Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-scale Disturbances over the Western North Pacific

The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the intraseasonal oscillation (ISO) and the asymmetric development of EKE during the ISO transitional phases are examined through the diagnoses of eddy energetics. As the summer mean flows always contribute positively toward the EKE, the synoptic eddies extract energy from the ISO during its active phase. This positive barotropic energy conversion occurs when the northeast-southwest tilted synoptic-scale disturbances interact with cyclonic and convergent ISO flows. This contrasts to the ISO dry phase during which anticyclonic and divergent low-frequency circulation results in a negative barotropic energy conversion. As a result, the synoptic eddies lose kinetic energy to the ISO flow. The upscale feedback of the synoptic-scale variability (SSV) to ISO is further investigated by diagnosing the atmospheric apparent heat source (Q1), apparent moisture sink (Q2), and eddy momentum transport. The synoptic Q1 and Q2 heating (cooling) anomalies are in phase with cyclonic (anticyclonic) vorticity disturbances, aligned in a southeast-northwest oriented wave-train pattern over the western North Pacific (WNP). The wave train is well-organized and strengthened during the ISO wet phase, whereas they are loosely organized and much weaker during the ISO dry phase. The SSV-induced Q1 and Q2 fields account for 15-25% of the total intraseasonal Q1 and Q2 variabilities over the WNP. During the ISO wet (dry) phase, the synoptic variability induces intraseasonal Q1 and Q2 heating (cooling) to the northwest of the ISO OLR center, favoring the northwestward propagation of ISO. The horizontal transports of eddy momentum fluxes act to generate an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) maximum during the ISO wet (dry) phase. As a result, the eddy momentum transports contribute to the northward propagation of the boreal summer ISO over the WNP.