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Observations of Upper-Tropospheric Influence on a Monsoon Trough over the Western North Pacific [Monthly Weather Review]
[April 24, 2014]

Observations of Upper-Tropospheric Influence on a Monsoon Trough over the Western North Pacific [Monthly Weather Review]


(Monthly Weather Review Via Acquire Media NewsEdge) ABSTRACT This study examined the synoptic evolution and internal structure of a monsoon trough in association with the deep equatorward intrusion of a midlatitude upper trough in the western North Pacific Ocean in June 2008. The study was based on data from routine synoptic observations and intensive observations conducted on board the research vessel Mirai at 12°N, 135°E. The monsoon trough was first observed to extend southeastward from the center of a tropical depression. It then moved northward, with its eastern edge moving faster and approaching a surface low pressure cell induced by the upper trough. The distinct northward migration caused the monsoon trough to become oriented from the southwest to the northeast. The monsoon trough merged with the surface low pressure cell and extended broadly northeastward. The passage of the monsoon trough over the Mirai was accompanied by lower pressure, higher air and sea surface temperature, and minimal rainfall. The monsoon trough extended upward to nearly 500 hPa and sloped southward with height. It was overlain by northwesterly winds, negative geopotential height and temperature anomalies, and extremely dry air in the upper troposphere. Precipitation systems were weak and scattered near the monsoon trough but were intense and extensive south of the surface monsoon trough, where intense low-level convergence and upper-level divergence caused deep and vigorous upward motion. It appears that the upper trough exerted important impacts on the development of both the monsoon trough and associated precipitation, which are discussed according to the observational results.



1. Introduction The monsoon trough is a directional shear line in a region of low atmospheric pressure at sea level, with southwesterly monsoonal winds on the equatorward side and easterly trade winds on the poleward side (Sadler 1964). The monsoon trough is one of the most important features of monsoon systems in various monsoon regimes and has a great impact on regional climate and weather phenomena. In the western North Pacific Ocean, the mean location of the monsoon trough advances north- eastward during the summer, which has a relationship to the northward progress of the monsoon onset (Ueda et al. 1995; Wu and Wang 2001; Wu 2002). In addition, the intraseasonal variation in the western North Pacific and East Asia is closely related to the extension and meridional shifts of the monsoon trough (Hsu 2005). Furthermore, the position, orientation, and strength of the monsoon trough exert important influences on the formation and track of typhoons in the western North Pacific (Chia and Ropelewski 2002; Harr and Chan 2005; Harr and Wu 2011).

The position, orientation, and shape of the monsoon trough in the western North Pacific are highly variable and episodic. The monsoon trough in this region un- dergoes extensive migration. During the summer, the monsoon trough in the western North Pacific may ap- pear between 58 and 258N (Lander 1994). A substantial change in the orientation of the monsoon trough over the western North Pacific can also occur. Over the long term, the monsoon trough is generally oriented from the northwest to the southeast (Sadler et al. 1987). How- ever, a southwest-northeast-oriented monsoon trough, called a reverse-oriented monsoon trough, can be found in the western North Pacific a significant fraction of the time (Lander 1994, 1996). The reverse-oriented mon- soon trough typically occurs during the summer and autumn. As indicated by Lander (1994), the monsoon trough of the western North Pacific can extend to 2000 km in length or disappear episodically. Sometimes the monsoon trough develops into a monsoon gyre, which is a large cyclonic vortex with a radius of 2500 km in the sea level pressure field. The impact of the mon- soon trough on the development of tropical cyclones in the western North Pacific is also variable. Although tropical cyclones can form simultaneously along the monsoon trough (Carr and Elsberry 1994; Lander 1996), there are inactive monsoon regimes that tend to be as- sociated with inactive tropical cyclone periods (Harr and Elsberry 1995).


Lander (1996) and Harr and Wu (2011) showed that variations in the position, orientation, and shape of the monsoon trough over the western North Pacific occur on interannual, intraseasonal, and synoptic time scales, and factors causing many types of variability in the monsoon trough exhibit primary influences on the characteristics of convective activities such as tropical cyclones. How- ever, although many studies have investigated inter- annual (e.g., Wu and Wang 2000; Wang et al. 2001) and intraseasonal (e.g., Maloney and Dickinson 2003; Hsu 2005) variations of the monsoon trough and their im- pacts on tropical cyclones over the western North Pacific (e.g., Chen et al. 2009; Li and Zhou 2013a,b; Wu et al. 2012; Molinari and Vollaro 2013), few studies have ex- amined the evolution of the monsoon trough in this re- gion on synoptic time scales. One exception is the study by Love (1985), who documented an enhancement of the monsoon trough over the western North Pacific with a cold surge from the Southern Hemisphere.

