Decreasing Indian summer monsoon on the northern Indian sub-continent during the last 180 years: evidence from five tree-ring cellulose oxygen isotope chronologies

Chenxi Xu,Ashok Priyadarshan Dimri,Zhengtang Guo,Feng Shi,Masaki Sano,Takeshi Nakatsuka,Rengaswamy Ramesh

doi:10.5194/cp-14-653-2018

Abstract

Abstract. We have constructed a regional tree-ring cellulose oxygen isotope (δ18O) record for the northern Indian sub-continent based on two new records from northern India and central Nepal and three published records from northwestern India, western Nepal and Bhutan. The record spans the common interval from 1743 to 2008 CE. Correlation analysis reveals that the record is significantly and negatively correlated with the three regional climatic indices: all India rainfall (AIR; r  =  −0.5, p  <  0.001, n  =  138), Indian monsoon index (IMI; r  =  −0.45, p  <  0.001, n  =  51) and the intensity of monsoonal circulation (r  =  −0.42, p  <  0.001, n  =  51). The close relationship between tree-ring cellulose δ18O and the Indian summer monsoon (ISM) can be explained by oxygen isotope fractionation mechanisms. Our results indicate that the regional tree-ring cellulose δ18O record is suitable for reconstructing high-resolution changes in the ISM. The record exhibits significant interannual and long-term variations. Interannual changes are closely related to the El Niño–Southern Oscillation (ENSO), which indicates that the ISM was affected by ENSO in the past. However, the ISM–ENSO relationship was not consistent over time, and it may be partly modulated by Indian Ocean sea surface temperature (SST). Long-term changes in the regional tree-ring δ18O record indicate a possible trend of weakened ISM intensity since 1820. Decreasing ISM activity is also observed in various high-resolution ISM records from southwest China and Southeast Asia, and may be the result of reduced land–ocean thermal contrasts since 1820 CE.

Highlights

The Indian summer monsoon (ISM) delivers a large amount of summer precipitation to the Indian continent, and has a major influence on economic activity and society in this densely populated region (Webster et al, 1998)
The oxygen isotopes of four individuals of Abies spectabilis in Ganesh (GE, central Nepal) and three individuals of Cedrus deodara in Jageshwar (JG, northern India) were measured for the 1801 to 2000 CE and 1621 to 2008 CE intervals, respectively
The five tree-ring δ18O records for the Himalayan region are significantly correlated with one another at interannual timescale during the common period, and in most cases 31-year running averages of five tree-ring δ18O chronologies show significant correlations at multi-decadal timescale (Table 2). These results indicate that five tree-ring δ18O records reflect a common controlling factor that may be related to regional climate

Summary

Introduction

The Indian summer monsoon (ISM) delivers a large amount of summer precipitation to the Indian continent, and has a major influence on economic activity and society in this densely populated region (Webster et al, 1998). Current research on the ISM is mainly concerned with the study of interannual and inter-decadal variations, using meteorological data and climate models. Climate model experiments indicate that there is a significant increase in mean ISM precipitation of 8 % under the doubling atmospheric carbon dioxide concentration scenario (Kripalani et al, 2007) and human-influenced aerosol emissions mainly resulted in observed precipitation decrease during the second half of the 20th century (Bollasina and Ramaswamy, 2011).

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