Introduction

The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the boreal summer intraseasonal oscillation (ISO) is examined through the construction of a new eddy energetic diagnostic tool that separates the effect of ISO and a low-frequency background state (LFBS, at periods longer than 90 days). As the summer LFBS always contributes positively toward the EKE, the synoptic eddies extract energy from ISO during its active phase. This contrasts to the ISO dry phase during which the synoptic eddies lose kinetic energy to the ISO flow.
The interactions between the boreal summer ISO and synoptic-scale variability (SSV) are further investigated by diagnosing the atmospheric apparent heat source (Q1), apparent moisture sink (Q2), and eddy momentum transport. The nonlinearly rectified Q1 and Q2 fields due to the eddy-mean flow interaction account for 10-30% of the total intraseasonal Q1 and Q2 variabilities over the western North WNP. During the ISO wet (dry) phase, the nonlinearly rectified intraseasonal Q1 and Q2 heating (cooling) appears to the northwest of the ISO convection center, favoring the northwestward propagation of ISO. A diagnosis of the zonal momentum budget shows that the eddy momentum flux convergence forces an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) center during the ISO wet (dry) phase. As a result, the eddy momentum transport may contribute to the northward propagation of the boreal summer ISO over the WNP.