Effects of the Tibetan Plateau on the Asian summer monsoon: a numerical case study using a regional climate model

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 30: (2010) Published online 20 April 2009 in Wiley InterScience ( DOI: /joc.1906 Effects of the Tibetan Plateau on the Asian summer monsoon: a numerical case study using a regional climate model Jee-Hey Song, a,b Hyun-Suk Kang, a,c * Young-Hwa Byun, c and Song-You Hong a a Department of Atmospheric Sciences and Global Environment Laboratory, Yonsei University, Seoul, Korea b Graduate School of Environmental Studies, Seoul National University, Seoul, Korea c Climate Research Laboratory, National Institute of Meteorological Research, Korea Meteorological Administration, Seoul, Korea ABSTRACT: To understand impacts of the Tibetan Plateau (TP) upon the Asian summer monsoon (ASM), including both the Indian summer monsoons (ISM) and East Asian summer monsoons (EASM), a series of numerical experiments using the National Centers for Environmental Prediction (NCEP) regional spectral model (RSM) are conducted with various TP heights ranging from the flat surface (1.2 km) to 140% of the actual height. It was found that an increase in the TP height leads to an increase in the simulated ISM precipitation over northern India and conversely a decrease in the height leads to a decrease in precipitation. This sensitivity is associated with both the thermal and dynamical effects of the TP; however, although the ISM precipitation over India is affected by the changes in atmospheric circulation, it is not directly affected by the thermal effect. The thermal effect of surface heating plays a role in developing positive vorticity with a consequent increase in monsoon precipitation over northern India. The width of the plateau also seems to be associated with the intensity of the sensitivity for the ISM region. For the EASM region, the orographic effect caused by changes in the lower-atmospheric circulation and its link with upper-atmospheric circulation are crucial to the monsoon circulation and precipitation. With increased TP height, the monsoon precipitation moves inland in a northwestward direction, which qualitatively follows the previous findings based on general circulation models (GCMs), but with a detailed dynamical mechanism in linkage between the lower- and upper-atmospheric circulation in the regional climate modelling (RCM) framework in this study. Copyright 2009 Royal Meteorological Society KEY WORDS Tibetan Plateau; Indian summer monsoon; East Asian summer monsoon; thermal and dynamical effects; regional climate model Received 9 June 2008; Revised 1 February 2009; Accepted 4 March Introduction Most countries in Asia suffer from severe floods and/or droughts in summer because of the inter-annual and intra-seasonal variability of the Asian summer monsoon (ASM) system. The ASM, which can be divided into two sub-systems, the Indian summer monsoon (ISM) or South Asian summer monsoon and the East Asian summer monsoon (EASM), is the largest, most energetic, and most complex monsoon system in the world. However, the EASM should not be considered simply as the eastward and/or northward extension of the ISM because these two monsoon systems are independent of each other, although they do interact (Ding and Chan, 2005). The monsoon system is basically caused by thermal contrasts between land and ocean (e.g. Tian and Yasunari, 1998; Zhao et al., 2007a,b). Tian and Yasunari (1998) have shown that the east-west thermal contrast in spring between the Indo China peninsula and the western North * Correspondence to: Hyun-Suk Kang, Climate Research Laboratory, National Institute of Meteorological Research, Korea Meteorological Administration, Seoul , Korea. hyunsuk@kma.go.kr Pacific plays the key role in persistent rainfall over central China during March and April. In addition, Zhao et al. (2007a,b) showed that EASM winds and rainfall are significantly associated with the thermal contrast between Asian continents and the western North Pacific, defined as the Asian Pacific Oscillation. Nevertheless, it is well recognized that the atmospheric response to the differential heating reveals quite complex behaviours due to the land atmosphere ocean interactions as well as multi-scale non-linear interaction among atmospheric disturbances from meso-γ to planetary scales (Ninomiya and Akiyama, 1992; Ding, 1994), which leads to difficulty in understanding the nature of the Asian monsoon system. In this context, the Tibetan Plateau (TP) has been considered as one of the major controlling factors influencing Asian monsoon activity through thermal and mechanical effects (Yanai and Wu, 2006). In the late 1940s, studies on the TP focused mainly on mechanical effects (e.g. Queney, 1948; Yeh, 1950) until the thermal effects were highlighted in the late 1950s (e.g. Flohn, 1957; Yeh et al., 1957). As an example of studies on mechanical forcing, Hahn and Manabe (1975) found Copyright 2009 Royal Meteorological Society

