气候变暖背景下北太平洋年代际涛动对我国西北干旱半干旱区东西部干湿反位相变化的影响机制

The arid and semi-arid region in northwest China is an important pasturing and irrigated agriculture areas which is very sensitive to global change. In such region due to the terrain with varying elevations, slopes and aspects, its climate is complex and is both influenced by the monsoon system and mid-latitude westerlies which struggle with each other. Therefore, the climate change responses are distinctly different in spatial distribution over the arid and semi-arid regions of northwest China——its East gets drying and its West gets wetting. Why the dry/wet trend is out of phase between the two parts of the arid and semi-arid regions in northwest China? The relevant studies are few and the variation characteristic of this "out of phase relationship" and its mechanism are still unclear. In this proposal, multi-scale characteristics of this "out of phase relationship between the eastern and western parts over the arid and semi-arid regions in northwest China" are investigated based on the in-situ, satellite remote sensing, reanalysis data and CMIP5 model outputs. Considering the important role of sea surface temperature anomalies, the mechanism of this "out of phase relationship" are explored in the view of the Pacific Decadal Oscillation (PDO) and the contribution of the PDO to this phenomena are quantitatively estimated here. Moreover, the proposal will evaluate the simulation capability of CMIP5 on the responses of the dry/wet variations to the PDO mode over the arid and semi-arid regions of northwest China and predict the trends of its eastern and western regions under different greenhouse gas emission scenarios in the future.

西北干旱半干旱地区是我国重要牧业区和和灌溉农业区，也是全球变化的敏感地区。然而由于地形差异较大，气候条件复杂，东亚季风和中纬度西风急流在该地区相互博弈，使得我国西北干旱半干旱地区对全球变化的响应存在空间格局的差异：东部地区干旱化，西部呈湿润化。目前，着重研究这一“跷跷板”式干湿变化关系的研究很少，对其变化规律及其原因机制的认识尚不清楚。本项目拟利用地面观测、卫星遥感、再分析资料及CMIP5多模式集合等多源数据，研究我国西北干旱半干旱区东、西部干湿反位相变化的演变特征，并基于太平洋年代际涛动（PDO）揭示气候变暖背景下西北干旱半干旱地区东、西部干湿变化反位相的原因机制，量化PDO在其中的贡献大小；评估CMIP5模式对我国西北干旱半干旱地区干湿变化对PDO模态响应的模拟能力，并预测未来不同排放情景下该地区东、西部的干湿变化趋势，为我国西北干旱半干旱区水资源的风险预估和人类有序适应提供科学依据。

我国西北干旱半干旱区是一个西风和季风相互博弈的地区，其东部地区更多是受季风影响，而其西部地区与中纬度西风关系更为密切，正是由于这种东、西气候背景差异加上不同的下垫面和人类活动，使得其对全球变化的响应也存在空间格局的差异。很多学者研究发现近几十年我国西北干旱半干旱区西部特别是新疆西北部降水增多，呈明显湿润化趋势，而其东部地区的降水则显著减少，出现显著干旱化特征。而什么原因造成近几十年我国西北干旱半干旱区东、西部的干湿变化呈截然相反趋势以及什么因素在维持这种反位相关系还不清楚。本项目就是在该背景下利用地面观测、卫星遥感、再分析资料及CMIP5多模式集合等多源数据，研究我国西北干旱半干旱区东、西部干湿反位相变化的演变特征，并基于太平洋年代际涛动（PDO）揭示气候变暖背景下西北干旱半干旱地区东、西部干湿变化反位相的原因机制。项目执行期间，我们紧扣项目研究内容，得到了主要结论有：1）理清了我国西北干旱半干旱区干湿变化的时空变化特征；发现其西部地区降水还具有显著的年代际和年际变化特征：1901-1944年的降水快速减少，之后到80年代又出现上升趋势，这和PDO的年代际转变又一定影响；此外，研究还指出近现代（1951-2013年）干旱半干旱区西部降水年际变化特征与ENSO关系密切，解释方差为17%，东部地区发生极端干旱的风险显著增加；2）揭示了年代际气候变率PDO通过减弱中低纬地区的夏季风环流、增强中高纬地区的西风急流来影响我国西北干旱半干旱地区干湿反位相变化的物理机制；3）指出全球变暖背景下强厄尔尼诺并不意味着我国北方夏季极端干旱发生，欧亚中纬度地区正的遥相关型是我国北方夏旱发生的关键，海气耦合的动力预测模式只有较好地捕捉到正位相欧亚遥相关型，才能预测出我国北方的极端干旱事件。