Dominant Processes for Dependence of Boreal Summer Intraseasonal Oscillation on El Nino 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 Nina than El Nino summers. However, BSISO dependence on El Nino-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 Nino 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.