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    (a) Ten-day-averaged precipitation of 35th and 36th pentad of CMAP data, (b) vertical accumulated water vapor transport, (c) geopotential height at 850 hPa, and (d) wind field at 200 hPa (vector) and wind speed (shadow) of ECMWF objective analysis data in late Jun 1998. The contour interval of the geopotential height is 10 m.

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    (a) Ten-day-averaged precipitation of 72d and 73rd pentad of CMAP data, (b) vertical accumulated water vapor transport, (c) geopotential height at 850 hPa, and (d) wind field at 200 hPa (vector) and wind speed (shadow) of ECMWF objective analysis data in late Dec 1998. The contour interval of the geopotential height is 10 m; only contours over 1400 m are drawn

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    Observed monthly SST distribution in (a) Jun 1998, (b) Dec 1997, (c) and Dec 1998 (Reynolds and Smith 1994). The shadow indicates the warm SST area more than 300 K (27°C). The contour interval is 2 K (2°C)

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    (a) Monthly averaged precipitation of CMAP data, (b) vertical accumulated water vapor transport, and (c) wind field at 200 hPa (vector) and wind speed (shadow) of ECMWF objective analysis data in Dec 1997

  • View in gallery

    The same as Fig. 4, except for Dec 1998

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    Conceptual figure of the zonal mean simulation: (a) 10-day-averaged zonal mean wind at 200 hPa of ECMWF data; (b) the interpolated zonal mean wind field to polar-stereographic coordinate as initial and boundary condition. (c) Wind field after 38 days from the start of simulation

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    Vertical cross section of 10-day-averaged zonal mean wind (shadow) and temperature (contour) in NH in (a) late Jun, and Southern Hemisphere in (b) mid-Dec 1998

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    (a) Thirty-day-averaged precipitation (shadow) and vertical accumulated water vapor transport (vector), (b) wind field (vector) and wind speed (shadow) at 200 hPa, (c) and geopotential height at 850 hPa of the result of the baiu-control run

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    (a) Thirty-day-averaged precipitation (shadow) and vertical accumulated water vapor transport (vector), (b) wind field at 200 hPa (vector) and wind speed (shadow), (c) and geopotential height at 850 hPa of the result of SPCZ-control run. Only contours over 1400 m are drawn

  • View in gallery

    Same as Fig. 9, except for SPCZ-SST9712 run

  • View in gallery

    Same as Fig. 9, except for 10-day-averaged results from 21st to 30th day from simulation start of SPCZ-control run

  • View in gallery

    Same as Fig. 9, except for ZM-NHZM run

  • View in gallery

    Same as Fig. 9, except for ZM-NHZM-topo run

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    Same as Fig. 9, except for ZM-NHZM-mirror run

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Formation Mechanism of the Simulated SPCZ and Baiu Front Using a Regional Climate Model

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  • 1 The Frontier Research System for Global Change, Yokohama Institute for Earth Science, Yokohama, Japan
  • | 2 Institute of Geoscience, University of Tsukuba, Tsukuba, Japan
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Abstract

The formation mechanisms of both the South Pacific convergence zone (SPCZ) and the baiu front are investigated using a regional climate model. Some idealistic numerical experiments are carried out assuming zonally uniform and temporally constant atmospheric fields obtained from ECMWF analysis data as initial and lateral boundary conditions. A rainfall zone similar to the SPCZ is reproduced using a zonal mean atmospheric field of the Southern Hemisphere (SH) summer. The simulated SPCZ in the idealized model framework is highly sensitive to the variation of SST during 1997–98 in a manner similar to observation. The SPCZ is extremely weak in an experiment under the zonal mean field of the Northern Hemisphere (NH) early summer. Experiments with a different intensity of zonal wind speed and baroclinicity suggest that a mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even occasionally suppresses precipitation when it is too weak. The heat contrast between the Australian continent and the South Pacific Ocean contributes to form another rainfall zone when the zonal flow is very weak. Under these conditions, the SPCZ becomes unclear, and the rainfall zone appears from the southeastern part of Australia to the east of New Zealand. The latter rainfall zone will be intensified if the orography in the Australian continent is magnified. This rainfall zone is formed by the heat contrast between land and ocean and is somewhat similar to the baiu front. A continent as large as Eurasia creates a better-defined rainfall zone, even under stronger zonal flow. The baiu front seems to be a rainfall zone caused by the heat contrast between land and ocean that differs from the SPCZ in its formation mechanism.

