Abstract
Indian Summer Monsoon (ISM) rainfall has a direct effect on the livelihoods of two billion people in the Indian-subcontinent. Yet, our understanding of the drivers of multi-decadal variability of the ISM is far from being complete. In this context, large-scale forcing of ISM rainfall variability with multi-decadal resolution over the last two millennia is investigated using new records of sea surface salinity (δ18Ow) and sea surface temperatures (SSTs) from the Bay of Bengal (BoB). Higher δ18Ow values during the Dark Age Cold Period (1550 to 1250 years BP) and the Little Ice Age (700 to 200 years BP) are suggestive of reduced ISM rainfall, whereas lower δ18Ow values during the Medieval Warm Period (1200 to 800 years BP) and the major portion of the Roman Warm Period (1950 to 1550 years BP) indicate a wetter ISM. This variability in ISM rainfall appears to be modulated by the Atlantic Multi-decadal Oscillation (AMO) via changes in large-scale thermal contrast between the Asian land mass and the Indian Ocean, a relationship that is also identifiable in the observational data of the last century. Therefore, we suggest that inter-hemispheric scale interactions between such extra tropical forcing mechanisms and global warming are likely to be influential in determining future trends in ISM rainfall.
Highlights
The Indian summer monsoon (ISM) is a large-scale coupled land-ocean-atmosphere phenomenon that plays a dominant role in transporting water vapour to the Indian subcontinent[1]
The Medieval Warm Period (MWP) from 1200 to 800 years BP is characterized by negative δ18Ow anomalies compared to the preceding period (i.e. 2000 to 1300 years BP), which reveals that Bay of Bengal (BoB) experienced highest Indian Summer Monsoon (ISM) rainfall and strong freshening during this MWP over the last 2000 years
During the Little Ice Age (LIA), positive δ18Ow anomalies indicate a decrease in ISM rainfall from 700 to 200 years BP
Summary
The Indian summer monsoon (ISM) is a large-scale coupled land-ocean-atmosphere phenomenon that plays a dominant role in transporting water vapour to the Indian subcontinent[1]. Example includes the famous break of July 2002, where rainfall was reduced by 50%9 These large year-to-year variations in rainfall associated with internal and external interactions pose a considerable challenge for Indian monsoon forecasting[10] and for attributing monsoon variability to large-scale forcings. This is true for multi-decadal and longer time scales given that observational data only covers few cycles and paleoclimate records span longer time scales which fill this gap. We present a new record of ISM rainfall and SST variability from the Northern Indian Ocean spanning the last 2,000 years at multi-decadal resolution and investigate the large-scale forcing mechanisms of ISM rainfall
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