Role of the Indochina Peninsula Narrow Mountains in Modulating the East Asian–Western North Pacific Summer Monsoon

Chi-Hua Wu Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan

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Huang-Hsiung Hsu Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan

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Abstract

Unrealistic topographic effects are generally incorporated in global climate simulations and may contribute significantly to model biases in the Asian monsoon region. By artificially implementing the Arakan Yoma and Annamese Cordillera—two south–north-oriented high mountain ranges on the coasts of the Indochina Peninsula—in a 1° global climate model, it is demonstrated that the proper representation of mesoscale topography over the Indochina Peninsula is crucial for realistically simulating the seasonality of the East Asian–western North Pacific (EAWNP) summer monsoon.

Presence of the Arakan Yoma and Annamese Cordillera helps simulate the vertical coupling of atmospheric circulation over the mountain regions. In late May, the existence of the Arakan Yoma enhances the vertically deep southwesterly flow originating from the trough over the Bay of Bengal. The ascending southwesterly flow converges with the midlatitude jet stream downstream in the southeast of the Tibetan Plateau and transports moisture across the Indochina Peninsula to East Asia. The existence of the Annamese Cordillera helps the northward lower-tropospheric moisture transport over the South China Sea into the mei-yu–baiu system, and the leeside troughing effect of the mountains likely contributes to the enhancement of the subtropical high to the east. Moreover, the eastward propagation of wave energy from central Asia to the EAWNP suggests a dynamical connection between the midlatitude westerly perturbation and mei-yu–baiu. Including the Annamese Cordillera also strengthens a Pacific–Japan (PJ) pattern–like perturbation in late July by enhancing the cyclonic circulation (i.e., monsoon trough) in the lower-tropospheric western North Pacific. This suggests the contribution of the mountain effects to the intrinsic variability of the summer monsoon in the EAWNP.

Corresponding author address: Chi-Hua Wu, Research Center for Environmental Changes, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei 115, Taiwan. E-mail: chhwu@gate.sinica.edu.tw

Abstract

Unrealistic topographic effects are generally incorporated in global climate simulations and may contribute significantly to model biases in the Asian monsoon region. By artificially implementing the Arakan Yoma and Annamese Cordillera—two south–north-oriented high mountain ranges on the coasts of the Indochina Peninsula—in a 1° global climate model, it is demonstrated that the proper representation of mesoscale topography over the Indochina Peninsula is crucial for realistically simulating the seasonality of the East Asian–western North Pacific (EAWNP) summer monsoon.

Presence of the Arakan Yoma and Annamese Cordillera helps simulate the vertical coupling of atmospheric circulation over the mountain regions. In late May, the existence of the Arakan Yoma enhances the vertically deep southwesterly flow originating from the trough over the Bay of Bengal. The ascending southwesterly flow converges with the midlatitude jet stream downstream in the southeast of the Tibetan Plateau and transports moisture across the Indochina Peninsula to East Asia. The existence of the Annamese Cordillera helps the northward lower-tropospheric moisture transport over the South China Sea into the mei-yu–baiu system, and the leeside troughing effect of the mountains likely contributes to the enhancement of the subtropical high to the east. Moreover, the eastward propagation of wave energy from central Asia to the EAWNP suggests a dynamical connection between the midlatitude westerly perturbation and mei-yu–baiu. Including the Annamese Cordillera also strengthens a Pacific–Japan (PJ) pattern–like perturbation in late July by enhancing the cyclonic circulation (i.e., monsoon trough) in the lower-tropospheric western North Pacific. This suggests the contribution of the mountain effects to the intrinsic variability of the summer monsoon in the EAWNP.

Corresponding author address: Chi-Hua Wu, Research Center for Environmental Changes, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei 115, Taiwan. E-mail: chhwu@gate.sinica.edu.tw

1. Introduction

The Asian summer monsoon system consists of two major and distinct components: the South Asian and the East Asian–western North Pacific (EAWNP) summer monsoons. The former is strongly influenced by high-rising topography in South Asia because of the anchoring effect of precipitation near the topography and mechanistically induced perturbations (Wu and Zhang 1998; Rodwell and Hoskins 2001; Kitoh 2002; Xie et al. 2006; Wu et al. 2007; Molnar et al. 2010; Watanabe and Yamazaki 2012; Wu et al. 2012). By contrast, the EAWNP summer monsoon is more strongly affected by the air–sea and extratropical–tropical interaction because of its location to the east of the high-rising topography in South Asia and over the region where the Asian continent massif and the North Pacific meet (Liang and Wang 1998; Wu and Wang 2001; Suzuki and Hoskins 2009).

Most of the previous studies focused on the topographic effect of the major mountain systems (e.g., the Tibetan Plateau and Himalayas). Recently, there has been a renewed interest in the topographic effect in understanding the role of the south–north-oriented narrow mountain ranges south of the Tibetan Plateau (e.g., the Western Ghats, the Arakan Yoma, and the Annamese Cordillera; Gadgil 1977; Xie et al. 2006). Regional and global model simulations have demonstrated the large-scale and downstream effect of these narrow mountain ranges (Qi and Wang 2012; Wang and Chang 2012; Wu et al. 2014). While these narrow mountains can be relatively well resolved in regional models, they are often missing in the global atmospheric model. Wu et al. (2014) demonstrated that the Asian summer monsoon onset over the Bay of Bengal during late May cannot be accurately simulated using a model with a 100-km resolution unless the narrow Arakan Yoma northwest of Myanmar, which is not resolved in the model, is artificially prescribed. The key improvements are the midtropospheric trough, mountain-anchoring precipitation, and enhanced near-surface southwesterly that are induced by this specific narrow mountain range.

The Indochina Peninsula is a region of rough terrain characterized by many meridionally oriented mesoscale mountain ranges. The Annamese Cordillera, with the highest point at approximately 3 km, is another major mountain range located along the east coast of Vietnam and is known to produce heavy rainfall in autumn (Chen et al. 2012) in the prevailing easterly flow or during landfall of tropical cyclones. The mountains also have a strong influence on the summer monsoon flow and air–sea interaction in the South China Sea (Xie et al. 2006; Qi and Wang 2012). In view of the significant effect of the Arakan Yoma demonstrated by Wu et al. (2014), one wonders how the Annamese Cordillera affects the Asian summer monsoon and how it intertwines with the effect of the Arakan Yoma. Whereas the substantial effect of the Annamese Cordillera has been demonstrated in Qi and Wang (2012), its role in a global model has not been explored. This study is the second in a series of papers in which we examine the role of the Indochina Peninsula topography in modulating the Asian summer monsoon. In addition to the Arakan Yoma, the effect of the Annamese Cordillera is also investigated, individually and combined with the Arakan Yoma. As will be shown later, the effect of the Arakan and Annamese mountain ranges on the EAWNP summer monsoon is considerable, particularly the complex monsoonal seasonality (Murakami and Matsumoto 1994).