Additional affiliation: The Frontier Research System for Global Change, Yokohama Institute for Earth Science, Yokohama, Japan

Corresponding author address: Dr. Takao Yoshikane, The Frontier Research System for Global Change, Yokohama Institute for Earth Science 3173-25, Showa-Machi, Kanazawa-Ku, Yokohama, Kanagawa, 236-0001, Japan. Email: yoshikat@jamstec.go.jp

Abstract

The formation mechanisms of both the South Pacific convergence zone (SPCZ) and the baiu front are investigated using a regional climate model. Some idealistic numerical experiments are carried out assuming zonally uniform and temporally constant atmospheric fields obtained from ECMWF analysis data as initial and lateral boundary conditions. A rainfall zone similar to the SPCZ is reproduced using a zonal mean atmospheric field of the Southern Hemisphere (SH) summer. The simulated SPCZ in the idealized model framework is highly sensitive to the variation of SST during 1997–98 in a manner similar to observation. The SPCZ is extremely weak in an experiment under the zonal mean field of the Northern Hemisphere (NH) early summer. Experiments with a different intensity of zonal wind speed and baroclinicity suggest that a mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even occasionally suppresses precipitation when it is too weak. The heat contrast between the Australian continent and the South Pacific Ocean contributes to form another rainfall zone when the zonal flow is very weak. Under these conditions, the SPCZ becomes unclear, and the rainfall zone appears from the southeastern part of Australia to the east of New Zealand. The latter rainfall zone will be intensified if the orography in the Australian continent is magnified. This rainfall zone is formed by the heat contrast between land and ocean and is somewhat similar to the baiu front. A continent as large as Eurasia creates a better-defined rainfall zone, even under stronger zonal flow. The baiu front seems to be a rainfall zone caused by the heat contrast between land and ocean that differs from the SPCZ in its formation mechanism.

Additional affiliation: The Frontier Research System for Global Change, Yokohama Institute for Earth Science, Yokohama, Japan

Corresponding author address: Dr. Takao Yoshikane, The Frontier Research System for Global Change, Yokohama Institute for Earth Science 3173-25, Showa-Machi, Kanazawa-Ku, Yokohama, Kanagawa, 236-0001, Japan. Email: yoshikat@jamstec.go.jp

1. Introduction

The South Pacific convergence zone (SPCZ) is a vast rainfall zone extending southeastward from the tropical region of the western South Pacific. Reviews of the SPCZ were introduced by Vincent (1994). Some studies have investigated the formation mechanism of the SPCZ. Using the general circulation model (GCM), Kiladis et al. (1989) indicated that the orientation of the SPCZ is more sensitive to interactions with the middle latitude westerly over the South Pacific than the distribution of sea surface temperature and surface process of land over the Southern Hemisphere. Matthews et al. (1996) indicated that the SPCZ is strongly influenced by the Madden–Julian oscillation (MJO) and explained the formation mechanism of the SPCZ using an idealized numerical model as follows. 1) Convection over the tropical region around Indonesia leads to an upper-tropospheric anticyclone above that region. 2) By interaction with the anticyclone, meandering of the subtropical jet is intensified and leads to the formation of an upper-tropospheric trough on the eastern flank of the anticyclone. 3) Deep convection develops over a line-shaped area extending southward from Indonesia caused by the baroclinic instability in the front of the trough, which is the SPCZ.

On the other hand, some previous studies have indicated that the feature of SPCZ is similar to that of the baiu front, which appears in southern China to the east of Japan in the early summer. Kodama (1993) indicated that both the SPCZ and the baiu front are characterized by a strong horizontal moisture convergence, front genesis in equivalent potential temperature fields, and remarkable convective instability. From the analysis, he concluded that the low-level poleward flows, which are induced by the heat sources of monsoon convection and by land surface heating over the continents, are essential to form rainfall zones such as the SPCZ and the baiu front. Using an aquaplanet GCM, Kodama (1999) also pointed out that subtropical convergence zones, such as the SPCZ and the baiu front, are closely related with the heat sources located in the tropical region.

