欧洲东部土壤湿度对东北亚初夏气温异常转折的影响及其可能物理过程

采用1979—2020年观测和再分析资料,研究了年际时间尺度上初夏(5—6月)东北亚气温异常月际转折的基本特征,以及欧洲东部土壤湿度异常对其的影响及可能物理过程。结果表明,年际时间尺度上东北亚初夏气温异常月际演变的主导模态为转折模态,即5月偏暖(冷)则6月偏冷(暖);转折模态的形成直接源于东北亚地区环流异常的转折。进一步分析发现,5月欧洲东部土壤湿度偏低往往导致东北亚5月偏暖而6月偏冷,可能的物理过程如下:5月土壤湿度偏低导致局地土壤温度和对流层低层增温,进而造成地中海地区(欧洲北部)对流层低层经向温度梯度和大气斜压性减弱(增强),相应地高频瞬变波活动减弱(增强),并通过瞬变涡度强迫有利于欧洲中东部形成异常高压和Rossby波波源;相关的Rossby波沿极锋急流东传,导致东北亚为准正压的异常高压,地表升温。土壤湿度异常可持续到6月,但强度减弱;类似地,其可通过瞬变涡度强迫有利于异常高压和Rossby波波源的形成,但中心西移至欧洲西部;相关Rossby波活动导致东北亚为准正压的异常低压,地表降温。5月和6月欧洲东部土壤湿度异常相关的 Rossby波的活动特征(波源、活动中心和传播路径)存在明显差异,这与两个月欧亚北部大气平均态的差异密切相关。当5月欧洲东部土壤湿度偏高时,上述物理过程则大致相反。

Impacts of eastern Europe soil moisture on the reversal of air temperature anomalies over Northeast Asia in early summer and its possible physical mechanisms

This study investigates the characteristics and underlying processes of the month-to-month reversal of air temperature anomalies in Northeast Asia during early summer (May to June) on the interannual timescales,focusing on the influences of soil moisture anomalies in eastern Europe.Utilizing observational and reanalysis data spanning from 1979 to 2020,the results reveal that the dominant mode of intermonthly variation in air temperature anomalies exhibits a reversal pattern,with warmer (colder) conditions in May and colder (warmer) conditions in June.This reversal mode is directly linked to circulation anomalies over Northeast Asia.Further analysis demonstrates that reduced soil moisture in eastern Europe during May contributes to a warmer May and a colder June in Northeast Asia.The potential physical processes driving these effects are explored.Lower soil moisture in May leads to local soil warming and subsequent warming of the lower troposphere.This,in turn,weakens (enhances) the lower tropospheric meridional temperature gradient and baroclinicity in the Mediterranean region (northern Europe).Consequently,high-frequency transient wave activity weakens (enhances),facilitating the formation of an anomalous high-pressure system over central and eastern Europe,thereby generating Rossby wave sources through transient vorticity forcing.These associated Rossby waves propagates eastward along the polar front jet,resulting in the formation of a barotropic anomalous high-pressure system over Northeast Asia and inducing surface warming.While the soil moisture anomalies persist until June,their intensity weakens.Similarly,this favors the formation of an anomalous high-pressure system and Rossby wave source;however,their centers shift westward towards western Europe.The associated Rossby wave activity leads to the formation of a barotropic anomalous low-pressure system,causing cooling over Northeast Asia.Notably,the activity characteristics of Rossby waves (wave source,activity centers,and propagation pathway) associated with the soil moisture anomalies in eastern Europe exhibit distinct differences between May and June,closely tied to the atmospheric climatology over northern Eurasia.When soil moisture is high in May,the aforementioned physical processes exhibit reverse tendencies.