Lander (1994) and Holland (1995) noted that the equatorward intrusion of an upper-tropospheric synoptic- scale trough plays important roles in modulating the monsoon circulation over the western North Pacific. The intrusion of midlatitude upper troughs into the tropics is associated with midlatitude-breaking Rossby waves and occurs most often over the North Pacific during the summer (Postel and Hitchman 1999). Recently, Wu et al. (2009) and Wu and Chou (2012) investigated the influence of midlatitude upper-tropospheric circulation variability on the rapid late-July summer monsoon tran- sition in East Asia and the western North Pacific. They found that the monsoon trough deepens and the surface southwesterly flow strengthens during the monsoon transition when the midlatitude upper trough intrudes southward and westward. These authors' findings de- scribing the upper-tropospheric influence on the mon- soon trough are consistent with the results of Davidson et al. (2007), who indicated that the amplification of an equatorward-extending midlatitude upper trough and tropical ridge triggers the development of the underlying monsoon trough within the Australian monsoon region. The results of these studies imply that the synoptic evolution and internal structure of the monsoon trough can be significantly modified by the intrusion of mid- latitude upper troughs into the tropics. Davidson et al. (2007) further argued that extratropical cyclogenesis can lead to an intensification of subtropical easterlies, which eventually enhances the low-level cyclonic vorticity of the monsoon trough within the Australian monsoon re- gion. However, the physical processes and mechanisms related to the influence of equatorward-intruding upper troughs on the development of the monsoon trough in the western North Pacific are still poorly understood.

Several studies have illustrated the internal structure of the monsoon trough observed in different geographic areas. Great similarities are found between the monsoon trough observed over India (Miller and Keshvarnurty 1968; Raghavan 1973) and that over western Africa (Bayo Omotosho 1985). The monsoon trough in these regions is characterized by westerlies in the lower tro- posphere and easterlies in the upper troposphere. It extends upward to the middle troposphere and slopes equatorward with height. Raghavan (1973) and Bayo Omotosho (1985) demonstrated that weak convection and minimal rainfall are usually found along the mon- soon trough, whereas deep convection and higher amounts of rainfall develop in the westerlies several hundred kilometers away from the trough axis at sea level. The oceanic monsoon trough observed over the south Indian Ocean (Ramage 1974) and that over the North Atlantic Ocean (Sadler 1975) also exhibit similar structures. The monsoon trough is overlain by relatively clear skies and coincides with the warmest temperatures in the near-surface layer. Ramage (1974) showed that the oceanic monsoon trough is a thermal trough co- inciding with a sea surface temperature maximum and that the monsoon trough can be maintained and en- hanced by insolation through relatively clear skies. However, observational studies examining the internal structure of the monsoon trough over the western North Pacific are lacking. Specifically, no studies have illus- trated the structure of the monsoon trough in this region under the influence of midlatitude upper troughs in- truding into the tropics.

In June 2008, a monsoon trough evolved with the equatorward intrusion of a midlatitude upper trough in the western North Pacific. The monsoon trough passed over the research vessel Mirai, which was conducting intensive observations at 128N, 1358E. The evolution of large-scale circulations surrounding the monsoon trough both in the lower and upper troposphere was captured well by routine synoptic observations. In addition, the internal structure of the monsoon trough was observed for the first time in the western North Pacific by the Mirai. The purpose of this paper is therefore to investigate the synoptic evolution of the migration and orientation and the kinematic and thermodynamic structure of the mon- soon trough under the influence of the upper trough. The observational results are used to illustrate the roles of the upper trough in the development of both the mon- soon trough and associated precipitation. This paper is organized as follows. In section 2, the data and analytical methods are described. Section 3 describes the synoptic evolution of the monsoon trough. The internal structure of the monsoon trough observed by the Mirai is pre- sented in section 4. The roles of the upper trough are discussed in section 5. Section 6 compares our findings with other studies, followed by a summary and conclu- sions in section 7.

2. Data and methods The synoptic evolution of the migration and orienta- tion of the monsoon trough and other large-scale cir- culations was analyzed using the Cross-Calibrated MultiPlatform (CCMP) product (Atlas et al. 2011), weather charts from the Japan Meteorological Agency (JMA), satellite wind data from the University of Wis- consin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS; Velden et al. 2005), and geosta- tionary satellite data from the Weather Satellite Image Archive at Kochi University. The gridpoint values (GPVs) of the global objective analysis data from the JMA were also used as supplementary material. In ad- dition, the internal structure of the monsoon trough was investigated using observations of the surface, radio- sonde, and a C-band Doppler radar on board the Mirai. The radiosonde observations were conducted at 3-h in- tervals, and the shipboard Doppler radar performed volume scans at 10-min intervals.