2 744 J-H. SONG ET AL. that the presence of mountains is partially responsible for maintaining the South Asian low pressure system, so that much warmer temperatures are produced in the middle and upper troposphere over the TP region. An et al. (2001) observed an increase in South and East Asian monsoon s activities associated with the higher TP, while Abe et al. (2003) also found strong ISM precipitation and zonal wind shear associated with the mountain uplift. Liu and Yin (2002), who investigated the sensitivity of the EASM climate to idealized progressive uplift of the TP, found little change in the ISM wind intensity but marginally somewhat significant sensitivity of the EASM. Magagi and Barros (2004) emphasized the role of the Himalayan range in providing suitable conditions for moisture transport between the TP and the Indian subcontinent, a result consistent with other studies (Lang and Barros, 2002; Barros et al., 2004). For more than three decades, numerical studies have supported the influence of the TP orographic forcing on the atmospheric circulation over the globe. For example, Manabe and Terpstra (1974) demonstrated a significant increase in cyclogenesis probability on the lee side of mountains, which implies the importance of mountain forcing on the global precipitation distribution. For the EASM region, Yoshikane et al. (2001) argued that orography, including the TP, plays a role in intensifying the low level jet (LLJ) and precipitation over the EASM front, but attributed the front genesis to only two factors: zonal mean atmospheric fields and land/sea contrast. Recently, Kitoh (2004) investigated the effects of mountain uplift on East Asian summer climate using an atmosphere ocean coupled general circulation model (GCM). According to his results, the precipitation area moves inland with the stronger subtropical Pacific High and its associated trade winds as mountain heights increase. Drastic change in the EASM circulation is apparent with a threshold value at 60% of mountain height. That is, with the TP height 60% or higher, the southwesterly monsoon flow from the Indian Ocean is strengthened by uplift and moisture transport towards East Asia, which helps maintain a quasi-stationary monsoon front. Meanwhile, with mountains higher than the actual height, an intensified subtropical high plays an important role in transporting moisture from the Pacific into the Asian land mass rather than the southwesterly monsoon flow. He also found larger sensitivity of the EASM to the mountain uplift than to the atmospheric GCM alone. As a heat source in the warm season, TP is the region of highest near-surface potential temperature which implies that the air moves inward and upward along the flanks of the plateau, producing cyclonic circulation tendency near the surface (Hoskins, 1991). Yanai et al. (1992) showed that the first stage of upper-tropospheric warming is primarily caused by diabatic heating and warm horizontal advection, whereas the second stage is caused by adiabatic warming because of large-scale subsidence. Hoskins and Rodwell (1995) have also shown that a prescribed diabatic heating alone without mountains is able to reproduce the upper-tropospheric anticyclonic circulation during summer. Li and Yanai (1996) showed that the ASM onset is concurrent with the reversal in meridional temperature gradient south of the TP, caused by warming of the Eurasian continent that is most pronounced over the TP area. Duan and Wu (2005) concluded that the thermal effect of the TP plays an important role in the subtropical summertime climate of south Asia because the TP acts as a strong heat source, which leads to a shallow and weak cyclonic circulation near the surface and a deep and strong anti-cyclonic circulation above. The Tibetan elevated heating plays an important role in inter-annual variability of the EASM activity as well. Zhao and Chen (2001a) found a remarkable negative correlation between the summer heat source of the Plateau and the convection over the ASM regions such as southeastern Plateau, the Bay of Bengal, the Indo China peninsula, southeastern China, etc. Furthermore, Zhao and Chen (2001b) evaluated quantitative heat sources over the TP and found a close correlation between the heat source and EASM rainfall. In nature, however, mechanical and thermal forcing cannot be separated from each other. As examples, Murakami and Ding (1982) found a northward displacement of the strong upper level jet(ulj)from30to40 N during the monsoon season and a consequent adiabatic heating over the TP caused by the uplift flow. Ringler and Cook (1999) demonstrated that the seasonal variability of the low-level stationary response can be understood only by analyzing combined effects of mechanical and thermal forcing. Liu