1月北大西洋东部-乌拉尔山阻塞高压频次年际尺度偶极子模态特征

基于观测和再分析资料,本文研究了近几十年来1月北大西洋东部-乌拉尔山阻塞高压频次年际变异主导模态特征及与其相联的大气背景场。结果表明,1980—2019年冬季(12月—次年2月)该地区阻塞高压频次年际变异的主导模态存在明显月际差异:12月表现为北大西洋东部-欧洲西部地区阻塞高压频次的显著同位相变化,1月为北大西洋东部-欧洲西部与乌拉尔山地区阻塞高压频次的具有显著的反位相变化即偶极子模态,2月则为北大西洋东部-乌拉尔山阻塞高压频次的显著同位相变化。进一步研究表明:1月北大西洋东部-乌拉尔山阻塞高压频次偶极子模态与同期局地纬向西风、纬向风垂直切变、经向位涡梯度等大气背景场异常偶极型变化相联系。当乌拉尔山地区关键大气背景场为负异常,而北大西洋东部-欧洲西部为正异常时,两个地区阻塞高压频次分别增加和减少,呈现偶极子模态;反之亦然。阻塞高压频次偶极子模态及相关的背景环流异常可通过调节水平温度平流、垂直运动和水汽输送等来影响1月欧亚北部气温和降水,以及它们的极端事件频次。当阻塞高压频次偶极子模态处于正位相时,乌拉尔山北部(南部)和欧洲南部增温(降温),极端暖、冷事件频次分别增加(减少)和减少(增加),斯堪的纳维亚半岛北部降温且极端暖事件减少,乌拉尔山及东北亚地区降水和极端降水频次减少而欧洲大陆部分地区增加;反之亦然。此外,在冬季仅1月阻塞高压频次年际变异主导模态表现为偶极子模态可能与关键大气背景场气候态及其变异的月际差异密切相关。

Dipole pattern characteristics of the blocking high frequency over eastern North Atlantic-Ural in January on an interannual scale

Based on observational and reanalysis data,this study investigated the dominant mode characteristics of interannual variation in the January blocking high frequency over the eastern North Atlantic-Ural region in recent decades,along with their associated key atmospheric background fields.The results show that there are significant inter-monthly differences in the dominant modes of the interannual variation of blocking high frequency in this region during the winter months (December to February) between 1980 and 2019.In December,the dominant mode is manifested as significant in-phase variation of blocking high frequency in the eastern North Atlantic-western Europe region.In January,it shows significant out-of-phase variation of the blocking high frequency between the eastern North Atlantic-western Europe and the Ural region.The dominant mode in February exhibits significant in-phase variation of the blocking high frequency in the eastern North Atlantic-Ural region.Further studies show that the dipole pattern of the blocking high frequency in the eastern North Atlantic-Ural region in January is associated with the dipole variation of the simultaneous atmospheric background field,such as zonal wind,vertical shear of zonal wind,and meridional gradient of potential vorticity.When anomalies of the key atmospheric background fields are negative in the Ural region and positive in the eastern North Atlantic-western Europe region,the blocking high frequency of the two regions respectively increases and decreases,showing a dipole pattern,and vice versa.The dipole pattern of the blocking high frequency and associated background atmospheric circulation anomalies can affect temperature and precipitation in northern Eurasia in January,and can also affect their frequency of their extreme events by modulating horizontal temperature advection,vertical motion and water vapor transport.When the dipole pattern of the blocking high frequency is in the positive phase,the north (south) and south of the Ural region warms (cools),and the frequency of extreme warm and cold events increases (decreases) and decreases (increases),respectively.In northern Scandinavia,which cools,the frequency of extreme warm events decreases.Moreover,the frequency of precipitation and extreme precipitation decreases in the Ural region and northeastern Asia,but increases in parts of continental Europe,and vice versa.In addition,this paper discusses why,among the winter months,the dominant mode of interannual variation of the blocking high frequency exhibits a dipole pattern only in January.This may in part be caused by the inter-monthly differences in the key atmospheric background fields'' climatology and variations.