Evidence for decreasing South Asian summer monsoon in the past 160 years from varved sediment in Lake Xinluhai, Tibetan Plateau

Abstract

[1] We report glacial varves in the sediment of Lake Xinluhai, Tibetan Plateau. Independent data of 137 Cs and 210 Pb indicate that these are annually deposited varves. Varves appear as rhythmic units of light-colored silt layer capped by a dark clay layer under microscope. Varve thickness in Lake Xinluhai is sensitive to precipitation because sediment accumulation is strongly affected by monsoon rainfall in the area. A general decreasing trend can be observed in the varve thickness over the past 160 years. Spectral analyses of the varve record are dominated by cycles which are similar to ENSO periodicities. It implies that the decreasing trend of the South Asia monsoon may be linking with ENSO. Spatially, the decreasing trend can be observed across different proxy records in the south of the Tibetan Plateau. Although arguments still remain for the dynamic mechanisms and spatial rainfall difference, the South Asian summer monsoon could be weakened due to rising temperatures.

Variability of summer humidity during the past 800 years on the eastern Tibetan Plateau inferred from δ<sup>18</sup>O of tree-ring cellulose

“…Moreover, decreasing varve thicknesses imply a weakening Asian summer monsoon over the past 160 years (Chu et al, 2011). The aforementioned analysis revealed a link to warm phases of ENSO and an anomalous regional Hadley circulation.…”

Deuterium excess record in a southern Tibetan ice core and its potential climatic implications

“…Weakened Indian summer monsoon circulation is also projected by climate models (Ueda et al 2006) and supported by observation results (Dash et al 2009) in the backdrop of global warming conditions, which will lead to weakened incursion of monsoonal air masses into the TP (Kaspari et al 2007) and thus the reduced monsoon precipitation, in despite of the increase in atmospheric water content from the warmer Indian Ocean. A recent study (Chu et al 2011) has linked the decreasing varve thickness in a lake on the southern TP in the past 160 years to the reducing South Asian summer monsoon rainfall due to rising temperatures. In this sense, accompanied by the significant increasing Indian Ocean summer SST from the beginning of the twentieth century, the gradual decreasing ratios of monsoon precipitation to annual accumulation in the NK ice core have also contributed to the increasing trend of the annual mean d values.…”

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

“…A reduction in the monsoon precipitation and relative humidity of the ISM in the last 200 years is also evident in other areas influenced by the ISM. Maar lake sediments in Myanmar exhibit a decreasing trend of monsoonal rainfall since 1840 CE (Sun et al, 2016); a tree-ring δ 18 O record from southeast Asia exhibits a drying trend since 1800 CE (Xu et al, 2013a); a stalagmite δ 18 O record from southwest China reveals an overall decreasing trend in monsoon precipitation since 1760 CE (Tan et al, 2016); tree-ring δ 18 O and maar lake records in southwest China indicate reduced monsoon precipitation, relative humidity and cloud cover since 1840 or 1860 CE (Chu et al, 2011;Grießinger et al, 2017;Liu et al, 2014;Wernicke et al, 2015;Xu et al, 2012). Monsoon precipitation in northwestern India shows a significant decreasing trend during the period of 1866(Bhutiyani et al, 2010.…”