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Zhao Y.,Institute of Desert Meteorology | Zhang H.,Bureau of Meteorology
Climate Dynamics | Year: 2015

Based on the historical and RCP8.5 experiments from 25 Coupled Model Intercomparison Project phase 5 (CMIP5) models, the impacts of sea surface temperature (SST) warming in the tropical Indian Ocean (IO) on the projected change in summer rainfall over Central Asia (CA) are investigated. The analysis is designed to answer three questions: (1) Can CMIP5 models reproduce the observed influence of the IO sea surface temperatures (SSTs) on the CA rainfall variations and the associated dynamical processes? (2) How well do the models agree on their projected rainfall changes over CA under warmed climate? (3) How much of the uncertainty in such rainfall projections is due to different impacts of IO SSTs in these models? The historical experiments show that in most models summer rainfall over CA are positively correlated to the SSTs in the IO. Furthermore, for models with higher rainfall-SSTs correlations, the dynamical processes accountable for such impacts are much closer to what have been revealed in observational data: warmer SSTs tend to favor the development of anti-cyclonic circulation patterns at low troposphere over north and northwest of the Arabian Sea and the Bay of Bengal. These anomalous circulation patterns correspond to significantly enhanced southerly flow which carries warm and moisture air mass from the IO region up to the northeast. At the same time, there is a cyclonic flow over the central and eastern part of the CA which further brings the tropical moisture into the CA and provides essential moist conditions for its rainfall generation. In the second half of twenty-first century, although all the 25 models simulate warmed SSTs, significant uncertainty exists in their projected rainfall changes over CA: half of them suggest summer rainfall increases, but the other half project rainfall decreases. However, when we select seven models out of the 25 based on their skills in capturing the dynamical processes as observed, then the model projected changes are much closer. Five out of the seven models predicted more rainfall over CA. Such a result is helpful for allowing us to attribute part of the observed upward rainfall trend in the CA region in the last several decades to the IO SST warming. © 2015 The Author(s) Source

Saumel I.,TU Berlin | Ziche D.,Johann Heinrich Von Thunen Institute | Yu R.,Institute of Desert Meteorology | Kowarik I.,TU Berlin | Overdieck D.,TU Berlin
Journal of Arid Environments | Year: 2011

The endangered river plain woodlands of semi-arid Central Asia provide numerous ecosystem services. Previous studies have focused mainly on changes in water supply and salinity as the underlying mechanisms of Populus euphratica woodland's decline. We tested whether vegetative regeneration of P. euphratica serves as an alternative pathway to propagation from seeds for reproduction in a degraded tree steppe in northwestern China and whether this method also works on grazed sites. We measured the effects of different grazing pressures on tree growth, survival, and vegetative regeneration. When subjected to high or moderate grazing pressure, P. euphratica populations failed to regenerate vegetatively, indicating recruitment limitation. A 25% increase of grazing pressure decreased the ramet density to 50% and the ramet height to 25%, as well as reducing the average age of ramets to one year. As overgrazing seriously limits the potential of natural recovery of P. euphratica a balance between livestock grazing and the regeneration of the Tugai forests is needed to sustain positive effects of increasing water tables and floods on tree vitality and regeneration. To restore population structure, and to support early vegetative regeneration long-term livestock management interventions to exclude livestock from degraded and recovered land need to be developed. © 2010 Elsevier Ltd. Source

Zhou Y.,Nanjing University | Jiang J.,Nanjing University | Huang A.,Nanjing University | La M.,Nanjing University | And 2 more authors.
Environmental Research Letters | Year: 2013

