Drought Variation Over The Past 500 Years In Asia As Inferred From Tree Rings

Reginal drought has been an important part in the research of global change. Under the background of global warming, the area of extreme drought in the world has expanded more than two times in half a century, which has seriously affected the social and economic development. It is of great significance to understand the spatial and temporal characteristics of historical drought change and its relationship with largescale ocean-atmosphere-land interaction in Asian. Using tree-ring records, the author reconstructed the drought history of the Taihe Mountains in Gansu province and Asia, and explored their contaction with large-scale climate change.1. A group of Chinese pine samples collected from the Taihe Mountains were surfaced, cross dated, measured and standardized to establish the standard(STD) chronology. Climate response anlysis showed that significantly positive correlations(at the 95% confidence level(C.L.)) between the STD chronology and precipitation were found during August and September of the previous year and during April of the current year. Significantly negative correlations were found between the STD chronology and temperature during September of the previous year and during January, February, March and May of the current year. After seasonal combinations, the strongest response of the STD chronology to precipitation was found from August of the previous year to May of the current year, and the strongest response of the STD chronology to temperature was found from January of the current year to May of the current year. The STD chronology responded positively to the Standardised PrecipitationEvapotranspiration Index(SPEI) irrespective of the analyzed time scales and the most significant correlations occurred at the time scales between 10 and 13 months, indicating that the Chinese pines in the study area can’t be affected significantly by drought unless they are exposed to sustained water deficits.2. Bsed on the results of climate response analysis, the author designed a transfer function and reconstructed the drought variation for the last 283 years in the Taihe Mountains. The results of the split-sample calibration-verification tests indicated that the transfer function was reliable and stable. The reconstructuin explained 51% of the total variance, and caught the real climatic signal at both low and high frequency. The reconstruction revealed long-term patterns of drought variability and captured 4 drought epoch(i.e. 1763–1766, 1831–1833, 1882–1884 and 1928–1930) and 6 wet spells(i.e. 1730–1733, 1784–1787, 1702–1704, 1919–1922, 1943–1946 and 1948–1952) that lasted over 3 years.The reconstruction showed good spatial agreement with the global gridded SPEIAJ dataset, and similar dry–wet fluctuations to the dryness/wetness indices and the tree-ring based precipitation recoonstrctioon in the Changling-Shoulu region and Liancheng region, indicating that precipitation may be a major contributor to drought in the study area.3. The multi-taper method(MTM) spectral analysis, the Wavelet Analysis and the ensemble empirical mode decomposition(EEMD) all showed significant highfrequency and low-frequency periods. The 2–7-year cycles were probably affectd by the ENSO and QBO, the multi-decade cycles were probably affectd by the solar effects or Pacific Decadal Oscillation(PDO), and the centurial cycles might suggest the influences of solar effects. The reconstruction showed sigficant correlations with SST of the Indian Ocean, the equatorial central and eastern Pacific and the northern Pacific, and showed significant inverse phase relationship with PDO, suggesting that large-scale ocean-atmosphere-land circulation systems may have strong effects on the drought variations in the study area.4. Using CRUST programe, the 389 tree-ring records were standardized to establish signal-free chronologies. Both the instrumental data and the proxy data were prewhitened before the reconstruction and the estimated autocorrelation as removed from the instrumental data over the calibration interval was added back to the prewhitened reconstructions. The study area was divided into 41 subregions, and 6 screening processes were used to prefilter the full network of proxy data. The Reg EM with Truncated Total Least Squares(TTLS) were used to reconstrut the summer Palmer drought severity index(PDSI) variation during the past 500 years of each subregion and the 41 subregions were combined to form the final reconstruction. A proper screening process could make the reconstruction skills better but more rigorous screening conditions were not necessarily able to produce better reconstruction skills. During the verification period, the RE was positive in 65.73% of the study area, significant R2(p<0.10) was found in 42.51% of the study area, and the CE was positive in 31.46% of the study area.5. The reliabliy and utility of the reconstructed gridded PDSI was demonstrated in a set of examples of several famous historial droughts(e.g. 1586–1589, 1638–1641, 1876–1878 and 1928–1931). The PDSI reconstruction compared well with the reconstruction based on PPR method. EOF analysis was used to the reconstructed PDSI data(1500–2014 AD), and the first four spatial modes compared well with the first four spatial modes of the instrumental data(1951–2014 AD). The principal components correlated significantly with the SST variations of the Equatorial Indian Ocean and the Equatorial Pacific Ocean, indicating that the drought in the study area had a close relationship with the intensity of the Asian summer monsoon and ENSO.