The rest of the paper is organized as follows: Model and data are described in section 2. In section 3, we introduce the seasonality of the EAWNP summer monsoon, discuss its correspondence with the South Asian monsoon, and point out potential model biases due to the representation of topography in the model. Model responses to the mountain ranges are explored in section 4. We further discuss in section 5 the individual effects of the two mountain ranges and the oceanic feedback. Conclusions and final remarks are presented in section 6.

2. Model and data

Data used in this study include 1) precipitation from the 3B42 version 7 of the Tropical Rainfall Measuring Mission (TRMM) in 1998–2014 (Kummerow et al. 1998), 2) ocean surface wind speed from the TRMM Microwave Imager (TMI) in 1998–2014 (Gentemann et al. 2010), and 3) atmospheric circulation fields from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) (Saha et al. 2010) in 1979–2010. Precipitation and ocean surface wind speed are available at a 3-hourly temporal resolution and a 0.25° latitude–longitude resolution; the 6-hourly circulation fields are at a 0.5° latitude–longitude resolution. Topography data were obtained from the U.S. Geological Survey (USGS) at an approximately 0.16° latitude–longitude resolution.

The Community Atmospheric Model (CAM) version 5.1 of the National Center for Atmospheric Research (NCAR) coupled with the slab ocean model (SOM) (http://www.cesm.ucar.edu) was used to simulate the atmospheric response to the artificially prescribed mountains. Parameterizations in CAM include moist processes (deep/shallow convection and large-scale condensation), cloud macrophysics/microphysics and radiation, surface models, and turbulent mixing (planetary boundary layer, vertical diffusion, and gravity wave drag). For more details on CAM, refer to Neale et al. (2013). The CAM–SOM simulations were integrated using the finite volume dynamical core with 30 vertical levels in CAM at a horizontal resolution of approximately 1°. Observed net heat transport by ocean current was prescribed (Q flux) to keep the simulated sea surface temperature (SST) close to the observed. The boundary and initial conditions of the simulations were taken from the Community Earth System Model (CESM) preindustrial control experiment (Vertenstein et al. 2010).

The 1° topography in the CAM–SOM control simulation does not resolve the Arakan Yoma and Annamese Cordillera. Thus, the Arakan Yoma (AR) and Arakan Yoma and Annamese Cordillera (AA) simulations were conducted by imposing the Arakan Yoma and both the Arakan Yoma and Annamese Cordillera, respectively, in the 1° model for comparison. Only the surface at the location of the prescribed mountains was artificially elevated, by replacing topography data in the model (also refer to the CESM website). The integration length is 50 years, and the outputs for years 31–50 were analyzed. The potential effect of spatial elevation discontinuity by the small width of the added mountains might be therefore minimized after long-term adjustment. The effectiveness of the CAM–SOM in simulating the monsoon is readily observed through a simple comparison with the observations (Danabasoglu and Gent 2009). Because the focus is on the mountain effects, detailed evaluation of the model performance is not discussed hereafter unless necessary.

3. Potential linkage between the Asian monsoon and topography in late May and late July

The South Asian summer monsoon is characterized by orographic precipitation, which often occurs in the windward side of terrain. The topographically induced precipitation can further affect the evolution of the monsoon circulation. Generally, the East Asian summer monsoon also begins in late May in the form of mei-yu–baiu, and its topographic dependence may be inferred from Figs. 1a and 1b. First, the deep southwesterly flow originating from the trough over the Bay of Bengal, which as demonstrated by Wu et al. (2014) is induced by the Arakan Yoma, transports moisture across the Indochina Peninsula to East Asia (Sampe and Xie 2010). Second, this ascending southwesterly flow meets the midlatitude jet stream in the southeast of the Tibetan Plateau (including the Yunnan–Guizhou Plateau). The latter reflects the mechanical effect of the plateau on the mei-yu–baiu (Wu and Chou 2013). Third, the low-level flow around the Annamese Cordillera is associated with the northward lower-tropospheric moisture transport over the South China Sea (Fig. 1c) into the mei-yu–baiu system. We will show later that the existence of the Annamese Cordillera enhances the near-surface southerly winds to the east of the mountain range seen in Fig. 1c. We will also demonstrate that the existence of the Annamese Cordillera also accelerates the midtropospheric southerly flows above the mountains (Fig. 1c) and contributes substantially to the northward moisture supply to the late May mei-yu–baiu that is usually located over the northern South China Sea.

Fig. 1.
Fig. 1.

(a) Precipitation (mm day−1; shadings) and 500-hPa winds (m s−1; red vectors for vertical velocity higher than 3 Pa min−1); (b) the 925-hPa moisture convergence (g kg−1 day−1; shadings) and flux (g kg−1 m day−1; vectors); (c) (top) vertical circulations (shadings for meridional wind speeds; unit: m s−1) and precipitation along the 24°N band in the Arakan Yoma region and the 15°N band in the Annamese Cordillera region, and (bottom) the 925-hPa wind vectors and ocean surface wind speeds (m s−1; shadings) from 21 May to 4 Jun. (d)–(f) As in (a)–(c), but from 25 Jul to 8 Aug. Gray and black (Arakan and Annamese Mountains) thick contour lines denote the elevation = 0.8 km, and black bars denote the mountain ranges.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

The South Asian summer monsoon reaches its mature phase in mid-June when precipitation expands from the Bay of Bengal to the southern Tibetan Plateau and the midlevel monsoon trough over the Bay of Bengal is replaced by the monsoon low (Fig. 1d). By contrast, the EAWNP summer monsoon enters another monsoon phase one month later (Wu and Chou 2012; Hsu et al. 2014). The EAWNP summer monsoon transition is characterized by the northward shift of the western North Pacific high and the deepened monsoon trough south of the high (Figs. 1d,e). This high–trough structure appearing from around 25 July to 8 August (hereafter late July) is associated with the end of the mei-yu–baiu rainy season and the western North Pacific monsoon onset (Wu 2002; Suzuki and Hoskins 2009). The regional circulation behaviors over the mountain area in late July (Fig. 1f) may only partly differ from that in late May (Fig. 1c). However, the large-scale monsoonal circulation has markedly changed in late July, particularly in the EAWNP. How the existence of the mountain ranges modulates the monsoon in this stage is an interesting issue that is not well understood and will be explored in later sections.

