Definitions of QBWO types and their propagation characteristics
Using the tracking method defined in Sect. 2, we identified a total of 88 events from the 1997–2015 rainfall data. We first obtained statistics about propagation speed, propagation range, and zonal scale, and then removed irrelevant events on this basis (see “Conclusions and discussion” below).
Most frequently the speed of tracked events (Fig. 2a) is between − 5 m s−1 and − 3 m s−1 with an average speed of about − 7 m s−1 (negative value means westward propagation), a bit faster than that of the MJO which has an average speed about + 5 m s−1 (see Fig. 3 of Zhang and Ling 2017). Second, the most frequent propagation range of events (Fig. 2b) is about 40°. Few events have a propagation range of more than 80°. For MJO events, the propagation range can exceed 120°. This result confirms that QBWO events are more regional compared to the MJO, which has a more global character. Further, the dominant zonal scale of QBWO precipitation (Fig. 2c) is between 10° to 20° and that of MJO events is about 30°, which is about twice that of the QBWO.
Based on the aforementioned QBWO attributes, we chose QBWO events with westward propagation speed between 2 m s −1 and 10 m s −1, which results in 21 events (23.9%) being excluded. We also choose events with propagation distance greater than 20°, which resulted in the exclusion of an additional 11 events (12.5%). A total of 59 QBWO events are thus retained in our analysis.
To further explore the characteristics of the selected QBWO events, such as the starting and ending longitudes, propagation speed, range, and strength, individual and joint frequency distributions are provided (see Fig. 3). The starting longitudes of QBWO events cluster in two different regions (Fig. 3a). One region is 110°–140°E (the South China Sea) and the other is from 150°E to the dateline (the western Pacific). The joint frequency distributions indicate that the starting and ending longitudes are strongly correlated, with a correlation coefficient of 0.69. As mentioned above, these events propagate on average about 40° of longitude (Fig. 2b). The QBWO events that originate in the South China Sea end primarily between 60° and 90°E (the Indian Ocean), while those originating in the western Pacific end primarily between 110° and 130°E (the South China Sea). In addition, a few QBWO events generated between 170°W and 160°W disappear in the western Pacific. The average strength is defined as the rain rate averaged along a track the starting to end point. The strength of QBWO events is distributed over a range of 2–5 mm/day (Fig. 3b), regardless of whether the events originate from the South China Sea or the western Pacific. However, the ones that originate between 170°W and 160°W are weaker. Further, a clear connection exists between QBWO strength and its zonal scale (Fig. 3c), although there is no well-defined connection between the propagation range and zonal scale (Fig. 3d).
Intraseasonal oscillations such as the QBWO are closely related to the East Asian monsoon rainfall (Lau et al. 1988). Therefore, to understand how QBWO events that originate in the western Pacific affect the climate in the East Asian monsoon region, we use the starting longitude (Fig. 3a) as a criterion to define two types of QBWO events for the subsequent analysis. Among the total of 59 QBWO events, the events with a starting longitude between 110°E and 140°E are defined as type 1, and those with a starting longitude between 150°E and the dateline are defined as type 2. Using these definitions, 22 type-1 events and 20 type-2 events are defined. To better illustrate the westward propagation of the two types of QBWOs, longitude-time sections of 10–20-day filtered precipitation composites are shown (Fig. 4). The composite type-1 events originate at 140°E and end to the east of 90°E 12 days later (Fig. 4a). The most prominent propagation occurs in the South China Sea. The main propagation region of type-2 events occurs over the Philippine Sea and lasts for about 10 days (Fig. 4b).
Three dimensional QBWO structure
Comparing the two types of QBWO indicates several important structural differences at initiation. At day 0 for the type-1 events (Fig. 5a), a negative rainfall anomaly near Vietnam is accompanied by an anti-cyclonic circulation. A positive rainfall anomaly occurs in the western Philippine Sea, accompanied by a cyclonic circulation. For the type-2 events (Fig. 5b), the center of positive rainfall anomaly is weighted more toward the east side of the cyclonic circulation than for the type-1 disturbances, associated with southwesterly anomalies. The northeasterly wind anomaly between the cyclonic and anti-cyclonic circulations that extends into the Philippines is associated with suppressed rainfall.
