Meixue Yang

Millennium tree-ring reconstruction of drought variability in the eastern Qilian Mountains, northwest China

Knowledge of natural long-term drought variability is essential for water resource management and planning, especially in arid and sub-arid regions of the world. In the eastern Qilian Mountains of China, long-term drought variability based on high-resolution proxy records such as tree-ring data are still scarce to date. Here we present a new tree-ring chronology from the eastern Qilian Mountains which provides a valuable 1,002-year record (1009–2010 CE) of drought variability. The new reconstruction of June–July 5-month scale standardized precipitation and evapotranspiration index is the first millennium tree-ring estimate of past climate developed in the eastern Qilian Mountains. The record shows that this region has experienced several persistent droughts and pluvials over the past millennium, with significantly drier climate during the fifteenth century and dramatic wetting since the nineteenth century. The low frequency generally agrees with other nearby studies based on both tree-ring data and other proxy data.

A comparison of tree-ring records and glacier variations over the past 700 years, northeastern Tibetan Plateau

Abstract The ecological environment of the headwater area of the Yellow River, west China, is seriously deteriorating because of the harsh natural environment, weakened ecological systems and intensified human activities as well as regional climate changes. Forests and glaciers coexist in this area. Glaciers in the area have retreated over the last decade because of climate change. Most glaciers on the Tibetan Plateau (TP) tend to retreat during warm intervals and advance during cold intervals. Tree-ring records provide an important index for examining past climate changes. A total of 139 core samples from 97 living cypresses (Juniperus przewalskii) in the central region of the Yellow River headwater area, the Animaqin mountains, northeastern TP, were sampled from three sites that are close to each other. The chronologies were developed using the ARSTAN program. Analyses indicate that these tree-ring width records reflect the summer maximum temperature of the study area over the past 700 years. The tree-ring records and the glacier advances recorded by terminal moraines are compared. Inferred summer maximum temperatures suggest three cold periods during the Little Ice Age, around AD1500, 1700 and 1850. These cold intervals are consistent with the glacier moraine record from the region.

Climate–growth analysis of Qilian juniper across an altitudinal gradient in the central Qilian Mountains, northwest China

In the context of global warming, it is of high importance to assess the influence of climatic change and geographic factors on the radial growth of high-elevation trees. Using tree-ring data collected from four stands of Qilian juniper (Juniperus przewalskii Kom.) across an altitudinal gradient in the central Qilian Mountains, northwest China, we compared the radial growth characteristics and climate–growth relationships at different elevations. Results indicated that there was little difference in the tree-ring parameters of the four chronologies. Correlation analyses both for unfiltered and 10-year high-passed data of monthly climatic variables and chronologies were presented to investigate the climatic forcing on tree growth, and results revealed that the correlation patterns were consistent among the four sites, especially for high-passed data. We employed the principal components analysis method to obtain the first principal component (PC1) of the four chronologies and computed the correlations between PC1 and climate factors. The PC1 correlated significantly with winter (November–January) temperature, prior August and current May temperature, and precipitation in the previous September and current January and April, indicating that tree growth in this region was mainly limited by cold winter temperature and drought in early growing season and prior growing season (prior August and September). However, the climate–growth relationships were unstable; with an increase in temperature, the sensitivity of tree growth to temperature had decreased over the past few decades. Considering the instability of the climate–growth relationships, climate reconstructions based on tree rings in the study area should be approached with more caution.

Water Recycling between the Land Surface and Atmosphere on the Northern Tibetan Plateau—A Case Study at Flat Observation Sites

ABSTRACT High-resolution soil moisture, temperature, and precipitation data from the northern part of the Tibetan Plateau provide the basis for analyzing the cycling of water between the land surface and atmosphere. Data analyzed come from the Intensive Observation Period (IOP) of the GEWEX (Global Energy and Water Cycle Experiment) Asian Monsoon Experiment (GAME) on the Tibetan Plateau (GAME-Tibet). Observations from July to August 1998 show that evaporation from flat land surfaces was 177 mm on the south side of the Tanggula Mountains, and 73 mm on the north side. These represent about 73% and 58%, respectively, of the precipitation in the same period. Evaporation not only transports considerable water but also considerable energy from the land surface to the atmosphere, which can slow the rising of soil temperatures. Differences in the evaporation between the south and north sides of Tanggula Mountains is mainly caused by differences in precipitation.

