Relative Impacts of the Orography and Land–Sea Contrast over the Indochina Peninsula on the Asian Summer Monsoon between Early and Late Summer

Moran Zhuang aLASG, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing, China

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https://orcid.org/0000-0002-3022-2582
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Anmin Duan bState Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
cCollege of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China

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Riyu Lu aLASG, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing, China
cCollege of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China

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Puxi Li dState Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing, China

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Jinglong Yao eState Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China

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Abstract

The Indochina Peninsula (ICP) has a critical effect in shaping the Asian summer monsoon (ASM). However, the seasonal responses of the ASM to the ICP are not fully understood. This study employs a 1° atmospheric general circulation model to examine the different contributions of the ICP’s orography and land–sea contrast to the ASM during the early and late summer. Results indicate that the orographic effect increases South Asian rainfall and reduces the rainfall over the South China Sea (SCS) and North China in early summer, but its influence on monsoonal circulation and rainfall is limited to East Asia in late summer. The impact of the ICP’s land–sea contrast is basically opposite in the two summer stages. With the presence of the ICP, SCS rainfall is enhanced but South Asian rainfall is weakened in early summer. In late summer, however, rainfall from the ICP to the northwestern Pacific is strikingly reduced, accompanied by intensified rainfall over South Asia. Relatively, the orographic effect seems to be more important in modulating the South Asian monsoon in early summer, while the land–sea contrast is dominant in strengthening the SCS monsoon and suppressing the northwest Pacific monsoon via the interaction between the induced local circulation and multilevel ASM subsystems. In late summer, the orographic effect on the ASM is much weaker compared to the land–sea contrast, which plays a critical role by shifting the subtropical high southwestward and through the “thermal adaption” feedback mechanism. Therefore, the orographic impact of the ICP on the ASM differs from that of the land–sea contrast in the two summer stages.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Anmin Duan, amduan@lasg.iap.ac.cn

Abstract

The Indochina Peninsula (ICP) has a critical effect in shaping the Asian summer monsoon (ASM). However, the seasonal responses of the ASM to the ICP are not fully understood. This study employs a 1° atmospheric general circulation model to examine the different contributions of the ICP’s orography and land–sea contrast to the ASM during the early and late summer. Results indicate that the orographic effect increases South Asian rainfall and reduces the rainfall over the South China Sea (SCS) and North China in early summer, but its influence on monsoonal circulation and rainfall is limited to East Asia in late summer. The impact of the ICP’s land–sea contrast is basically opposite in the two summer stages. With the presence of the ICP, SCS rainfall is enhanced but South Asian rainfall is weakened in early summer. In late summer, however, rainfall from the ICP to the northwestern Pacific is strikingly reduced, accompanied by intensified rainfall over South Asia. Relatively, the orographic effect seems to be more important in modulating the South Asian monsoon in early summer, while the land–sea contrast is dominant in strengthening the SCS monsoon and suppressing the northwest Pacific monsoon via the interaction between the induced local circulation and multilevel ASM subsystems. In late summer, the orographic effect on the ASM is much weaker compared to the land–sea contrast, which plays a critical role by shifting the subtropical high southwestward and through the “thermal adaption” feedback mechanism. Therefore, the orographic impact of the ICP on the ASM differs from that of the land–sea contrast in the two summer stages.

© 2022 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Anmin Duan, amduan@lasg.iap.ac.cn

1. Introduction

The land–sea heating contrast induced by the seasonal evolution of solar radiation and large-scale orography are the key factors affecting the Asian summer monsoon (ASM). Numerous studies have revealed the importance of planetary-scale land–sea thermal contrast and vast mountain ranges, such as the Tibetan Plateau (Hahn and Manabe 1975; Kitoh 2004, 2017; Wu et al. 2007, 2012; Boos and Kuang 2010; Hu and Duan 2015) and East African Highlands (Krishnamurti et al. 1976; Rodwell and Hoskins 1995; Chakraborty et al. 2002, 2009; Johnson et al. 2016), in inducing and maintaining the ASM. In addition to these factors, it is generally considered that the extension of the Asian subtropical continent into the tropics, including the Indian Peninsula and Indochina Peninsula (ICP), is also an important agent for organizing monsoonal winds and precipitation (Gadgil 1977; Chen and Chen 1991; Chang et al. 2005; Jin et al. 2006; Xie et al. 2006; Wang and Wu 2009; Liu et al. 2010). Chow et al. (2006) pointed out that the ICP has a much more significant impact on the East Asian summer monsoon (EASM) than that of the Indian Peninsula where the surface-heating effect is relatively local and limited to the Bay of Bengal (BOB).

By altering the land–sea distribution in regional or global simulations, many researchers have investigated how the thermal differences between the ICP and its surrounding oceans modulate the onset and evolution of the ASM in early summer. Numerical modeling studies (Xu et al. 2002; Liang et al. 2005) in which experiments are conducted with and without the ICP have revealed that the ICP may affect the establishment of the South China Sea (SCS) summer monsoon through the “secondary thermal adaption” mechanism. The land–sea thermal contrast between the ICP and its surrounding oceans in spring can be a precursory signal of the SCS summer monsoon onset (Li et al. 2020). Specifically, the strong surface sensible heat over the ICP in early spring starts earlier than that over the Tibetan Plateau and induces a low-level anomalous cyclone, causing the subtropical high to break over the ICP in April (Wu et al. 1999). The resulting convergent wind and the low-level southwesterly are conducive to the heavy precipitation over the SCS (Zhang and Qian 2002). The increased release of the latent heat due to the intensified precipitation further promotes the development of cyclonic circulation there and then causes the SCS summer monsoon to break out in May. Besides, the deep convection over the BOB also helps to intensify the atmospheric instability over the SCS and induce the SCS summer monsoon onset (Wu et al. 2005).

