Comparison of impacts of intraseasonal oscillation on tropical cyclogenesis over the western North Pacific based on two methods

The present study compares the relative contributions of intraseasonal variations of different environmental factors to the western North Pacific (WNP) tropical cyclone (TC) genesis during boreal summer-fall estimated using two methods. The first method is composite based on the phase of the intraseasonal oscillation (ISO). The second method is composite based on local and instantaneous TC genesis. The genesis potential index (GPI) is used as a measure for contributions of different factors to TC genesis. The two methods show consistently that the mid-level relative humidity has a major contribution to and the absolute vorticity has a secondary effect on positive GPI anomalies in the active ISO phase. However, the two methods obtain different results during the inactive ISO phase. The composite based on the ISO phase obtains negative GPI anomalies, whereas the composite based on the TC genesis location obtains positive GPI anomalies in the regions where TCs are formed. The absolute vorticity and vertical wind shear have a nearly comparative contribution to positive GPI anomalies during the inactive ISO phase. This indicates that composite based on local and instantaneous perspective can better capture the actual environmental conditions that set up the background for TC genesis.