北京地区汛期降水季节循环异常的典型特征及其成因

Typical Characteristics and Causes of Anomalous Seasonal Cycle of Precipitation during Flood Season in Beijing Area

中文关键词:  北京地区  降水季节循环异常  自组织映射神经网络  多样性  年际变化

英文关键词:Beijing area  Anomalies in the seasonal cycle of precipitation  Self-organizing mapping neural networks  Diversity  Interannual variability

中文摘要:

      北京地区面积大、人口密度高，汛期降水的季节循环特征具有明显年际差异，对首都经济建设和人民生活产生重要影响。基于1981–2022年北京地区20个气象观测站点的逐日降水观测数据，使用自组织映射神经网络和统计诊断方法，分析了当地汛期降水季节循环异常时空变化的典型特征及其成因。结果表明，北京地区汛期降水的季节循环异常具有4种独立类型，包括“持续旱涝”和“旱涝急转”两种典型特征，分别占比45.2%和54.8%，其直接原因是日本海上空的具有相当正压结构的环流异常。在持续旱涝年，当日本海上空出现高压（低压）异常时，中纬度蒙古气旋和高空南亚高压异常耦合联动，局地盛行低空北风（南风）异常，抑制（促进）了来自西北太平洋的水汽输送，令降水持续偏少（偏多）。而在旱涝急转年，当7月日本海上空盛行低压（高压）异常、西太副高异常南移（北抬）时，北京地区降水异常偏少（偏多）；8月，伴随着蒙古气旋的异常加强（减弱），日本海上空转变为高压（低压）异常，北京地区降水随之异常偏多（偏少）。进一步研究表明，持续旱涝型异常和热带外海温存在显著相关，而旱涝急转型异常不仅与热带ENSO事件联系紧密，还和局地降水显著的10-30天季节内振荡信号有关。

英文摘要:

      Beijing, a prominent megacity in China, experiences notable interannual variations in its seasonal precipitation cycle, exerting substantial influences on both economic activities and societal well-being. Based on the daily precipitation observations spanning the period from 1981 to 2022 from 20 meteorological stations in Beijing, employing self-organizing mapping neural networks and statistical diagnostic methods, it is shown that spatiotemporal variability characteristics of local precipitation seasonal cycle anomalies and elucidate their underlying causes. The seasonal cycle of precipitation anomalies during the flood season in Beijing manifests four distinct types. Notably, two archetypal features, namely "persistent drought or flood" and "rapid change of drought and flood," are directly linked to circulation anomalies in the barotropic structure over the Sea of Japan. These two types account for 45.2% and 54.8% in the total samples, respectively. In years characterized by persistent droughts or floods, the occurrence of a high-pressure (low-pressure) anomaly over the Sea of Japan triggers a coupling of mid-latitude Mongolian cyclones and the South Asia high. Consequently, a low-altitude northerly (southerly) anomaly prevails in the local area, inhibiting (facilitating) moisture transport from the Northwest Pacific and resulting in a sustained low (high) level of precipitation. In the years featuring rapid changes in drought and flood, when an anomalous low (high) pressure system prevails over the Sea of Japan in July and the Western Pacific Subtropical High shifts anomalously southward (northward), precipitation in the Beijing region is significantly reduced (increased). In August, with the abnormal strengthening (weakening) of the Mongolian cyclone and the transition to an anomalous high (low) pressure system over the Sea of Japan, precipitation in the Beijing region increases (decreases) subsequently. Furthermore, persistent drought or flood-type anomalies exhibit a significant correlation with extratropical sea surface temperatures. In contrast, drought-flood rapid transition anomalies are strongly associated not only with tropical El Ni?o-Southern Oscillation (ENSO) events but also with 10-30 days intraseasonal oscillations in localized precipitation.