The previous studies mentioned above suggested that the precipitation zone of the baiu front is formed by convective instability. Details of the baiu front were indicated by Ninomiya (1984). The instability is induced by the differential advection of moist static energy, namely, the warm and wet air in the lower layer from the south and the cold and dry air in the upper layer from the north (Ninomiya 2000). The low-level and upper-level jets are intensified on the southern and northern sides of the rainfall zone, respectively, because the release of latent heat modifies the temperature distribution (Chen 1982; Yoshikane et al. 2001). The features of the baiu front were also simulated by Kawatani and Takahashi (2003) and Ninomiya et al. (2002) using an AGCM. Kawatani and Takahashi (2003) showed that the simulated baiu front differs in its precipitation pattern between two cumulus convective parameterizations assumed in the model.

However, there have been few numerical studies on the differences in the formation mechanisms of the SPCZ and the baiu front. This study focuses on those differences by using a regional climate model and by considering the geographical features and differences in hemispheric circulation between the Northern and Southern Hemispheres. The observed characteristics of the SPCZ and the baiu front are described in section 2, the model and experiments are in section 3, the idealistic simulation in section 4, the sensitivity experiments in section 5, the discussion in section 6, and the conclusions in section 7.

2. Observed characteristics of the SPCZ and the baiu front

Figure 1 shows the 10-day-averaged precipitation given by (a) Climate Prediction Center (CPC) merged analysis of precipitation (CMAP; Xie and Arkin 1997) data, (b) vertical accumulated water vapor transport, (c) wind velocity at 200 hPa, (d) and geopotential height at 850 hPa in late June 1998. The latter three are derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) data. A strong rainfall zone and large water vapor transport are found east of the Eurasian continent. The distribution of the wind velocity in the upper troposphere indicates that an anticyclonic circulation is formed over the Tibetan Plateau and that a trough exists on the eastern flank of the plateau. The strong wind zone appears on the northern side of the rainfall zone, extending east from the eastern side of the trough, parallel to the low-level jet at 850 hPa.

In order to inspect the detailed structure of the SPCZ, the 10-day-averaged data in late December 1998 are shown in Fig. 2. A large active area of cumulus convection appears in the north of the Australian continent. In the upper troposphere over the cumulus convection area, anticyclonic circulation (2C in Fig. 2) occurs, corresponding to the cyclonic circulation at the lower troposphere (2A in Fig. 2). The trough is formed on the eastern flank of the anticyclonic circulation (2D in Fig. 2), and the rainfall zone occurs and extends along the upper-level strong wind to the east of the trough (2B in Fig. 2). This feature of the observed SPCZ is similar to that of Matthews et al. (1996). There is also a small rainbelt along the eastern coast of Australia. It seems to trace some disturbances; however, the details of the small rainband are not mentioned in this study.

Sea surface temperature (SST) distributions in June 1998 and December 1997 and 1998 are shown in Fig. 3. The World Meteorological Organization (WMO) reported that the El Niño event had continued in December 1997 and that the warm pool had extended into the tropical area of the eastern Pacific. Some previous studies indicate that the location of SPCZ during an El Niño event tends to move northeastward in comparison with that during a non–El Niño term (e.g., Trenberth and Shea 1987).

Figure 4 shows the observed precipitation by CMAP, the vertical accumulated water vapor transport, and the wind velocity at 200 hPa by ECMWF in December 1997. Figure 5 is the same as Fig. 4, except that the data correspond to 1998. These figures indicate that the active area of the intertropical convergence zone (ITCZ) extended further east and the SPCZ was located farther northeast in 1997 (2E in Fig. 4) than in 1998 (2F in Fig. 5). The structures of the SPCZ were clearer in 1998 than in 1997. Both the monthly mean and the 10-day mean precipitation are discussed in the present study. While the monthly mean is enough for demonstrating the SPCZ formation, the 10-day mean is suitable to demonstrate the impacts of the fluctuation of the ITCZ.

Some previous studies pointed out that the SPCZ is greatly influenced by the Madden–Julian oscillation (e.g., Matthews et al. 1996). In the figures showing the monthly mean precipitation, the SPCZ seems to be zonally broader due to the variability of its location during the month.

3. Model and experiments

The Regional Atmospheric Modeling System (RAMS; Pielke et al. 1992), which was developed at Colorado State University, is utilized in this study. The details of the model are shown in Table 1. The radiation scheme in the original RAMS version 3b is replaced by a precise band scheme presented by Nakajima et al. (2000). A brief description of the model and the assumed conditions are shown in Table 2. The cloud microphysics in a precipitation process is utilized for the precipitation process instead of a simple (dump bucket) scheme to simulate large-scale precipitation.

