Abstract
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.
Highlights
Tropical atmospheric intraseasonal oscillation (ISO) is a prominent component of tropical atmosphere variations and is dominated by the Madden–Julian Oscillation (MJO) near the equatorial regions (Madden and Julian 1971, 1972)
All above mechanisms highlight the importance of the asymmetric vorticity with respect to the convection center, while shallow convection plays a crucial role in conveying the asymmetric vorticity effect to inducing the northward propagation (NP) of boreal summer intraseasonal oscillation (BSISO) (Liu et al 2018).The convergence in the boundary layer cannot develop into the deeper convection if the detrainment of shallow convection is turned off in a general circulation model (Liu et al 2018)
Vigorous BSISO activity and its interannual variations are observed over the western Pacific
Summary
Tropical atmospheric intraseasonal oscillation (ISO) is a prominent component of tropical atmosphere variations and is dominated by the Madden–Julian Oscillation (MJO) near the equatorial regions (Madden and Julian 1971, 1972). Vertical shear flow and meridional gradient of baroclinic divergence could lead to positive barotropic vorticity in the free atmosphere to the north of convection center. Under a background state flow with easterly vertical wind shear, the effects of convective momentum transport can enhance the low-level troposphere convergence north of convection center by inducing a baroclinic secondary circulation and positive barotropic vorticity tendency (Kang et al 2010; Liu et al 2015). The meridional advection of baroclinic vorticity induced by the mean flow with northerly vertical wind shear could induce barotropic vorticity in the free atmosphere north of the convection center, which in turn creates convergence in the boundary layer (Bellon and Sobel 2008). 2. The influences of ENSO on the activity and NP of BSISO over the western Pacific in decaying summer are explored in Sect.
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