In recent years, many modeling studies aimed mainly at the local and regional effect of the narrow mountains over the ICP in both early and late summer. Regional simulations with and without the Annamese Cordillera in the southeastern ICP have indicated that the terrain-related descending southwesterly jet on the leeward side has a strong impact on suppressing the SCS summer monsoon from June to July through the air–sea interaction (Xu et al. 2008). The decreased release of the latent heat associated with the weakened mountain-anchoring rainfall further aggravates negative rainfall anomalies over the SCS and western North Pacific (WNP; Qi and Wang 2012) but promotes the convective activities from South China to the East China Sea. Wang and Chang (2012) highlighted the importance of local wind–terrain–precipitation interaction over the ICP and showed that the onset of the ICP monsoon could be delayed until June when mountains were removed from this region, although it was found that the onset of the Indian monsoon is not sensitive to the ICP’s topography. Recent studies have further verified the role of the Annamese Cordillera and Arakan Mountains (in the northwestern ICP) in the ASM between early and late summer. For instance, it was found that the Arakan Mountains could deepen the low-level southwesterly and midlayer trough and facilitate the BOB monsoon in late May (Wu et al. 2014). The terrain-forced ascending southwesterly converges with the midlatitude westerly jet, helping moisture to be transported across the ICP to the mei-yu region (Wu and Hsu 2016; Wu et al. 2018). Meanwhile, the leeside troughing effect of the Annamese Cordillera also likely reinforces the mei-yu and shifts an intensified subtropical high to the east in early summer, thereby intensifying the SCS and WNP summer monsoons, which is inconsistent with previous findings (Xu et al. 2008; Qi and Wang 2012). In late summer, the Annamese Cordillera exerts an essential effect on enhancing the East Asian and WNP monsoons by regulating the monsoon trough in the WNP, and causes a tripole rainfall pattern in East Asia by triggering a Pacific–Japan pattern (PJ)-like perturbation (Wu and Hsu 2016).

As is known, the Asian monsoonal rainfall and circulation undergo notable changes from June to July (Yu and Zhou 2007; Wang et al. 2009; Wu and Hsu 2016). Considering the distinct temporal and spatial structures in the mean state during early (May–June) and late (July–August) summer, it is thus reasonable to divide the boreal summer season into two stages. The responses of the Asian climate to the ICP also likely vary before and after the onset of the ASM. From the abovementioned studies, the importance of the ICP in modulating the ASM circulation and precipitation has been studied extensively. Early studies in this regard mainly focused on the role of the land–sea thermal contrast over the ICP in late May, and consensus has been reached on its role in triggering the SCS monsoon onset. However, relatively less attention has been paid to its influence in late summer. Besides, the experiments with and without the ICP in previous studies actually include the contribution from the mesoscale terrain and are thus insufficient to explain the individual influence of the land–sea contrast. As for the ICP’s orography, although recent studies have paid far more attention to its impact on scales much larger than its size in both early and late summer, its complex role in the SCS and WNP summer monsoons has not been explored extensively. Furthermore, despite plenty of numerical simulations having been conducted, few have focused on the relative contributions of the narrow mountains and land–sea thermal contrast over the ICP to the ASM.

In this study, by analyzing the results from a series of numerical experiments with an atmospheric general circulation model, we therefore aim at exploring the differences in the response of the ASM to the land–sea contrast and orography over the ICP between early and late summer, and attempt to identify the relative importance of the narrow mountains and land–sea contrast of the ICP to the ASM in the two summer stages. We will also focus on the underlying physical process behind the impact of the ICP on the ASM in terms of the interaction between the forced local circulation and large-scale ASM circulation subsystems, as well as the positive feedback between the monsoonal circulation and precipitation.

The rest of the paper is structured as follows: The datasets, methods, model, and experiment design are described in section 2. Section 3 reports the general features of the ASM in the two boreal summer stages and evaluates the model performance. The effects of the ICP on the mean state of the ASM from different aspects, such as monsoonal precipitation and moisture transport, in the two monsoon stages, are presented in section 4. The responses of the regional and large-scale circulation and the possible mechanisms involved are discussed in section 5. Finally, section 6 summarizes the main conclusions.

2. Data, methods, and experimental design

a. Data

The data used in this study include 1) precipitation with a horizontal resolution of 2.5° × 2.5° from the Global Precipitation Climatology Project, version 2.1 (Adler et al. 2003), and 2) atmospheric variables with a horizontal resolution of 1.25° × 1.25° from the Japanese 55-year Reanalysis project (Ebita et al. 2011). All dataset variables are available at daily and monthly temporal resolution and cover the boreal summer season (May–August) for the period 1985–2004. The topography data on a 5-min latitude/longitude grid are derived from the Global Digital Elevation Model dataset (ETOPO5; NGDC 1993). Data of the standard variables were interpolated to the same horizontal resolution as the model grid at surface and vertical layers.

b. Methods

The apparent heat source Q1 (Yanai et al. 1973) is computed as
Q1=Cp(pp0)R/Cp[θt+Vθ+ωθp],
where θ, V, and ω represent the potential temperature, horizontal velocity, and vertical velocity, respectively, and p, R, and Cp are the pressure, gas constant, and specific heat at constant pressure of dry air, respectively (p0 = 1000 hPa). The diabatic heating rate, Q = Q1/Cp, can be used to diagnose the regional thermal condition. If Q is less than zero, the condition is diabatic cooling; otherwise, it is diabatic heating.
The complete form of the vertical vorticity tendency equation (Wu et al. 1999) is
ζtA+VζB+βυC=(f+ζ)VD+f+ζθzQzE1θz(υzQxuzQy)F+ρddt(PEθzCD)G+1θz FυθH,
where ζ and f are the vertical components of the relative vorticity and geostrophic vorticity, respectively; θz is the vertical gradient of θ; and β, ρ, and CD are the Rossby parameter, air density and thermal parameter, respectively; also, PE and Fυ represent the Ertel potential vorticity and friction dissipation, respectively. Term A is the time derivative of ζ, which can generally be ignored as its seasonal mean value is much smaller than other terms. Terms B and C on the left-hand side of Eq. (2) are the horizontal advection of ζ and the β effect term, respectively. Terms D, E, and F on the right-hand side represent the divergence and the vertical and horizontal inhomogeneous distribution of Q, respectively.
According to the scale analysis (Wu et al. 1999), term E is one order of magnitude larger than terms D and F. In tropical and subtropical regions, the contribution of the thermal change in the atmosphere (i.e., term G) is negligible. Ignoring the friction dissipation of term H for the long-time-scale processes, Eq. (2) can be simplified to
Vζ+βυf+ζθzQz.
In the subtropical region, where the prevailing easterly converges with westerly flow, the zonal wind is nearly zero. Thus, the horizontal advection effect of term B is negligible. Therefore, Eq. (3) can be further simplified to
βυf+ζθzQz,θz0.

c. Model and experimental design

The Finite-volume Atmospheric Model (FAMIL; Zhou et al. 2012, 2015; Yu et al. 2014), version 2.2, developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences, enables relatively realistic interactions among different atmospheric processes and has been widely employed to study the climate dynamics of the ASM (Liu et al. 2004; Wu et al. 2012; Hu and Duan 2015; Liu and Duan 2017; Zhuang and Duan 2019). The details of the physical parameterizations and model settings for FAMIL are the same as in Zhuang and Duan (2019). FAMIL has 32 vertical levels, with the model top at 2.16 hPa. A high horizontal resolution of C96 (approximately 0.9875° × 0.9875°) is chosen to resolve the major topographical structure of the mountain ranges on the ICP. Figure 1 displays the land–sea fraction and orography, indicating that at the C96 resolution FAMIL is able to depict the outlines and rough shapes of the terrain over the ICP, despite underrepresenting the mountain height and steepness to a certain degree (Figs. 1a,b,e,f).