Figure 6 schematically shows the concept of the zonal mean (ZM) simulation “ZM simulation,” which is an idealistic experiment, and called after Hoskins and Rodwell (1995) in this study. The vector indicates the horizontal wind velocity, and the light-, semilight-, and dark-shadowed areas indicate wind speeds in excess of 10, 20, and 30 m s−1, respectively. In the ZM simulation, the initial and the lateral boundary conditions, which were obtained from the analysis data by ECMWF, were assumed to be zonally uniform and temporally constant. The global zonal mean data are obtained by a 10-day average of ECMWF data from 11 to 20 December 1998 (Fig. 6a). The initial condition is given by interpolation of the global zonal mean data, as in Fig. 6b. The boundary condition is assumed to be stationary. Figure 6c indicates the circulation after 38 days of integration. These simulations permit an evaluation of the influence of the interaction between the zonal flow and the surface boundary conditions on the regional-scale atmospheric circulation, avoiding any intruding disturbances from outside the calculation domain. The ZM simulations were also utilized in the numerical study on the baiu front by Yoshikane et al. (2001).

Numerical experiments are listed in Table 3. In the baiu-control run and SPCZ-control run, the zonal mean field is estimated by the analysis data from late June and mid-December, respectively. The vertical cross sections of wind velocity and temperature of the zonal mean field are shown in Fig. 7. The monthly mean SST, which is obtained from Reynolds and Smith (1994), is applied throughout the integration period. Generally, the nonhydrostatic system is not necessary to conduct the simulation with a 150-km grid space. A hydrostatic model could also be conducted in this type of experiment. Some sensitive experiments listed in Table 3 are also conducted to investigate the formation mechanism of the rainfall zone. The purpose of these simulations will be described in section 5 in detail.

The ZM simulation in the present study differs from that in the study by Hoskins and Rodwell (1995). Their model was initialized by a zonal mean of the climatology, and the zonally mean components of all major variables are fixed throughout the integration period. The diabatic heating rate is assumed to be stationary and is estimated from the residual in the thermodynamic equation applied to the time series of the ECMWF analysis data. On the contrary, the ZM simulation in the present study assumes fixed zonal mean variables only for the lateral/top boundaries and initial conditions. There is no nudging or artificial diabatic heating in the inner region of the model domain. Most of the diabatic heating in the model is induced by the precipitation process. Therefore, subregional-scale disturbances are generated only by the physical process in the inner model region. It is difficult to remove the influence of disturbances from the outside of the subject region on the simulated atmospheric circulation of inside region by the studies using a GCM. On the other hand, it is not necessary to consider the influence of disturbances more than a wavenumber of one from the outside of the calculation region because of the fixed lateral boundary condition with the zonal mean field. The factors of the formation mechanism of simulated atmospheric circulation could be simply investigated in the ZM simulation. This is the one of the advantage points of the study using a regional climate model as compared with a GCM.

4. Idealistic simulation

Figures 8 and 9 show the 30-day mean field of the baiu-control and the SPCZ-control runs, respectively. The top indicates the precipitation and the vertical accumulated water vapor transport, the middle, the wind vector and speed at 200 hPa, and the lower, the geopotential height at 850 hPa. These results are the average for the period of the 11th to 40th day of simulation time. The features of the baiu front and SPCZ can be confirmed around the 10th day from the start of the simulation. The simulation results will be discussed only in the interior region except for four grid points near the lateral boundaries, because those results may be inconsistently influenced by the boundary conditions. The simulated water vapor transport corresponds well with the wind field at 850 hPa. The fields of the water vapor transport, upper-tropospheric wind, and lower-tropospheric geopotential height of the baiu-control and the SPCZ-control are qualitatively similar to observations. The synoptic structures of the simulated baiu front (Fig. 8) have close similarity to those of the observations (Fig. 1), including the water vapor transport, the location of the rainfall, and the ridge and trough of the upper-level jet. The large-scale features of the simulated baiu front are almost the same as those presented in Yoshikane et al. (2001), where the features of the baiu front are discussed in detail except for the small-scale features less than mesoscale, as mentioned in section 1.

