Impact of the cross-tropopause wind shear on tropical cyclone genesis over the Western North Pacific in May

Abstract

The role of the 50- to 200-hPa zonal wind shear in modulating tropical cyclone (TC) genesis over the western North Pacific (WNP) in May is investigated in this study. Concurrent with the strong cross-tropopause shear over the key region (0°–5°N, 160°–180°E), suppressed convection was observed over the tropical WNP, especially over the South China Sea and the Philippines. The monsoon trough (MT) was confined westward. However, enhanced convection occurred in the weak shear years and the MT extended eastward. This cross-tropopause wind shear is negatively correlated with TC genesis in May, with a decreased (increased) number of TCs corresponding to strong (weak) cross-tropopause wind shear. This cross-tropopause wind shear can be treated as the combined impacts of the El Nino-Southern Oscillation (ENSO) events and the stratospheric quasi-biennial oscillation (QBO). When decaying El Nino events coupled with the easterly phase of the QBO were noted, the cross-tropopause wind shear was stronger with weakened convection, and an enhanced western Pacific subtropical high was observed. TCs are rarely generated during these years. In contrast, the modulation of the QBO westerly phase on decaying La Nina events is limited. Affected by the QBO westerly phase, TC genesis in the May following La Nina events is only slightly enhanced. The energy analysis indicates that the combined impacts of the decaying El Nino events and the QBO easterly phase might suppress the barotropic eddy kinetic energy conversion in May, whereas the decaying La Nina events and the QBO west phase act in an opposite manner.