Fig. 1.
Fig. 1.

The (top) grid point land fraction, (middle) topography (shaded; km), and (bottom) sea surface temperature averaged from May to August from the (a),(e) observations, (b),(f),(i) CTRL, (c),(g),(j) ICnoTP, and (d),(h),(k) ICnoLD.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

To simulate the atmospheric responses to the orography and land–sea thermal contrast of the ICP, three sets of numerical experiments were conducted: realistic ICP (CTRL), flat ICP (ICnoTP), and aqua ICP (ICnoLD). Realistic orography and land–sea contrast in the ICP are retained in the control experiment CTRL (Figs. 1b,f,i), whereas the land grid points of the ICP (8°–22°N, 90°–110°E) in ICnoLD are replaced by ocean grid points and are forced by the climatological annual-cycle AMIP II SST (Atmospheric Model Intercomparison Project, phase 2; Figs. 1d,h,k). The SST in the ICP land grids of ICnoLD is obtained by linear interpolation from the SST in the surrounding SCS and BOB (Fig. 1k). In ICnoTP, the land–sea distribution is retained but the terrain height is reduced to zero over the ICP (Figs. 1c,g,j). CTRL differs from ICnoTP only in the presence of the topography over the ICP, and ICnoTP differs from ICnoLD only in the land–sea thermal contrast. The changes in precipitation and atmospheric circulation thus can be directly attributed to the individual effect of the orography or the land–sea contrast by comparing the differences among CTRL, ICnoTP, and ICnoLD. The length of simulation of these three experiments is 22 years (1983–2004), and the daily outputs of the last 20 years are analyzed.

It is generally recognized that the initial stage of the ASM is characterized by the establishment of strong convection and the reversal of the prevailing wind over the BOB, ICP, and SCS. The ASM onset first occurs in the eastern BOB and the ICP around the middle of May, which is followed by the SCS summer monsoon onset in late May and the Indian summer monsoon onset in early June on average (Matsumoto 1992; Lau and Yang 1997; Wu and Zhang 1998; Wang and LinHo 2002). Therefore, 16 May–10 June (MJ) was selected as the early summer period in this study, and 16 July–15 August (JA) as the late summer. Note that the model results were found not be sensitive to the chosen time periods of two summer stages (figure not shown). Student’s t test was applied to check whether the differences among the three experiments were significant.

3. Distinct characteristics of the ASM in two summer stages and model performance

It is commonly recognized that the ASM exhibits remarkable differences in subseasonal mean states between early and late summer (Wang et al. 2009; Wu and Hsu 2016). Before depicting the differences among numerical simulations, the performance of FAMIL in reproducing the evolution of the ASM is evaluated.

The observed climatological precipitation, multilevel omega, horizontal winds, and geopotential height, along with the simulated counterparts, from both early and late summer are shown in Fig. 2. In MJ, as the southwest monsoon impinges on the coastal mountains, the major rainy regions are confined to the west coast of the Indian Peninsula, the ICP, the eastern SCS and South China (Fig. 2a), along with a subtropical high located east of the Philippines, a strong trough at 500 hPa over the BOB (Fig. 2b) and a 200-hPa anticyclone located in South Asia. From MJ to JA, the 500-hPa subtropical high moves northwest to East China and the BOB trough is replaced by the monsoonal low (Fig. 2b vs Fig. 2d); meanwhile, the South Asia high shifts northwest to the foothills of the Himalayas and the low-level WNP monsoon trough over the Philippine Sea becomes well established (Fig. 2c). This monsoonal circulation configuration provides a favorable condition for the vertical ascending motion and enables the South Asian summer monsoon to mature with a rainfall belt extending to the southern Tibetan Plateau. It also promotes the onset of the WNP summer monsoon and makes the rainy region in South China move northward to north of the Yangtze River in JA.

Fig. 2.
Fig. 2.

Climatological (a) precipitation (shaded; mm day−1), horizontal winds (vectors; m s−1) at 850 hPa, and geopotential height (green contours; gpm) at 200 hPa, and (b) vertical velocity (shaded; hPa h−1), horizontal winds, and geopotential height (pink contours) at 500 hPa from 16 May to 10 Jun (MJ) from the observation. (c),(d) As in (a) and (b), but from 16 Jul to 15 Aug (JA) from the observation. (e)–(h) As in (a)–(d), but from CTRL.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

In comparison with the mean state presented in Figs. 2a–d, it is evident that the model can generally capture the distinct temporal and spatial structures of the ASM rainfall and circulation, as well as the abrupt changes in climatological mean fields from MJ to JA (Figs. 2e–h), albeit with some biases in terms of the magnitude of the rainfall and circulation maxima. For example, the simulated South Asian monsoonal rainfall and low-level westerly near 15°N are notably stronger than observed in both MJ and JA (Figs. 2e,g). The subtropical rainfall zone near the Yangtze River in JA is significantly underestimated in CTRL, which is closely related to the longer and narrower subtropical high at 500 hPa, being located farther north and west than observed.

In summary, the Asian monsoonal rainfall and circulation at multiple vertical levels in the two boreal summer stages in the reanalysis data and FAMIL exhibit considerable consistency. This indicates the model is capable of realistically reproducing the seasonal march of the ASM and could thus be used with confidence to conduct sensitivity experiments to investigate the role played by the ICP in the ASM.

