Changes in extended boreal summer tropical cyclogenesis associated with large-scale flow patterns over the western North Pacific in response to the global warming hiatus

This study examines extended boreal summer (May–October) tropical cyclogenesis events (TCGEs) associated with large-scale flow patterns (LFPs) over the western North Pacific (WNP) between 1979–1997 and 1998–2012. WNP TCGEs are objectively identified to be associated with five LFPs [e.g., monsoon shear line (SL), monsoon confluence region (CR), monsoon gyre (GY), Pacific easterly wave (EW) and preexisting tropical cyclone (PTC)]. Results show that an apparent decrease in TCGEs from 1998–2012 was due to the significant decrease in TCGEs associated with the PTC pattern and to a somewhat lesser degree, TCGEs associated with the GY pattern. In contrast, TCGEs associated with the SL pattern show a small increase, which seems to contradict the weakened monsoon circulation since 1998 but corresponds well to cyclonic anomalies over the Philippines region. Decreased TCGEs associated with the GY pattern and increased TCGEs associated with the EW pattern are closely related to the strengthening of Pacific easterly waves in response to the Mega La Nina-like pattern that predominated during 1998–2012. Weakened easterly shear over the eastern WNP is not conducive to the development and propagation into the southeastern WNP of Rossby wave trains induced by preexisting TC energy dispersion. Consequently, there is a significant reduction of TCGEs associated with the PTC pattern and a weakening in the contribution of TCGEs associated with the PTC pattern to TCGEs associated with the EW pattern. An increased correlation between TCs associated with the SL/GY/EW patterns and central Pacific (CP)-type ENSO during 1998–2012 is observed. A stable and robust association between TCGEs associated with the CR pattern and tropical North Atlantic sea surface temperature is observed regardless of decadal climate regime shifts. However, there is no significant link between TCGEs associated with the PTC pattern and more CP ENSO events during 1998–2012, but there is a strong association between the Pacific meridional mode and TCGEs associated with the PTC pattern during 1979–1997. More observational analyses and numerical simulations are needed to further investigate the underlying physical mechanism.