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Final Report Summary - MONSOON (Role of the Indian Monsoon on Global Climate Change)

Executive Summary:

Role of the Indian Monsoon on Global Climate Change:

The Indian summer monsoon accounts for more than 80-90% of the annual rainfall in the Indian subcontinent providing sustenance for over a billion people. Understanding and predicting its behavior in response to global change is a high scientific priority. Most of the monsoon reconstructions from the Arabian Sea are based on the upwelling indices (Prell, 1984; Naidu and Malmgren, 1996; Overpeck et al., 1996, Anderson et al., 2001), which essentially represent the SW monsoon wind strength rather than precipitation. Bay of Bengal is a highly suitable region to evaluate the SW monsoon precipitation for the following reasons: i) The four major rivers Irrawaddy, Brahmaputra, Ganges and Godavari discharge annually approximately 1.5x1012m3 of fresh water into the Bay of Bengal (Martin et al., 1981), ii) annual rainfall over the bay varies between 1 m off the east coast of India to more than 3 m in the Andaman (Baumgartner and Reichel, 1975), and iii) the peak discharge of rivers and rainfall over the bay occurs during the SW monsoon season. Hence Bay of Bengal is a highly suitable region to evaluate the SW monsoon precipitation. We therefore, investigated two sediment cores from the Bay of Bengal in order to reconstruct monsoon variability at decadal and centennial time scale with particular emphasis on the last 12 kyrs. We have used the paired measurements of oxygen and carbon isotopes and Mg/Ca to reconstruct the monsoon rainfall variability. Previously these proxies were successfully used to reconstruct the monsoon rainfall variability from the India Ocean (Anand et al., 2008; Govil and Naidu, 2010). Significant results of our studies are presented in figure 1.

Figure 1. Reconstruction of Indian monsoon rainfall by using the δ18Ow (blue line) derived from Globigerinoides ruber from a sediment cores in the Bay of Bengal and compared with Elnino Events numbers (redline). Elnino data is from Li et al (2o11). (a) Monsoon rainfall reconstruction at decadal scale shows that more monsoon rainfall during Roman Warm Period (RWP) and Medieval Warm Period (MWP) coincides with less Elnino Events. More fluctuations monsoon rainfall during Little Ice Age (LIA). (b) Monsoon rainfall reconstructions at subcentennial time scale comapred with the Elnino Events. An active mode of ENSO inititated around 7000 years and more number of ENSO from 5000 to 2000 years which coincides with decsending phase of monsoon rainfall.

Highlights and Conclusons:

Reconstruction of decadal time scale variability of the Indian Monsoon rainfall based on the δ18Ow reveals that increased monsoon rainfall during Roman Warm Period (RWP) and Medieval Warm Period (MWP) and greater monsoon rainfall fluctuations during the Little Ice Age (LIA). Coincidence of increased monsoon rainfall during RWP and MWP with less number ENSO events suggests a possible link between climate and ancient civilization over the globe.

Climate models are projected that the mean summer rainfall for India will increase slightly by the end of the century, largely because global warming due to the fact that warmer air can hold more water. Monsoon reconstructions based on the abundance of Globigerina bulloides have inferred an increase in the strength of monsoon winds off Oman during last four centuries as a consequence of global warming (Anderson et al., 2003) Such wind strengthening is expected to reflect in the Indian summer (June through September) monsoon rainfall (hereafter monsoon rainfall). By contrast δ18Ow reconstructions in the Bay of Bengal do not show an increase of monsoon rainfall over last four centuries, on other hand greater fluctuations of monsoon rainfall is noticed for last four centuries (1600 to 2000 years).

Scientific consensus exist that the inverse relationship between the El Nino southern oscillation (ENSO) and the Indian Monsoon. Conversely recent historical records of 140 years revealed that the relationship between Indian Monsoon and ENSO has broken down (Kumar et al., 1999). A comparison of monsoon rainfall variability on decadal time scale with the number of ENSO events over last 2000 years suggest a strong link between the monsoon rainfall in the Indian Subcontinent and ENSO. Furthermore, our evaluation of Indian monsoon rainfall variability at sub-centennial time scale with ENSO events over last 10000 years also reveals a strong influence of ENSO and Indian monsoon rain fall. Therefore, we conclude that on decadal and sub-centennial time scale ENSO controls the Indian Monsoon Rainfall through the stronger Walker circulation and associated tropical convection process.

A comparison of δ18Ow values between northern and southern Bay of Bengal cores during the Holocene reveals that North- South salinity gradient increase and decrease coincides with weak and strong monsoon rainfall respectively.

An establishment of a relationship between ENSO and Indian monsoon rainfall at decadal time scale has wide implication in reliable forecasting of the monsoon rainfall in the Indian subcontinent.

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