Mechanisms of northward propagating ISOs in the northeast Asian region

The composite structures of northward propagating intraseasonal oscillations (NPISO) in the northeast Asian region are investigated using NCEP/NCAR reanalysis in order to examine mechanisms for NPISO in the mid-latitude region. There are several mechanisms for NPISO in the tropical area. In particular, the vertical shear process is one of the most important mechanisms for NPISO in easterly shear zone. However, the vertical shear mechanism is no longer significant in the mid-latitude area. An observation analysis shows that NPISO can penetrate to mid-latitude region where prevailing winds have a westerly shear structure. Also, the moisture advection mechanism is not functioning in the Northeast Asian summer monsoon area for NPSIO. This study suggests that air-sea interaction process can contribute to NPISO in the Northeast Asian region.