Hydroclimatic variability on the Indian subcontinent in the past millennium: Review and assessment

Abstract

Paleoclimate records spanning the past millennium show manifestations of two distinct climate anomalies – the Medieval Warm Period (MWP), also known as the Medieval Climate Anomaly (MCA) in the context of tropical and sub-tropical regions, followed by the Little Ice Age (LIA). The occurrence of these warm and cool periods differs from region to region, in terms of timing, duration and magnitude of the temperature anomalies. PAGES 2K consortium (2013) compiled global temperature estimates for the last two millennia; however, the Indian subcontinent remained under-represented in this work. A substantial body of evidence and insights exist, based on traditional and novel proxy data as well as modeling, which has revealed intriguing new aspects of the reconstructed climate of the last millennium in the Indian subcontinent. Here, we present a synthesis of the past millennium hydroclimate variability in India inferred from proxy records from regions affected by the Indian Summer Monsoon (ISM) and the Westerlies. The possible physical mechanisms linked to the moisture variations during the past millennium are also discussed. The aim of this work is to improve our current understanding and stress the gaps that exist in the knowledge of climate variability in the last millennia in one of the most populous regions of the world. We find that although there were no globally synchronous warm or cold intervals that define a MCA or LIA on the Indian subcontinent, a pattern of generally coherent regional precipitation variation during MCA and LIA period can be observed. The reconstructions from the ISM regime show generally wet conditions between 900 and 1350CE (MCA), punctuated in some regions of India by megadroughts, which was followed by relatively dry period during the global LIA interval from 1500 to 1800CE, punctuated by ‘wet rain spells’ in the Central Himalayas. In the Westerlies-dominated regions, very limited data restricts our understanding of rainfall variability during the MCA period; however, the LIA period was characterized by increased precipitation in these regions. The summer temperature reconstructions closely follow the ASIA 2K temperature reconstructions and also the global temperature trends. On the contrary, spring temperatures in the western Himalayas show an opposite trend with a rapid cooling during the 20th century. Changes in local sea surface temperature (SST) fields and other external boundary conditions like ENSO are suggested to play a major role for summer monsoon variations during the MCA-LIA period. Cooler oceans and continental temperatures are suggested to have pushed the low pressure systems associated with the Westerlies further south to be carried by the southern winter jet stream along the southern margin of the Tibetan Plateau bringing more precipitation in northern India during the LIA period. For the 20th century decreasing trend of ISM, increased anthropogenic atmospheric aerosol loading over south Asia is a possible causal factor.