Impact of Boreal Summer Intraseasonal Oscillation on Environment of Tropical Cyclone Genesis over the Western North Pacific

The impact of intraseasonal oscillation on tropical cyclone genesis (TCG) over the western North Pacific during boreal summer was examined in association with categorized synoptic-scale low-level flow patterns. Five synoptic-scale flow patterns were considered as synoptic-scale environment connecting the large-scale intraseasonal oscillation and meso-scale TCG: a shear line, a confluence region, an easterly wave, a monsoon gyre, and a pre-existing tropical cyclone. The phase and amplitude of the boreal summer intraseasonal oscillation (BSISO) were used. It is found that the phase 7 was favorable for TCG associated with the shear line, and the phases 7 and 8 were also favorable for TCG associated with the confluence region. During the favorable phases for each flow pattern associated with TCG, the BSISO affected the large-scale zonal wind distribution, and the large-scale zonal wind forms synoptic-scale flow patterns over the suitable environment for TCG. The link between the intraseasonal oscillation and synoptic-scale flow patterns is an important factor in TCG.