长江中游和下游夏季降水季节内振荡的差异

Differences in Intraseasonal Summer Rainfall Oscillation between the Middle and Lower Reaches of the Yangtze River

利用1979~2013年中国站点逐日降水资料和NCEP/NCAR再分析资料,对长江中下游夏季降水的季节内振荡最显著周期进行了分析研究。结果表明长江中游最显著周期为10~30天,长江下游最显著周期为30~60天。为了揭示这种差异产生的物理原因,进一步利用位相合成的方法对这两个区域不同周期的季节内振荡降水、高低空风场和高度场以及垂直结构和水汽等循环过程的演变特征进行分析。在200 h Pa环流场上,长江中游的降水主要受到高纬度自西向东传播的波列影响,而长江下游的降水与鄂霍次克海的高度场的变化相关。在风场的垂直涡度和散度的位相结构演变过程中,10~30天的垂直涡度和散度有自北向南的移动,30~60天的垂直涡度和散度在长江以南地区有自南向北的传播。水汽输送的位相发展过程表明,长江中游的水汽分别来自于南海的向北输送和长江以北地区向南的水汽输送;长江下游地区的水汽则主要来自于热带东印度洋经孟加拉湾的向东输送并在南海的北向输送,以及西太平洋水汽向西输送到南海再向长江下游的输送。从高层大尺度环流场和整层积分的水汽通量输送上解释了长江中游10~30天降水的自北向南移动,和长江下游30~60天降水自南向北传播的原因。

The most significant climatological intraseasonal oscillation（ISO） periods of summer rainfall over the middle and lower reaches of the Yangtze River（M-LYR） are investigated by using the gauge-based daily precipitation analysis data of China and the NCAR/NCEP reanalysis data from 1979 to 2013. It is found that the oscillation periods of summer rainfall at the intraseasonal timescale show remarkable difference between the MYR and LYR. The 10-30-day oscillation is the primary mode over the MYR, while the longer period of 30-60-day dominates the LYR. The phase composite analysis is applied to reveal evolutions of intraseasonal circulation and vertical structure associated with these two periodsof climatological ISO precipitation over the Yangtze River. In the upper troposphere at 200 h Pa, it shows that a wave train over the mid-high latitudes propagates eastward, affecting the summer rainfall over MYR, while the southwestward propagation of the weakened Okhotsk blocking high is associated with the 30-60-day rainfall oscillation over LYR. The vertical structures of wind vorticity and divergence/convergence exhibit different features during their moving toward the Yangtze River. 10-30-day anomalous vorticity and divergence/convergence move southward toward the MYR; however, the 30-60-day anomalies move northward from the tropics. Atmospheric moisture from the South China Sea and from north of the Yangtze River converges over MYR, leading to favorable condition for the occurrence of rainfall. In contrast, the moisture in the LYR primarily comes from the tropics. Large amounts of atmospheric moisture originate from the eastern Indian Ocean, move through the Bay of Bengal and the South China Sea, converge with moisture transported westward from the western tropical Pacific, and then move northward, reaching the LYR. The results explain from the perspective of upper-level circulation and vertically integrated moisture flux why the 10-30-day oscillation mode of rainfall over MYR propagates southward, while the 30-60-day oscillation mode of rainfall over LYR propagates northward.