华南汛期旱涝急转特征及其与海温异常的关系

Characteristics of Flood-drought Shift in Southern China and Their Relationship with SST Anomaly

【目的】探究华南汛期旱涝异常时空特征,旱涝急转前/后大气环流和水汽输送的变化,及其与赤道太平洋、南海海温异常的关系。【方法】基于华南地区129个站点的降水资料、NCEP/NCAR再分析资料和海温资料,采用合成分析、相关分析等统计学方法,拉格朗日后向气流轨迹模式(HYSPLIT_4.9),分析华南汛期旱涝急转特征及其成因。【结果与结论】涝转旱事件旱期相比于涝期,西太平洋副热带高压西伸加强,水汽辐散,来源于洋面的水汽贡献率减少,不利于降水的形成。旱转涝期涝期相较于旱期,副热带高压强度无太大变化,但副热带高压位置南落至华南地区南侧,水汽辐合,来自洋面的水汽通道向西偏移至印度洋,来自印度洋的水汽贡献增多,西南到偏南风向华南输送充足水汽,有利于降水产生。赤道中东太平洋海温正异常引起华南地区气旋式环流异常,有利于将暖湿气流输送至华南地区,使得华南地区降水异常偏多,而ENSO正位相减弱渐变为负位相的过程中,华南地区降水相应减少,会发生涝转旱事件。反之,赤道中东太平洋海温负异常易造成旱转涝事件的发生。

【Objective】To explore the spatial and temporal characteristics of flood and drought anomalies in southern China,and the changes in atmospheric circulation and water vapour transport before/after a rapid shift of drought-flood,and their causes.【Methods】Based on the precipitation data from 129 stations in South China,NCEP/NCAR reanalysis data and sea surface temperature(SST)data,statistical methods such as synthetic analysis,correlation analysis and post-Lagrangian directional airflow trajectory model(HYSPLIT_4.9)were used to analyse the characteristics and causes of rapid shift of drought-flood in southern China.【Result and Conclusion】During drought period for flood to-drought events,the westward extension of the western Pacific subtropical high intensifies compared to the flooding phase,and water vapour dissipates,reducing the contribution of water vapour from the ocean surface,which is not conducive to the formation of precipitation.During flood period for drought flood events,there is no significant change in the intensity of the subtropical high compared to the dry period,but the subtropical high moves southwards to the south of South China,water vapour convergence,water vapour channels from the ocean surface shift westwards to the Indian Ocean,the contribution of water vapour from the Indian Ocean increases,and southwester and southerly winds deliver sufficient water vapour to South China,which is conducive to precipitation generation.The positive SST anomaly in the equatorial central eastern Pacific causes subsequent cyclonic circulation anomalies in southern China,which facilitates the transport of warm and humid air currents to southern China,resulting in unusually high precipitation in southern China,while the weakening of the positive phase of ENSO to a negative phase results in a corresponding decrease in precipitation in southern China and a flood-to-drought event.Conversely,negative SST anomalies in the equatorial central eastern Pacific are likely to cause subsequent drought-to-flood events.