Orbital insolation forcing of the Indian Monsoon – a motor for global climate changes?
Abstract Both modern and ancient Indian summer monsoons are driven by transequatorial pressure differences, directly coupled with the insolation difference between the Northern and Southern subtropical Hemispheres. A high-resolution record of upwelling and dust flux from the western Arabian Sea resembles an insolation-based Indian Summer Monsoon Index. This index and the observed monsoonal climate variations share major elements on the orbital obliquity and precessional band with the Specmap marine oxygen isotope record, representing global ice volume. The long-term evolution of the index mirrors almost exactly the insolation changes at 65°N, showing that the forcing of low latitude climate variability has a structure similar to that of the insolation forcing in the high northern latitudes. Moreover, insolation forcing in the low latitudes directly controls atmospheric processes in the African, Indian and Asian Monsoon, being responsible for a huge amount of transequatorial water vapour and therefore latent heat transport. Millennial-scale variability of the monsoonal climate is concentrated at periodicities near 1100, 1450, 1750 and 2300 years. These cycles are not strictly periodic, but occur in bands, with specific activity phases and amplitude increases during warm stages and interstadials for the 1100- and 1750-yr cycles, whereas the 1450-yr cycle dominates the cold intervals.