One of the crucial aspects of the observed SPCZ is how it differs from a normal year to an El Niño year. In order to validate the similarity between the simulated and the real SPCZ, the variability corresponding to the differences of SST distribution between a normal year and an El Niño year was investigated. Figure 10 shows the water vapor transport and precipitation, the wind speed at 200 hPa, and the geopotential height of a sensitivity experiment, an SPCZ-9712SST run, in which the SST is replaced by that observed in December 1997 instead of 1998. In December 1997, a remarkable influence of the El Niño event was observed in the rainfall distribution over the tropical western Pacific. These results show that the ITCZ is reproduced in the tropical area of the central and the eastern Pacific and that the rainfall zone, which corresponds to the SPCZ, is located farther northeast than in the SPCZ-control run. These features are similar to those observed and to the findings in the previous studies. Vincent (1994) indicated that the SST gradients impose pressure gradients and drive low-level winds, which result in moisture convergence. This mechanism seems to be important to induce the difference in the location of the SPCZ between 1997 and 1998.

Figure 11 shows the 10-day-averaged water vapor transport and precipitation, the wind speed at 200 hPa, and the geopotential height of the SPCZ-control run during the 10 days between the 21th and 30th days. These figures allow a clearer evaluation of the features of the SPCZ. Large cumulus activity was observed in the ITCZ north of the Australian continent in mid-December 1998 (Fig. 7); however, the convective activity area is located southeast of Papua New Guinea in the SPCZ-control run (marked by 4A in Fig. 11). The anticyclonic circulation over the cumulus convection (4C in Fig. 11), a trough on the eastern flank of the anticyclonic circulation (4D in Fig. 11), a zone of rainfall stretching southeastward, and a large water transport zone (4B in Fig. 11) also appear in the SPCZ-control run. These numerical simulations agree with the formation mechanism of the SPCZ explained by Matthews et al. (1996), although the location of ITCZ is different between them.

5. Sensitivity experiments

a. Effects of a zonal mean

The SPCZ-NHZM experiment was conducted assuming the zonal mean field of the Northern Hemisphere (NH) in late June 1998, instead of the zonal mean field of the Southern Hemisphere (SH) in the SPCZ-control run. This run will show the role of the upper-tropospheric wind in the formation process of the SPCZ. The assumed zonal mean field is shown in Fig. 7a, which corresponds to those during the mid- to late baiu seasons. The figure shows some differences in the zonal wind between the NH and SH. In the NH summer, the westerly wind is much weaker than that in the SH summer, and the upper-tropospheric jet is located in the higher latitudes. These differences in zonal winds are very important in the formation of the SPCZ and the baiu front, as explained in a later section.

The water vapor transport and precipitation, the wind speed at 200 hPa, and the geopotential height of the SPCZ-NHZM run are shown in Fig. 12. In the weak westerly run (SPCZ-NHZM), the SPCZ is unclear in the same region as in the strong westerly run (SPCZ-control). However, the rainfall zone, which seems to be a coastal rainfall zone, appears from the southeastern part of the Australian continent to New Zealand. The formation mechanism of the rainfall zone mentioned above can be explained as follows.

The continental heat low intensified because the ventilation effects were weaker in the NH summer field in late June. A poleward low-level jet then appeared on the eastern side of the Australian continent, forming a coastal rainfall zone. The characteristics of the continental-scale coastal rainfall zone are discussed in the following section.

b. Effects of orography

The “coastal rainfall zone” reproduced in the SPCZ-NHZM run may be formed by the heat low around the Australian continent. However, the simulated coastal rainfall zone is so weak that the formation mechanism is unclear. If high mountains such as the Tibetan Plateau existed in Australia, the continental heat low would be reinforced, and the rainfall zone might then be intensified. Actually, there are few mountains higher than 2000 m in Australia. To investigate the dependency of the rainfall zone on the continental heat low, a run that was identical to the SPCZ-NHZM, except that the orography was enhanced 5 times in the Australian continent (SPCZ-NHZM-topo), was conducted. The maximum height of the mountain in the calculation region was still less than 3000 m because the topography was smoothed in the 150-km spacing grid.