4. Effects of the Arakan and Annamese Mountains

Figure 2 displays simulated precipitation in the local mountain regions (box regions in Figs. 2a,b) and the downstream EAWNP region along the 125°–145°E longitudinal band. Compared with the control simulation, inclusion of the more realistic mountain ranges in the model better simulates the strength and temporal evolution of precipitation over the mountain regions (Figs. 2c,d). The downstream EAWNP precipitation characteristic during late May (Fig. 2e) and late July (Fig. 2f) is also simulated more closely to the observed in the simulation that considers more realistic mountain ranges. In Figs. 2e and 2f, the control simulation underestimates the late May mei-yu–baiu precipitation across 20°–30°N and the late July western North Pacific monsoon precipitation over 15°–25°N while overestimating the precipitation across 25°–35°N in late July. The improvement in late July is particularly evident because of the better-simulated subtropical high and monsoon trough in the western North Pacific. It will be shown later that this improvement may be associated with a stronger Pacific–Japan (PJ) pattern–like circulation, which is considered as an intrinsic dynamical mode (Nitta 1987; Kosaka and Nakamura 2006, 2010).

Fig. 2.
Fig. 2.

(a),(b) Topography (km; only higher than 0.8 km is plotted); shadings for 0.16° grid size, red contours for 1° control simulation, and black contours for the mountains added for the AA simulation. (c),(d) Time series of area-mean precipitation (mm day−1) across 88°–96°E, 18°–26°N and 105°–120°E, 14°–20°N [box regions in (a) and (b)]. (e),(f) Precipitation in the 125°–145°E band from 21 May to 4 Jun and from 25 Jul to 8 Aug, respectively; black lines for TRMM data, orange dashed lines for the CAM–SOM control simulation, and blue lines for the CAM–SOM AA simulation.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

To reveal the mountain effects on the EAWNP monsoon, particularly the large-scale circulation, we explore in the following the response of the local and synoptic circulation, moisture transport, and the planetary-scale upper-tropospheric circulation to the existence of more realistic mountain ranges.

a. Synoptic-scale circulation and moisture response

In late May, the simulated mei-yu–baiu precipitation band in the control simulation is located farther north than the observed (Figs. 3a and 1a), resulting in a dry bias over the northern South China Sea and south of Taiwan and a wet bias south of Japan. These precipitation biases are reduced to a certain extent in the AA simulation (Figs. 3a,b). Inclusion of the two mountain ranges helps the reproduction of mei-yu–baiu precipitation by enhancing 1) the midlevel troughing effect southeast of the Tibetan Plateau (trough anomaly embedded in the westerly flow; blue dashed line in Fig. 3b) and 2) the low-level moisture transport over the Indochina Peninsula and the South China Sea (Fig. 3d). The enhanced mei-yu–baiu precipitation southeast of the plateau (east of 110°E) is closely related to the increased low-level moisture convergence. The enhanced rainband over the northern South China Sea and the subtropical western North Pacific is associated with the enhanced monsoon trough and subtropical high in the 5°–25°N band. It is particularly interesting to note that low-level cyclonic circulation and precipitation (and moisture convergence) are near the two mountain ranges and are enhanced (Fig. 3d), a feature similar to leeside troughing. The one near the Arakan Yoma enhances the moisture transport over the northern Indochina Peninsula, whereas the one near the Annamese Cordillera enhances the northward transport over the South China Sea. The leeside troughing induced by the Annamese Cordillera likely enhances cyclonic circulation over the western South China Sea, which in turn induces stronger anticyclonic flow (and subtropical high) to the east. This improvement confirms again that the local narrow mountains have a strong and remote effect on a scale much larger than its size.

Fig. 3.
Fig. 3.

(left) CAM–SOM control simulation and (right) AA minus control simulations: (a),(b) precipitation (mm day−1; shadings) and 500-hPa winds (m s−1; red vectors for vertical velocity higher than 3 Pa min−1); (c),(d) 925-hPa moisture convergence (g kg−1 day−1; shadings) and flux (g kg−1 m day−1; vectors) from 21 May to 4 Jun. (e)–(h) As in (a)–(d), but from 25 Jul to 8 Aug. Gray and black (Arakan and Annamese Mountains) thick contour lines denote the elevation = 0.8 km. Dots denote regions where the precipitation or moisture convergence difference attains a confidence level of 90%. Red and thick vectors indicate upward air motion.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

According to the above results, we speculate that the vertical coupling of atmospheric circulation (i.e., midlevel westerly and low-level southwesterly) is crucial for the early summer mei-yu–baiu, and the existence of the Arakan and Annamese mountain ranges has a substantial influence in the local and synoptic scales. Nevertheless, we also find that the effects of the two mountains individually might be of the opposite sense. We discuss this later in section 5a.

To further pinpoint the topographic effects, Fig. 4 shows the vertical structure of atmospheric circulation near the mountains (cf. observations in Fig. 1c and Fig. 1f). In late May, the existence of the mountain ranges splits and perturbs both zonal and meridional winds. The AA simulation (by including the mountains in it) simulates the local and synoptic circulation more realistically than the control simulation. Across the Arakan Yoma (24°N; Figs. 4a–c) the mountain-induced precipitation associated with the expected ascent/descent is evident, and lower-tropospheric southerly winds east of the mountain are markedly enhanced because of the leeside troughing. These are consistent with the increase of southwesterly flow over the Indochina Peninsula and moisture transport to East Asia. Across the Annamese Cordillera (15°N; Figs. 4d–f), in addition to the mountain-induced precipitation, the southwesterly flow over the South China Sea is stronger and deeper from the lower to midtroposphere in the AA simulation. The westerly and southerly winds leeside of the Annamese Cordillera occur only below 700 hPa in the control simulation, whereas they occur at much higher levels (reaching 400 hPa) when the mountain is present.

Fig. 4.
Fig. 4.

Streamlines and meridional wind speeds (m s−1; shadings) in a pressure–longitude plane and precipitation (mm day−1; blue lines) along the (top) 24°N band and (bottom) 15°N band from 21 May to 4 Jun. (a),(d) Control simulation, (b),(e) AA simulation, and (c),(f) AA minus control simulations. Black bars denote topography. Circles denote that the meridional wind difference has a 90% confidence level.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

In late July, the control simulation has a marked dry bias in the subtropical Asian monsoon region—that is, the Bay of Bengal, the Indochina Peninsula, and the subtropical western North Pacific (Figs. 3e and 1d). In the EAWNP, the underestimated precipitation is related to the simulated weak monsoon over the subtropical western North Pacific, as indicated by the overestimated subtropical high and insignificant monsoon trough in Figs. 3e and 3g. Again, the precipitation bias is reduced when the mountains are included. Inclusion of the two mountains increases the monsoon precipitation notably near the mountains and over the subtropical western North Pacific and decreases the precipitation in East Asia south of 5°N and north of 30°N. This rainfall anomaly and cyclonic/anticyclonic circulation pair exhibit a PJ-like or a rainfall tripole pattern, which are believed to arise because of intrinsic dynamics (Hsu and Lin 2007; Kosaka and Nakamura 2006, 2010). In Figs. 3f and 3h, the cyclonic circulation anomalies in the lower troposphere and midtroposphere exhibit an equivalent barotropic structure.

