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  • View in gallery

    Surface weather charts showing sea level pressure (hPa, contours) and horizontal winds (kt, half barb = 5 kt, full barb = 10 kt, and flag = 50 kt; 1 kt = 0.5144 m s−1) from the JMA at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The location of the Mirai (12°N, 135°E) is shown by a filled star.

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    300-hPa weather charts showing geopotential height (m, solid contours), horizontal winds (kt, wind barbs as in Fig. 1), and isotach (kt, dashed contours) from the JMA at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The bold dashed line A–B in (b) depicts the cross section shown in Fig. 3. The location of the Mirai (12°N, 135°E) is shown by a filled star.

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    Cross section along line A–B in Fig. 2b derived from the global objective analysis data at 0000 UTC 19 Jun 2008 showing geopotential height anomalies (m, shaded). The anomaly of geopotential height at a level is defined as the deviation of geopotential height from its mean value at that level.

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    Satellite-derived 350–250-hPa winds (m s−1, half barb = 2.5 m s−1, full barb = 5 m s−1, and flag = 20 m s−1) from the UW-CIMSS at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The original data are interpolated to Cartesian grids with a grid spacing of 1.25°. The distribution of divergence (10−5 s−1, shaded) is superimposed. The dashed line in each figure outlines the outer edge of the upper trough. The location of the Mirai (12°N, 135°E) is shown by a filled star.

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    Sea surface winds (m s−1, wind barbs as in Fig. 4) from the CCMP product at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The distribution of the infrared brightness temperature (°C, shaded) from the geostationary satellite image is superimposed. The dashed line in each figure indicates the position of the monsoon trough defined as a shear line with equatorial southwesterlies on its equatorward side and easterlies on its poleward side. The location of the Mirai (12°N, 135°E) is shown by a filled star. The capital “L” represents the surface low pressure cell from the surface weather chart.

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    Positions of the monsoon trough at 12-h intervals. The location of the Mirai (12°N, 135°E) is shown by a filled star.

  • View in gallery

    Time series of (a) surface zonal wind (m s−1, solid curve) and meridional wind (m s−1, dashed curve), (b) sea level pressure (hPa, solid curve) and hourly rainfall amount (mm, vertical bars), and (c) surface air temperature (°C, solid curve) and sea surface temperature (°C, dashed curve) measured on board the Mirai. All fields except rainfall have been filtered with a 1-day running mean. The vertical arrow in each figure indicates the approximate passing time of the surface monsoon trough.

  • View in gallery

    Time–height diagram of winds (m s−1, wind barbs as in Fig. 4) and isotach (m s−1, shaded) from the radiosonde observation. All fields have been filtered with a 1-day running mean. The monsoon trough axis is indicated by the bold dashed line.

  • View in gallery

    (a) As in Fig. 8, but for geopotential height anomalies (m, shaded). The anomaly definition is as in Fig. 3. (b) As in Fig. 8, but for absolute vorticity (10−5 s−1, contours) below 500 hPa associated with the zonal flow derived from the radiosonde observations, which are based on the time–space translation corresponding to the motion of the monsoon trough. Hatching indicates absolute vorticity greater than 10 × 10−5 s−1 and shading indicates regions where the meridional gradients of absolute vorticity associated with the zonal flow are negative.

  • View in gallery

    (a) As in Fig. 8, but for temperature anomalies (°C, contours). Shading indicates negative temperature anomalies. The anomaly definition is as in Fig. 3. (b) As in Fig. 8, but for relative humidity (%, contours). Shading indicates relative humidity lower than 50%.

  • View in gallery

    Reflectivity (dBZ, shaded) at 3 km from the measurements of the Doppler radar at (a) 1200 UTC 19 Jun, (b) 1800 UTC 19 Jun, (c) 0000 UTC 21 Jun, and (d) 0600 UTC 21 Jun 2008. Sea surface winds (m s−1, wind barbs as in Fig. 4) from the CCMP product are superimposed on each figure. The position of the surface monsoon trough is illustrated by the dashed line. The open circle in each figure indicates the coverage of the Doppler radar.

  • View in gallery

    As in Fig. 11, but for 10-dBZ echo top (km, shaded).

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    Time series of radar-observed area-mean hourly rainfall amount (mm, vertical bars) within 100 km of the radar and 300-hPa zonal wind (m s−1, solid curve) from the radiosonde observation. The zonal wind has been filtered with a 1-day running mean. The vertical arrow indicates the approximate passing time of the surface monsoon trough. Distances relative to the surface monsoon trough are also shown according to the time–space translation corresponding to the motion of the monsoon trough. Positive and negative distances represent positions north and south of the surface monsoon trough, respectively.