Details on the postprocessing of the radiosonde and Doppler radar data can be found in Geng et al. (2011). Reflectivity data at an altitude of 1 km were used to calculate the mean rainfall within a range of 100 km from the radar according to the reflectivity-rainfall rate (Z-R) relations of Tokay and Short (1996). Because of a lack of validation, the rainfall derived from the radar data was analyzed and compared only in a relative sense. The velocity-azimuth display (VAD) method described by Mapes and Lin (2005) was used to analyze the me- soscale kinematics observed by the Doppler radar.

The monsoon trough passed over the Mirai as it moved from the south to the north. The position and movement of the monsoon trough were estimated form the CCMP data. A time-space translation corresponding to the motion of the monsoon trough was used to determine the distance of the Mirai relative to the surface monsoon trough. This method was applied to the radiosonde data to derive the meridional gradients of meteorological fields in the layer below 500 hPa, where the monsoon trough was observed, which were mainly used to calcu- late the absolute vorticity associated with the zonal flow and its meridional gradient surrounding the monsoon trough.

3. Synoptic observations a. Weather chart analysis Figure 1 provides the relevant surface weather charts. At 0000 UTC 18 June (Fig. 1a), there was a tropical depression located to the southwest of the Mirai. The tropical depression moved west-northwestward and in- tensified into a typhoon at 0000 UTC 19 June (Fig. 1b). At the same time, a low pressure cell developed at the surface around 208N, 1528E. The low pressure cell moved west-southwestward and could still be recognized at 0000 UTC 20 June (Fig. 1c). By 0000 UTC 21 June (Fig. 1d), the low pressure cell had disappeared, al- though a pressure trough still extended broadly east- northeastward from the center of the typhoon.

In the upper troposphere, there was a midlatitude trough near 1508E at 0000 UTC 18 June (Fig. 2a). This upper trough was advancing southwestward from the midlatitudes; its southern edge had approached 248Nby 0000 UTC 19 June (Fig. 2b). As evidenced by isotachs, the upper trough continued to move southwestward and intruded deeply into the tropics (Figs. 2c,d). A small low pressure center could be recognized to the northeast of the Mirai at 0000 UTC 20 June (Fig. 2c).

The surface low pressure cell had developed in front of the southwestward-advancing upper trough (Figs. 1b and 2b), suggesting that the development of the surface low pressure cell was closely related to the equatorward intrusion of the upper trough. Such a relationship is much clearer in a supplementary figure, which shows a cross section of geopotential height anomalies over the surface low pressure cell from the objective analysis data (Fig. 3). The anomaly of geopotential height is defined as the deviation of geopotential height from its mean value at each level. It is evident that the development of the surface low pressure cell was associated with the upper trough, which sloped southward with decreasing height and penetrated to the surface.

b. Upper-level wind analysis Satellite-derived upper-level winds are shown in Fig. 4. The deep equatorward intrusion of the midlatitude upper trough can be seen more clearly in this figure. The upper trough was tilted along a northeast-southwest orienta- tion. Furthermore, the upper trough evolved in the downstream of an upper-level anticyclone to the west and was advected equatorward anticyclonically. These facts suggest that the equatorward intrusion of the upper trough is linked to a breaking Rossby wave on the southern periphery of the upper-level anticyclone (Thorncroft et al. 1993; Postel and Hitchman 1999). Accompanying the equatorward intrusion of the up- per trough, divergent northwesterly winds strength- ened around the Mirai (Figs. 4c,d).

c. Sea surface wind analysis Figure 5 shows the evolution of sea surface winds during the study period. At 0000 UTC 18 June, there was a vortex centered near 9 8N, 1338E (Fig. 5a). The vortex was located to the southwest of the Mirai and was as- sociated with the tropical depression (Fig. 1a). To the south of the vortex center, the cross-equatorial flow from the winter hemisphere turned from southeasterly to southwesterly. A monsoon trough extended south- eastward from the center of the depression to 1458E, with equatorial southwesterly winds to the south and easterly trade winds to the north. The monsoon trough exhibited the typical northwest-southeast orientation at this time.

