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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. [ABSTRACT FROM AUTHOR]