  • View in gallery

    (a) Time–height diagram of divergence (10−5 s−1, shaded) and horizontal winds (m s−1, wind barbs as in Fig. 4) derived from the VAD method within a 100-km radius of the Doppler radar. All fields have been filtered with a 1-day running mean. (b) As in (a), but for vertical velocity (cm s−1, shaded). Distances are relative to the surface monsoon trough, and their definitions are as in Fig. 13.

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Observations of Upper-Tropospheric Influence on a Monsoon Trough over the Western North Pacific

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  • 1 Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
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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.

Corresponding author address: Dr. Biao Geng, Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, 237-0061, Japan. E-mail: bgeng@jamstec.go.jp

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.

Corresponding author address: Dr. Biao Geng, Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, 237-0061, Japan. E-mail: bgeng@jamstec.go.jp

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 northeastward 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 undergoes extensive migration. During the summer, the monsoon trough in the western North Pacific may appear between 5° and 25°N (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). However, 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 monsoon 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 monsoon 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 associated 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. However, although many studies have investigated interannual (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 impacts 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 examined the evolution of the monsoon trough in this region 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 transition 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 describing the upper-tropospheric influence on the monsoon 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 midlatitude 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 region. 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 troposphere 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 monsoon 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 coinciding with a sea surface temperature maximum and that the monsoon trough can be maintained and enhanced 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 illustrated the structure of the monsoon trough in this region under the influence of midlatitude upper troughs intruding 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 12°N, 135°E. 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 monsoon 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 monsoon 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 presented 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 conclusions in section 7.

2. Data and methods

The synoptic evolution of the migration and orientation of the monsoon trough and other large-scale circulations 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 Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS; Velden et al. 2005), and geostationary 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 addition, the internal structure of the monsoon trough was investigated using observations of the surface, radiosonde, and a C-band Doppler radar on board the Mirai. The radiosonde observations were conducted at 3-h intervals, 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 mesoscale 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 calculate 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 intensified into a typhoon at 0000 UTC 19 June (Fig. 1b). At the same time, a low pressure cell developed at the surface around 20°N, 152°E. 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, although a pressure trough still extended broadly east-northeastward from the center of the typhoon.

Fig. 1.
Fig. 1.

Surface weather charts showing sea level pressure (hPa, contours) and horizontal winds (kt, half barb = 5 kt, full barb = 10 kt, and flag = 50 kt; 1 kt = 0.5144 m s−1) from the JMA at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The location of the Mirai (12°N, 135°E) is shown by a filled star.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

In the upper troposphere, there was a midlatitude trough near 150°E at 0000 UTC 18 June (Fig. 2a). This upper trough was advancing southwestward from the midlatitudes; its southern edge had approached 24°N by 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).

Fig. 2.
Fig. 2.

300-hPa weather charts showing geopotential height (m, solid contours), horizontal winds (kt, wind barbs as in Fig. 1), and isotach (kt, dashed contours) from the JMA at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The bold dashed line A–B in (b) depicts the cross section shown in Fig. 3. The location of the Mirai (12°N, 135°E) is shown by a filled star.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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.

Fig. 3.
Fig. 3.

Cross section along line A–B in Fig. 2b derived from the global objective analysis data at 0000 UTC 19 Jun 2008 showing geopotential height anomalies (m, shaded). The anomaly of geopotential height at a level is defined as the deviation of geopotential height from its mean value at that level.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 orientation. 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 upper trough, divergent northwesterly winds strengthened around the Mirai (Figs. 4c,d).

Fig. 4.
Fig. 4.

Satellite-derived 350–250-hPa winds (m s−1, half barb = 2.5 m s−1, full barb = 5 m s−1, and flag = 20 m s−1) from the UW-CIMSS at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The original data are interpolated to Cartesian grids with a grid spacing of 1.25°. The distribution of divergence (10−5 s−1, shaded) is superimposed. The dashed line in each figure outlines the outer edge of the upper trough. The location of the Mirai (12°N, 135°E) is shown by a filled star.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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°N, 133°E (Fig. 5a). The vortex was located to the southwest of the Mirai and was associated 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 southeastward from the center of the depression to 145°E, 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.

Fig. 5.
Fig. 5.