et al. (2007) also investigated both the orographic and thermal effect of the TP on summer flow over Asia, and have shown qualitative similarity in the responses to the two forcings when they were applied separately. Despite the many efforts devoted to understanding the role of the TP on weather and climate events over Asia, most conclusions are still qualitative due to various limitations, such as lack of observations and limited knowledge about land atmosphere ocean interactions (Yanai and Wu, 2006). Furthermore, most numerical studies of the influence of the TP on the ASM have been made within a GCM framework and/or idealized simple configuration. However, there are two major limitations to understanding mechanisms responsible for Asian monsoon using GCMs. One is the predictability problem. The simulation result from the GCMs loses predictability rapidly after a few days. The other limitation is the coarse-resolution covering the globe. It seems likely that GCM s skill in simulating the proper amplitude and phase of the mean circulation and precipitation patterns over the monsoon region may be diminished by an inadequate representation of mesoscale processes. To circumvent the resolution problem in GCMs, the regional climate modelling (RCM) approach has been used. There are some examples that show that high-resolution models of the summer monsoon are better produced by the regional model nested in GCM or reanalysis data than by GCMs.

3 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 745 Please refer to several review papers for more information on applications of regional climate models such as Giorgi and Mearns (1999); Leung et al. (2003), and Wang et al. (2004). Sato and Kimura (2007) conducted a numerical study with a simplified RCM configuration, in which a few key physical processes were omitted, to separate the thermal and mechanical forcings over the TP. They found that both have a great impact on the transition of the ISM rainfall by changing the mid-tropospheric circulation. By and large, however, the quantitative impacts of the TP, especially in terms of linkage between the changes in monsoon precipitation and atmospheric circulation, have rarely been investigated in either the GCM or the RCM frameworks. In this context, our study seeks to investigate the impacts of TP height both on the ISM and EASM precipitation and associated atmospheric circulation using a RCM, i.e. the National Centers for Environmental Prediction (NCEP) regional spectral model (RSM). We evaluate the RSM s capability to reproduce the seasonal mean climate and investigate the sensitivity of the ISM and EASM climate to the TP height. Particularly, we analyze how the changes in atmospheric circulation due to the presence of TP alter the spatial distribution and precipitation of the ASM. Our study follows the methodology of Kitoh (2004) but uses a RCM approach, which has the advantages of refined physical processes and spatial resolution, as well as the ability to isolate regional-scale feedbacks from large-scale environments in an open system, by maintaining realistic large-scale forcing, which is not possible in the GCM approach (Hong and Kalnay, 2000; Park and Hong, 2004). A brief description of the model, experimental setup, and the selected case are presented in Section 2. Evaluation of the control experiment is given in Section 3. Sections 4 and 5 deal with the changes in the ISM and EASM, respectively, caused by different TP heights in terms of the seasonal mean circulation and precipitation. Finally, we give a summary of our findings and concluding remarks in Section 6. longwave and shortwave radiative transfer by considering clouds radiation interaction, the planetary boundary layer (PBL) process, deep and shallow convections, largescale precipitation, gravity wave drag, simple hydrology model, and vertical and horizontal diffusions. The soil physics utilizes a two-layer soil model based on Mahrt and Pan (1984). The PBL physics is a non-local diffusion scheme (Hong and Pan, 1996). Precipitation is produced by both large-scale condensation and deep convection schemes. The large-scale precipitation scheme used in this study does not include prognostic clouds; however, the evaporation of rain water in unsaturated condition below the condensation level is taken into account. The parameterization scheme for deep convection follows the one developed by Pan and Wu (1995) and Hong and Pan (1998) Experimental setup The model domain includes the whole TP and covers both the EASM and ISM regions as well as the Indian and western North Pacific Oceans centred on 27.5 N and 105 E (Figure 1). The model grids consist of 151 (west east) by 110 (north south) grid lines at approximately 60 km horizontal separation and 28 sigma layers in the vertical, which is the typical resolution for most RCM community. The RSM topography is made on grid-points