Simulation of the Indian monsoon and its variability during the last millennium

Abstract. The general circulation model ECHAM5 has been used to simulate the Indian monsoon and its variability during the Medieval Warm Period (MWP; 900–1100 AD), the Little Ice Age (LIA; 1515–1715 AD) and for recent climate (REC; 1800–2000 AD). The focus is on the analysis of external drivers and internal feedbacks leading to extreme rainfall events over India from interannual to multidecadal time scale. An evaluation of spatiotemporal monsoon patterns with present-day observation data is in agreement with other state-of-the-art monsoon modeling studies. The simulated monsoon intensity on multidecadal time scale is weakened (enhanced) in summer (winter) due to colder (warmer) SSTs in the Indian Ocean. Variations in solar insolation are the main drivers for these SST anomalies, verified by very strong temporal anticorrelations between Total Solar Irradiance and All-India-Monsoon-Rainfall in summer monsoon months. The external solar forcing is coupled and overlain by internal climate modes of the ocean (ENSO and IOD) with asynchronous intensities and lengths of periods.

In addition, the model simulations have been compared with a relative moisture index derived from paleoclimatic reconstructions based on various proxies and archives in India. In this context, the Lonar record in Central India has been highlighted and evaluated the first time. The simulated relative annual rainfall anomalies in comparison to present-day climate are in agreement (disagreement) with the reconstructed moisture index for MWP (LIA) climate.

In order to investigate the interannual monsoon variability with respect to monsoon failures, dry summer monsoon composites for 30-yr-long periods of MWP, LIA and REC have been further analysed. Within dry years of LIA, the summer rainfall over India and surrounding oceans is less than in MWP indicating stronger drying conditions due to a stronger summer solar insolation forcing coupled with variations in ENSO. To quantify the ECHAM5 simulated long-term drought conditions within Monsoon Asia, the Palmer Drought Severity Index has been additionally estimated for recent climate showing strong pattern correlation between global SST anomalies and EOF variability signal of the drought index, whereas the temporal relationship is weak.