Sea surface winds (m s−1, wind barbs as in Fig. 4) from the CCMP product at 0000 UTC (a) 18 Jun, (b) 19 Jun, (c) 20 Jun, and (d) 21 Jun 2008. The distribution of the infrared brightness temperature (°C, shaded) from the geostationary satellite image is superimposed. The dashed line in each figure indicates the position of the monsoon trough defined as a shear line with equatorial southwesterlies on its equatorward side and easterlies on its poleward side. The location of the Mirai (12°N, 135°E) is shown by a filled star. The capital “L” represents the surface low pressure cell from the surface weather chart.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

By 0000 UTC 19 June, the southwesterly winds intensified south of the typhoon (Figs. 1b and 5b), consistent 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 pressure cell after 0000 UTC 21 June (Fig. 5d), less northeastward 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 s−1 toward 299°.

Fig. 6.
Fig. 6.

Positions of the monsoon trough at 12-h intervals. The location of the Mirai (12°N, 135°E) is shown by a filled star.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 5°N, 128°E to 10°N, 136°E, 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 surface, upper-air, and Doppler radar observations conducted on board the Mirai is analyzed.

a. Surface analysis

Figure 7 shows time series of surface elements observed 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 s−1.

Fig. 7.
Fig. 7.

Time series of (a) surface zonal wind (m s−1, solid curve) and meridional wind (m s−1, dashed curve), (b) sea level pressure (hPa, solid curve) and hourly rainfall amount (mm, vertical bars), and (c) surface air temperature (°C, solid curve) and sea surface temperature (°C, dashed curve) measured on board the Mirai. All fields except rainfall have been filtered with a 1-day running mean. The vertical arrow in each figure indicates the approximate passing time of the surface monsoon trough.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 characteristics 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 upward approaching 500 hPa and sloped leftward (i.e., southward), with height, similar to the vertical extension and slope of the monsoon trough in other regions described in previous studies (Miller and Keshvarnurty 1968; Raghavan 1973; Bayo Omotosho 1985). Southwesterly winds intensified south of the monsoon trough and reached a speed greater than 12 m s−1 between 900 and 650 hPa.

Fig. 8.
Fig. 8.

Time–height diagram of winds (m s−1, wind barbs as in Fig. 4) and isotach (m s−1, shaded) from the radiosonde observation. All fields have been filtered with a 1-day running mean. The monsoon trough axis is indicated by the bold dashed line.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 northwesterly 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 penetration of intense negative geopotential height anomalies from the upper troposphere was observed south of the surface monsoon trough.

Fig. 9.
Fig. 9.

(a) As in Fig. 8, but for geopotential height anomalies (m, shaded). The anomaly definition is as in Fig. 3. (b) As in Fig. 8, but for absolute vorticity (10−5 s−1, contours) below 500 hPa associated with the zonal flow derived from the radiosonde observations, which are based on the time–space translation corresponding to the motion of the monsoon trough. Hatching indicates absolute vorticity greater than 10 × 10−5 s−1 and shading indicates regions where the meridional gradients of absolute vorticity associated with the zonal flow are negative.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 (~−10 m) was observed early on 20 June. This continuous trend of falling geopotential height over the monsoon trough apparently resulted from the equatorward 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 divergent 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 characterized by cyclonic vorticity. A region of intense cyclonic vorticity (~10 × 10−5 s−1) was observed immediately south of the monsoon trough approximately between 800 and 650 hPa. This intense cyclonic vorticity was beneath stronger negative geopotential height anomalies 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 (~0.4°C) (Fig. 10a), consistent with the surface observation indicating that the monsoon trough exhibited characteristics of a thermal trough. Over the monsoon trough, there were cores of negative temperature anomalies (~−0.4°C) 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 equatorward intrusion of the upper trough and the intensification of northwesterly winds in the upper troposphere (Figs. 2c,d; 4c,d; and 8).

Fig. 10.
Fig. 10.

(a) As in Fig. 8, but for temperature anomalies (°C, contours). Shading indicates negative temperature anomalies. The anomaly definition is as in Fig. 3. (b) As in Fig. 8, but for relative humidity (%, contours). Shading indicates relative humidity lower than 50%.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 distribution of radar echoes. At 1200 and 1800 UTC 19 June (Figs. 11a,b), the surface monsoon trough was approaching and had just passed the Mirai, respectively. Consequently, the Doppler radar captured the distribution 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).

Fig. 11.
Fig. 11.

