ENSO Modulation of 10-30-day and 30-60-day Intraseasonal Oscillation and the associated rainfall events

Extreme weather and climate events over the Asian summer monsoon regions, such as torrential rainfalls and heat waves, are significantly affected by the Boreal summer intraseasonal oscillation (BSISO), which strongly interacts with background mean fields. It has been suggested that BSISO on 30–60-day time scales is influenced by El Niño and Southern Oscillation (ENSO), with more pronounced intensity and propagation in La Niña than El Niño summers. However, the dependency of 10–30-day BSISO on ENSO has not been well understood. This study aims to comprehensively understand how ENSO modulates 10–30-day and 30–60-day BSISO and the associated heavy rainfall events. First, the intensity and northward propagation of the 30–60-day BSISO tend to be stronger and more organized in La Niña summers than El Niño summers due to the Indian-Ocean-driven vertical wind shear mechanism as indicated by previous studies. It is further noted that 30–60-day BSISO is stronger in El Niño developing (E-DV) than decaying (E-DC) summers over the western North Pacific (WNP)-East Asia (EA) region. The easterly vertical shear, responsible for the next vorticity generation north of the BSISO convection, in the E-DV (E-DC) remarkably increases (decreases) due to the enhanced (weakened) upper-level easterly anomaly. The air-sea interaction in E-DV gets more substantial due to the more significant meridional gradient of sea surface temperature, amplifying northward propagation than in E-DC. Second, the intensity and northward propagation of the 10–30-day BSISO convection are more vigorous and more organized in E-DV summers than the other summers, including E-DC, La Niña, and neutral summers over the WNP-EA region. During the E-DV summers, the BSISO-induced precipitation and low-level circulation tend to exhibit a stronger meridional tripolar pattern than those during the other summers. The strengthening of 10–30-day BSISO northward propagation and the associated rainfall anomalies over EA in the E-DV summers is contributed by not only the stronger air-sea interaction with a larger meridional gradient of sea surface temperature but also the enhanced dynamic process with the stronger relative vorticity and moisture convergence. The ENSO modulation has an important implication on the predictability and prediction skills of BSISO and the associated rainfall events.