影响中国北方强降雪事件年际变化的典型环流背景和水汽收支特征分析

基于1961-2014年中国台站观测资料和NECP/NCAR再分析资料，对影响中国北方强降雪事件（日降雪量5 mm及以上，包括大到暴雪）年际变化的典型大尺度环流特征和水汽条件进行了综合分析.结果表明：中国北方强降雪事件主要集中在新疆北部和东北两个地区，而且强降雪日数和降雪量具有高度一致的年际变化特征.中国北方强降雪事件偏多时，对应北大西洋涛动（NAO）和北极涛动（AO）负位相；贝加尔湖上空维持异常低槽区，有利于冷空气的爆发南下；热带印度洋至热带西太平洋上空维持一条异常反气旋带，有利于暖湿气流向北输送；中国北方及以北区域高空为异常西风气流，提供有利的动力抬升条件，使得强降雪易于在中国北方发生；反之亦然.水汽收支分析显示，中国北方西边界和南边界水汽入流增强在强降雪偏多中起着主要贡献.异常西风水汽输送利于新疆北部大到暴雪偏多，异常西南风水汽输送则利于东北地区大到暴雪的发生.进一步研究揭示：与小雪相比，影响中国北方大到暴雪年际偏多的中高纬环流特征相类似，但环流经向度更大；而且大到暴雪与NAO和AO的关系更密切，并更多的受到来自中低纬地区的水汽输送影响.

Large-scale atmospheric circulations and water vapor transport influencing interannual variations of intense snowfalls in northern China

Based on the observation data from meteorological stations of China and the reanalysis dataset from the NECP/NCAR for the period 1961-2014, the large-scale atmospheric circulations and water vapor transport associated with the interannual variations of intense snowfall events in northern China are analyzed. The results indicate that intense snowfall events (defined as daily snowfall no less than 5 mm, including large and heavy snowfall) mainly occur in northern Xinjiang and Northeast China, and the interannual variation of the intense snowfall frequency is highly consistent with that of the intense snowfall amount. Associated with more intense snowfall events in northern China, the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) tend to be in a negative phase and the Lake Baikal trough tends to be deeper, which favors the southward outbreak of cold air. Meanwhile, a cluster of anomalous anticyclones prevail over the tropics from the Indian Ocean to the western Pacific, which is beneficial for warm and moist airflows to be transported northward. In addition, the upper troposphere of northern China and its north flank is dominated by westerly anomalies, which provides a dynamically uplift condition favorable for the occurrence of intense snowfall events. The opposite atmospheric circulation anomalies are applicable for fewer intense snowfall events in northern China. Further analysis on water vapor budget suggests that the intensification of westerly and southerly water vapor inflows in northern China have a major contribution to more occurrences of intense snowfall events. Anomalous westerly water vapor transport is favorable for more intense snowfall events in northern Xinjiang, and anomalous southwesterly water vapor transport is favorable for more intense snowfall events in Northeast China. Compared with light snowfall events, the atmospheric circulation anomalies at mid-high latitudes that are associated with more intense snowfall events in northern China are similar but with larger meridional meander. Moreover, the intense snowfall has a closer relationship with the NAO and AO. Besides, water vapor transport from the mid-lower latitudes is much more important for the occurrence of intense snowfall events.