Dominant Processes for Dependence of Boreal Summer Intraseasonal Oscillation on El Niño Phases

Boreal summer intraseasonal oscillation (BSISO) strongly interacts with background mean fields and tends to be stronger and longer in its northward propagation during La Niña than El Niño summers. However, BSISO dependence on El Niño‐Southern Oscillation phase has not been well understood. Here, we show that BSISO‐related convections are stronger and more organized with northward propagation on 30–60‐day timescales during El Niño developing (E‐DV) than decaying (E‐DC) summers over the western Pacific. 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 stronger due to the larger meridional gradient of sea surface temperature, amplifying northward propagation than in E‐DC. Plain Language Summary In boreal summer, convective activity over the western Pacific on the intraseasonal time scale is primarily located to the north of the equator and exhibits characteristics of propagating northward. The northward propagation of convective activity can modulate the subtropical variability and typhoon activity and has significant impacts on the extreme weather and climate in Asia. The intensity and propagation of the convective activity are considerably influenced by the El Niño‐Southern Oscillation. In this study, we find that the northward propagation of convective activity over the western Pacific is significantly strengthened by easterly wind shear related with the positive vorticity generation as well as air‐sea interaction associated with the meridional gradient of the warm Sea Surface Temperature anomalies in the El Niño developing summers than in the decaying summers. Better understanding the mechanisms of the complicated convective activity is the key to reduce the climate‐related socio‐economic risks and disasters by improving our capability for accurate prediction of BSISO and the related risks. Key Points Convective activity and northward propagation of boreal summer intraseasonal oscillation (BSISO) in El Niño developing summers are stronger and more distinct than decaying summers Dynamical and air‐sea interaction processes leading to northward propagation of BSISO are more active in El Niño developing summers Key factors driving the phase dependence are the increase in upper‐level easterly and meridional Sea Surface Temperature (SST) gradient during de