4. Effect of the ICP on ASM precipitation and moisture transport

a. ASM precipitation

The simulated precipitation, together with the differences among the three numerical experiments during early and late summer, is illustrated in Figs. 3 and 4. Despite the overall rainfall patterns over the Asian monsoon region remaining unchanged in Figs. 3a, 3b, 3d, and 3e, the rainfall magnitude does show some differences under the orographic effect. In the MJ period, the rainband (rain shadow) on the windward (leeward) sides of the Asian tropical terrain is intensified due to the presence of the ICP’s orography, with the maximum value being larger than 5 mm day−1 in the offshore Arabian Sea, eastern BOB, and western SCS (Fig. 4b). However, the rainfall near equatorial Asia is reduced considerably. Note that a noticeably anomalous tripole rainfall pattern appears in China, with positive extension from central China to the East China Sea and negative extension over North and South China. As we can see from the percentage change relative to the mean rainfall in CTRL (Fig. 4g; regions are defined in Table 1), the rainfall over South Asia increases by about 20%, while that over the tropical Indian Ocean, the SCS, and North China is reduced by as much as 10%–55% with the ICP’s orography added into the model. In the JA period, the monsoonal precipitation over the complex terrain of the ICP changes by more than 5 mm day−1, while the rainfall from the Arabian Sea to the BOB remains almost unchanged irrespective of whether the ICP’s orography is present or not (Figs. 3d,e and 4e). The complex orography also has a restraining impact on the rainfall from central China to the WNP, with rainfall there reduced by about 30%. In contrast, the ICP’s topography exerts profound local and remote impacts on the upstream South Asian and downstream SCS and East Asian summer monsoons in MJ. However, the local effect is prominent in late summer, with the response of the Asian rainfall to the complex orography mainly being limited to the area with surface property changes.

Fig. 3.
Fig. 3.

Precipitation (shaded; mm day−1) in (a) CTRL, (b) ICnoTP, and (c) ICnoLD from 16 May to 10 Jun. (d)–(f) As in (a)–(c), but from 16 Jul to 15 Aug.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

Fig. 4.
Fig. 4.

Differences in precipitation (shaded; mm day−1) (a) between CTRL and ICnoLD, (b) between CTRL and ICnoTP, and (c) between ICnoTP and ICnoLD, and (g) the precipitation percentage averaged over several key areas defined in Table 1, from 16 May to 10 Jun. (d)–(f) As in (a)–(c), but from 16 Jul to 15 Aug; (h) as in (g), but from 16 Jul to 15 Aug. Dotted areas exceed the 5% significance level of the t test.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

Table 1

Definitions of the geographic regions. TropIO and ArabIDP denote the tropical Indian Ocean and the region from the Arabian Sea to the Indian Peninsula, respectively. BOB, ICP, SCS, and WNP denote the Bay of Bengal, Indochina Peninsula, South China Sea, and western North Pacific, respectively. NorCh indicates North China.

Table 1

Comparing ICnoTP and ICnoLD in the MJ period (Figs. 3b,c and 4c) reveals that the SCS summer monsoon is significantly intensified, with the maximum monsoon rainfall increased by about 5 mm day−1 due to the land–sea thermal contrast. Meanwhile, the monsoonal rainfall over North China increases by about 2 mm day−1, which is a considerable amount considering that the climatological rainfall there is less than 4 mm day−1. However, the rainfall extending from the Arabian Sea to the southern foothills of the Tibetan Plateau, especially the northwestern ICP, is weakened considerably. In the JA period, the distribution of the ASM in ICnoLD is markedly different from that in ICnoTP (Figs. 3e,f). There is a southwest–northeast negative precipitation anomaly zone extending from the ICP to the WNP, and an east–west-oriented positive precipitation anomaly band in tropical Asia, with the presence of the land–sea thermal contrast over the ICP (Fig. 4f). The rainfall over North China is also enhanced. In contrast, the responses of South Asian and SCS monsoons to the land–sea contrast are basically opposite in the two summer stages. For the downstream WNP and East Asian summer monsoons, the impact of the land–sea contrast is similar between the two summer stages, but the influence on the former is much stronger in late summer than in early summer, reducing the WNP rainfall by more than 25% (Fig. 4h).

By further comparing the differences among CTRL, ICnoTP, and ICnoLD, it can be seen that the reinforcing influence of the land–sea contrast on the SCS summer monsoon outweighs the weakening effect of the topography over the ICP in MJ, but for South Asia the remote effect of the ICP’s terrain is more substantial (Figs. 4a–c and 4g). In JA, the land–sea contrast is dominant in the ASM rather than the orography, strikingly weakening the summer monsoon from the northern Indian Peninsula to the WNP but intensifying the rainfall near the equator and in North China (Figs. 4d–f and 4h). However, both the topography and land–sea contrast are important to the ICP monsoon.

b. Regional moisture budget

The transportation of water vapor is closely linked to precipitation in the Asian monsoon region; it is also the energy source for the latent heat of condensation. It is considered one of the critical components of the ASM. To diagnose how the atmospheric water vapor transport changes when the ICP’s orography and land–sea contrast are removed, we calculated the vertically integrated moisture transport (VIMT) following the procedure of Zhuang and Duan (2019). The moisture budget can be generally described by calculating the VIMT into and out of a region.

The climatological mean VIMT and moisture flux divergence from CTRL, along with the differences among sensitivity experiments, are presented in Fig. 5. To further pinpoint the VIMT into and out of a region, Fig. 6 shows the VIMT integrated across several key boundaries and the regional net moisture budget. Both the low-level westerly and cross-equatorial flows play an important role in transporting moisture from the Southern to the Northern Hemisphere. In the CTRL run (Fig. 5a), net moisture convergence can be found in the ASM region during the early summer. The southerly VIMT contributes more to the maximum rainfall centers in South Asia and South China, but the zonal VIMT carries much more moisture from the Indian and Pacific Oceans to the ICP and SCS than the meridional component (Fig. 6a). In the JA period, similar spatial distributions can also be found (Figs. 5b and 6b), but the moisture transport into South and East Asia is stronger than that in MJ, mainly due to the distinct pattern of the ASM circulation.

Fig. 5.
Fig. 5.

Vertically integrated moisture transport (vectors; 107 kg m−1 s−1) and vertically integrated moisture flux divergence (shading; mm day−1) in (a) CTRL, and the differences (c) between CTRL and ICnoLD, (e) between CTRL and ICnoTP, and (g) between ICnoTP and ICnoLD, from 16 May to 10 Jun. (b),(d),(f),(h) As in (a), (c), (e), and (g), but from 16 Jul to 15 Aug. Black vectors and dotted areas in (c)–(h) exceed the 5% significance level of the t test.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

Fig. 6.
Fig. 6.