By 0000 UTC 19 June, the southwesterly winds in- tensified south of the typhoon (Figs. 1b and 5b), con- sistent with previous studies' findings of the enhancement of the monsoon by typhoons (e.g., Beattie and Elsberry 2012). The eastern edge of the monsoon trough was in close proximity to the cyclonic circulation related to the surface low pressure cell. The monsoon trough merged with the surface low pressure cell at 0000 UTC 20 June (Fig. 5c). As a result of the stronger southwesterlies, the monsoon appeared to flow downstream without much obstruction, and the monsoon trough extended broadly northeastward. With the decay of the surface low pres- sure cell after 0000 UTC 21 June (Fig. 5d), less north- eastward extension of the monsoon trough was observed.

The monsoon trough was moving northward and had passed the Mirai after 0000 UTC 20 June (Fig. 6). The eastern edge of the monsoon trough moved northward much faster than the western edge of the monsoon trough before 1200 UTC 19 June. Consequently, the monsoon trough became oriented approximately from the west to the east by 1200 UTC 18 June. After 1200 UTC 19 June, the orientation of the monsoon trough changed to southwest-northeast (Fig. 6), similar to the reverse- oriented monsoon trough described by Lander (1994, 1996). With the continuous poleward propagation, the monsoon trough was caught up in the midlatitude flow from 22 June (not shown). The average motion of the monsoon trough was 4.5 m s21 toward 2998.

d. Satellite image analysis Geostationary satellite images are shown in Fig. 5. Although deep convective clouds with lower brightness temperatures were observed over some parts of the monsoon trough, they were related to the typhoon and the surface low pressure cell. The segment of the monsoon trough that had approached and passed the Mirai was characterized by few deep convective clouds (Figs. 5b-d). At 0000 UTC 20 June, deep convection, aligned approximately from 58N, 1288Eto108N, 1368E, developed south of the segment of the monsoon trough that had passed the Mirai (Fig. 5c). The north portion of the deep convection was located in the region of upper- level divergent northwesterly winds intensified south of the upper trough (Fig. 4c). The deep convection south of the monsoon trough was passing the Mirai at 0000 UTC 21 June (Fig. 5d).

4. Mirai observations As shown in the previous section, the monsoon trough passed over the Mirai. In this section, the internal structure of the monsoon trough captured by the sur- face, upper-air, and Doppler radar observations con- ducted on board the Mirai is analyzed.

a. Surface analysis Figure 7 shows time series of surface elements ob- served on board the Mirai. The zonal wind shifted from easterlies to westerlies at approximately 1300 UTC 19 June (Fig. 7a), indicating that the surface monsoon trough passed over the Mirai around this time. Southerly winds intensified during the passage of the monsoon trough, reaching a speed of greater than 5 m s21.

Rainfall during the passage of the monsoon trough was minimal, but intense rainfall events were observed one and half a days later (Fig. 7b). This result is consistent with the satellite observation showing the development of the deep convection south of the monsoon trough (Figs. 5c,d). As will be shown in section 4c, the intense rainfall events were associated with strong and deep precipitation systems that developed approximately 450-680 km south of the monsoon trough.

The monsoon trough was located within a region of relatively low sea level pressure (Fig. 7b) and relatively high air and sea surface temperatures (Fig. 7c). This result is consistent with the findings of Ramage (1974) and Sadler (1975) for monsoon troughs observed over other oceanic regions. In line with these studies, the monsoon trough observed here exhibited marked char- acteristics of a thermal trough.

b. Upper-air analysis A time-height diagram of horizontal winds from the radiosonde observations conducted on board the Mirai is shown in Fig. 8. The horizontal axis of the figure is time, which increases leftward. Because the monsoon trough moved from the south to the north, using a time- space translation, the left side of the monsoon trough in the figure can be considered as the south side of the monsoon trough. The monsoon trough extended up- ward approaching 500 hPa and sloped leftward (i.e., southward), with height, similar to the vertical exten- sion and slope of the monsoon trough in other regions described in previous studies (Miller and Keshvarnurty 1968; Raghavan 1973; Bayo Omotosho 1985). South- westerly winds intensified south of the monsoon trough and reached a speed greater than 12 m s21 between 900 and 650hPa.

A deep layer of northwesterly winds was observed over the monsoon trough. This feature is different from the findings of the above-mentioned previous studies that showed the domination of easterly winds over the monsoon trough. The axis of maximum northwesterly winds in the upper troposphere was observed early on 20 June, consistent with the time when the upper trough intruded deeply into the tropics (Figs. 2c and 4c). This behavior indicates that the intensification of northwest- erly winds over the monsoon trough is closely related to the deep equatorward intrusion of the upper trough.

