Mid-Holocene to present-day evolution of the Indian monsoon in transient global simulations
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Mid‑Holocene to present‑day evolution of the Indian monsoon in transient global simulations Julien Crétat1,2,5 · Pascale Braconnot1 · Pascal Terray3 · Olivier Marti1 · Fabrizio Falasca4 Received: 28 February 2020 / Accepted: 7 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The low-frequency evolution of Indian rainfall mean-state and associated interannual-to-decadal variability is discussed for the last 6000 years from a multi-configuration ensemble of fully coupled global transient simulations. This period is marked by a shift of Indian Summer Monsoon Rainfall (ISMR) distribution towards drier conditions, including extremes, and a contraction of the rainy season. The drying is larger in simulations with higher horizontal resolution of the atmosphere and revised land surface hydrology. Vegetation–climate interactions and the way runoff is routed to ocean modulate the timing of the monsoon onset but have negligible effects on the evolution of seasonal rainfall amounts in our modeling framework in which carbon cycling is always active. This drying trend is accompanied by changes in ISMR interannual-to-decadal variability decreasing over north and south India but increasing over central India (20°–25° N). The ISMR interannual-todecadal variability is decomposed into six physically consistent regimes using a clustering technique to further characterize its changes and associated teleconnections. From 6 to 3.8 kyr bp, the century-to-century modulations in the frequency of occurrence associated to the regimes are asynchronous between the simulations. Orbitally-driven trends can only be detected for two regimes over the whole 6–0 kyr bp period. These two regimes reflect increased influence of ENSO on both ISMR and Indian Ocean Dipole as the inter-hemispheric energy gradient weakens. Severe long-term droughts are also shown to be a combination of long-term drying and internally generated low-frequency modulations of the interannual-to-decadal variability. Keywords Holocene · Indian summer monsoon · Internal variability · Orbital forcing · Rainfall mean-state and variability · Transient simulations
1 Introduction
* Julien Crétat julien.cretat@science‑partners.com 1
IPSL/Laboratoire des Sciences du Climat et de l’Environnement, CEA‑CNRS‑UVSQ, Université Paris Saclay, Gif‑sur‑Yvette, France
2
Biogéosciences/CRC, CNRS‑UB, Université de Bourgogne, Dijon, France
3
IPSL/Laboratoire d’océanographie et du climat : expérimentations et approches numériques, Sorbonne Universités-CNRS-IRD-MNHN, Université Paris 6, Paris, France
4
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
5
Present Address: Science Partners, Paris, France
The Indian Summer Monsoon Rainfall (ISMR) provides about 80% of the annual rainfall amounts over India from June to September (JJAS) with significant variability on a wide range of timescales (Roxy and Chaithra 2018). Since ISMR variations profoundly impact on livelihood, which mostly relies on rain-f
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