Monsoon trend and multi-scale variability changes over the last 6000 years

The Holocene started about 10000 years before present and is the period during civilizations as we know them today emerged. However, during that time several regions such as Sahel-Sahara or the Indus valley in the tropics experienced severe aridification and dramatic environmental changes for ecosystems and humans. There is general agreement that this has been caused by the southward shift of the boreal monsoon rain belt and that slow variations of Earth’s orbital parameters are the long-term driver. In addition to insolation forcing, several feedbacks involving the ocean, sea-ice, or vegetation have had a profound impact on regional changes and on the multiscale monsoon variability, including extreme monsoon years. They have shaped the magnitude and the timing of environmental changes depending on monsoon systems. Although these feedbacks have been widely discussed, their relative strength is still under debate. These unknows prevent proper anticipation and simulation of future monsoon behavior. Long transient simulations of the Holocene climate allow us to revisit these questions by shedding light on monsoon multiscale variability and the representation of vegetation feedbacks. Using a set of transient mid to late Holocene simulations (last 6000 years), we will discuss the relative evolution of the global monsoons. Highlights will be on the relative responses to changes in insolation seasonality between African and Indian monsoons, the role of dynamical versus thermodynamical atmospheric feedbacks in monsoon precipitation, and on the relationship between long term trends, interannual to multicentennial variability and periods of extreme dry or wet monsoon seasons. Comparisons of model results with proxy reconstruction of climate variability over land and ocean from speleothems, coral and shells has been done considering the chaotic nature of multiscale monsoon variability. They provide us with indication of the consistency of model inferred trends in monsoon variability and the real climate trajectory.