Interannual Shift of the Tropical Upper-Tropospheric Trough and Its Influence on Tropical Cyclone Formation over the Western North Pacific

The east-west migration of the tropical upper-tropospheric trough (TUTT) on the interannual time scale and its influence on tropical cyclone (TC) formation over the western North Pacific (WNP) is investigated in this study. Climatologically, the TUTT can be identified from 100 to 400 hPa with a relative vorticity maximum between 150 and 200 hPa. In addition to the strong westerly vertical wind shear in the south flank of the TUTT, this study shows that the cold-core system is associated with low relative humidity and subsidence to the east of the trough axis. As a result, the TC formation is enhanced (suppressed) in the eastern portion of the WNP when the TUTT shifts eastward (westward) on the interannual time scale. The interannual TUTT shift is closely associated with the SST anomalies in the central and eastern tropical Pacific or ENSO phases. The warm (cold) phase of ENSO corresponds to the eastward (westward) shift of the TUTT. The physical factors found to be responsible for the influence of ENSO on TC formation can be associated with the east-west shift of the TUTT. It is shown that the interannual variations of TC formation in the eastern part of the WNP basin are closely associated with the east-west shift of the TUTT due to the associated environmental conditions that are generally not favorable for TC formation.