As in late May, the existence of the mountain ranges splits and perturbs the monsoon flows in late July, and the synoptic circulation is better reproduced. Nevertheless, the mountain effect on synoptic circulation in this period differs from that in late May. Across the Arakan Yoma (24°N; Figs. 5a–c), although the mountain-induced precipitation and vertical motion in local scale is considerably large, the mountain effect plays a relatively minor role in improving the simulation of a well-developed South Asian monsoon circulation (Figs. 3f,h). By contrast, the response to the Annamese Cordillera is in a much larger scale and closely related to the downstream change in the EAWNP—namely, an enhanced monsoon trough over the South China Sea and a weakened subtropical high over the subtropical western North Pacific (Figs. 3f,h and 5d–f).

Fig. 5.
Fig. 5.

As in Fig. 4, but from 25 Jul to 8 Aug.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

b. Planetary-scale and monsoonal circulation response

We furthermore explore the large-scale circulation response by starting with the upper-tropospheric perturbation. In late May, the upper-tropospheric high (anticyclone) is located over the subtropical region with its high ridge over approximately 20°N. The center of the anticyclone is over the Indochina Peninsula, closely related to the vertical circulation coupling during the Bay of Bengal monsoon onset. To the north of the anticyclone the jet stream is strong over the Asian continent and North Pacific (Fig. 6a). Without the mountain ranges in the control simulation (Fig. 6b), the upper-level jet stream in central Asia and the east of Japan is weaker than observed, consistent with the underestimated upper-level subtropical high (Fig. 6b). Compared with their state in the control simulation, including the mountains enhances the midlatitude jet stream and the high in the upper troposphere, which are still weaker than but more comparable to the observed (Figs. 6a,c).

Fig. 6.
Fig. 6.

Streamlines and zonal wind speeds (m s−1; shadings) of (a),(e) reanalysis data, (b),(f) CAM–SOM control simulation, (c),(g) AA simulation, and (d),(h) AA simulation minus control simulation at 200 hPa (left) from 21 May to 4 Jun and (right) from 25 Jul to 8 Aug. Contours denote the 200-hPa geopotential heights of 12 400–12 480 m and 12 450–12 550 m. Dots denote regions where the wind speed difference attains a confidence level of 90%.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

The upper-tropospheric circulation response to the mountains in late May is a wavelike pattern in the 30°–50°N latitudinal band, which is characterized by anomalous cyclones over 50°–80° and 110°–140°E and an anticyclone in between (Fig. 6d). This result is consistent with previous studies that related similar wavelike perturbation to mountain forcing (Krishnan and Sugi 2001; Wu 2002; Ding and Wang 2007; Cheng et al. 2008). Interestingly, the mountain-induced wavelike perturbation evidently has a strong effect on the circulation and precipitation in the EAWNP to produce a more realistic simulation. The process leading to this impact can be demonstrated by the wave activity flux (WAF) shown in Fig. 7. The WAF formulated by Takaya and Nakamura (1997, 2001) is useful in identifying the origin and energy propagation of the large-scale circulation perturbation (Honda et al. 1999; Hsu and Lin 2007). The horizontal components of WAF for stationary eddies (Takaya and Nakamura 1997) are defined as p/2U, where ψ denotes the geostrophic streamfunction, subscripts denote partial derivatives, p = pressure/1000 hPa, (u, υ) are the time mean of horizontal geostrophic velocities, and U is the magnitude of basic state wind speed. In our calculation of WAF the control simulation was taken as the basic state, and the perturbation is defined as the difference between the mountain and control simulations. In late May, the wavelike pattern existing from the upper to lower levels exhibits an equivalent barotropic structure in central Asia and a slight tilt in the EAWNP region (Figs. 7a–c). The WAF reveals an eastward propagation of wave energy from central Asia to the EAWNP and suggests a dynamical connection between the midlatitude westerly perturbation and the EAWNP monsoon.

Fig. 7.
Fig. 7.

Wave activity fluxes and streamfunctionlike geopotential heights (10−6 m s−2; shadings) at (top) 200, (middle) 500, and (bottom) 850 hPa, (a)–(c) from 21 May to 4 Jun and (d)–(f) from 25 Jul to 8 Aug.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

The mountain effect induces three-dimensional circulation perturbations in the EAWNP that are vertically coupled in late May. One example is evident across 20°–30°N over East Asia. The existence of mountains enhances the lower-level southwesterly flow over the South China Sea (Fig. 3d), which increases moisture transport to the region below the midlevel trough (Fig. 3b) and the upper-level westerly flow (Fig. 6d) over the northern South China Sea and the western North Pacific across 20°–30°N. This enhanced vertical circulation coupling would create a favorable condition for the development of the mei-yu–baiu rainband in late May.

In late July, the upper-level circulation response displays primarily a meridionally wavelike pattern, rendering the weakening and northward shift of the East Asian jet stream (Figs. 6e–h). The wave source in the upper troposphere is located over northeastern Asia, apparently as a consequence of the shift of the East Asian jet stream (Fig. 7d); the wave source in the lower troposphere (Fig. 7f) can be related to a deeper monsoon trough and enhanced precipitation (Figs. 3f,h) over the northern Philippine Sea. These circulation responses consistently confirm that the mountainous effect can be substantial on the midsummer EAWNP monsoon transition. This finding also implies that the mountain effects demonstrated in this study contribute to enhancing the formation of the PJ pattern and the tripole rainfall pattern in East Asia, which are suggested as an intrinsic mode of the summer EAWNP monsoon.

5. Individual mountain effect and ocean feedback

a. Effects of the individual mountains

By comparing the atmospheric response between the AA simulation and AR simulation, the individual effect of the two mountain ranges is further explored. Comparing Figs. 3b and 3d and Fig. 8 indicates that in late May, the Arakan mountain effect contributes mainly to the improvement in South Asia (e.g., vertical coupling of atmospheric circulation south/southeast of the Tibetan Plateau). By contrast, the inclusion of the Annamese Cordillera induces the lower-level cyclonic circulation over the Indochina Peninsula, lower-level anticyclonic circulation in the subtropical western North Pacific, and stronger westerly flow and a rainband between 20° and 30°N in the EAWNP.

Fig. 8.
Fig. 8.

