The influence of environmental conditions on the intensity changes oftropical cyclones (TCs) over the western North Pacific (WNP) is investigatedthrough examination of 37 TCs during 2000–2011 that interacted directly withthe western North Pacific subtropical high (WNPSH). Comprehensive compositeanalysis of the environmental conditions is performed for two stages ofstorms: one is categorized as intensifying events (maximum wind speedincreases by 15 kn over 48 h) and the other is categorized as weakeningevents (maximum wind speed decreases by 15 kn over 48 h). Comparison of thecomposite analysis of these two cases show that environmental conditionsassociated with the WNPSH play important roles in the intensity changes ofTCs over the WNP. When a TC moves along the southern periphery of the WNPSH,the relatively weaker easterly environmental vertical wind shear helps bringwarm moist air from the south and southeast to its southeast quadrant within500 km, which is favorable for the TC to intensify. However,when a TC moves along the western edge of the WNPSH, under the combinedinfluences of the WNPSH and an upper-level westerly trough, a strongwesterly vertical shear promotes the intrusion of dry environmental airassociated with the WNPSH from the north and northwest, which may lead tothe inhibition of moisture supply and convection over the western half of theTC and thus its weakening. These composite results are consistent with thosewith additional geographic restrictions, suggesting that the dry airintrusion and the vertical wind shear (VWS) associated with the WNPSH,indeed affect the intensity changes of TCs over the WNP beyond thedifference related solely to variations in geographical locations. Theaverage sea surface temperature (SST) of 27.6 °C for the weakeningevents is also lower than an average of 28.9 °C for the strengtheningevents, but remains above the critical value of 27 °C for TCintensification, suggesting that the SST may be regarded as a less positivefactor for the weakening events.
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