of 60-km resolution, and transformed to spectral waves. After that, a spectral filter for smoothing is used to remove the shorter waves, similar to the finite-difference grid points mesoscale models. Initial conditions and large-scale forcing are obtained from the 6-hourly NCEP-Department of Energy (DOE) reanalysis (R-2) data (Kanamitsu et al., 2002). Observed sea surface temperatures (SST) are updated daily from the optimum interpolation sea surface temperature (OISST) weekly dataset (Reynolds and Smith, 1994). Following the approach in Kitoh (2004), four experiments are designed to investigate the role of the TP height 2. Model and experimental setup 2.1. Model The spectral representation of the NCEP RSM is a twodimensional cosine series for perturbations of pressure, divergence, temperature, and mixing ratio, but a twodimensional sine series for the perturbation of vorticity. Linear computations of horizontal diffusion and semiimplicit adjustment are considered only for perturbations, so that the error due to the re-evaluation of the linear forcing from the base fields is eliminated. Details on spectral representation and perturbation methods of the NCEP RSM are given in Juang and Kanamitsu (1994) and Juang et al. (1997). The physical package employed in the RSM is well documented in Hong and Leetmaa (1999). It includes Figure 1. The model domain with orography with 500-m interval. The inner solid box indicates the TP region for which the heights are changed for sensitivity experiments. The two dotted boxes are the Indian and East Asian summer monsoon regions used for skill score calculations in Table I.

4 746 J-H. SONG ET AL. in ASM simulations. The control experiment (TP10) uses the realistic orography obtained from the United States Geographical Survey (USGS) global digital elevation model (DEM) with a horizontal grid spacing of 30 arc s ( 1 km). To isolate the effect of TP height, we change the topography height over the TP region only (Figure 1) but maintain the real orography for other regions, which differs from the Kitoh s (2004) experimental design. In the FLAT experiment, the TP region has a flat topography with a height of 1.2 km, which is the approximate average height of the surrounding areas. The TP06 and TP14 experiments use 60 and 140% of the TP height, respectively. In summary, the topography height for each experiment is obtained as given in Equation (1): H = α(h 1.2) + 1.2, only if H>1.2 (1) where H is the modified height (km), H is the original height (km), and α is the factor to alter the topography height: 0.0, 0.6, 1.0 and 1.4 for FLAT, TP06, TP10, and TP14 experiments, respectively, in which the corresponding maximum values of TP height are 1.2, 3.6, 5.2, and 6.8 km. Large-scale base fields are different among the four experiments over the TP region because they are reinterpolated vertically according to modified topography height. To do this, the surface pressure is first calculated using the modified topography with the hydrostatic assumption. Then, the zonal and meridional wind components, potential temperature, and specific humidity are interpolated vertically from the R-2 pressure levels to the RSM sigma levels by a cubic spline method. The atmospheric perturbations caused by vertical reinterpolation among four experiments may dissipate in a few days, so their impacts on seasonal averages are not significant (Park and Hong, 2004). Nevertheless, to remove natural variability of the model, all experiments consist of three ensemble members. Each member was initialized on 30 May, 31 May, and 1 June of 2004 and run for 3 months [June July August, (JJA)]. Preliminary experiments revealed that the magnitude of sensitivities of each ensemble member does not differ in terms of monthly and seasonal climatology, which is consistent with the results of Park and Hong (2004). Thus, the significance of the ensemble mean for each experiment will not be shown except when necessary Case description A typical summer of East Asia is characterized by longlasting rainy days during June and July accompanied by a quasi-stationary monsoon front, i.e. Mei-yu in China, Changma in Korea, and Baiu in Japan, and a hot spell from late July to mid-august. Meanwhile, over South Asia, the rainy and hot weather has a relatively longer period from May to September. The climatological features of monsoon evolution are well described in Ding and Chan (2005). In this study, we choose the 2004 JJA, comprehensively reviewed by Levinson et al. (2005), because, compared to the long-term climatology, this year had typical summer monsoon behaviours in East Asia but with a slightly dry condition. In addition, there were only limited regional-scale impacts associated with El-Niño and the southern oscillation (ENSO) event in this year. China, which covers the major part of the East Asian monsoon area, experienced a drier ( 22 mm) and warmer (+0.8 C) year in 2004 compared with the period of However, the precipitation anomaly, compared to the 26 years from 1979, shows relatively small magnitudes in China, Korea, and Japan, which implies that the EASM behaviour in 2004 seems to be close to normal (Figure 2(a)) but with a slight dry condition. Meanwhile, the Indian monsoon produced deficient rainfall during 2004, at about 84% of the average, leading to a drought condition particularly in the Indian Ocean and the Bay of Bengal. In the 2004 JJA, besides the persistent rain band associated with the inter-tropical convergence zone (ITCZ) near 15 N over the western North Pacific, two monsoonal precipitation bands are distinct, as shown in the Global Precipitation Climatology Project (GPCP) dataset (Huffman et al., 2001) (Figure 2(b)). One is the ISM, which extends northward from tropical monsoon areas near the Equator to the Indo China peninsula, Bay of Bengal, and the western coast of India. The other one is the precipitation associated with the EASM, which includes rainfall over southeastern China, the Mei-yu over the Yangtze River Valley, the Changma in Korea and the Baiu in Japan. The beginning of the local rainy season is highly associated with the strengthening of the spring southwesterly wind, the tropospheric upward motion, and the convergence of low-level water vapour over southeastern China (Zhao et al., 2007a). In addition to the GPCP precipitation data, which has 1 resolution, we also used stationbased precipitation measurements to obtain a detailed pattern of observed precipitation over land (Figure 2(c)). This data set was obtained from the summary of the day and month dataset from the NCEP Climate Prediction Center (CPC) ( which extracts surface synoptic weather observations from the Global Telecommunications System (GTS) and performs limited automated validation of the parameters. The data is then summarized for all reporting stations on a daily basis to meet current operational requirements related to the assessment of crop and energy production. The number of stations used for the current model domain is approximately 1200 per day. 3. Control experiment We first evaluate the simulation results of the control experiment (TP10) before looking into the results from the various sensitivity experiments. In the following, only the seasonal mean averages over JJA are shown and discussed. Overall capability of the model will be assessed over the whole domain. Large-scale model features will be evaluated against the R-2 data and precipitation against the GPCP and CPC observation data.

5 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 747 Figure 2. (a) Precipitation anomaly (mm/month) of the 2004 JJA from the period of ; (b) and (c) are GPCP analyzed and CPC station measured precipitation (mm/month) for 2004 JJA, respectively. Table I. Statistics of the bias, the root mean square errors (RMSE), and the pattern correlation for the M10 experiment over the whole domain. Observed data are the GPCP for precipitation and R-2 reanalysis for others. India and East Asia regions are the inner boxes shown in Figure 1 and the entire area indicates the region covering E, EQ 55 N. Variable Bias RMSE Pattern correlation All India EA All India EA All India EA Precipitation (mm day 1 ) hpa Temperature ( C) hpa Temperature ( C) hpa Temperature ( C) hpa Wind speed (m s 1 ) hpa Wind speed (m s 1 ) Large-scale features The TP10 experiment captures the large-scale features associated with the ASM circulation (Figure 3 and Table I). At 850 hpa (Figure 3(a) and (d)), the westerly Somalia jet, which is important for warm and moist air transport toward India and South Asia, is simulated fairly well compared to the R-2 data. The slightly stronger low-level flow in the TP10 experiment near southern India, the Bay of Bengal, and the South China Sea

6 748 J-H. SONG ET AL. Figure 3. Average of the R-2 reanalysis for 2004 JJA (left panel) and from which the difference by the TP10 experiment (right panel) for (a, d) the 850 hpa winds and relative humidity (RH), (b, e) the 500 hpa geopotential height (GPH) and temperature, and (c, f) the 200 hpa winds. Units are m s 1 for winds, % for RH, gpm for GPH, and oc for temperature. Shadings indicate (a) RH values greater than 70%, (c) wind speed greater than 20 m s 1, (d) RH difference, (e) temperature difference, and (f) wind speed difference. contributes to the northward displaced and wet biased simulated precipitation (Figure 4). General patterns of the 500 hpa geopotential height (GPH), temperature fields of the TP10 results and R-2 data are largely consistent with one another (Figure 3(b), (e)). For example, the TP10 experiment reproduces the two planetary scale troughs in the mid-latitudes and the northwestern Pacific High, as well as the thermal low over the TP (not shown). Although distinctive differences in the 500 hpa temperature appear over the TP region and northeastern China,