As in Figs. 3b,d, but for (a),(b) AR minus control simulations and (c),(d) AA minus AR simulations.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

We also note that in some regions the atmospheric responses to the Arakan Yoma and Annamese Cordillera individually are likely to be in opposite phase. In late May, inclusion of only the Arakan Yoma enhances and better simulates the Bay of Bengal monsoon onset (Figs. 8a,b), whereas inclusion of only the Annamese Cordillera induces 500-hPa flow in the reverse direction but much weaker (Figs. 8c,d). In the EAWNP domain, although including the Annamese Cordillera simulates the late May mei-yu–baiu closer to the observed (20°–25°N), the East Asian precipitation is overestimated near 30°N. This overestimation is somewhat canceled out by the response to the Arakan Yoma. Overall, the mei-yu–baiu precipitation and circulation can be better reproduced when the Arakan and Annamese mountain effects are included than when either of them is considered alone.

Unlike their marked effect on the late May South Asian monsoon circulation, the Arakan Yoma may play a minor role in modulating the circulation in late July (Figs. 3f,h and 9a). The most prominent atmospheric responses in the EAWNP domain in the AA simulation can be primarily attributed to the inclusion of the Annamese Cordillera.

Fig. 9.
Fig. 9.

As in Figs. 3f,h, but for (a),(b) AR minus control simulations and (c),(d) AA minus AR simulations.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

b. Oceanic feedback

Similar simulations, but without coupling to the SOM (i.e., forced by prescribed SST), were also conducted (figure not shown). Comparing the results between the CAM–SOM and CAM-alone simulations, the oceanic feedback could be roughly evaluated. It is found that the atmospheric model with the oceanic feedback, even only from the SOM, produces a better simulation of EAWNP monsoon precipitation and circulation. Without air–sea coupling, the improvement in the simulation of the midlevel trough southeast of the Tibetan Plateau in late May (as seen in Fig. 3b) is insignificant, rendering no improvement in the vertical coupling of atmospheric circulation. We also find that the atmospheric response to the mountains is in opposite phase over the EAWNP region in late July compared to the coupled simulation. This finding is consistent with previous studies in which the important role of oceanic feedback in the EAWNP monsoon simulation was identified (Okajima and Xie 2007; Lau and Ploshay 2009). Further budget analysis is needed for evaluating the exact effect of oceanic feedback in response to the mountains.

6. Conclusions and final remarks

By simulations with and without the narrow Arakan and Annamese mountain ranges in the 1° global climate model, we demonstrate that the representation of mesoscale topography over the Indochina Peninsula, with air–sea interaction, is crucial for realistically simulating the seasonality of the EAWNP summer monsoon. The results further suggest the strong connection between the mountain-induced perturbations in South Asia and the EAWNP, perhaps also the monsoon–midlatitude interaction.

Figure 10 summarizes the major atmospheric response to the two narrow mountain ranges. In the beginning of the summer monsoon around late May (Fig. 10a), inclusion of the two mountain ranges helps the reproduction of the mei-yu–baiu precipitation and circulation, and the improvement is better when the mountains are both included than when either of them is considered alone. The existence of the Arakan Yoma benefits the vertical coupling of atmospheric circulation by enhancing the midtropospheric troughing effect and lower-tropospheric southwesterly flow. The Bay of Bengal monsoon onset can be reproduced much better when the Arakan Yoma are included (Wu et al. 2014). Corresponding to the well-simulated Bay of Bengal monsoon onset, the deep southwesterly flow originating from the trough over the Bay of Bengal can transport moisture across the Indochina Peninsula to East Asia more realistically in the model. The ascending southwesterly flow converging with the midlatitude jet stream in the southeast of the Tibetan Plateau is simulated more closely to the observed as a result. This might be also related to the more realistically simulated mechanical effect of the Tibetan Plateau on mei-yu–baiu. The existence of the Annamese Cordillera furthermore helps the northward lower-tropospheric moisture transport over the South China Sea into the mei-yu–baiu system. The accelerated near-surface southerly flow in the lee side of the mountain and midtropospheric southerly flow above the mountain are better simulated when the Annamese Cordillera are included.

Fig. 10.
Fig. 10.

Schematic diagram of the atmospheric response (200, 500, and 850 hPa) to the mountains for (a) late May and (b) late July. Orange color denotes the general features in each period, and blue color denotes the mountain-induced changes. H denotes high pressure and AC/C denotes the anticyclonic/cyclonic circulation anomaly.

Citation: Journal of Climate 29, 12; 10.1175/JCLI-D-15-0594.1

The well-simulated vertical circulation coupling in the westerly and southwesterly flows southeast of the Tibetan Plateau, when including the narrow mountain ranges, results in the improvement of the late May mei-yu–baiu precipitation. In terms of the large-scale circulation, the wave activity flux reveals an eastward propagation of wave energy from central Asia to the EAWNP, suggesting a dynamical connection between the midlatitude westerly perturbation and the mei-yu–baiu system (Fig. 10a).

The effect of the narrow mountains (mainly the contribution of the Annamese Cordillera) on the late July EAWNP monsoon transition is also confirmed. Before the monsoon transition, the leeside troughing effect of the Annamese Cordillera likely enhances cyclonic circulation over the western South China Sea, which in turn induces stronger anticyclonic flow (and subtropical high) to the east (Takahashi and Yasunari 2006). After the monsoon transition, the response to the Annamese Cordillera is in a much larger scale and is closely related to the downstream perturbations in the EAWNP—namely, an enhanced monsoon trough over the South China Sea and Philippine Sea and a weakened subtropical high over the subtropical western North Pacific. The cyclonic circulation anomalies in the lower, mid-, and upper troposphere, due to the presence of the mountains, exhibit an equivalent barotropic structure (Fig. 10b). The finding also implies that the mountain effects contribute to the formation of the PJ pattern and the tripole rainfall pattern in East Asia, which are suggested as an intrinsic mode of the summer EAWNP monsoon.

Acknowledgments

This work was supported by the Consortium for Climate Change Study (CCliCS) under the auspices of the Ministry of Science and Technology (MOST), Taiwan, under Grant NSC 100-2119-M-001-029-MY5 (CHW and HHH). CHW was also supported under Grant MOST 104-2111-M-001-001-. The authors thank Dr. Wei-Liang Lee and Dr. Chein-Jung Shiu from the Research Center for Environmental Changes, Academia Sinica, for guidance on modeling, Ms. Hai-Wei Lin for preparing the schematic diagram, and the National Center for High-Performance Computing (NCHC) for computer time.

REFERENCES

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    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Rodwell, M. J., and B. J. Hoskins, 2001: Subtropical anticyclones and summer monsoons. J. Climate, 14, 31923211, doi:10.1175/1520-0442(2001)014<3192:SAASM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Saha, S., and Coauthors, 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor. Soc., 91, 10151057, doi:10.1175/2010BAMS3001.1.