Figure 13 shows the water vapor transport and precipitation, the wind speed at 200 hPa, and the geopotential height of the SPCZ-NHZM-topo run. The coastal rainfall zone, which also appeared in the SPCZ-NHZM run, is formed from east of the continent to the far east, and a strong wind zone clearly appears at 200 hPa over the enhanced topography of Australia. This rainfall zone is different from the SPCZ.

c. Effect of the eastern part of the Eurasian continent

It has been speculated that the baiu front is formed as a result of the influence of various factors, such as the Indian monsoon, the cumulus convection in the tropical region, and the dynamic and thermal effects of the Tibetan Plateau. The complex and huge geography of East Asia may possibly be related to the regional climate. To compare the formation system between the coastal rainfall zone and the baiu front, a run which was identical to the SPCZ-NHZM, except for the replacement of land in the NH around East Asia, was conducted by shifting land in the NH 40° east and mirroring it symmetrically with respect to the equator (SPCZ-NHZM-mirror). This run is nearly a mirror symmetric conversion of the baiu front in late June in the NH, except for the SST and the global zonal mean, which are kept the same as they are assumed in the SPCZ-NHZM run. By these simulations, the differences in the formation mechanism between the SPCZ and the baiu front were investigated, considering the different features between the Eurasian and Australian continents.

Figure 14 shows the water vapor transport and precipitation, the wind speed at 200 hPa, and the geopotential height of the SPCZ-NHZM-mirror run. In comparison with the SPCZ-NHZM run, the poleward water vapor transport is extremely large along the coastal rainfall zone due to the large pressure gradient around the east of the continent (Fig. 14a). In the upper layer at 200 hPa (Fig. 14b), the upper-level jet meanders as a result of the interaction with the anticyclonic circulation over the Tibetan Plateau, and a trough is formed in the eastern flank of the Tibetan Plateau. In the lower layer at 850 hPa (Fig. 14c), the pressure gradient is extremely large over the continent on the eastern side of the Tibetan Plateau.

6. Discussion

The MJO is considered an important factor to form the SPCZ by many studies (e.g., Matthews et al. 1996). In the SPCZ-control run, the fluctuation of the ITCZ is confirmed. Generally, the fluctuation of the ITCZ is also known as one of the typical features of MJO. However, it is impossible to indicate whether the numerical model could produce the MJO or not because the mechanism of MJO is still unknown. More studies will be necessary to explain this point. Therefore, this study does not mention the relationship between the MJO and the SPCZ but only discusses the relationship between the fluctuation of the ITCZ and the SPCZ.

The SPCZ is weak in the SPCZ-NHZM run. The difference in the zonal mean field between the NH and the SH in the early summer should be an important factor for the formation of the SPCZ. The suggested reasons are as follows. In the SPCZ-NHZM run, 1) the heat low over the Australian continent is mostly formed due to the decrease of the ventilation effect of the lower-tropospheric wind; 2) the cold air advection, which enhances convective instability on the eastern flank of the cumulus convection in the ITCZ, is weakened due to the decrease of the baroclinicity of the zonal mean field; and 3) the vorticity in the lower troposphere is large at the latitudes near the Australian continent; therefore, the poleward low-level wind can possibly be quite strong. As shown in Fig. 7, the baroclinicity in the SH summer zonal mean field is much stronger than that in the NH summer in the subtropical area (20°–30°N). The strong baroclinicity is very important in the formation of the SPCZ, as mentioned in section 1. The importance of the subtropical jet for the formation of the SPCZ has also been indicated by Kiladis et al. (1989).

In the SPCZ-NHZM run, the poleward flow appears in the lower troposphere above eastern Australia associated with a weak continental heat low. A rainfall zone is formed along the poleward flow through the southeastern part of Australia to New Zealand. This poleward flow and rainfall zones are intensified by the enhanced topography run (SPCZ-NHZM-topo). In the latter run, the upper-tropospheric wind meanders remarkably inducing anticyclonic circulation over the western part of the continent, while a strong heat low is observed in the low-level. A clear coastal rainfall zone is formed on the eastern flank of the anticyclonic circulation extending to the east. These circulation and precipitation systems seem to be formed by the continental heat low rather than by the heat caused by the cumulus convection in the ITCZ.

This rainfall zone and the SPCZ have some different features. The intensity and location of the baiu front are quasi stationary, since it is formed by an almost stationary continental heat low, while those of the SPCZ are varying, in association with the fluctuation of heating by the ITCZ. The fluctuation of ITCZ is necessary to form the SPCZ. These differences also imply that the formation mechanism of the SPCZ and the rainfall zone mentioned above are different and that the latter is formed by the stationary continental heat low.

