NOAA/WDS Paleoclimatology - Manali, NW India 242 Year Tree Ring Oxygen Isotope Data

The oxygen isotope ratio (d18O) of tree-ring cellulose is known to be a reliable proxy for hydroclimate in monsoon Asia. However, tree-ring d18O data are still spatially and temporally limited, so a denser tree-ring network in Asia is required to better understand the dynamics of the monsoon circulation and its past variability. Here, we present a 242-year d18O chronology calculated from five silver fir trees collected in the western (Indian) Himalaya, a region located on the northwestern periphery of the summer monsoon incursions. Response analyses using regionalized climatic data revealed that tree-ring d18O is controlled by hydroclimatic variables, including precipitation, relative humidity, and the drought index, during the summer monsoon season. In addition, spatial correlation analyses with gridded climatic parameters showed that the strongest correlations of tree-ring d18O are not observed with the climatic parameters at the sampling site, but with those in a region several hundred kilometers to the southwest, indicating that water vapor originating in the Arabian Sea is transported to the study site. Based on these results, we reconstructed the self-calibrating Palmer Drought Severity Index (scPDSI) for the summer monsoon season (June-September) over the past 242 years (1767-2008 CE), using a linear regression model that accounts for 45.0% of the actual scPDSI variance. Our chronology showed significant correlations with other tree-ring d18O data from Nepal and Bhutan, indicating that common signals related to the moisture supply from the Bay of Bengal are also recorded in the present reconstruction. However, the tree-ring record from India often showed weak correlations with that from Bhutan, especially when the summer monsoon was relatively weak. This result, together with the fact that the water vapor at the tree site was also derived from the Arabian Sea, implies that a weaker monsoon circulation enhances the flux of Arabian Sea moisture and reduces the flux of Bay of Bengal moisture to the study region.

树木年轮纤维素的氧同位素比值（d18O）已被证实为亚洲季风区水文气候的可靠代用指标。然而，当前树木年轮d18O数据在空间与时间维度上仍存在局限性，因此亟需构建更为密集的亚洲树木年轮网络，以深入解析季风环流的动态变化及其过去的变异性。本研究基于采自西（印度）喜马拉雅——一处位于夏季季风入侵西北边缘的区域——的5株银冷杉，建立了一段时长242年的d18O年表。利用区域化气候数据开展的响应分析显示，在夏季季风季期间，树木年轮d18O受降水、相对湿度与干旱指数等水文气候变量调控。此外，针对格点气候参数的空间相关分析表明，树木年轮d18O的最强相关并非来自采样点本地的气候参数，而是来自西南方向数百公里外的区域，这意味着源自阿拉伯海的水汽被输送至研究区域。基于上述结果，本研究利用可解释45.0%实际自校准帕默尔干旱强度指数（self-calibrating Palmer Drought Severity Index, scPDSI）方差的线性回归模型，重建了过去242年（公元1767-2008年）夏季季风季（6-9月）的自校准帕默尔干旱强度指数。本研究建立的年表与尼泊尔、不丹的其他树木年轮d18O数据呈现出显著相关性，表明源自孟加拉湾的水汽供应相关的共同气候信号也被记录在了本次重建结果中。然而，印度的树木年轮记录与不丹的记录往往相关性较弱，尤其是在夏季季风相对偏弱的时期。这一结果，结合研究区域的水汽同样源自阿拉伯海这一事实，表明偏弱的季风环流会增强阿拉伯海水汽的输送通量，并减少孟加拉湾水汽向研究区域的输送通量。