    • Search Google Scholar
    • Export Citation
  • Sampe, T., and S. P. Xie, 2010: Large-scale dynamics of the meiyu-baiu rainband: Environmental forcing by the westerly jet. J. Climate, 23, 113134, doi:10.1175/2009JCLI3128.1.

    • Search Google Scholar
    • Export Citation
  • Suzuki, S., and B. Hoskins, 2009: The large-scale circulation change at the end of the baiu season in Japan as seen in ERA40 data. J. Meteor. Soc. Japan, 87, 8399, doi:10.2151/jmsj.87.83.

    • Search Google Scholar
    • Export Citation
  • Takahashi, H. G., and T. Yasunari, 2006: A climatological monsoon break in rainfall over Indochina—A singularity in the seasonal march of the Asian summer monsoon. J. Climate, 19, 15451556, doi:10.1175/JCLI3724.1.

    • Search Google Scholar
    • Export Citation
  • Takaya, K., and H. Nakamura, 1997: A formulation of a wave-activity flux for stationary Rossby waves on a zonally varying basic flow. Geophys. Res. Lett., 24, 29852988, doi:10.1029/97GL03094.

    • Search Google Scholar
    • Export Citation
  • Takaya, K., and H. Nakamura, 2001: A formulation of a phase-independent wave-activity flux for stationary and migratory quasigeostrophic eddies on a zonally varying basic flow. J. Atmos. Sci., 58, 608627, doi:10.1175/1520-0469(2001)058<0608:AFOAPI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Vertenstein, M., T. Craig, A. Middleton, D. Feddema, and C. Fischer, 2010: CESM1.0.3 user’s guide. CESM, 146 pp. [Available online at http://www.cesm.ucar.edu/models/cesm1.0/cesm/cesm_doc_1_0_3/ug.pdf.]

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    • Search Google Scholar
    • Export Citation
  • Watanabe, T., and K. Yamazaki, 2012: Influence of the anticyclonic anomaly in the subtropical jet over the western Tibetan Plateau on the intraseasonal variability of the summer Asian monsoon in early summer. J. Climate, 25, 12911303, doi:10.1175/JCLI-D-11-00036.1.

    • Search Google Scholar
    • Export Citation
  • Wu, C. H., and M. D. Chou, 2012: Upper-tropospheric forcing on late July monsoon transition in East Asia and the western North Pacific. J. Climate, 25, 39293941, doi:10.1175/JCLI-D-11-00343.1.

    • Search Google Scholar
    • Export Citation
  • Wu, C. H., and M. D. Chou, 2013: Tibetan Plateau westerly forcing on the cloud amount over Sichuan Basin and the early Asian summer monsoon. J. Geophys. Res. Atmos., 118, 75587568, doi:10.1002/jgrd.50580.

    • Search Google Scholar
    • Export Citation
  • Wu, C. H., H. H. Hsu, and M. D. Chou, 2014: Effect of the Arakan Mountains in the northwestern Indochina Peninsula on the late May Asian monsoon transition. J. Geophys. Res. Atmos., 119, 10 76910 779, doi:10.1002/2014JD022024.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., and Y. S. Zhang, 1998: Tibetan Plateau forcing and the timing of the monsoon onset over South Asia and the South China Sea. Mon. Wea. Rev., 126, 913927, doi:10.1175/1520-0493(1998)126<0913:TPFATT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., and Coauthors, 2007: The influence of mechanical and thermal forcing by the Tibetan Plateau on Asian climate. J. Hydrometeor., 8, 770789, doi:10.1175/JHM609.1.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., Y. Liu, X. Liang, A. Duan, Q. Bao, and J. Yu, 2012: Revisiting Asian monsoon formation and change associated with Tibetan Plateau forcing: I. Formation. Climate Dyn., 39, 11691181, doi:10.1007/s00382-012-1334-z.

    • Search Google Scholar
    • Export Citation
  • Wu, R., 2002: A mid-latitude Asian circulation anomaly pattern in boreal summer and its connection with the Indian and East Asian summer monsoons. Int. J. Climatol., 22, 18791895, doi:10.1002/joc.845.

    • Search Google Scholar
    • Export Citation
  • Wu, R., and B. Wang, 2001: Multi-stage onset of the summer monsoon over the western North Pacific. Climate Dyn., 17, 277289, doi:10.1007/s003820000118.

    • Search Google Scholar
    • Export Citation
  • Xie, S. P., H. Xu, N. H. Saji, Y. Wang, and W. T. Liu, 2006: Role of narrow mountains in large-scale organization of Asian monsoon convection. J. Climate, 19, 34203429, doi:10.1175/JCLI3777.1.

    • Search Google Scholar
    • Export Citation
Save
  • Chen, T., J. Tsay, M. Yen, and J. Matsumoto, 2012: Interannual variation of the late fall rainfall in central Vietnam. J. Climate, 25, 392413, doi:10.1175/JCLI-D-11-00068.1.

    • Search Google Scholar
    • Export Citation
  • Cheng, H., T. Wu, and W. Dong, 2008: Thermal contrast between the middle-latitude Asian continent and adjacent ocean and its connection to the East Asian summer precipitation. J. Climate, 21, 49925007, doi:10.1175/2008JCLI2047.1.

    • Search Google Scholar
    • Export Citation
  • Danabasoglu, G., and P. R. Gent, 2009: Equilibrium climate sensitivity: Is it accurate to use a slab ocean model? J. Climate, 22, 24942499, doi:10.1175/2008JCLI2596.1.

    • Search Google Scholar
    • Export Citation
  • Ding, Q., and B. Wang, 2007: Intraseasonal teleconnection between the summer Eurasian wave train and the Indian monsoon. J. Climate, 20, 37513767, doi:10.1175/JCLI4221.1.

    • Search Google Scholar
    • Export Citation
  • Gadgil, S., 1977: Orographic effects on the southwest monsoon: A review. Pure Appl. Geophys., 115, 14131430, doi:10.1007/BF00874416.

  • Gentemann, C. L., T. Meissner, and F. J. Wentz, 2010: Accuracy of satellite sea surface temperatures at 7 and 11 GHz. IEEE Trans. Geosci. Remote Sens., 48, 10091018, doi:10.1109/TGRS.2009.2030322.

    • Search Google Scholar
    • Export Citation
  • Honda, M., K. Yamazaki, H. Nakamura, and K. Takeuchi, 1999: Dynamic and thermodynamic characteristics of atmospheric response to anomalous sea-ice extent in the Sea of Okhotsk. J. Climate, 12, 33473358, doi:10.1175/1520-0442(1999)012<3347:DATCOA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Hsu, H. H., and S. M. Lin, 2007: Asymmetry of the tripole rainfall pattern during the East Asian summer. J. Climate, 20, 44434458, doi:10.1175/JCLI4246.1.