Influences of Two Land-Surface Schemes on RegCM4 Precipitation Simulations over the Tibetan Plateau

The effects of different RegCM4 land-surface schemes on Tibetan Plateau (TP) precipitation simulations were investigated. Two groups of ten-year (1992–2001) simulation experiments (hereafter referred to as BATS and CLM) were performed based on two land-surface schemes (BATS and CLM3.5, resp.) and were compared with observed data using the same domain, initial, and lateral boundary conditions, cumulus convective scheme, and spatial resolution. The results showed that the CLM monthly precipitation more closely matched the observed data compared with BATS. BATS and CLM both overestimated summer precipitation in the northern TP but underestimated summer precipitation in the southern TP. However, CLM, because of its detailed land-surface process descriptions, reduced the overestimated precipitation areas and magnitudes of BATS. Compared to CN05, the regional average summer precipitation in BATS and CLM was overestimated by 34.7% and underestimated by 24.7%, respectively. Higher soil moisture, evapotranspiration, and heating effects in the BATS experiment triggered changes in atmospheric circulation patterns over the TP. Moreover, BATS simulated the lower atmosphere as warmer and more humid and the upper atmosphere (~150 hPa) as colder than the CLM simulations; these characteristics likely increased the instability for moist convection and produced more summer precipitation.

Precipitation Distribution along the Qinghai-Xizang (Tibetan) Highway, Summer 1998

Geographic variations in summer (July–August) precipitation data for the northern Qinghai-Xizang (Tibetan) Plateau, collected during the IOP (Intense Observation Period) of GAME-Tibet, were examined. Results indicated that, basically, the daily precipitation amount consistently varies temporally at all sites. An increase in precipitation amount is evident from north to south. The distribution of summer monsoon precipitation is affected by latitude, altitude, and other factors (i.e. atmosphere circulation, water vapor transportation). The latitude effect of precipitation in July and August was significant. August precipitation varied with both latitude and altitude. The site with higher elevation received more precipitation. Topoclimatic controls operating over precipitation on slopes include elevation. The distribution of the precipitation during July–August mainly showed the effects of latitude, but this was modified by the altitude effect. Redistribution processes caused by topography induce increased precipitation with altitude at some sites. Owing to the complexity of surface conditions on the Qinghai-Xizang (Tibetan) Plateau, precipitation is controlled by many factors simultaneously. Without benefit of an expanded observation network, it is difficult to separate the effects of the many factors influencing the distribution of precipitation in the region.

Effects of modified soil water‐heat physics on RegCM4 simulations of climate over the Tibetan Plateau

To optimize the description of land surface processes and improve climate simulations over the Tibetan Plateau (TP), a modified soil water-heat parameterization scheme (SWHPS) is implemented into the Community Land Model 3.5 (CLM3.5), which is coupled to the regional climate model 4 (RegCM4). This scheme includes Johansens soil thermal conductivity scheme together with Nius groundwater module. Two groups of climate simulations are then performed using the original RegCM4 and revised RegCM4 to analyze the effects of the revised SWHPS on regional climate simulations. The effect of the revised RegCM4 on simulated air temperature is relatively small (with mean biases changing by less than 0.1°C over the TP). There are overall improvements in the simulation of winter and summer air temperature but increased errors in the eastern TP. It has a significant effect on simulated precipitation. There is also a clear improvement in simulated annual and winter precipitation, particularly over the northern TP, including the Qilian Mountains and the source region of the Yellow River. There are, however, increased errors in precipitation simulation in parts of the southern TP. The precipitation difference between the two models is caused mainly by their convective precipitation difference, particularly in summer. Overall, the implementation of the new SWHPS into the RegCM4 has a significant effect not only on land surface variables but also on the overlying atmosphere through various physical interactions.