Moisture flux convergence for the two types of QBWO disturbances is examined next. The most intense centers of both types of QBWOs occur mainly over the band of 15°–20°N (Fig. 5). Hence, Fig. 6 shows a longitude-height cross-section of the moisture flux convergence and wind anomalies averaged between 15° and 20°N at day 0. Consistent with previous observations, at the initiation of type-1 events, total moisture flux convergence (Fig. 6a) is mainly focused in the eastern Philippines (120°–130°E) and accompanied by strong upward motion, while the corresponding sinking branch is near the Indo-China Peninsula (110°E). This moisture flux convergence anomaly is mainly provided by its zonal (Fig. 6b) and vertical (Fig. 6d) flux convergence components. However, the positive moisture convergence anomaly of type-2 disturbances (Fig. 6e) occurs over the ocean in the region of 140°–160°E, accompanied by strong upward motion, while the corresponding sinking branch is located near 130°E. The moisture flux convergence of type-2 QBWOs is dominated by the meridional (Fig. 6g) and vertical (Fig. 6h) components. We note that cause and effect regarding precipitation and moisture flux convergence anomalies is difficult to determine, since the heating associated with convection can drive tropical convergence anomalies (e.g. Neelin and Held 1987). However, dry dynamical considerations have been documented to be important for driving vertical motion that focuses convection in some disturbances (e.g. Kiladis et al. 2006; Rydbeck and Maloney 2015).
Note that significant difference in positive anomalies occurs near 120°E east of the Philippines, where the type-1 QBWO originates (Fig. 5a) and propagates to mainland China, and the type-2 QBWO weakens and disappears (Fig. 5b). We plot the 105°–115°E averaged latitude-height cross-section to present the downward branch of the type 1 QBWO, and the 120°–130°E averaged latitude-height cross-section to present the upward branch (Fig. 7). For the type 2 QBWO, we plot the 125°–135°E and 120°–130°E averages to present the downward and upward branches, respectively (Fig. 8). The leading moisture convergence is mainly provided by the zonal component for type 1 (Fig. 7f), and by the meridional component for type 2 (Fig. 8g) QBWO disturbances. It is worth noting that the source of water vapor in the middle-upper atmospheres for both type 1 and type 2 are provided by vertical moisture flux (Figs. 7h, 8h), although cause and effect relative to precipitation anomalies is difficult to determine from this analysis. In general, the structures of type-1 and type-2 QBWOs are different, and the generation mechanisms of the two types may also be different.
Life cycles of QBWO events
A lagged composite analysis of 10–20-day filtered precipitation and 850-hPa wind anomalies illustrates the life cycles of the two types of QBWOs. The left column of Fig. 9 presents the life cycle of the type-1 QBWOs. On day 0, the negative anomaly of rainfall occurs in the Beibu Gulf, while the positive anomaly appears to the east of the Philippines (Fig. 9a). This positive anomaly propagates westward and becomes stronger over the South China Sea with an associated cyclonic circulation (Fig. 9b–c). The positive anomaly then reaches the Indo-China Peninsula and southern China (Fig. 9d). On day 8 (Fig. 9e), the positive anomaly weakens over northern Vietnam, and a negative rainfall anomaly appears near 120°E. The evolution shown in Fig. 9e–h for days 8 to 14 is quite similar to that from day 0 to day 6 (Fig. 9a–d), but with opposite sign. The right column of Fig. 9 shows the life cycle of type 2 QBWOs. The starting longitude for type 2 (Fig. 9i) QBWO events is further eastward than that for type 1 (Fig. 9a). On day 0 (Fig. 9i), a negative anomaly of rainfall occurs at 140°–160°E, while a positive anomaly emerges east of the Philippines. From day 0 to day 6 (Fig. 9i–l), the positive rainfall center moves westward from the western Pacific. It strengthens and eventually reaches the South China Sea. From day 8 onward (Fig. 9m), the positive rainfall center continues to propagate westward but gradually weakens. The positive rainfall anomaly fades and disappears when the disturbance propagates to the south of Japan (Fig. 9n–p). The southwest–northeast tilted wind structure of type 2 QBWO disturbances in this study also bear some resemblance to the synoptic-scale disturbances described in Lau and Lau (1992).
Previous results using EOF-based methods have indicated that QBWOs originate mainly near 160°E and strong QBWOs can propagate to 110°E. An important finding from our study is that the QBWO events generated near 160°E are not necessarily linked to those events that initiate and propagate near 110°E. There are two kinds of QBWO events with different origins, propagation paths, and ranges of influence.
The positive anomaly of the type-1 QBWO propagates to East Asia where it affects precipitation before decaying. However, the positive anomaly of the type-2 QBWO decays over the topography of the Philippine Islands, and has little effect on mainland Asia. Comparing these two types of QBWO events, topography and interaction with land may be important factors in determining the propagation of QBWOs.