    • Search Google Scholar
    • Export Citation
  • Hsu, H. H., T. Zhou, and J. Matsumoto, 2014: East Asian, Indochina and western North Pacific summer monsoon—An update. Asia-Pac. J. Atmos. Sci., 50, 4568, doi:10.1007/s13143-014-0027-4.

    • Search Google Scholar
    • Export Citation
  • Kitoh, A., 2002: Effects of large-scale mountains on surface climate—A coupled ocean-atmosphere general circulation model study. J. Meteor. Soc. Japan, 80, 11651181, doi:10.2151/jmsj.80.1165.

    • Search Google Scholar
    • Export Citation
  • Kosaka, Y., and H. Nakamura, 2006: Structure and dynamics of the summertime Pacific–Japan teleconnection pattern. Quart. J. Roy. Meteor. Soc., 132, 20092030, doi:10.1256/qj.05.204.

    • Search Google Scholar
    • Export Citation
  • Kosaka, Y., and H. Nakamura, 2010: Mechanisms of meridional teleconnection observed between a summer monsoon system and a subtropical anticyclone. Part I: The Pacific–Japan pattern. J. Climate, 23, 50855108, doi:10.1175/2010JCLI3413.1.

    • Search Google Scholar
    • Export Citation
  • Krishnan, R., and M. Sugi, 2001: Baiu rainfall variability and associated monsoon teleconnections. J. Meteor. Soc. Japan, 79, 851860, doi:10.2151/jmsj.79.851.

    • Search Google Scholar
    • Export Citation
  • Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and J. Simpson, 1998: The Tropical Rainfall Measuring Mission (TRMM) sensor package. J. Atmos. Oceanic Technol., 15, 809817, doi:10.1175/1520-0426(1998)015<0809:TTRMMT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Lau, N. C., and J. J. Ploshay, 2009: Simulation of synoptic- and subsynoptic-scale phenomena associated with the East Asian summer monsoon using a high-resolution GCM. Mon. Wea. Rev., 137, 137160, doi:10.1175/2008MWR2511.1.

    • Search Google Scholar
    • Export Citation
  • Liang, X. Z., and W. C. Wang, 1998: Associations between China monsoon rainfall and tropospheric jets. Quart. J. Roy. Meteor. Soc., 124, 25972623, doi:10.1002/qj.49712455204.

    • Search Google Scholar
    • Export Citation
  • Molnar, P., W. Boos, and D. Battisti, 2010: Orographic controls on climate and paleoclimate of Asia: Thermal and mechanical roles for the Tibetan Plateau. Annu. Rev. Earth Planet. Sci., 38, 77102, doi:10.1146/annurev-earth-040809-152456.

    • Search Google Scholar
    • Export Citation
  • Murakami, T., and J. Matsumoto, 1994: Summer monsoon over the Asian continent and western North Pacific. J. Meteor. Soc. Japan, 72, 719745.

    • Search Google Scholar
    • Export Citation
  • Neale, R. B., J. Richter, S. Park, P. H. Lauritzen, S. J. Vavrus, P. J. Rasch, and M. Zhang, 2013: The mean climate of the Community Atmosphere Model (CAM4) in forced SST and fully coupled experiments. J. Climate, 26, 51505168, doi:10.1175/JCLI-D-12-00236.1.

    • Search Google Scholar
    • Export Citation
  • Nitta, T., 1987: Convective activities in the tropical western Pacific and their impact on the Northern Hemisphere summer circulation. J. Meteor. Soc. Japan, 65, 373390.

    • Search Google Scholar
    • Export Citation
  • Okajima, H., and S. P. Xie, 2007: Orographic effects on the northwestern Pacific monsoon: Role of air-sea interaction. Geophys. Res. Lett., 34, L21708, doi:10.1029/2007GL032206.

    • Search Google Scholar
    • Export Citation
  • Qi, L., and Y. Wang, 2012: The effect of mesoscale mountain over the East Indochina Peninsula on downstream summer rainfall over East Asia. J. Climate, 25, 44954510, doi:10.1175/JCLI-D-11-00574.1.

    • Search Google Scholar
    • Export Citation
  • Rodwell, M. J., and B. J. Hoskins, 2001: Subtropical anticyclones and summer monsoons. J. Climate, 14, 31923211, doi:10.1175/1520-0442(2001)014<3192:SAASM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Saha, S., and Coauthors, 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor. Soc., 91, 10151057, doi:10.1175/2010BAMS3001.1.

    • Search Google Scholar
    • Export Citation
  • Sampe, T., and S. P. Xie, 2010: Large-scale dynamics of the meiyu-baiu rainband: Environmental forcing by the westerly jet. J. Climate, 23, 113134, doi:10.1175/2009JCLI3128.1.

    • Search Google Scholar
    • Export Citation
  • Suzuki, S., and B. Hoskins, 2009: The large-scale circulation change at the end of the baiu season in Japan as seen in ERA40 data. J. Meteor. Soc. Japan, 87, 8399, doi:10.2151/jmsj.87.83.

    • Search Google Scholar
    • Export Citation
  • Takahashi, H. G., and T. Yasunari, 2006: A climatological monsoon break in rainfall over Indochina—A singularity in the seasonal march of the Asian summer monsoon. J. Climate, 19, 15451556, doi:10.1175/JCLI3724.1.

    • Search Google Scholar
    • Export Citation
  • Takaya, K., and H. Nakamura, 1997: A formulation of a wave-activity flux for stationary Rossby waves on a zonally varying basic flow. Geophys. Res. Lett., 24, 29852988, doi:10.1029/97GL03094.

    • Search Google Scholar
    • Export Citation
  • Takaya, K., and H. Nakamura, 2001: A formulation of a phase-independent wave-activity flux for stationary and migratory quasigeostrophic eddies on a zonally varying basic flow. J. Atmos. Sci., 58, 608627, doi:10.1175/1520-0469(2001)058<0608:AFOAPI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Vertenstein, M., T. Craig, A. Middleton, D. Feddema, and C. Fischer, 2010: CESM1.0.3 user’s guide. CESM, 146 pp. [Available online at http://www.cesm.ucar.edu/models/cesm1.0/cesm/cesm_doc_1_0_3/ug.pdf.]

  • Wang, Z., and C. P. Chang, 2012: A numerical study of the interaction between the large-scale monsoon circulation and orographic precipitation over South and Southeast Asia. J. Climate, 25, 24402455, doi:10.1175/JCLI-D-11-00136.1.

    • Search Google Scholar
    • Export Citation
  • Watanabe, T., and K. Yamazaki, 2012: Influence of the anticyclonic anomaly in the subtropical jet over the western Tibetan Plateau on the intraseasonal variability of the summer Asian monsoon in early summer. J. Climate, 25, 12911303, doi:10.1175/JCLI-D-11-00036.1.

