The Role of Multiscale Interaction in Synoptic-Scale Eddy Kinetic Energy over the Western North Pacific in Autumn

Abstract

This study formulates a synoptic-scale eddy (SSE) kinetic energy equation by partitioning the original field into seasonal mean circulation, intraseasonal oscillation (ISO), and SSEs to examine the multiscale interactions over the western North Pacific (WNP) in autumn. In addition, the relative contribution of synopticmean and synoptic-ISO interactions to SSE kinetic energy was quantitatively estimated by further separating barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CK S2M ) and synoptic-ISO barotropic energy conversion (CK S2ISO ) components.The development of tropical SSE in the lower troposphere is mainly attributed to CK associated with multiscale interactions. Mean cyclonic circulation in the lower troposphere consistently provides kinetic energy to SSEs (CK S2M . 0) during the ISO westerly and easterly phases. However, CK S2ISO during the ISO westerly and easterly phases differs considerably. During the ISO westerly phase, the enhanced ISO cyclonic flow converts energy to SSEs (CK S2ISO . 0). The magnitude of the downscale energy conversion from mean and ISO to SSEs is related to the strength of the SSEs. During the ISO westerly phase, a stronger SSE extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE-ISO interactions causes further strengthening of SSEs during the ISO westerly phase.By contrast, upscale energy conversion from SSEs to ISO anticyclonic flow (CK S2ISO , 0) was observed during the ISO easterly phase. The weaker SSE activity during the ISO easterly phase occurred because the mean circulation provides less energy to SSEs and, at the same time, SSEs lose energy to ISO during the ISO easterly phase. The two-way interaction between the ISO and SSEs has considerable effects on the development of tropical SSEs over the WNP in autumn.

Present Simulation and Future Typhoon Activity Projection over Western North Pacific and Taiwan/East Coast of China in 20-km HiRAM Climate Model

“…3a). The dynamic link between large-scale circulation and TS formation in autumn is similar to that in summer (not shown) except that the monsoon trough in summer is replaced by a strong cyclonic circulation over the WNP in autumn (Tsou et al 2014). These results indicate that the monsoon trough and low-level convergence are vital factors for TS formation and development (Holland 1995 The TS formation frequency spatial distribution and gross monsoon trough features, subtropical high and convergence region are well captured by HiRAM (Fig.…”

Subtropical High Affects Interdecadal Variability of Tropical Cyclone Genesis in the South China Sea

“…Our study indicates a close correlation between SH and ISV at decadal time scales. Noticeably, TC may feedback to large‐scale environment and ISV (e.g., Straub & Kiladis, ; Tsou et al, ). The relation between SH and ISV is worth further investigation in the near future.…”

Effect of ISO‐SSE Interaction on Accelerating the TS to Severe TS Development in the WNP Since the Late 1990s

“…Additionally, a recent study reported that the ISO‐associated dynamic and oceanic fields may substantially increase the number of TDs that develop into C5 typhoons (Weng & Hsu, ). Based on the findings of Tsou et al () and Weng and Hsu (), we will demonstrate in section that the enhancement of the ISO‐SSE interaction played a critical role in accelerating the TS‐to‐STS development since the late 1990s.…”

“…In particular, the variance of SSE in the western WNP (west 140°E) was also enhanced in the IP2 (Figure c). The enhancement of the ISO and SSE provided a favorable large‐scale environment for ISO‐SSE interaction, which was identified as an important physical process for TS development (Tsou et al, ). Additionally, a recent study reported that the ISO‐associated dynamic and oceanic fields may substantially increase the number of TDs that develop into C5 typhoons (Weng & Hsu, ).…”

“…In the kinetic energy framework, scale interaction occurs only through CK, whereas CE involves only the eddy. A scale analysis reveals that the right hand side of equation is primarily determined by CK and CE, which have maximum magnitudes in the lower and upper levels, respectively (Maloney & Hartmann, ; Tsou et al, ). For the sake of brevity, we focus on the analysis of CK at 850 hPa and CE at 200 hPa.…”