Modulation of 10–30-day Boreal Summer Intraseasonal Oscillation and the Associated Rainfall Events by El Niño

Daily precipitation anomalies in the western North Pacific (WNP) and East Asia (EA) exhibit significant intraseasonal variability, peaking at 10–30-day time scales. It has been suggested that boreal summer intraseasonal oscillation (BSISO) on 30–60-day time scales is strongly modulated by El Niño and Southern Oscillation (ENSO) with stronger intensity and propagation during La Niña compared to El Niño summers, but the dependency of 10–30-day BSISO on ENSO has not been well understood. Here we show that the intensity and northward propagation of the 10–30-day BSISO convection over the WNP-EA region are stronger and more organized during El Niño developing summers than the other summers, including neutral summers. During El Niño developing summers, the BSISO-induced precipitation and low-level circulation tend to exhibit a stronger meridional tripolar pattern than those during neutral summers. We highlight that the strengthening of 10–30-day BSISO northward propagation and the associated rainfall anomalies over EA in the El Niño developing summers is contributed by not only the stronger air-sea interaction with a larger meridional gradient of sea surface temperature previously proposed but also the enhanced dynamic process with the stronger relative vorticity and moisture convergence.