Figure 9a shows a time-height diagram of geopotential height anomalies. The monsoon trough was overlain by negative geopotential height anomalies from the upper to the lower troposphere. In particular, a downward pene- tration of intense negative geopotential height anomalies from the upper troposphere was observed south of the surface monsoon trough.

In the upper troposphere, negative geopotential height anomalies began to appear from approximately 0600 UTC 19 June, and a core of negative geopotential height anomalies (;210 m) was observed early on 20 June. This continuous trend of falling geopotential height over the monsoon trough apparently resulted from the equa- torward intrusion of the upper trough (Figs. 2c and 4c). Beginning at approximately 1500 UTC 20 June, positive geopotential height anomalies were observed in the upper troposphere. As will be shown in section 4c, this increase in geopotential height was associated with di- vergent outflow atop the deep convection south of the monsoon trough.

Figure 9b shows the absolute vorticity below 500 hPa associated with the zonal flow based on the time-space translation corresponding to the motion of the monsoon trough. It is evident that the monsoon trough was char- acterized by cyclonic vorticity. A region of intense cyclonic vorticity (;10 3 1025 s21) was observed immediately south of the monsoon trough approximately between 800 and 650 hPa. This intense cyclonic vorticity was beneath stronger negative geopotential height anoma- lies penetrating downward from the upper troposphere (Fig. 9a).

Time-height diagrams of temperature anomalies and relative humidity are shown in Fig. 10. The monsoon trough near the surface was located in a core of positive temperature anomalies (;0.48C) (Fig. 10a), consistent with the surface observation indicating that the mon- soon trough exhibited characteristics of a thermal trough. Over the monsoon trough, there were cores of negative temperature anomalies (;20.48C) in the middle and upper troposphere. At the same time, the air over the monsoon trough became extremely dry, with relative humidity as low as 25% observed near 300 hPa (Fig. 10b). The decreases in temperature and humidity over the monsoon trough were associated with the deep equa- torward intrusion of the upper trough and the intensi- fication of northwesterly winds in the upper troposphere (Figs. 2c,d; 4c,d; and 8).

c. Doppler radar analysis As the monsoon trough passed over the Mirai,an organization of precipitation systems associated with the monsoon trough were observed by the Doppler radar on board the Mirai. Figure 11 shows the horizontal distri- bution of radar echoes. At 1200 and 1800 UTC 19 June (Figs. 11a,b), the surface monsoon trough was ap- proaching and had just passed the Mirai, respectively. Consequently, the Doppler radar captured the distri- bution of precipitation close to the monsoon trough. Small radar echoes were scattered in the radar coverage. Moreover, radar echoes were absent over most parts of the monsoon trough line covered by the Doppler radar. Except for a few radar echoes, the majority of radar echoes were weak, with intensity lower than 30 dBZ. The heights of the small and weak radar echoes were generally lower than 10 km (Figs. 12a,b).

After 0000 UTC 21 June (Figs. 11c,d), the Doppler radar observed the distribution of precipitation evolving in the southwesterlies south of the monsoon trough. Figure 6 shows that the monsoon trough was located approximately 58 north of the Mirai at 0000 UTC 21 June. Radar echoes in the southwesterlies as far as 58 south of the monsoon trough were more consolidated than were the scattered and small radar echoes close to the monsoon trough (Figs. 11a,b). More precipitation developed with radar echoes greater than 30-dBZ in- tensity. Precipitation was also much more extensive and spread over the radar coverage. These results indicate that intense precipitation systems had developed south of the monsoon trough. Accompanying the consolidation of radar echoes, the area of towering convection with radar echo tops higher than 10 km increased (Figs. 12c,d).

Figure 13 shows a time series of area-mean rainfall derived from the radar observations. Distances relative to the surface monsoon trough are also shown in this figure based on the time-space translation correspond- ing to the motion of the monsoon trough. The radar- observed rainfall evolved in agreement with the surface rainfall observed on board the Mirai (Fig. 7b). The evolution of rainfall corresponded closely to the de- velopment of precipitation systems (Figs. 11-12). In accordance with the small and weak radar echoes, only light rain was observed near the monsoon trough. Co- pious rainfall appeared approximately 450-680 km south of the surface monsoon trough, consistent with the strong and deep precipitation systems developed in these re- gions. The weak convection and rainfall that occurred near the monsoon trough and the deep convection and intense rainfall that developed several hundred kilome- ters south of the monsoon trough are consistent with the distribution of convection and rainfall around the mon- soon trough found by previous studies (Raghavan 1973; Bayo Omotosho 1985).

