Influences of ENSO on boreal summer intraseasonal oscillation over the western Pacific in decaying summer

Characteristics of boreal summer intraseasonal oscillation (BSISO) over the western Pacific in ENSO decaying summer are revealed in this study. BSISO activity over the western Pacific is significantly weakened (strengthened) in El Niño (La Niña) decaying summer. Coherent and robust northward propagation (NP) of BSISO can be observed from the equator to the north of 25° N in La Niña decaying summer, while the intensity of BSISO NP is rapidly weakened to the north of 15° N in El Niño decaying summer. ENSO modulates BSISO activity by regulating circulation and moisture anomalies. Large-scale atmospheric circulation undergoes radical changes between El Niño and La Niña decaying summer. In El Niño (La Niña) decaying summer, anomalous anticyclonic (cyclonic) circulation and descending (ascending) motion are observed over the western Pacific, which provides an unfavorable (favorable) background state for the activity of BSISO. Low-level moistening north of BSISO convection center is crucial to its activity and NP, and the stronger low-level moistening in La Niña decaying summer induces the stronger NP. Differences in moisture variations between El Niño and La Niña decaying summer mainly stem from the meridional moisture advection, especially the meridional advection of intraseasonal moisture caused by low-frequency background state meridional wind. The stronger air-sea interaction over 15°–25° N in La Niña decaying summer could also promote the stronger NP of BSISO via surface heat fluxes exchanges. Zonal and meridional vertical wind shears can also influence the boundary convergence to the north of BSISO convection center and promote the NP of BSISO via vertical wind shear and vorticity advection mechanisms, but they are not the dominant reasons for the differences in the NP of BSISO between El Niño and La Niña decaying summer.