Distinct Influences of Two Boreal Summer Intraseasonal Oscillation Modes on Synoptic-Scale Eddies over the Western North Pacific

Abstract

Abstract The boreal summer intraseasonal oscillation (BSISO), which consists of a 10–30-day component and a 30–90-day component, is vigorous over the East Asian and the western North Pacific (WNP) monsoon regions where synoptic-scale eddies are also active. In this study, we systematically compare and diagnose the modulating effects of the 10–30- and 30–90-day BSISO modes on the development and maintenance of the WNP synoptic-scale eddies in terms of eddy energetics. During the developing phase of the synoptic eddies, the eddies tend to grow faster under the background conditions associated with the 10–30-day BSISO mode, compared with that associated with the 30–90-day mode, indicating the importance of the 10–30-day mode in eddy development. In contrast, the 30–90-day BSISO mode shows a larger contribution to the maintenance of the synoptic eddy intensity after the eddies reach their maximum amplitude. On the basis of the diagnoses of the eddy kinetic energy (EKE) budget equation, we find that the positive EKE advection induced by the 10–30-day flow is the leading process resulting in the increased EKE for eddy development. However, the 30–90-day circulation anomalies provide long-lasting favorable conditions (relative to those of the 10–30-day anomalies) to maintain the eddy intensity by generating a positive barotropic energy conversion from 30- to 90-day kinetic energy to EKE. These results not only advance our understanding of multiscale interactions over the WNP but also help toward better monitoring and prediction of synoptic-scale eddies, including tropical cyclones, using the background BSISO information. Significance Statement The western North Pacific is a region with active but complex multiscale interactions among synoptic-scale disturbances, intraseasonal oscillation, and low-frequency background monsoonal activity. The relative effects of 10–30- and 30–90-day intraseasonal variability on the synoptic-scale disturbances were quantitatively examined based on the eddy kinetic energy budget analysis. The results show that the 10–30-day (30–90-day) mode of intraseasonal oscillations plays a key role in the growth (maintenance) of synoptic-scale disturbances through the advection (barotropic energy conversion) process. The results suggest the implication of monitoring and predicting the activity of synoptic-scale disturbances using the information of two different intraseasonal modes.