Time-height diagrams of the mesoscale kinematics derived from Doppler radar observation based on the VAD method are shown in Fig. 14. The horizontal winds observed by the Doppler radar were similar to those observed by the radiosondes, as shown in section 4b. In particular, the vertical kinematic structure of the deep convection south of the monsoon trough was well cap- tured by the Doppler radar. Around 21 June, when the Mirai was located approximately 538 km south of the surface monsoon trough, there was convergence ex- tending from the surface to 6 km (Fig. 14a). Strong convergence (magnitude greater than 1 3 1025 s21) was associated with intense southwesterly winds. In contrast, a deep layer of divergent northwesterly flow existed above 6 km. A strong divergence over 10 km (magnitude greater than 2 3 1025 s21) corresponded closely to the positive geopotential height anomalies in the upper tro- posphere measured by the radiosondes (Fig. 9a). Such a structure of intense low-level convergence and upper- level divergence resulted in deep and vigorous upward motion extending from the surface to the upper tropo- sphere (Fig. 14b). The appearance of intense rainfall (Figs. 7b and 13) coincided well with this strong upward motion. It is also evident that the vigorous upward mo- tion and intense rainfall occurred after upper-level di- vergent westerly winds over the Mirai had intensified and reached their maximum magnitude (Figs. 13-14).

5. Roles of the upper trough The synoptic evolution and internal structure of the monsoon trough in association with the equatorward intrusion of the midlatitude upper trough have been analyzed in previous sections. The observational results indicate that the upper trough significantly modulated the atmospheric structure surrounding the monsoon trough. It appears that the upper trough exerted impor- tant influences on the development of the monsoon trough and associated precipitation, which is discussed in this section.

a. Migration and orientation of the monsoon trough It has been shown that before the merging of the monsoon trough with the surface low pressure cell, the eastern edge of the monsoon trough moved poleward much faster than the western edge of the monsoon trough (Figs. 5-6). This result implies that there are processes facilitating the fast northward transport of westerly momentum in the eastern edge of the monsoon trough. One possible process can be attributed to the intensification of the meridional pressure gradient near the eastern edge of the monsoon trough due to the de- velopment of the surface low pressure cell to the north- east of the monsoon trough (Fig. 1). This process is similar to the findings of Love (1985), who indicated that a pressure rise at the equator leads to enhanced monsoon westerly flow. The rapid northward transport of westerly momentum in the eastern edge of the monsoon trough evidently contributes to a change in the orientation of the monsoon trough from southeast-northwest to southwest- northeast (Figs. 5-6), inducing the formation of a reverse- oriented monsoon trough. This northward movement also facilitates the merging of the monsoon trough with the surface low pressure cell and promotes the broad northeastward extension of the monsoon trough.

The surface low pressure cell developed in the tropical latitudes in front of the equatorward-intruding upper trough that penetrated to the surface (Figs. 1b, 2b, and 3). This behavior indicates that the surface low pressure cell can be induced by the upper trough, which is con- sistent with the relationship between the tropical upper- tropospheric trough (TUTT) and lower-tropospheric circulation illustrated by Sadler (1967). Sadler argued that the TUTT can penetrate into the surface layer and induce a surface trough or vortex, whose intensity de- pends on the areal extent, intensity, and penetration depth of the upper trough. Consequently, it appears that the upper trough acted to induce the development of the surface low pressure cell, which, in turn, acted to regu- late the migration and orientation of the monsoon trough by modulating the meridional pressure gradient around the monsoon trough.

b. Intensity of the monsoon trough The intensity of the monsoon trough is usually evalu- ated according to the vorticity around it. The equatorward- intruding upper trough acted to reduce atmospheric pressure over the intense cyclonic vorticity around the monsoon trough (Figs. 2c,d; 4c,d; and 9), which implies that the process for enhancing lower-tropospheric cir- culation by the TUTT, as discussed by Sadler (1967), can also be applied here. Consequently, the upper trough acted to enhance low-level cyclonic vorticity and, hence, also enhanced the intensity of the monsoon trough by lowering the atmospheric pressure over it.