Regional net vertically integrated moisture budget (VIMT; blue/red large-sized numbers; 107 kg s−1) and the VIMT across key boundaries (black small-sized numbers) defined in Table 1 in (a) CTRL, and the differences (c) between CTRL and ICnoLD, (e) between CTRL and ICnoTP, and (g) between ICnoTP and ICnoLD from 16 May to 10 Jun. (b),(d),(f),(h) As in (a), (c), (e), and (g), but from 16 Jul to 15 Aug.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

When the ICP’s orography is included in MJ (CTRL minus ICnoTP; Fig. 5e), the moisture convergence is significantly enhanced over South Asia and South China, but strikingly restrained over the tropical Indian Ocean, SCS, and North China. The positive net VIMT anomalies over the ICP, WNP, and South China are contributed by both the zonal and meridional VIMT anomalies, but the increased VIMT from the Arabian Sea to the Indian Peninsula and decreased VIMT over the tropical Indian Ocean and North China are mainly attributable to the moisture transport from the meridional boundaries (Fig. 6e). Compared with the remarkable difference in MJ, the moisture convergence in the Asian monsoon region remains almost unchanged in ICnoTP compared with that in CTRL in JA, except for North China and the SCS (Fig. 5f). About 0.8 × 107 and 2.1 × 107 kg s−1 of water vapor is carried out of these two regions, accounting for about 15.1% and 20.4% of the total in the CTRL run, respectively (Fig. 6f).

The differences in moisture convergence between ICnoTP and ICnoLD in the two summer stages (Figs. 5g,h) are consistent with the overall precipitation changes in Figs. 4c and 4f. As indicated in Fig. 6g, the region from the Arabian Sea to the BOB and the WNP lose about 6.0 × 107 and 4.5 × 107 kg s−1 during the early summer, respectively, mainly in relation to the weakened low-level westerly with the presence of the land–sea contrast (Fig. 5g). However, about 4.2 × 107 and 5.0 × 107 kg s−1 of water vapor is transported zonally from the Indian Ocean and WNP into the ICP and SCS, respectively, which can promote the intensified convective activities there. Both South and North China receive more moisture from the southern boundary owing to the accelerated southerly wind, accounting for about 25.0% and 130.0% of the total in CTRL. From MJ to JA, much more water vapor is transported from the western boundaries of the ICP to the Indian Ocean, giving rise to profound convergent moisture anomaly in South Asia and divergent moisture anomaly from the SCS to WNP. North China obtains more meridional VIMT from South China, and thereby the net VIMT in North China increases by about 15.1%.

5. Responses of ASM circulation and the possible underlying physical mechanisms

It is widely known that the regional and planetary-scale atmospheric circulation plays a vital role in regulating the moisture transportation and ASM rainfall. The changes of the moisture convergence could modulate the release of latent heat and lead to a cooling/warming of the air, which in turn shapes the circulation over the ASM. Therefore, it is meaningful to investigate the interaction between the circulation and precipitation in relation to the orography and land–sea contrast over the ICP. To further reveal the mechanisms behind the ICP’s effect on the ASM, we next investigate the relative responses of the local and large-scale circulation, as well as the “thermal adaption” effect to the existence of orography and land–sea contrast.

a. Response of ASM circulation to the orography of the ICP

Figure 7 illustrates the climatological mean horizontal winds, omega, and geopotential height at different levels during the early summer. To further pinpoint the ICP’s effect, Fig. 8 shows vertical cross section along the red-line segment in Fig. 7b. In the MJ period, the terrain over the ICP forces anomalous ascending and sinking air along the mountain slope. The active (suppressed) convection on the windward (leeward) side of the ICP is strengthened owing to the interaction of the wind and terrain (Figs. 7c,g,k and 8a,b,e). In addition, the deep uplifting southwesterly anomaly on the windward side of the ICP originating from the BOB converges with the midtroposphere westerly jet near 25°N in the southeast of the Tibetan Plateau, which helps to transport moisture to the mei-yu region and promote the development of convection there (Figs. 7g and 8e). This result is consistent with the findings of Wu and Hsu (2016), who suggested that this might be related to more realistically simulating the mechanical effect of the Tibetan Plateau on mei-yu rainfall. Inclusion of the mountains of the ICP also weakens the westerly and southerly in the lower-middle layer in the SCS, which results in the west ridge of the 500-hPa subtropical high in the WNP extending southwestward from the SCS in ICnoTP (22°N, 110°E; purple lines in Fig. 7e) to the southeastern ICP in CTRL (15°N, 105°E; red lines in Fig. 7e). Thus, an anticyclonic anomaly and a cyclonic anomaly occupy the SCS and East Asia, respectively (Fig. 7g). In the upper troposphere, a stronger South Asia high moves northwestward (purple lines in ICnoTP versus red lines in CTRL in Fig. 7i), resulting in an anticyclonic/cyclonic anomaly in the western Tibetan Plateau/East China and an anomalous northerly wind from South Asia to the SCS (Fig. 7k).

Fig. 7.
Fig. 7.

Horizontal winds (vectors; m s−1), vertical velocity (shading; hPa h−1), and geopotential height (contours; gpm) at (left) 850 hPa, (center) 500 hPa, and (right) 200 hPa, respectively, in the (a),(e),(i) CTRL, and the differences (b),(f),(j) between CTRL and ICnoLD, (c),(g),(k) between CTRL and ICnoTP, and (d),(h),(l) between ICnoTP and ICnoLD from 16 May to 10 Jun. Red, purple, and green contours in (a), (e), and (i) are the geopotential height from CTRL, ICnoTP, and ICnoLD, respectively. Black vectors and dotted areas exceed the 5% significance level of the t test.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

On the one hand, this anomalous vertical configuration can markedly weaken the upward motion in the SCS region and is not conducive to water vapor convergence and convective activity. On the other hand, a quasi-barotropic cyclonic anomaly is induced in East China. The anomalous northerly in the west of the cyclonic anomaly prohibits the transportation of water vapor from central China to North China, while the ascending southerly anomaly in the southeast of the cyclonic anomaly enhances the WNP monsoon rainfall. Moreover, the southward retreat and westward extension of the midtropospheric subtropical high further cause the shrinking of the anticyclone at 500 hPa over the northern Arabian Sea and a resultant cyclonic anomaly appears. The enhanced southwesterly jet in South Asia and the deepened BOB trough in the lower troposphere facilitate the ascent there and transport more moisture from the tropical Indian Ocean to South Asia, giving rise to a significant increase (decrease) in the precipitation over South Asia (the tropical Indian Ocean). Therefore, the ICP’s orography can notably shape the large-scale monsoon circulation and enhance the vertical coupling of the ASM circulation during the early summer.

