A tree-ring based reconstruction of summer relative humidity variability in eastern Mongolian Plateau and its associations with the Pacific and Indian Oceans
A regional chronology (RC) based on tree-ring-width data at two sites was developed for the southern Hulun Buir Sandy Land, a marginal area of the eastern Mongolian Plateau, China. This chronology significantly and positively correlated with the relative humidity from the previous December to September, except in May. The highest correlation of the RC with the mean June–August relative humidity suggested that the growth of Pinus sylvestris var. mongolica strongly reflects seasonal moisture conditions. Consequently, the variation in the mean June–August relative humidity was reconstructed for the reliable time period of 1835–2013; 41.6% of the variance occurred during the calibration period of 1953–2012. Similar patterns of drought and pluvial fluctuations were found in other tree-ring-based moisture reconstructions. Our reconstruction implied that the characteristics of regional hydroclimatic variations, such as the drought and low humidity of the late 1990s to 2000s, were not unusual from the long-term perspective, as suggested by the hydrological records obtained from tree-ring material. Spectrum analyses results and significant spatial correlations of our relative humidity reconstruction with the sea surface temperature (SST) in the eastern equatorial Pacific Ocean and tropical Indian Ocean implied close linkages between moisture in the eastern Mongolian Plateau and remote oceans. When the SST related to the El Nino–Southern Oscillation was high (low), the intensity of the Indian summer monsoon at the northern extremities weakened (strengthened). Thus, moisture decreased (i.e., the relative humidity was low) in North China and in our study area. The low land-sea thermal contrast between Asia and the Indian Ocean due to the enhanced SSTs in the tropical Indian Ocean also influenced the strength of the moisture transfer via the Indian summer monsoon, thereby reducing the moisture variations in the eastern Mongolian Plateau.