The spatially heterogeneous distribution of precipitation of the Anduo area, Tibetan Plateau, in summer 1998

Abstract Examination of precipitation in the Tibetan Plateau is important to understanding the regional water cycle processes and the plateau-scale energy budget. Based on hourly precipitation data obtained during the GAME-Tibet intensive observation period at four sites, the spatial distribution of precipitation in summer 1998 within the Anduo area was examined. The results show that, between 1 July and 11 September 1998, the precipitation that occurred simultaneously (at the same hours) at the sites accounted for 6.9–15.3% of the total precipitation at each site during the study period. Even at the two observation sites that are only 20 km apart, the percentage of precipitation that occurred simultaneously was quite small. This indicates that precipitation occurred not only frequently but also very locally, except on several days with very strong monsoon precipitation. The limited observations highlight that the precipitation distribution is quite complex, and large-scale intensive precipitation observations are needed in the future to clarify the heterogeneity of precipitation on the Tibetan Plateau.

Correlation between precipitation and temperature variations in the past 300 years recorded in Guliya ice core, China

Abstract The Guliya ice cap, on the crest of the Kunlun Shan, central Asia, is an ideal site for acquiring ice cores for climate-change studies. Detailed analyses of the precipitation index (glacier accumulation) and the temperature proxy (δ18O) recorded in the Guliya ice core since 300 years BP show that precipitation correlates with temperature in this region. Climate conditions in the Guliya region since 300 years BP can be separated into three periods: warm and wet from AD 1690 to the end of the 18th century; cold and dry from the 19th century to the 1930s; and warm and wet again since the 1940s. During this period, the climate exhibits just two phases: warm/wet and cold/dry. Comparison of the temperatures and the precipitation recorded in the Guliya ice core shows that variations of temperature and precipitation in the region correlate quite well. However, changes in the precipitation regime appear to lag behind those of the temperature by 20–40 years. We believe this results from the larger heat capacity of the ocean relative to that of the land. Hence, ocean temperatures and corresponding evaporation rates change more slowly than do continental conditions. Additionally, however, positive feedback processes, such as increasing temperatures and precipitation improving vegetation, moisture retention and, hence, local convective precipitation probably play an important role. In this paper, we explain how the timescale of evolving vegetation and the feedback mechanism between precipitation and the temperature could help explain why the changes in precipitation lag those of temperature by 20–40 years over long periods. Taking this time lag into account, we should be able to predict future precipitation trends, based on observed temperature trends.

Assessing the influences of tree species, elevation and climate on tree-ring growth in the Qilian Mountains of northwest China

Key messageThe tree species, elevation and climate change jointly influenced the tree-ring growth,directing us to carefully consider the comprehensive effects of these variables when performing large-scale and multi-species-based dendroclimatic studies.AbstractThe rate of tree growth varies by species and is influenced by elevation and climate gradients. Assessing the influence of these variables on tree-ring growth can improve projections for tree growth under future climate conditions. Here, we evaluate the relationships between tree species, elevation, climate and tree-ring growth in the Qilian Mountains of northwest China by using the tree-ring records of Qinghai spruce (Picea crassifolia) and Qilian juniper (Juniperus przewalskii) trees. The rotated principal component analysis was applied to examine the individual tree growth and the associated influencing factors. The tree-ring chronologies were used to compare tree growth patterns and the climate–growth relationships between the two species and elevations. The results indicated that trees from the Qinghai spruce sites showed similar growth patterns and climate–growth relationships, even though the elevation differed. The Qilian juniper chronologies from the high and low elevation sites revealed different growth patterns during recent decades that were closely related to the discrepant climate–growth relationships with the temperatures and the recent warming trend. The differences in tree growth patterns between both species and elevation could be attributed to plant physiological traits. The results of this study demonstrate the impact of tree species, elevation and climate change on tree-ring growth, suggesting that accounting for the influences of these variables could improve large-scale and multi-species-based dendroclimatic studies.