In the JA period, although the large-scale circulation in South Asia is not sensitive to the ICP’s mountains, the orographic impact on the East Asian monsoon circulation is much stronger than that in MJ (Figs. 9c,g,k). The troughing effect of the ICP enhances the southwesterlies in the western SCS and shifts the subtropical high in the WNP westward to the eastern Tibetan Plateau (Fig. 9e). Accordingly, it gives rise to an obvious anticyclonic anomaly appearing from East China to the WNP in the lower to middle layers, along with positive geopotential height and anomalous descending flow (Fig. 9c,g). In the upper layer, a stronger South Asia high extends eastward and an anticyclonic anomaly is thus located over East Asia. The significant downward flow anomaly in the middle to upper layers in the SCS outweighs the low-level upward motion related to the troughing effect of the ICP, which is ultimately not conducive to the development of the SCS summer monsoon (Figs. 8g,h,k). At the same time, the enhanced southeasterly in the northern SCS and southwesterly in East Asia transports more water vapor from the SCS across the center of China to the region near Japan, further suppressing the convective activity over the SCS and East Asia but intensifying the rainfall near Japan. Besides, the terrain-induced ascent (descent) and rain belt (rain shadow) anomalies on the windward (leeward) slopes of the ICP are also large.

Fig. 8.
Fig. 8.

Zonal circulation (vectors; m s−1), meridional wind (shading; m s−1), and precipitation (blue solid line; units: mm day−1) along the red-line segment shown in Fig. 7b, from (a) CTRL, (b) ICnoTP, and (c) ICnoLD, and the differences (d) between CTRL and ICnoLD, (e) between CTRL and ICnoTP, and (f) between ICnoTP and ICnoLD from 16 May to 10 Jun. (g)–(l) As in (a)–(f), but from 16 Jul to 15 Aug. Black vectors and dotted areas in (d)–(f) and (j)–(l) exceed the 5% significance level of the t test.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

Fig. 9.
Fig. 9.

As in Fig. 7, but from 16 Jul to 15 Aug.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

b. Response of ASM circulation to the land–sea contrast of the ICP

Next, we further explore the large-scale circulation response to the land–sea contrast of the ICP in both early and late summer. As we can see from Fig. 7, the monsoonal trough over the SCS gets stronger in the MJ period (Fig. 7d) and the west ridge of the subtropical high in the middle layer moves northeastward from the eastern ICP in ICnoLD (15°N, 105°E; green lines in Fig. 7e) to South China in ICnoTP (22°N, 110°E; purple lines in Fig. 7e) with the land–sea contrast presented in the model. As a result, a cyclonic anomaly at 500 hPa appears over the SCS (Fig. 7h) and an anomalous divergent circulation exists in the SCS in the upper layer (Fig. 7l), which provides a favorable condition for the ascent and intensifies the water vapor convergence there. However, the region from East China to the WNP is controlled by an anticyclonic anomaly throughout the entire troposphere, thus promoting (restraining) the ascending motion and moisture convergence in East China (the WNP) (Figs. 8b,c,f). Besides, Fig. 7e also indicates that the 500-hPa anticyclone in the northwestern Arabian Sea becomes strengthened slightly and extends eastward with the presence of land–sea contrast. The resultant anticyclonic anomaly in the middle layer provides an adverse condition for the ascent in South Asia.

Comparison between the atmospheric responses to the orography and land–sea contrast in the MJ period (Figs. 7 and 8a–f) enables us to identify the key processes of the surface properties over the ICP in regulating the ASM. The responses of the monsoon circulation over South Asia to the ICP during the early summer are controlled by the orography through modulating the anticyclone over the northern Arabian Sea, while the changes over the SCS, WNP, and East Asia are mainly associated with the midlevel subtropical high moving northeastward with the presence of the land–sea contrast.

The land–sea contrast also has a strong and remote impact on a monsoonal scale in late summer, but the effect of the land–sea contrast in JA differs from that in MJ. From early summer to late summer, the anticyclonic circulation anomaly over East China and the WNP (Figs. 7d,h) becomes continuously strengthened and merges with the anticyclonic circulation anomaly over the northeastern ICP (Figs. 9d,h). Meanwhile, the cyclonic anomaly over the SCS becomes weakened and retreats southward, along with the intensified subtropical high extending southwestward in JA (Fig. 9e; green lines in ICnoLD versus purple lines in ICnoTP). At the upper level, a convergent anomaly can be seen from the ICP to WNP, as the South Asia high extends westward (Fig. 9i). The region within 100°–150°E, 10°–20°N is controlled by a strong southerly anomaly and the region from the Indian Peninsula to the southern BOB is occupied by a strong northerly anomaly (Fig. 9l). In short, the land–sea thermal contrast can induce a quasi-baroclinic circulation anomaly over both South and East Asia. This circulation configuration greatly weakens the ascent and modifies the moisture convergence from the northern ICP to WNP, causing a substantial decrease in rainfall by more than 6 mm day−1 over the SCS and WNP (Figs. 8h,i,l). However, it strengthens the upward motion in the equatorial Indian Ocean and southern SCS, which is conducive to the occurrence and development of convection. In North China, the enhanced westerly jet anomaly near 40°N in the mid-to-upper layers is notably strengthened, leading to an increase in precipitation in this region by more than 25% (Fig. 4h).

In comparison, the responses of the monsoonal circulation associated with the orography are much weaker than those forced by the land–sea contrast in the JA period, as shown in Figs. 8j–l and 9. It indicates that the ASM is related more to the land–sea thermal contrast than to the narrow mountains of the ICP during the late summer. However, the uplifting and sheltering effect of the ICP’s orography is more substantial in modulating the rainfall over the topographically complex areas of the ICP and western SCS.

c. Possible mechanisms behind the changes in the ASM rainfall related to the ICP

The Asian monsoonal circulation and precipitation change considerably when the orography and land–sea contrast are modified. But what is the possible mechanism behind this feature? Based on the above diagnostic analysis, we speculate that the local and large-scale impacts of the orography of the ICP on the ASM in the two boreal summer stages are mainly mechanical. This is coherent with previous studies (Chang et al. 2005; Xie et al. 2006; Qi and Wang 2012) that highlight the profound interaction between onshore winds and narrow mountains. The terrain–wind interaction promotes the occurrence of orographic convection over the ICP. The local response to the forcing of the terrain can further modulate the larger-scale moisture transport and has a remote effect on the South and East Asian monsoons, with significantly increased rainfall over the onshore BOB in MJ and decreased rainfall over the SCS and North China in both the MJ and JA periods (sections 4a, 4b, and 5A).