7 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 749 Figure 4. The seasonal precipitation (mm month 1 ) for 2004 JJA simulated by the TP10 experiment. their magnitude is less than or equal to about 1 2 C. The GPH differences are profound over East Asia, which is consistent with the warm bias in temperature. Accordingly, a difference in upper-atmospheric circulation also shows enhanced anti-cyclonic circulation over East Asia with a difference around 4 m s 1 in wind speed. Table I summarizes the basic statistics for systematic bias, root mean square error (RMSE), and pattern correlation between the TP10 experiment and the R-2 reanalysis. Bias of the simulated temperature indicates that the RSM tends to stabilize the simulated climate by presenting cold bias in the lower level but warm bias in the upper level, except for the East Asia region, where the warm bias is evident at the mid-troposphere. In general, the RSM produces better scores in terms of pattern correlation for the upper-atmosphere than for the lower-atmospheric variables and precipitation, which indicates that the largescale R-2 reanalysis does not include regional scale characteristics in the lower-atmosphere while the TP10 experiment does, as shown in other models (e.g. Fu et al., 2005) Precipitation The TP10 experiment, which uses the actual TP height, reproduces the seasonal mean precipitation fairly well (Figures 4 and 2(b)). The RSM is able to capture the major precipitation areas of the Indian Ocean tropical monsoon near the Equator, the western North Pacific near the Philippines, the ISM (western coast of India, northeastern India and Bay of Bengal), and the East Asian subtropical monsoon. In spite of a wet bias of about 1.4 mm day 1 for the entire domain, the spatial pattern of the simulated precipitation agrees well with the GPCP data, with a correlation of 0.63 (Table I). Some detailed spatial features in the simulated precipitation seem to be more realistic than the GPCP data. For example, several local maxima are apparent in the simulated precipitation over the Indian subcontinent, the Indo China peninsula, and southern China, which are consistent with the station-measured observations shown in Figure 2(c). Despite the overall agreement in the large-scale features between the TP10 and R-2 reanalysis, significant discrepancies exist over the ISM region in terms of precipitation amount and positions of local maxima, which result in pattern correlation worse than the domain average (Table I). For example, the maximum observed JJA precipitation near the western coast of India is about 300 mm month 1 in GPCP (Figure 2(b)) and about mm month 1 in the CPC archives (Figure 2(c)), while the simulated precipitation is greater than 1000 mm month 1 (Figure 4). In addition, there are several local maxima of more than 600 mm month 1 over northeastern India (along the southern flank of TP), which are not shown at all in the GPCP precipitation, but still apparent in the CPC observation. For the EASM region, the RSM tends to overestimate the summer precipitation over land, which is a systematic characteristic of the model (Kang and Hong, 2008), particularly over southeastern China, where the simulated amount is about mm month 1 but the observed amount is only about mm month 1.Therainfall associated with the Mei-yu over the Yangtze River Valley appears to be shifted northwestward so that the amount of rainfall in northeastern China is overestimated by about 50 mm month 1 compared to the GPCP. However, the existence of local maxima in precipitation extended from southern China to the northern border of Korean peninsula can be confirmed clearly from the CPC station-measured precipitation data (cf Figures 4 and 2(c)). The simulated precipitation over the Korean peninsula and Japan looks quite close to the observed precipitation. Figure 5 compares the lower-atmospheric circulation between the TP10 simulation and the R-2 reanalysis over the EASM region, which is the large-scale forcing to drive the RSM. Compared to the R-2 data, the TP10 experiment reproduces the LLJ in association with monsoonal precipitation and anti-cyclonic circulation along the western periphery of the subtropical high fairly well. However, the intensity of the southerly and southwesterly monsoon flow that transports warm and moist air toward the mid-latitudes is overestimated, especially over southern China and its nearby ocean. The relatively intense monsoon flows simulated by the TP10 experiment are attributed to the thermal and pressure gradients between land and ocean, caused by warm bias over land and cold bias over ocean, which are about +1 2 C and 1 C on average, respectively. This feature of lower-atmospheric circulation is consistent with wet bias and northwestward displacement of the precipitation over land, as simulated by the TP10 experiment.

