Proxy Climatic Records of Past Monsoons
R. Ramesh, H. Boragaonkar, S. Band and M.G. Yadava
Modeling of climate variability on interdecadal to century timescale is a major research area in the field of climatology (e.g., Masson-Delmotte et al. 2013). The shortness of instrumental climate records that cover the past * 100 years at the most renders this research rather difficult. In the Himalayan/Tibetan Plateau region, this problem is more severe due to lack of long instrumental records. For a better understanding of climate variability on different timescales, it is important to expand the geographical and temporal spans of climatic records by including historical records and climate proxies such as tree rings, ice-cores, speleothems, marine, and lake sediments. By understanding the past monsoonal variability and its relationship to various forcing factors, it is possible to improve our knowledge of the dynamics of the climate system. This will help develop more reliable predictive models of Asian monsoon variability.
Any well-dated, high-resolution paleoclimatic proxy offers a means of extending the climate record back in time. The Asian monsoon, one of the most important components of the climate system that impacts almost half of the world’s population, has recently been the focus of international attention to decipher its driving mechanisms and feedbacks. A time series of Indian monsoon rainfall from instrumental records prepared from a dense network of meteorological stations covering a period of more than a century is available on regional scale as well as for India as a whole (Parthasarathy et al. 1995; Rajeevan et al. 2006; Guhathakurta et al. 2014). Optimizing the number of stations, appropriate statistical model has
R. Ramesh (H) • S. Band • M.G. Yadava
Indian Institute of Tropical Meteorology, Earth System Science Organization, Pune,
Maharashtra 410008, India
© Springer Science+Business Media Singapore 2017 271
M.N. Rajeevan and S. Nayak (eds.), Observed Climate Variability and Change Over the Indian Region, Springer Geology, DOI 10.1007/978-981-10-2531-0_15
helped to extend the series further back to early nineteenth century (Sontakke et al. 1993). These indicate a highly variable, trend-less behavior of Indian summer monsoon rainfall with epochal changes. Climate variability studies over the Indian Himalayan region, Nepal, and Tibetan Plateau (Li and Tang 1986; Seko and Tahahashi 1991; Borgaonkar 1996) are mostly based on very specific localities and short data periods. Reasonably, long records of about a century from about 15 stations over the Indian Himalaya, however, do not show any long-term trend in rainfall in the instrumental period.
Climatic anomalies such as the occurrence of glacial phases, transitions of glacial and interglacial periods, and variability of monsoon as reflected in the frequency and intensity of droughts need to be examined with data sets much longer than the instrumental records. There are some records of paleomonsoon in India and other parts of Asia, which have been useful in bringing out some features of long-term climate change (see for a review, Tiwari et al. 2009; Ramesh et al. 2010; Sanyal and Sinha 2010). In this paper, we summarize our current knowledge about monsoons in the past from quantifiable proxies with unambiguous dates only. We mainly discuss results based on marine (618O of planktic foraminifera) and terrestrial proxies such as tree rings and speleothems (cave deposits such as stalactites and stalagmites), because these are well dated and used for high-resolution monsoon reconstructions. Stable oxygen isotopic (618O) variations in these proxies help quantify monsoon changes with uncertainties of the order of *13 and 8 mm, respectively. These uncertainty estimates are based on (a) the ‘amount effect’ in tropical rainfall, i.e., a change of 1.5 %o per 100 mm change of monthly rain, and (b) the experimental uncertainty of 618O measurements, i.e., ±0.2 and ±0.1 %, respectively, for tree-ring cellulose and speleothem carbonates.
However, the 618O values recorded in continental proxies at some sites may not necessarily reflect local rainfall variability; instead, they could be useful in the reconstruction of large-scale convective systems or upstream rainfall (Lekshmy et al. 2014, 2015; Midhun and Ramesh 2015). Figure 1 shows the locations of marine and terrestrial records discussed below in detail.