Deputy Project Director: Dr. Supriyo Chakraborty
Information on past climate, extending back to thousands of years and even longer, is vital to understand the various external and internal forcing on the climate and thereby make reliable future projections. In this time frame, the tree-rings, corals and speleothem with their interannual to sub-annual resolution, have been recognized to be reliable sources of proxy climate data. Past climate information, for example, high resolution monsoon rainfall data prior to the instrumental records for a few thousand years is crucial to understand the natural modes of monsoon variability. If such kind of records are retrieved from a wide geographical area of the Asian monsoon region it may be possible to determine whether the current declining trend in Indian monsoon rainfall is part of a natural climate variability, or it is driven by human induced climate change. We are involved in analyzing various natural archives that provide proxy records of climate state parameters, such as, monsoon rainfall, temperature, relative humidity, drought index, atmospheric pCO2 etc. in order to address such kind of scientific issues. We mainly rely on the analysis of tree ring records (i.e. ring , ring density) and the isotopic analysis of tree rings, sediments, such as, carbonate deposits etc. Since quantification of past rainfall variability using the isotopic records of tree ring, speleothem requires the knowledge of isotopic variability of modern rainwater, a significant amount of work consists of the collection and analysis of modern rainwater from different locations across India. Isotopic analysis of rainwater also helps study the hydro-meteorological processes and some aspects of short term monsoon variability.
Isotopic analysis of rainwater at the Andaman Islands:
Isotopic analysis of precipitation over the Andaman Islands in the Bay of Bengal was carried out for the year 2012 and 2013 in order to study the atmospheric controls on rainwater isotopic variations. Figure 1 shows the oxygen isotopic records (d18O) of Port Blair rain water for the years 2012 and 2013. The oxygen and hydrogen isotopic compositions are typical of the tropical marine sites but show significant variations depending on the ocean-atmosphere conditions; maximum depletion was observed during the tropical cyclones. The isotopic composition of rainwater seems to be controlled by the dynamical nature of moisture rather than the individual rain events. Precipitation isotopes undergo systematic depletions in response to the organized convection occurring over a large area and are modulated by the integrated effect of convective activities. Precipitation isotopes appear to be linked to the 10-20 day mode of monsoon intraseasonal variability in addition to synoptic scale fluctuations. During the early to mid monsoon, the amount effect arose primarily due to rain re-evaporation but in the later phase, it was driven by moisture convergence rather than evaporation. Amount effect had distinct characteristics in these two years, which appears to be modulated by the intraseasonal variability of monsoon. It is shown that the variable nature of amount effect limits the ability to reconstruct the past-monsoon rainfall variability on annual to sub-annual time scale. [Chakraborty S., Sinha N., Chattopadhyay R., Sengupta S., Mohan P.M., Datye A., Atmospheric controls on the precipitation isotopes over the Andaman Islands, Bay of Bengal, Scientific Reports, 6:19555, January 2016, doi:10.1038/srep19555]
Figure 1. Oxygen isotopic records of rainfall at the Andaman Island for the summer monsoon period. Solid line represents the year 2012 and the broken line is for 2013.
Heat index reconstruction based on tree–ring records of western Himalaya in India
Tree ring- index chronology based on a well replicated tree core samples (Cedrus deodara and Pinus roxburghii) of the western Himalaya have been carried out in relation to study climate variability / change. The first principal component (PC1) prepared by using multiple-sites tree ring- chronologies of western Himalaya is seen to be negatively correlated with heat index and positively with the Palmer drought Severity Index (PDSI) and moisture index during February to May as representative of regional climate. The correlation coefficient of PC1 with heat index, PDSI and moisture index for the period 1901-1988 is found to be -0.60, 0.37 and 0.59 respectively which are highly significant at 0.1% level. The result shows that increasing heat index may enhance transpiration and evaporation over western Himalaya; which may cause insufficient moisture at root zone of the trees. Based on the tree ring data, heat index of spring season (February to May) has been reconstructed back to AD 1839 (Figure 2). The reconstructed heat index showed the widest warm periods during 1952-1963 and 1966-1976, in 20th century. [Somaru Ram and H.P. Borghaonkar, 2016, Reconstruction of heat index based on tree-ring records of western Himalaya in India Dendrochronologia, 40, 64-71].
Figure-2 a: Comparison between actual heat index for the period 1901-2014 (dashed line) and estimated heat index (solid line) for the period 1901-1988 during February to May. The A.D. 1901-1988 regression model was used for calibration; CC is the correlation coefficient in upper panel for the period 1901-1988. b: A 31- year sliding correlation coefficients between actual and reconstructed heat index, correlation coefficients are plotted against the central year of the 31 –yr period, with the significant at 5% level. c: February to May heat index reconstruction (A.D. 1839-1988). The smoothed line is a 10 year cubic spline fit in lower panel.
Deputy Project Director: Dr. Supriyo Chakraborty, Scientist-E