Climatological Intraseasonal Oscillation In The Middle-Upper Troposphere And Its Effect On The Northward Migration Of The East Asian Westerly Jet And Rain Belt Over Eastern China

Using daily atmospheric circulation data for 1981-2010, we identify the climatological intraseasonal oscillation (CISO) in the mid-upper troposphere and reveal its critical role in anchoring the Meiyu season to the period from mid-June to mid-July and the rainfall episode of northeastern China to begin in late July. Wavelet analysis shows a significant 20-90-day period in the climatological 200- and 500-hPa zonal winds during boreal summer, indicating the existence of the zonal wind CISO in the mid-upper troposphere. The leading mode of zonal wind CISO features a dipole pattern along the coast of East Asia controlled by convection over the southeastern China-western North Pacific (SC-WNP). In mid-June, suppressed convection over the SC-WNP induces the negative phase of intraseasonal Pacific-Japan pattern, leading to the strong anticyclone with a poleward tilt with height to dominate over the WNP. Consequently, more moisture are transported to the middle and lower reaches of the Yangtze River Valley (YRV), and the mid-tropospheric (upper-tropospheric) East Asian westerly jet (EAWJ) is shifted north of 32 degrees N (36 degrees N), which further increases the YRV rainfall through enhancing warm advection and upper-tropospheric divergence, thus facilitating the initiation of Meiyu season. In mid-July, active convection occupies the SC-WNP instead, with the reversed circulation pattern terminating the Meiyu season. In late July, active convection shifts from the equator to the tropical western Pacific, leading to the second mode of zonal wind CISO, which is characterized by a quasi-barotropic Mongolian anticyclone in the mid-upper troposphere. The upper-tropospheric EAWJ is further shifted to 45 degrees N, displacing the induced upper-tropospheric divergence northward, thus initiating the rainfall episode of northeastern China.