Reflectivity (dBZ, shaded) at 3 km from the measurements of the Doppler radar at (a) 1200 UTC 19 Jun, (b) 1800 UTC 19 Jun, (c) 0000 UTC 21 Jun, and (d) 0600 UTC 21 Jun 2008. Sea surface winds (m s−1, wind barbs as in Fig. 4) from the CCMP product are superimposed on each figure. The position of the surface monsoon trough is illustrated by the dashed line. The open circle in each figure indicates the coverage of the Doppler radar.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

Fig. 12.
Fig. 12.

As in Fig. 11, but for 10-dBZ echo top (km, shaded).

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 5° north of the Mirai at 0000 UTC 21 June. Radar echoes in the southwesterlies as far as 5° 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 intensity. 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 corresponding 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 development of precipitation systems (Figs. 1112). In accordance with the small and weak radar echoes, only light rain was observed near the monsoon trough. Copious rainfall appeared approximately 450–680 km south of the surface monsoon trough, consistent with the strong and deep precipitation systems developed in these regions. The weak convection and rainfall that occurred near the monsoon trough and the deep convection and intense rainfall that developed several hundred kilometers south of the monsoon trough are consistent with the distribution of convection and rainfall around the monsoon trough found by previous studies (Raghavan 1973; Bayo Omotosho 1985).

Fig. 13.
Fig. 13.

Time series of radar-observed area-mean hourly rainfall amount (mm, vertical bars) within 100 km of the radar and 300-hPa zonal wind (m s−1, solid curve) from the radiosonde observation. The zonal wind has been filtered with a 1-day running mean. The vertical arrow indicates the approximate passing time of the surface monsoon trough. Distances relative to the surface monsoon trough are also shown according to the time–space translation corresponding to the motion of the monsoon trough. Positive and negative distances represent positions north and south of the surface monsoon trough, respectively.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 captured by the Doppler radar. Around 21 June, when the Mirai was located approximately 538 km south of the surface monsoon trough, there was convergence extending from the surface to 6 km (Fig. 14a). Strong convergence (magnitude greater than 1 × 10−5 s−1) 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 × 10−5 s−1) corresponded closely to the positive geopotential height anomalies in the upper troposphere 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 troposphere (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 motion and intense rainfall occurred after upper-level divergent westerly winds over the Mirai had intensified and reached their maximum magnitude (Figs. 1314).

Fig. 14.
Fig. 14.

(a) Time–height diagram of divergence (10−5 s−1, shaded) and horizontal winds (m s−1, wind barbs as in Fig. 4) derived from the VAD method within a 100-km radius of the Doppler radar. All fields have been filtered with a 1-day running mean. (b) As in (a), but for vertical velocity (cm s−1, shaded). Distances are relative to the surface monsoon trough, and their definitions are as in Fig. 13.

Citation: Monthly Weather Review 142, 4; 10.1175/MWR-D-13-00233.1

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 important 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. 56). 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 development of the surface low pressure cell to the northeast 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. 56), 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 consistent 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 depends 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 regulate 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 evaluated 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 circulation 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 equatorward 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 insolation through relatively clear skies over the monsoon 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 development of the deep convection south of the monsoon trough.

One role of the upper trough is related to the divergent 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 furnished by the deep troughs in the midlatitude westerlies. The deep convection that developed south of the monsoon 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 associated with a deep layer of stronger divergent northwesterly flow in the upper troposphere and developed after upper-level westerly winds had been enhanced by the upper trough (Figs. 4c,d and 1314). 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 reversal 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 appeared south of intense cyclonic vorticity between approximately 850 and 650 hPa, which implies that the barotropic instability criterion was met there. A negative meridional gradient of absolute vorticity was also found below 850 hPa south of the monsoon trough. This negative meridional gradient appears to have been induced by precipitation and did not influence the barotropic 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 implies 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 monsoon 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. 56), which is consistent with the formation process of a reverse-oriented monsoon trough described by Lander (1996). In addition, the reverse-oriented monsoon 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 cyclone 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 associated 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 geostationary 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 12°N, 135°E.

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 typhoon. To the northeast of the monsoon trough, a surface 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 northeast and merged with the surface low pressure cell. 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 surface 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 overlain by a deep layer of northwesterly winds, negative geopotential height and temperature anomalies, and extremely dry air in the upper troposphere, which implies 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 immediately south of the monsoon trough and was located beneath 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 indicated that precipitation systems were weak and scattered 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 cyclonic 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 barotropic 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 equatorward 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 research vessel Mirai and to the technical staffs of Global Ocean Development Inc. and Marine Works Japan, Ltd., for their support in obtaining the intensive observation data. We would also like to thank Kochi University for providing the geostationary satellite data.

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