As for the distinct impact of the land–sea thermal contrast, the thermal gradient, both zonally and meridionally, is one of the key factors active in shaping and maintaining the ASM (Liang et al. 2005). Figure 10 shows the temporal evolution of the surface sensible heat and latent heat along latitudes and longitudes. In ICnoTP, notable surface sensible heat (more than 35 W m−2) dominates the ICP from 1 to 31 May (Figs. 10b,e). The sensible heat flux then decreases rapidly as the monsoonal rainfall appears in early June, and the surface latent heat correspondingly increases rapidly and blocks the shortwave radiation heating the surface (Figs. 10h,k). In ICnoLD, the zonal and meridional thermal gradients of the ICP are nearly zero from early May to August (Figs. 10c,f), but stronger latent heat, with values exceeding 160 W m−2, generally occurs over the ICP from the middle of June to late August (Figs. 10i,l). Therefore, the enhanced SCS monsoon in the MJ period is closely related to the profound zonal and meridional thermal contrast due to the presence of the ICP. The weaker SCS monsoon in the JA period involves atmospheric processes associated with the markedly reduced latent heat over the ICP and SCS from ICnoLD to ICnoTP (Figs. 10h,i,k,l).

Fig. 10.
Fig. 10.

Longitude–time cross section of the surface sensible heat flux (W m−2) along 10°–22.5°N in (a) CTRL, (b) ICnoTP, and (c) ICnoLD. Latitude–time cross section of the surface sensible heat flux along 95°–110°E in (d) CTRL, (e) ICnoTP, and (f) ICnoLD. (g)–(i),(j)–(l) As in (a)–(c) and (d)–(f), respectively, but for the surface latent heat flux (W m−2).

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

Moreover, it has been suggested in previous studies that the “thermal adaption” effect exerts a vital influence in shaping the ASM. As such, we next diagnose the relationship between the circulation and moist convection using the complete form of the vertical vorticity tendency equation, and analyze the possible mechanisms of the ICP’s land–sea contrast affecting the monsoonal circulation and precipitation in the two summer stages (Wu et al. 1999). The simplified form [Eq. (4)] has already been used to study the mechanism behind the modulation of monsoon by the orography and land–sea contrast (Wu et al. 2008; Hu et al. 2020). As we know from the analysis in section 4a, the land–sea contrast of the ICP significantly enhances the precipitation over the SCS in the MJ period and leads to a substantial increase in the condensational latent heat released to heat the middle-to-upper atmosphere. Figure 11 shows the vertical profile of the diabatic heating source Q1 averaged over the SCS and WNP. The diabatic heating anomaly between ICnoTP and ICnoLD is positive over the SCS in MJ, and the increase in Q1 reaches a maximum at about 400 hPa (Figs. 11a,b). Therefore, the Q1 anomaly increases with altitude below 400 hPa (Q/z>0), but decreases with altitude above 400 hPa (Q/z<0; solid orange line in Fig. 11b). This, according to Eq. (4), will help the southerly (northerly) wind anomaly appearing in the lower (upper) troposphere over the SCS (Figs. 7d,l) and induce a cyclonic anomaly generated in the west of the heating center in the lower level. It could further promote upward movement and moisture convergence over the SCS.

Fig. 11.
Fig. 11.

Vertical profile of the apparent heat (Q1; 10−2 W kg−1) averaged over (a),(b) the South China Sea from 16 May to 10 Jun and (c),(d) the northwestern Pacific from 16 Jul to 15 Aug.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

During the late summer, the monsoon rainfall from the northern ICP to WNP is notably reduced due to the presence of the ICP’s land–sea contrast, with a negative cooling center at 400 hPa. The relevant Q1 in ICnoTP is much smaller than that in ICnoLD (green line vs orange line in Fig. 11c), with the Q1 anomaly decreasing (increasing) with altitude below (above) 400 hPa (solid orange line in Fig. 11d). The vertical distribution of the Q1 anomaly is favorable for weakening the low-level southwesterly and high-level northeasterly, which in turn restrains the vertical wind shear over the SCS and WNP (Figs. 9a,i). Consequently, the ascending motion is weakened to the point that it and inhibits moisture convergence and convective activity over the SCS and WNP. Besides, it also induces anticyclonic (cyclonic) circulation anomalies in the lower (upper) troposphere (Figs. 9d,l), which further suppress the convection from the northern ICP to WNP. The decrease in precipitation there will in turn strengthen this anomalous circulation configuration.

d. Contribution of the land surface processes over the ICP to the ASM

Although the difference between ICnoTP and ICnoLD is regarded as the land–sea thermal contrast in this study, it also involves the contribution from the land surface processes, such as the terrestrial vegetation, soil moisture, and so on. We further performed another idealized experiment named ICnoLDts to estimate the actual contribution of the land–sea thermal contrast over the ICP and address the potential impact of the land surface processes on the ASM. ICnoLDts is identical to ICnoLD except that the ICP in ICnoLDts was driven by the surface temperature of the ICnoTP. The atmospheric responses between ICnoLDts and ICnoLD represent the effect of the pure land–sea thermal contrast of the ICP.

In both early and late summer, the rainfall and horizontal wind anomalies between ICnoLDts and ICnoLD are quantitatively similar to those between ICnoTP and ICnoLD. However, the atmospheric responses over the Arabian Sea, which are due to the pure land–sea contrast over the ICP (Figs. 12a,c), are almost in the opposite phase compared to those in Figs. 4c, 7d, and 7h in early summer. Besides, the cyclonic (anticyclonic) circulation anomaly over the SCS (WNP) in early summer is weaker (stronger) in Figs. 12a and 12c. This could be related to the contribution of the land processes. In late summer, the differences between Figs. 12b and 12d and Figs. 4f, 9d, and 9h are subtle, which indicates the ASM is not sensitive to the land surface processes and the effect of the land–sea thermal contrast is dominant during the late stage of the ASM. This is consistent with Yang and Lau (1998), who suggested that the land surface wetness mainly affects the early stage of the ASM.

Fig. 12.
Fig. 12.

Differences of (a),(b) precipitation (shaded; mm day−1) and 850-hPa horizontal winds (vectors; m s−1) and (c),(d) differences of vertical motion (shaded; hPa h−1) and 500-hPa winds between ICnoLDts and ICnoLD (left) from 16 May to 10 Jun and (right) from 16 Jul to 15 Aug. Black vectors and dotted areas exceed the 5% significance level of the t test.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

6. Summary and discussion

This study investigates the relative roles played by the orography and land–sea thermal contrast of the ICP in the seasonal variation of the ASM during two summer stages, with the aid of an atmospheric general circulation model. By comparing the model results from three sensitivity experiments (i.e., with the ICP, with the ICP but without its orography, and with ocean only), it is suggested that the robust interaction between the terrain-forced local circulation and large-scale ASM circulation, as well as the positive feedback between the local and large-scale circulation and monsoonal precipitation associated with the subcontinental surface heating, enables the ICP’s orography and land–sea contrast to exert a profound effect on the ASM.

