2000—2015年华北地区8月极端干旱的异常环流型及其波活动特征

利用1961—2015年华北地区54个国家级气象观测站夏季8月逐日降水资料和2000—2015年欧洲中期天气预报中心(ECMWF)再分析数据,研究了华北地区8月极端干旱的异常环流型及原因。结果表明:华北地区极端干旱事件以年际变化为主,2000—2015年华北地区极端干旱年在中高纬500 hPa高度距平场上存在“-、+、-、+”的异常波列结构,乌拉尔山西部地区上空存在较强的反气旋性距平使得暖脊加强向北收缩,同时贝加尔湖上空存在较强的反气旋性距平使得此处的高空脊强度异常加强,影响范围加大,这种异常环流形势导致高空脊前干冷西北气流输送到华北地区。欧洲西部地区是北半球中高纬波作用通量的关键区,该地区地表2 m温度呈明显增加趋势,同时该关键区对应的500 hPa垂直波作用通量TNZ也呈现明显增加趋势。由于异常热力强迫作用激发出EU波列,低层能量向上传输,高层能量向外频散影响下游乌拉尔山西部地区暖脊加强,造成中高纬环流异常,导致华北地区极端干旱事件频发。

Anomalous circulation patterns and wave activity characteristics of extreme drought in North China in August from 2000 to 2015

Based on the daily precipitation data of 54 national meteorological observation stations in North China in August from 1961 to 2015 and reanalysis data from the European Center for Medium-Range Weather Forecasts(ECMWF)from 2000 to 2015,this paper investigates the anomalous circulation patterns and causes of the extreme drought in August in North China.The results are as follows.The extreme drought events in North China show interannual variations.During the extreme drought years in North China from 2000 to 2015,there is an abnormal wave train structure of“-,+,-,+”in the geopotential height anomaly field at 500 hPa in the mid-high latitudes.There is a strong anticyclonic anomaly over the western Ural Mountains which makes the warm ridge move northward,the strong anticyclonic anomaly over Lake Baikal strengthens the upper-level ridge and enlarges its influence range at the same time,and such anomalous circulation situation impels the cold and dry northwest airflow in front of the upper-level ridge to be transported to North China.Western Europe is a key area for the wave activity flux in the mid-high latitudes of the Northern Hemisphere,where the surface temperature at 2 m presents a significantly increasing trend,and the vertical wave activity flux TNZ at 500 hPa corresponding to this key area also increases significantly.Due to the abnormal thermal forcing,EU wave trains are excited,low-level energy is transmitted upward,and high-level energy is dispersed outward,which strengthens the warm ridge over the western Ural Mountains,causing anomalous circulation patterns in the mid-high latitudes and frequent extreme drought events in North China.