The SPCZ-NHZM-mirror and the SPCZ-NHZM-topo runs show the following similarities between the baiu front and the rainfall zone formed by the continental heat low. 1) Poleward wind is generated in the lower troposphere by the heat low over the continent. 2) The upper-tropospheric anticyclonic circulation is formed above the heat low. These systems are inconsistent with the system of the SPCZ. On the other hand, the water vapor transport along the east of the continent in the SPCZ-NHZM-mirror is much larger than that in the SPCZ-NHZM-topo. This difference is due to the difference in the horizontal scale of the continents and orography. Particularly, the Tibetan Plateau has an important role in intensifying the poleward lower-tropospheric flow.

In the SPCZ control run, the poleward part of the baiu front and SPCZ seem to be merged with the polar frontal zone. The SPCZ is simulated corresponding to the polar front, although the relationship between the baiu front and the polar front could not be clearly found. One of the reasons is speculated that the baroclinicity of the NH summer is smaller than that of SH summer season. However, details of the relationship are still unknown.

Additional experiments with fixed zenith angle are conducted to investigate the influences of the solar radiation. It is confirmed that the features of the simulated atmospheric circulation by additional experiments are the same as those of standard experiments, which is indicated in the section 4 and 5.

Some problems concerning the method used for the numerical experiments in the present study have been discussed in Yoshikane et al. (2001). The most severe problem is the estimation of the impacts of an artificially assumed lateral boundary condition on the physical process of the target phenomena. It is confirmed that the unnatural precipitation, temperature field, and wind field are caused around the lateral boundary in the ZM simulations. Those should be produced by the mismatch between the inner calculation region and the fixed lateral boundary. The internal gravity wave, which is produced by the unnatural precipitation around the lateral boundary, and reflected wave from the boundary are able to influence the atmosphere in the calculation region. In order to estimate the influence of the artificial boundary on the formation of the SPCZ, an additional experiment was conducted in the region extending about 20° zonally and 10° meridionally (figure not shown). The results indicated that the SPCZ is formed in almost the same location as the SPCZ-control run. The problems of the lateral boundary, such as the computational mode of internal gravity wave are not an essential issue in this study.

7. Conclusions

Sensitivity tests were conducted using a regional climate model to investigate the difference in the formation mechanisms between the SPCZ and the baiu front, which take place in the South Pacific during the austral summer and in East Asia during the boreal early summer, respectively. Zonally uniform and temporally constant atmospheric fields obtained from ECMWF data are assumed for the initial and lateral boundary conditions in the numerical experiments. These idealistic experiments can simulate the disturbances that are generated only by the interaction between surface boundary conditions and the zonal mean flow. This could be one of the advantage points of the study using a regional climate model as compared with a GCM.

Our experiment showed that a rainfall zone similar to the SPCZ is formed by the interaction between the zonal mean field and the surface boundary conditions. The primary factor for the formation mechanism of the SPCZ is the horizontal distribution of SST in the tropical Western Pacific. The land surface process over the Australian continent is not essential.

Other experiments were conducted assuming initial/boundary conditions with different intensity of zonal wind speed and baroclinicity, which are assumed to be zonal mean fields of the NH in the boreal summer. Results of these experiments suggested that the mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even that the SPCZ is not always formed when the zonal wind is too weak. On the other hand, the heat contrast between the Australian continent and the South Pacific Ocean contributes to form another rainfall zone when the zonal flow is very weak. Under these conditions, the SPCZ becomes unclear, and the rainfall zone appears from the southeastern part of Australia to the east of New Zealand. The latter rainfall zone intensifies if the orography in the Australian continent is magnified. This rainfall zone is formed by the heat contrast between land and ocean, having some similarity to the baiu front. A continent as large as Eurasia creates a clearer rainfall zone even when the zonal flow is stronger.

The baiu front seems to be one of the rainfall zones caused by the heat contrast between land and ocean that is different from the SPCZ in the formation mechanism. The baiu front is generated by an upper-level wind moving toward the equator and a low-level poleward wind on the eastern flank of the continent caused by the heat low due to the orographic thermal effects of the Eurasian continent and the Tibetan Plateau. The upper-level wind moving toward the equator and the low-level poleward wind generating the SPCZ in the SH are influenced by the heating due to the release of the latent heat from the cumulus convection in the tropical region.

