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Liguang Wu and Jingjing Duan


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.

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Liguang Wu, Zhongping Ni, Jingjing Duan, and Huijun Zong


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.

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