8 750 J-H. SONG ET AL. Figure 5. Comparison of the 850-hPa geopotential height (solid), temperature (dashed), wind vectors, and relative humidity greater than 70% between (a) the R-2 reanalysis and (b) the TP10 experiment averaged over 2004 JJA. 4. Impacts of the Tibetan Plateau on Indian summer monsoon 4.1. The FLAT experiment To examine the TP effects on the ISM region, we analyze the differences in seasonal precipitation between the TP10 and FLAT runs. Changes in the simulated precipitation over the Indian subcontinent are discernable (Figure 6(a)); these show increases in central and northeastern India but a decrease in southern India with statistical significance of 90% (not shown). An increase in surface heating because of the presence of the TP, as shown in Figure 6(a), is partially responsible for the development of thermal low and positive vorticity circulation near the surface at 500 hpa (Figure 6(c)) and upper-level anti-cyclonic circulation (Figure 6(d)) in this region. However, differences in precipitation seem to be more associated with the lower-atmospheric circulation rather than with surface heating because differences in surface sensible heat flux are marginal over both northeastern and southern India, where differences in precipitation are statistically significant. Meanwhile, the TP obstructs the southwesterly Somalia jet in the loweratmosphere (Figure 6(b)), which results in generation of the relative convergence over the southern flank of the TP centred over Pakistan and northeastern India. The spatial pattern in the horizontal convergence/divergence is largely consistent with that in the simulated precipitation. The magnitude of differences in surface heating of about 20 W m 2 (Figure 6(a)), and the corresponding changes in atmospheric condition seem to be underestimated because the RSM response to the changes in local forcing, i.e. the TP height, tends to be suppressed by the large-scale external forcing obtained from the real TP height. However, the direction of the response is consistent with previous studies on its mechanism. That is, the surface heat source is transferred to the atmospheric column by diffusion and produces a maximum heating near the surface. Then, it decreases gradually with increasing height and the atmospheric thermal adaptation to such a heating should produce positive vorticity in the lower-layer but negative vorticity in the upper layers aloft (Yanai and Wu, 2006). This mechanism can be confirmed in differences of the vertical structure between the TP10 and FLAT experiments. Figure 7(a) shows the changes in the vertical structure of the GPH and zonal winds, averaged over E, because of the presence of the TP. In the upperatmosphere, the TP plays a role in developing anticyclonic circulation, which leads to a decrease of zonal winds in the south and an increase in the north. Meanwhile, in the lower atmosphere, the changes in zonal winds and GPH show opposite signs to those in the upper-level because of the relative cyclonic circulation in northern India, as shown in Figure 6(b). The thermal effects of the presence of TP are shown in Figure 7(b), which shows prevailing surface heating over the TP with maximum values greater than 3 C. As discussed in Yanai and Wu (2006), the surface heating near the TP surface produces cyclonic (positive) vorticity in the shallow lower layer, but anti-cyclonic (negative) vorticity in the deep upper layers aloft. The results obtained from a previous composite analysis between strong and weak heat sources over the TP (Zhao and Chen, 2001a) also support the validity of our simulation results. Because the FLAT experiment cuts off the TP (where the height is greater than 1.2 km) for somewhat wide areas between N, the atmospheric columns with strong negative vorticity appear over corresponding widths in a north south direction. However, over the ISM region south of the TP, changes in temperature are not significant (Figure 7(b)). Therefore, it is difficult to claim that the relatively cyclonic circulation and decreases of GPH in the lower-troposphere in northern India are directly caused by the thermal effect of the TP. Meanwhile, orographic forcing of the TP as an obstacle for the southwesterly monsoonal flow across the plateau may cause