c. Maintenance of the monsoon trough As noted above, the coincidence of the monsoon trough with lower pressure, higher air and sea surface temperatures, and minimal rainfall (Fig. 7) implies that the monsoon trough observed here resembles a thermal trough. Steady insolation through relatively clear skies is necessary to maintain the thermal trough (Ramage 1971, 1974). The appearance of extremely dry air over the monsoon trough corresponded with the deep equator- ward intrusion of the upper trough from the midlatitudes and the intensification of upper-level northwesterly winds (Figs. 2c,d; 4c,d; 8; and 10), which implies that the equatorward intrusion of the upper trough enhanced the intrusion of colder and drier air into the tropics. This result is consistent with the findings of previous studies regarding the enhancement of the dry intrusion into the tropics by breaking Rossby waves (Yoneyama and Parsons 1999; Parsons et al. 2000; Allen et al. 2009). Similar to these studies, the entrainment of dry air into the tropics helped to suppress convective activities around the monsoon trough (Figs. 11a,b). Therefore, the upper trough enhanced the dry intrusion and convective suppression over the monsoon trough, facilitating the maintenance of the monsoon trough by promoting in- solation through relatively clear skies over the mon- soon trough.

d. Deep convection south of the monsoon trough The upper trough that deeply intruded into the tropics potentially played two important roles in the devel- opment of the deep convection south of the monsoon trough.

One role of the upper trough is related to the di- vergent flow over the deep convection. Sadler (1976) indicated that a facilitated outflow in the northern sector of deep convection in the tropics requires a channel to a westerly current and that this channel is often fur- nished by the deep troughs in the midlatitude westerlies. The deep convection that developed south of the mon- soon trough also occurred to the south of the upper trough (Figs. 2c,d; 4c,d; and 5c,d). Notably, the intense updraft and rainfall from the deep convection were as- sociated with a deep layer of stronger divergent north- westerly flow in the upper troposphere and developed after upper-level westerly winds had been enhanced by the upper trough (Figs. 4c,d and 13-14). These results imply that the upper trough observed here might play a role similar to that described by Sadler (1976). Consequently, the upper trough furnished an outflow channel to the north of the deep convection into the large-scale westerlies, facilitating the development of the deep convection south of the monsoon trough.

Another role of the upper trough is associated with barotropic instability. As shown in section 5b, the upper trough acted to enhance cyclonic vorticity immediately south of the monsoon trough (Fig. 9). Previous studies have shown that barotropic instability can be found in the monsoon trough region with intense vorticity (Hung and Yanai 2004; Mao and Wu 2010). A necessary condition for barotropic instability in zonal currents is the re- versal of the meridional gradient of absolute vorticity (Kuo 1949). It can be seen from Fig. 9b that a region of negative meridional gradient of absolute vorticity ap- peared south of intense cyclonic vorticity between ap- proximately 850 and 650 hPa, which implies that the barotropic instability criterion was met there. A nega- tive meridional gradient of absolute vorticity was also found below 850 hPa south of the monsoon trough. This negative meridional gradient appears to have been in- duced by precipitation and did not influence the baro- tropic instability.

The intense upward motion observed by the Doppler radar south of the monsoon trough was coincident with the negative meridional gradient of absolute vorticity between 850 and 650 hPa (Figs. 9b and 14b), which im- plies that barotropic instability can contribute to the development of intense upward motion south of the monsoon trough. Consequently, the upper trough that had deeply intruded into the tropics also facilitated the development of the deep convection south of the monsoon trough by enhancing barotropic instability in the mon- soon trough region.

6. Comparison with other studies on the reverse-oriented monsoon trough In this study, the structure and evolution of a reverse- oriented monsoon trough observed in the western North Pacific has been illustrated. The establishment of the reverse-oriented monsoon trough was associated with the northeastward transport of southwesterly monsoon winds (Figs. 5-6), which is consistent with the formation process of a reverse-oriented monsoon trough described by Lander (1996). In addition, the reverse-oriented mon- soon trough observed in this study was an episodic event, which is also consistent with the finding of Lander (1996). However, the life span of the observed reverse-oriented monsoon trough was approximately 4 days, which is shorter than those of the reverse-oriented monsoon troughs shown by Lander (1996), who noted that a reverse- oriented monsoon trough can persist for weeks. The breakdown of the monsoon trough in this study was associated with the continuous poleward propagation of the monsoon trough.

