The zonal oscillation of the western North Pacific subtropical high (WNPSH) is one of the key climate variabilities to the abnormal summer rainfall over East Asia (EA). This thesis scrutinizes two extreme phases of the zonal oscillation, defined by the top 10% strongest (positive phase) and weakest (negative phase) WNPSH index days in summers during 1979–2016. Key findings include the moisture re-allocation as viewed from a tripole pattern featuring differential precipitation responses across EA, Maritime Continent and western North Pacific. The onset of WNPSH-induced precipitation subsequently occurs in central Japan, the Korean Peninsula and eastern China at different time lags. An OLR–vorticity pattern is found and can be explained by the atmospheric response to diabatic heat...[ Read more ]

The zonal oscillation of the western North Pacific subtropical high (WNPSH) is one of the key climate variabilities to the abnormal summer rainfall over East Asia (EA). This thesis scrutinizes two extreme phases of the zonal oscillation, defined by the top 10% strongest (positive phase) and weakest (negative phase) WNPSH index days in summers during 1979–2016. Key findings include the moisture re-allocation as viewed from a tripole pattern featuring differential precipitation responses across EA, Maritime Continent and western North Pacific. The onset of WNPSH-induced precipitation subsequently occurs in central Japan, the Korean Peninsula and eastern China at different time lags. An OLR–vorticity pattern is found and can be explained by the atmospheric response to diabatic heating, which plays a key role in the zonal propagation of WNPSH. Local air–sea feedbacks and their competitions are highlighted in the life cycle of WNPSH phases.

In addition, subseasonal to interannual cycles of WNPSHI are evident based on wavelet analysis. An interdecadal shift of dominant variabilities of WNPSHI is deemed to synchronize with that of Niño3.4 index, from 3-6-yr to 2-3-yr time scales after mid-1990s and lastly to 1-2-yr time scale after late-2000s. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) phase sometimes occurs during a decaying El Niño (La Niña) in the preceding summer/autumn, and/or a developing La Niña (El Niño) in the current summer/autumn. The quasi-biennial WNPSH-ENSO relationship is considered to be non-linear as more than half of the extreme WNPSHI days do not follow the relationship. Undeniably the zonal oscillation of WNPSH plays a central role in EA summer climate, improved understanding on its variabilities may contribute to summer rainfall predictability and advance the hazard adaptations for extreme events in EA.