Analysis of the anomalous signals around the tropopause and in the stratosphere before the Meiyu onset.

Using the NCEP/NCAR reanalysis data and the ERA Interim reanalysis data together with the Meiyu data from the National Climate Center of China during 1989-2008, the tropopause anomalies and the NAM index over the Yangtze River and the Huaihe River valleys before Meiyu are analyzed. It is found that the tropopause height over the Yangtze River and the Huaihe River valleys decreases 3-5 days before the Meiyu onset， and the meridional change of the tropopause height is more significant than that in the zonal direction. It is proposed to that the lowering of the tropopause before the Meiyu onset is caused by frequent tropopause fold events associated with frequent cold surges from the north, the onset of the eastern Asia monsoon as well as the northward movement of the westerly jet. The decrease of the tropopause height before the Meiyu onset is accompanied by the descent of the cold air of high potential vorticity (PV) from the stratosphere. Positive PV anomalies can be noted over the Yangtze River and the Huaihe River valleys before the Meiyu onset， and the establishment and the maintenance of those PV anomalies are associated with PV anomalies over Lake Baikal, Western Siberia and near Sea of Okhotsk. The stratospheric temperatures over the Meiyu area reach a peak value before the Meiyu onset while the zonal winds in the stratosphere show a transition from westerly to easterly. After the onset of Meiyu, the stratospheric temperatures over the Meiyu area are decreased. The total precipitation during the Meiyu season has a consistent positive correlation with the NAM index in the upper troposphere and the lower stratosphere, which is most significant 15-30 days before the Meiyu onset. The results suggest that the NAM index near the tropopause region is one of good factors for forecasting the precipitation in the Meiyu season.