In early summer, inclusion of the ICP’s mountains weakens the low-level trough over the southwestern SCS along the eastern coast of the ICP, thereby causing the subtropical high in the WNP to extend southwestward and the weaker midlevel anticyclone over the Arabian Sea to retreat westward (Fig. 13a). The resultant cyclonic anomalies over South and East Asia intensify the South Asian monsoon and weaken the monsoonal rainfall over the SCS and North China. In the JA period, the orographic effect of the ICP is mainly limited to the SCS and East Asia. Although the enhanced rain belt (rain shadow) on the windward (leeward) slope of the ICP is similar to that during the early summer, the response in East Asia is almost opposite (Fig. 13b). The troughing effect of the mountains induces a low-level cyclonic anomaly over the SCS and a stronger subtropical high at 500 hPa in the WNP extends westward. The resultant divergent wind in the middle level restrains the low-level upward movement, resulting in a decrease of approximately 15% in precipitation over the SCS and from central China to the WNP.

Fig. 13.
Fig. 13.

Schematic diagram of the atmospheric response to the (a),(b) orography and (c),(d) land–sea contrast of the Indochina Peninsula during the (left) early and (right) late summer. SH and SAH denote the subtropical high and South Asia high, respectively. The red, purple, and green 500-hPa anticyclones and subtropical high are from CTRL, ICnoTP, and ICnoLD, respectively.

Citation: Journal of Climate 35, 10; 10.1175/JCLI-D-21-0576.1

With the presence of the ICP, the strong and persistent thermal gradient of the ICP in early summer promotes the convection over the SCS by enhancing the SCS trough. Meanwhile, the induced anticyclonic anomalies in South Asia and the WNP weaken the monsoonal rainfall there by as much as 10%–20%, as the zonal and meridional land–sea contrast of the ICP favors the subtropical high moving northeastward and a stronger anticyclone at 500 hPa over the Arabian Sea extending eastward. Besides, the significant rainfall-related latent heat release acts to intensify and maintain the vertical wind shear and summer rainfall over the SCS according to the “thermal adaption” effect (Fig. 13c). From MJ to JA, the anticyclonic anomaly from East China to the WNP becomes continuously strengthened and integrates with that over South Asia in late summer, thereby shifting the intensified subtropical high southwestward and significantly suppressing the convection from the northern ICP to the WNP. Furthermore, the decreased rainfall-related latent heat release at 400 hPa restrains the low-level southwesterly and high-level northeasterly and causes the South Asia high to extend westward (Fig. 13d). The weakened vertical wind shear and the resultant divergent anomaly further weaken the monsoonal rainfall from the ICP to the WNP, which in turn strengthens this anomalous circulation configuration. Moreover, the precipitation in North China and the southern SCS increases by about 30% owing to the enhanced meridional moisture transport. Further model results also demonstrated that the land surface processes play an essential role in the ASM, which can reverse the monsoonal circulation and precipitation anomalies over the Arabian Sea and partly modulate the atmospheric response over the SCS and WNP due to the land–sea contrast in early summer.

In general, the orography and land–sea contrast over the ICP exert profound but different impacts on the Asian monsoon in the two summer stages, and this is because of the interaction among the induced local circulation, planetary-scale monsoonal system, and rainfall-related condensational latent heat release. In contrast, the orographic effect over the ICP is nearly opposite to that of the land–sea contrast in early summer (Figs. 13a,c), but the topography is dominant in shaping the South Asian monsoon and the rainfall in North China. However, the land–sea contrast plays an essential role in the intensified SCS monsoon, with the rainfall increased by more than 20% owing to the enhanced zonal moisture transport. For the mei-yu region, both the mountains and land–sea thermal gradient are important. Compared with the orography, the ICP’s land–sea contrast in late summer governs the noticeable changes in the Asian monsoonal rainfall and circulation by modulating the multilevel ASM subsystems, except for the complex topographic region of the ICP.

Previous modeling studies have suggested that the mesoscale orography over the ICP helps shape the ASM in the two boreal summer stages through the air–sea interaction (Qi and Wang 2012) and enhance the vertical coupling of the midlayer troughing effect and lower-tropospheric southwesterly flow (Wu and Hsu 2016). However, the emphasis of this study was on exploring the distinct remote effects of the ICP’s mountains from the perspective of the interaction among terrain-induced local circulation and large-scale monsoon circulation (e.g., the SCS trough, the subtropical high in the WNP, and the South Asian high) and the resultant water vapor transport. The responses of the lower to midlevel atmospheric circulation over East Asia are consistent with those found in Wu and Hsu (2016) in early summer. The anomalous anticyclone or cyclone at multiple levels in late summer related to the mechanical effect of the orography in our modeling results shifted more to the southwest than that in Wu and Hsu (2016), but was similar to that in Qi and Wang (2012). Considering that current climate models have significant biases when simulating both the South and East Asian summer monsoons (Zhou et al. 2018), a multimodel approach is needed in the future to clarify the influence of the ICP on the ASM. In addition, this study also highlights the distinct impacts of the land–sea contrast of the ICP on the ASM between the MJ and JA periods and quantitatively compares the relative contributions of the topography and land–sea thermal contrast to the ASM.

Considering that the model employed in this study (i.e., FAMIL) is an atmospheric general circulation model without oceanic feedback and air–sea interaction, which are important to realistically reproduce the ASM (Kitoh 2004; Duan et al. 2008; Song and Zhou 2014), more simulations with air–sea coupled processes included are needed to reach unified conclusions as to the local and remote effects of the ICP’s surface properties on the seasonality of the ASM. Besides, it has been suggested that simulation of the ASM is sensitive to the model resolution, and a higher horizontal resolution helps to improve the model biases to some extent (Kitoh et al. 2010; Sabin et al. 2013). In view of the fact that the C96 horizontal resolution of FAMIL is still not precise enough to perfectly resolve the terrain and land–sea distribution of the ICP, further numerical modeling simulations at a higher resolution are required.

Acknowledgments.

This work was jointly supported by Guangdong Major Project of Basic and Applied Basic Research (2020B0301030004), the National Natural Science Foundation of China (41725018, 42105026, 42030602, and 42005039), and the State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences (Project LTO2117). We thank the three anonymous reviewers for their suggestions that greatly improve this manuscript and Dr. Yu Zhao for helpful discussions.

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