    • Search Google Scholar
    • Export Citation
  • Wu, C. H., and M. D. Chou, 2012: Upper-tropospheric forcing on late July monsoon transition in East Asia and the western North Pacific. J. Climate, 25, 39293941, doi:10.1175/JCLI-D-11-00343.1.

    • Search Google Scholar
    • Export Citation
  • Wu, C. H., and M. D. Chou, 2013: Tibetan Plateau westerly forcing on the cloud amount over Sichuan Basin and the early Asian summer monsoon. J. Geophys. Res. Atmos., 118, 75587568, doi:10.1002/jgrd.50580.

    • Search Google Scholar
    • Export Citation
  • Wu, C. H., H. H. Hsu, and M. D. Chou, 2014: Effect of the Arakan Mountains in the northwestern Indochina Peninsula on the late May Asian monsoon transition. J. Geophys. Res. Atmos., 119, 10 76910 779, doi:10.1002/2014JD022024.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., and Y. S. Zhang, 1998: Tibetan Plateau forcing and the timing of the monsoon onset over South Asia and the South China Sea. Mon. Wea. Rev., 126, 913927, doi:10.1175/1520-0493(1998)126<0913:TPFATT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., and Coauthors, 2007: The influence of mechanical and thermal forcing by the Tibetan Plateau on Asian climate. J. Hydrometeor., 8, 770789, doi:10.1175/JHM609.1.

    • Search Google Scholar
    • Export Citation
  • Wu, G. X., Y. Liu, X. Liang, A. Duan, Q. Bao, and J. Yu, 2012: Revisiting Asian monsoon formation and change associated with Tibetan Plateau forcing: I. Formation. Climate Dyn., 39, 11691181, doi:10.1007/s00382-012-1334-z.

    • Search Google Scholar
    • Export Citation
  • Wu, R., 2002: A mid-latitude Asian circulation anomaly pattern in boreal summer and its connection with the Indian and East Asian summer monsoons. Int. J. Climatol., 22, 18791895, doi:10.1002/joc.845.

    • Search Google Scholar
    • Export Citation
  • Wu, R., and B. Wang, 2001: Multi-stage onset of the summer monsoon over the western North Pacific. Climate Dyn., 17, 277289, doi:10.1007/s003820000118.

    • Search Google Scholar
    • Export Citation
  • Xie, S. P., H. Xu, N. H. Saji, Y. Wang, and W. T. Liu, 2006: Role of narrow mountains in large-scale organization of Asian monsoon convection. J. Climate, 19, 34203429, doi:10.1175/JCLI3777.1.

    • Search Google Scholar
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  • Fig. 1.

    (a) Precipitation (mm day−1; shadings) and 500-hPa winds (m s−1; red vectors for vertical velocity higher than 3 Pa min−1); (b) the 925-hPa moisture convergence (g kg−1 day−1; shadings) and flux (g kg−1 m day−1; vectors); (c) (top) vertical circulations (shadings for meridional wind speeds; unit: m s−1) and precipitation along the 24°N band in the Arakan Yoma region and the 15°N band in the Annamese Cordillera region, and (bottom) the 925-hPa wind vectors and ocean surface wind speeds (m s−1; shadings) from 21 May to 4 Jun. (d)–(f) As in (a)–(c), but from 25 Jul to 8 Aug. Gray and black (Arakan and Annamese Mountains) thick contour lines denote the elevation = 0.8 km, and black bars denote the mountain ranges.

  • Fig. 2.

    (a),(b) Topography (km; only higher than 0.8 km is plotted); shadings for 0.16° grid size, red contours for 1° control simulation, and black contours for the mountains added for the AA simulation. (c),(d) Time series of area-mean precipitation (mm day−1) across 88°–96°E, 18°–26°N and 105°–120°E, 14°–20°N [box regions in (a) and (b)]. (e),(f) Precipitation in the 125°–145°E band from 21 May to 4 Jun and from 25 Jul to 8 Aug, respectively; black lines for TRMM data, orange dashed lines for the CAM–SOM control simulation, and blue lines for the CAM–SOM AA simulation.

  • Fig. 3.

    (left) CAM–SOM control simulation and (right) AA minus control simulations: (a),(b) precipitation (mm day−1; shadings) and 500-hPa winds (m s−1; red vectors for vertical velocity higher than 3 Pa min−1); (c),(d) 925-hPa moisture convergence (g kg−1 day−1; shadings) and flux (g kg−1 m day−1; vectors) from 21 May to 4 Jun. (e)–(h) As in (a)–(d), but from 25 Jul to 8 Aug. Gray and black (Arakan and Annamese Mountains) thick contour lines denote the elevation = 0.8 km. Dots denote regions where the precipitation or moisture convergence difference attains a confidence level of 90%. Red and thick vectors indicate upward air motion.

  • Fig. 4.

    Streamlines and meridional wind speeds (m s−1; shadings) in a pressure–longitude plane and precipitation (mm day−1; blue lines) along the (top) 24°N band and (bottom) 15°N band from 21 May to 4 Jun. (a),(d) Control simulation, (b),(e) AA simulation, and (c),(f) AA minus control simulations. Black bars denote topography. Circles denote that the meridional wind difference has a 90% confidence level.

  • Fig. 5.

    As in Fig. 4, but from 25 Jul to 8 Aug.

  • Fig. 6.

    Streamlines and zonal wind speeds (m s−1; shadings) of (a),(e) reanalysis data, (b),(f) CAM–SOM control simulation, (c),(g) AA simulation, and (d),(h) AA simulation minus control simulation at 200 hPa (left) from 21 May to 4 Jun and (right) from 25 Jul to 8 Aug. Contours denote the 200-hPa geopotential heights of 12 400–12 480 m and 12 450–12 550 m. Dots denote regions where the wind speed difference attains a confidence level of 90%.

  • Fig. 7.

    Wave activity fluxes and streamfunctionlike geopotential heights (10−6 m s−2; shadings) at (top) 200, (middle) 500, and (bottom) 850 hPa, (a)–(c) from 21 May to 4 Jun and (d)–(f) from 25 Jul to 8 Aug.

  • Fig. 8.

    As in Figs. 3b,d, but for (a),(b) AR minus control simulations and (c),(d) AA minus AR simulations.

  • Fig. 9.

    As in Figs. 3f,h, but for (a),(b) AR minus control simulations and (c),(d) AA minus AR simulations.

  • Fig. 10.

    Schematic diagram of the atmospheric response (200, 500, and 850 hPa) to the mountains for (a) late May and (b) late July. Orange color denotes the general features in each period, and blue color denotes the mountain-induced changes. H denotes high pressure and AC/C denotes the anticyclonic/cyclonic circulation anomaly.

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