Impact of Multiscale Variability on Last 6,000 Years Indian and West African Monsoon Rain

Particularly dry or wet boreal summer monsoon seasons are major hazards affecting societal vulnerability in India and Africa. Several factors affect monsoon rainfall amount and limit the understanding of possible linkages between monsoon variability and mean climate changes. Here we characterize the multiscale variability of Indian and West African monsoon rain from two simulations of the last 6,000 years. Changes in Earth's orbit cause long-term monsoon drying trend in India and Africa, but the Indian monsoon is more sensitive to anthropogenic CO2. Variability is characterized by two major ranges of chaotic variability, each related to specific ocean-atmosphere modes present throughout the period. Combination of random 50- to 500- and 2- to 20-year variability leads to large events occurring at millennium scale. However, the two regions exhibit opposite trends in rainfall variability due to changes in teleconnection with Pacific sea surface temperature for India and Atlantic sea surface temperature for West Africa at interannual to decadal timescales.