Tropical cyclones (TCs) over the western North Pacific (WNP) are usually embedded in the multitime-scale summer monsoon circulation and occasionally experience sudden track changes, which are currently a challenge in TC forecasting. A composite analysis of 15 sudden north-turning cases and 14 west-turning cases that occurred during the period 2000–10 was conducted with a focus on influences of low-frequency monsoon circulations. It is found that TCs in the two specific categories of track changes are embedded in a monsoon gyre of about 2500 km in diameter on the quasi-biweekly oscillation (QBW) time scale, which is also embedded in a larger-scale cyclonic gyre or monsoon trough on the Madden–Julian oscillation (MJO) time scale. The two types of track changes are closely associated with interaction between low-frequency and synoptic flows. Two different types of asymmetric flow patterns are identified on the synoptic time scale in the vicinity of these TCs. In the north-turning case, enhanced winds lie mainly on the southeast side of TCs due to strong ridging associated with interactions between low-frequency and synoptic flows. In the west-turning case, the westward extension of the subtropical high leads to ridging on the northwest side of TCs and the enhanced winds can largely offset the steering of enhanced southwesterly winds on the synoptic time scale. Thus the north-turning (west turning) sudden track changes are affected primarily by the synoptic-scale (low frequency) steering. This may be one of the reasons for the larger forecasting errors in the north-turning case than in the west-turning case.

Previous studies suggest that the low-frequency background makes an important contribution to the predictability of tropical cyclone (TC) activity on the intraseasonal time scale by providing large-scale conditions favorable for TC formation. Extended numerical experiments were conducted to demonstrate additional low-frequency influence on TC activity, which results from the development of a synoptic-scale wave train. The cyclonic circulation of the wave train provides low-level synoptic-scale disturbances for TC formation. The observed TC formation events over the western North Pacific during 14 August–10 September 2004 were first successfully simulated with the initial and lateral conditions derived from the National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis. Then the 27-day extended experiment was repeated only with the initial and lateral boundary conditions derived from the FNL low-frequency (longer than 20 days) background. It is found that the development of the synoptic-scale wave train can be well simulated with TCs forming in the cyclonic circulations of the wave train although the wavelength of the simulated wave train is substantially reduced in the absence of higher-frequency influences with periods shorter than 20 days. Sensitivity experiments indicate that the development of wave trains is sensitive to the initial monsoon trough structure. This study suggests that the synoptic-scale wave train can develop in situ and does not need upstream precursors.

It is often difficult to predict tropical cyclone (TC) tracks for the case of binary or even multiple TCs. To investigate this issue, two TCs over the western North Pacific basin in September 2016, Meranti and Malakas, were explored in this study. The two TCs formed close to each other, and their tracks were similar at an early stage but then deflected northward over different regions. The results indicate that the Meranti track change may be due to the asymmetric highfrequency flows induced by topography, while the Malakas track change is attributed to the formation and strengthening of low-frequency southerlies. It was found that the formation and evolution of low-frequency monsoon circulation is the main cause for the emergence of the low-frequency flows, and the process is accompanied by the enhancement and movement of low-frequency positive vorticity. Therefore, based on the diagnostic results using the lowfrequency vorticity budget equation, it is shown that the low-frequency vorticity advection and convergence induced by the low-frequency flows play varying roles in the development of low-frequency positive vorticity. More noteworthy is that the high-frequency positive vorticity of TCs can trigger the enhancement of low-frequency positive vorticity through nonlinear interactions, which have rarely been discussed in previous studies. It can be concluded that the interactions between high-and low-frequency flows and vorticity may affect the development and evolution of the TCs at different time scales, finally leading to TC track deflection. K E Y W O R D S different time scale steering flows, low-frequency vorticity, tropical cyclone track

Changes in the duration of tropical cyclones (TCs) and the embedded large‐scale environment along with TC tracks have a substantial impact on basin‐wide TC intensity. However, their relative importance in changes of basin‐wide TC intensity remains unclear. This study focuses on their contributions to change in TC intensity over the western North Pacific (WNP) basin using a TC intensity model. First, simulations using a TC intensity model with inputs of TC tracks and the embedded environment adequately capture the climate variability of the observed basin‐wide WNP TC intensity. It suggests that the climate variability of basin‐wide TC intensity largely depends on the synergistic changes in large‐scale environment and TC tracks. Sensitivity tests indicated that changes in TC tracks generally appear to be the most important factor affecting basin‐wide TC intensity on the interannual and interdecadal timescales. Changes in large‐scale environment play a limited role especially for weak‐to‐moderate TCs (i.e., tropical storms and Cat.1–3 TCs), while comparable impacts of change in TC tracks and large‐scale environment are found for intense TCs (i.e., Cat.4–5 TCs). Given that TC tracks remain unchanged, sensitivity simulations further show that TC duration plays a more crucial role in controlling basin‐wide TC intensity than the environmental fields. Moreover, weak‐to‐moderate TCs appear to be more sensitive to duration than intense TCs, in consistent with the dramatic changes in basin‐wide TC intensity for westward TCs that form in the South China Sea and the Philippine Sea with short TC duration. Results of this study would be helpful for climate prediction of basin‐wide TC intensity over the WNP basin.