From the present study, it can be concluded that the mechanism of the baiu front is different from that of the SPCZ and that the former is greatly influenced by the heat low over the Asian continent. The intensity and location of baiu front is quasi stationary, since it is formed by the fixed continental heat low, while those of SPCZ do change with the fluctuating heating over the ITCZ, which is necessary to form the SPCZ. These characteristics of the reproduced baiu front and the SPCZ in the model are similar to the actual ones.

Acknowledgments

We are grateful to Prof. T. Yasunari, Dr. T. Tomita, Dr. S. Emori, and Dr. T. Enomoto of the Frontier Research System for Global Change for discussing our study. We would also express our thanks to Prof. T. Nakajima of the University of Tokyo, and Dr. A. Numaguti for providing valuable technical comments. The data used are provided by the European Centre for Medium-Range Weather Forecasts (ECMWF).

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Fig. 1.
Fig. 1.

(a) Ten-day-averaged precipitation of 35th and 36th pentad of CMAP data, (b) vertical accumulated water vapor transport, (c) geopotential height at 850 hPa, and (d) wind field at 200 hPa (vector) and wind speed (shadow) of ECMWF objective analysis data in late Jun 1998. The contour interval of the geopotential height is 10 m.

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 2.
Fig. 2.

(a) Ten-day-averaged precipitation of 72d and 73rd pentad of CMAP data, (b) vertical accumulated water vapor transport, (c) geopotential height at 850 hPa, and (d) wind field at 200 hPa (vector) and wind speed (shadow) of ECMWF objective analysis data in late Dec 1998. The contour interval of the geopotential height is 10 m; only contours over 1400 m are drawn

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 3.
Fig. 3.

Observed monthly SST distribution in (a) Jun 1998, (b) Dec 1997, (c) and Dec 1998 (Reynolds and Smith 1994). The shadow indicates the warm SST area more than 300 K (27°C). The contour interval is 2 K (2°C)

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 4.
Fig. 4.

(a) Monthly averaged precipitation of CMAP data, (b) vertical accumulated water vapor transport, and (c) wind field at 200 hPa (vector) and wind speed (shadow) of ECMWF objective analysis data in Dec 1997

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 5.
Fig. 5.

The same as Fig. 4, except for Dec 1998

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 6.
Fig. 6.

Conceptual figure of the zonal mean simulation: (a) 10-day-averaged zonal mean wind at 200 hPa of ECMWF data; (b) the interpolated zonal mean wind field to polar-stereographic coordinate as initial and boundary condition. (c) Wind field after 38 days from the start of simulation

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 7.
Fig. 7.

Vertical cross section of 10-day-averaged zonal mean wind (shadow) and temperature (contour) in NH in (a) late Jun, and Southern Hemisphere in (b) mid-Dec 1998

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 8.
Fig. 8.

(a) Thirty-day-averaged precipitation (shadow) and vertical accumulated water vapor transport (vector), (b) wind field (vector) and wind speed (shadow) at 200 hPa, (c) and geopotential height at 850 hPa of the result of the baiu-control run

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 9.
Fig. 9.

(a) Thirty-day-averaged precipitation (shadow) and vertical accumulated water vapor transport (vector), (b) wind field at 200 hPa (vector) and wind speed (shadow), (c) and geopotential height at 850 hPa of the result of SPCZ-control run. Only contours over 1400 m are drawn

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 10.
Fig. 10.

Same as Fig. 9, except for SPCZ-SST9712 run

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 11.
Fig. 11.

Same as Fig. 9, except for 10-day-averaged results from 21st to 30th day from simulation start of SPCZ-control run

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 12.
Fig. 12.

Same as Fig. 9, except for ZM-NHZM run

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 13.
Fig. 13.

Same as Fig. 9, except for ZM-NHZM-topo run

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Fig. 14.
Fig. 14.

Same as Fig. 9, except for ZM-NHZM-mirror run

Citation: Journal of the Atmospheric Sciences 60, 21; 10.1175/1520-0469(2003)060<2612:FMOTSS>2.0.CO;2

Table 1.

Model features

Table 1.
Table 2.

Calculation condition

Table 2.
Table 3.

Descriptions of zonal mean field, SST, and orography for different types of model runs

Table 3.
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