It is well known that the area near the axis of the monsoon trough is a favorable region for the genesis of tropical cyclones (Harr and Chan 2005; Harr and Wu 2011). Carr and Elsberry (1994) and Lander (1996) have shown that tropical cyclones form simultaneously along the reverse-oriented monsoon troughs studied by them. In contrast to their findings, however, no tropical cy- clone developed along the reverse-oriented monsoon trough investigated in this study. Harr and Elsberry (1995) illustrated that active (inactive) tropical cyclone periods in the western North Pacific tend to be related to active (inactive) monsoon regimes. Consequently, in contrast to the active reverse-oriented monsoon troughs studied by Carr and Elsberry (1994) and Lander (1996), the reverse-oriented monsoon trough shown in this study may be an inactive monsoon trough, which is not conducive to tropical cyclone genesis. Whether such inactiveness is related to the modulation associ- ated with the equatorward intrusion of the midlatitude upper trough is unclear and needs to be investigated further.

7. Summary and conclusions The synoptic evolution of a monsoon trough and other large-scale circulations observed in the western North Pacific in June 2008 was analyzed using sea surface winds, weather charts, upper-level winds, and geosta- tionary satellite images. In addition, the kinematic and thermodynamic structure of the monsoon trough were investigated using the data from the surface, radiosonde, and C-band Doppler radar observations on board the research vessel Mirai at 128N, 1358E.

The monsoon trough was first observed to the south of the Mirai and extended southeastward from the center of a tropical depression that later intensified into a ty- phoon. To the northeast of the monsoon trough, a sur- face low pressure cell developed in front of an upper trough that intruded from the midlatitudes into the tropics and approached the Mirai. The monsoon trough moved northward, with its eastern edge moving faster than its western edge. As a result, the monsoon trough became oriented from the southwest to the north- eastandmergedwiththesurfacelowpressurecell. Subsequently, the monsoon trough extended broadly northeastward.

As it moved northward, the monsoon trough passed over the Mirai. The passage of the monsoon trough was accompanied by lower pressure, higher air and sea sur- face temperatures, and minimal rainfall, which implies that the monsoon trough resembled a thermal trough. The monsoon trough extended upward approaching 500 hPa and sloped southward with height. It was over- lain by a deep layer of northwesterly winds, negative geopotential height and temperature anomalies, and extremely dry air in the upper troposphere, which im- plies that the upper troposphere over the monsoon trough was modulated significantly by the upper trough that had deeply intruded into the tropics. A region of low-level intense cyclonic vorticity appeared immedi- ately south of the monsoon trough and was located be- neath intense negative geopotential height anomalies penetrating downward from the upper troposphere.

As determined based on the satellite data, convective clouds along the monsoon trough were generally weak, except those associated with the typhoon and the surface low pressure cell. The Doppler radar observation in- dicated that precipitation systems were weak and scat- tered near the monsoon trough. Intense and extensive precipitation systems developed approximately 450- 680 km south of the surface monsoon trough and were characterized by convergent southwesterly flow at low levels and divergent northwesterly flow at upper levels. Deep and vigorous upward motion occurred south of the surface monsoon trough, where intense precipitation systems and copious rainfall were observed.

It appears that the midlatitude upper trough deeply intruding into the tropics exerted important influences on the development of both the monsoon trough and the associated precipitation. The upper trough modulated the migration and orientation of the monsoon trough by promoting the development of the low pressure cell, which, in turn, strengthened the meridional pressure gradient and facilitated the rapid northward transport of westerly momentum in the eastern edge of the monsoon trough. The upper trough also enhanced the intensity of the monsoon trough by strengthening the low-level cy- clonic vorticity around the monsoon trough, facilitated the maintenance of the monsoon trough by enhancing the dry intrusion and convective suppression over the monsoon trough, and triggered intense precipitation systems and rainfall south of the monsoon trough by enhancing upper-level divergence and low-level baro- tropic instability.

This study has identified complex modulations of the synoptic evolution and internal structure of a monsoon trough over the western North Pacific by the deep equa- torward intrusion of a midlatitude upper trough. Further studies, such as more comprehensive observations and a numerical investigation, are needed to resolve the detailed processes of these interesting modulations and their relationships to the genesis of tropical cyclones in the western North Pacific.

Acknowledgments. The authors would like to express their deep appreciation to the entire crew of the re- search vessel Mirai and to the technical staffs of Global Ocean Development Inc. and Marine Works Japan, Ltd., for their support in obtaining the intensive obser- vation data. We would also like to thank Kochi Uni- versity for providing the geostationary satellite data.

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BIAO GENG,KUNIO YONEYAMA, AND RYUICHI SHIROOKA Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan (Manuscript received 19 July 2013, in final form 11 November 2013) Corresponding author address: Dr. Biao Geng, Research In- stitute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, 237-0061, Japan.

E-mail: [email protected] DOI: 10.1175/MWR-D-13-00233.1 (c) 2014 American Meteorological Society

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