A regional climate model (RegCM4.1) is used to conduct two sets of sensitivity experiments. There are 20-year runs in each set of experiments. The experiments in each set include the variation of anthropogenic SO2 emissions during 1989-2008 but exclude natural variations of the atmosphere and sea surface temperature. The model results suggest that the high-speed emission of SO2 and its uneven distribution over eastern China can contribute to the change in May-August rainfall over eastern China between the two decades of 1999-2008 and 1989-1998, especially to the decrease (increase) of rainfall in the Yangtze River valley (Huang-Huai River region). Furthermore, the areas of decreasing (increasing) rainfall correspond to the downward (upward) currents of the anomalous atmospheric circulation, which is caused by the two humps of the difference in sulfate concentration between the two decades. © 2013 IOP Publishing Ltd. Source

Chen Y.,CAS Institute of Tibetan Plateau Research | Chen Y.,University of Chinese Academy of Sciences | Yang K.,CAS Institute of Tibetan Plateau Research | He J.,CAS Institute of Atmospheric Physics | And 4 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2011

The parameterization of thermal roughness length z0h plays a key role in land surface modeling. Previous studies have found that the daytime land surface temperature (LST) on dry land (arid and semiarid regions) is commonly underestimated by land surface models (LSMs). This paper presents two improvements of Noah land surface modeling for China's dry-land areas. The first improvement is the replacement of the model's z0h scheme with a new one. A previous study has validated the revised Noah model at several dry-land stations, and this study tests the revised model's performance on a regional scale. Both the original Noah and the revised one are driven by the Global Land Data Assimilation System (GLDAS) forcing data. The comparison between the simulations and the daytime Moderate Resolution Imaging Spectroradiometer- (MODIS-) Aqua LST products indicates that the original LSM produces a mean bias in the early afternoon (around 1330, local solar time) of about -6 K, and this revision reduces the mean bias by 3 K. Second, the mean bias in early afternoon is further reduced by more than 2 K when a newly developed forcing data set for China (Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS) forcing data) is used to drive the revised model. A similar reduction is also found when the original Noah model is driven by the new data set. Finally, the original Noah model, when driven by the new forcing data, performs satisfactorily in reproducing the LST for forest, shrubland and cropland. It may be sensible to select the z0h scheme according to the vegetation type present on the land surface for practical applications of the Noah LSM. Copyright 2011 by the American Geophysical Union. Source

Huang A.,Nanjing University | Zhou Y.,Nanjing University | Zhang Y.,Nanjing University | Huang D.,Nanjing University | And 2 more authors.
Journal of Climate | Year: 2014

Based on the outputs of historical and future representative concentration pathway (RCP) experiments produced by 28 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), future changes in climatic mean, interannual standard deviation (ISD), and long-term trends of the annual precipitation over central Asia (CA) have been estimated. Under different emission scenarios during the twenty-first century, the climatic mean and ISD (long-term trends) of the annual precipitation over CA projected by the five best models' ensemble mean show very similar (quite different) spatial patterns to those in the twentieth century. Relatively stronger increasing rates (over 3mmdecade-1 in RCP2.6 and over 6mmdecade-1 in RCP4.5 and RCP8.5) are located over northern CA and the northeastern Tibetan Plateau. Compared to the situations in the twentieth century, the climatic mean, ISD, and long-term trends of the projected annual precipitation over most of CA under different emission scenarios exhibit robust increasing changes during the twenty-first century. The projected increasing changes in the climatic mean (ISD) of theCAannual mean range from 10% to 35% (10%-90%) under different emission scenarios with relatively large increases over Xinjiang, China (northern CA and Xinjiang). The increasing trends of the annual precipitation over most of CA are projected to intensify with relatively large increases (over 3-9mmdecade-1) located over northern CA, the Tian Shan Mountains, and northern Tibet during the twenty-first century. In addition, the intensities of the increasing changes in the climatic mean, ISD, and trends of CA annual precipitation are intensified with the emissions increased correspondingly. Further analyses of the possible mechanisms related to the projected changes in precipitation indicate that the increases of the annual precipitation over CA in the twenty-first century are mainly attributed to the enhanced precipitable water that results from strengthened water vapor transport and surface evaporation. © 2014 American Meteorological Society. Source

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