The Characteristics Of The Intraseasonal Oscillation Of Summer Precipitation Between The Middle Reaches And The Lower Reaches Of The Yangtze River

Using the China gauge-based daily preciptation data and the NCEP/NCAR reanalysis data from 1981-2013 to investigate the distribution of the most significant climatological intraseasonal oscillation(ISO)periods of the summer rainfall over in nationwide.Emphasis is paid to contrast the middle reaches of the Yangtze River basin(MYRB)with the lower reaches of the Yangtze River basic(LYRB)under the differences ISO periods.Firstly,statistical analyses are carried out on the special distribution of summer rainfall and ISO precipitation of 824 gauge-based datasets.It is pointed out that the maximum ISO precipitation intensity is located in the South of the Yangtze River.The 10-90 day Lanczos band-pass filter was utilized to extract the component of the precipitation intraseasonal oscillation component.Using the spectrum analysis to obtain the maximum ISO period,we found that the distributions of summer ISO rainfall period have some interesting features.The maximum ISO precipitation period is lower than 30-day in most part of China.The 10–30-day oscillation is the primary mode over the MYRB,and the longer period with 30–60-day mode dominates the LYRB.The 20-30-day mode controls the South China.It is found that the oscillation periods of summer rainfall on intraseasonal timescale show remarkable difference between the MYRB and LYRB.The phase composite analysis is applied to reveal the evolutions of intraseasonal circulation and vertical structures associated with these two periods climatological ISO precipitation over the Yangtze River.In the upper troposphere of 200 hPa,it shows that a wave train over mid-high latitude propagates eastward to impact on the summer rainfall over MYRB,while the southwestward propagation of weakened Okhotsk blocking high is associated with the 30-60-day rainfall over LYRB.The vertical structures of wind vorticity and divergence/convergence exhibit different movement toward the Yangtze River.10-30-day anomalous vorticity and divergence/convergence show southward movement toward the MYRB,however the 30-60-day anomalies show northward from the tropics.The moisture convergence over MYRB,which comes from the South China Sea and the north of the Yangtze River,is favorable for the occurrence of rainfall.By comparison,the moisture in the LYRB primarily comes from tropics.Much more moisture originates from the eastern Indian Ocean,moving through the Bay of Bengal to the South China Sea and converge with the westward-propagating moisture 3from the western tropical Pacific,and then move northward to the LYRB.The results reveal why it is southward movement of 10-30-day rainfall over MYRB and northward movement of 30-60-day rainfall over LYRB from the upper-level circulation and vertically integrated moisture flux.