is both chaotic in the high-frequency time domain and spatially unpredictable, but that the sustainability of the agricultural water-harvesting infrastructure is tied to excess rain from NEM cyclones. Cyclonic activity has declined sharply since 1977. Based on twentieth-century analogues, we propose a qualitative model in which storminess decreases when NEM wind intensities increase and Eurasian winter temperatures are anomalously low. Extrapolating to the Holocene time frame, model expectations for the palaeorainfall record correlate with the independently established chronologies of Indian runoff-irrigation development and the ‘Mediaeval Warm Period’ ‘Little Ice Age’ oscillations, and further suggest that NEM and SWM rainfall anomalies have covaried with the same sign from seasonal to millennial timescales. The fortunes of the reservoir system are therefore tangibly climate-driven.