9 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 751 Figure 6. Differences between the TP10 and FLAT experiments (TP10 minus FLAT) for (a) the precipitation (shaded; mm month 1 ) and sensible heat flux (contour; W m 2 ), (b) the 850 hpa moisture flux (vector; 10 2 ms 1 ) and convergence (shaded; 10 5 s 1 ), (c) the 500 hpa relative vorticity (contour; 10 5 s 1 ) and temperature (shaded; C), and (d) the 200 hpa absolute vorticity ( 10 5 s 1 ) and streamline. cyclonic circulation and convergence of the warm and moist air in northeastern India (Figure 6(b)) Sensitivity to TP height In this subsection, we focus on the TP06 and TP14 experiments, which adopt a 40% lower- and higher-tp height, respectively. Figure 8 shows the differences in simulated precipitation and 850 hpa circulation of these two sensitivity experiments from the control experiment, i.e. the TP10. The TP06 experiment tends to decrease and the TP14 tends to increase the precipitation over the northern Indian region, north to the 20 N (Figure. 8(a) and (b)) with a 90% statistical significance level. In the south

10 752 J-H. SONG ET AL. Figure 7. Differences in vertical structure averaged over E between the TP10 and FLAT experiments (TP10 minus FLAT)for (a) the geopotential height (contour; gpm) and zonal winds (shaded; m s 1 ), and (b) the relative vorticity (contour; 10 5 s 1 ) and temperature (shaded; C). to 20 N, differences in the simulated precipitation have opposite signs compared to the north. The reduced precipitation in the TP06 experiment is associated with the relative anti-cyclonic circulation in that region (Figure 8(c)), whereas the increase in the TP14 experiment is associated with the relative cyclonic circulation over northern India (Figure 8(d)). It is also interesting to note that the magnitude of differences in the simulated precipitation is larger or comparable to that of the differences between the TP10 and FLAT experiments. This seems to be associated with the width of the TP s height difference. That is, a narrower width of the TP s height difference leads to more intensive response to the changes in local forcing, which is explained below. Changes caused by different TP heights in the vertical structure of the atmospheric circulation and temperature averaged over E are shown in Figure 9. The opposite signs in differences of the zonal wind and GPH are apparent between TP06 minus TP10 (Figure 9(a)) and TP14 minus TP10 (Figure 9(d)). Because of the relatively smaller difference in TP height between the TP10 and both the TP06 ( 40%) and TP14 (+40%), magnitudes of response to the upper-tropospheric circulation are also smaller than that between the TP10 and FLAT experiments. Nevertheless, the narrower width of TP height difference than that between the TP10 and FLAT experiments leads to more compressed changes of the vertical structure. Compared to the TP10 experiment, the TP06 (TP14) experiment shows a relatively strong and negative (positive) vorticity over and in the northern slope of the TP because of surface cooling (heating) (Figure 9(b) and (e)). On the contrary, in its north and above, there exist positive (negative) vorticity in deep atmospheric layers, which leads to the upper-level low (high) in the TP06 (TP14) experiment. Accordingly, the compensating negative (positive) relative vorticity is apparent in northern India in the TP06 (TP14) experiment, resulting in relatively anticyclonic (cyclonic) circulation in the lower-atmosphere (Figure 9(b) and (e)), and consequently decreased (increased) simulated precipitation (Figure 9(c) and (f)). Differences in temperature between these two sensitivity experiments (Figure 9(b) and (e)) may be attributed in part to diabatic heating because of changes in vertical motion and the simulated precipitation. Meanwhile, the magnitude of changes in the GPH, zonal winds, and temperature are smaller in the TP14 than in the TP06 experiment; however, changes in vorticity and precipitation are larger in the TP14 experiment because of the squeezed width of the TP height difference. This feature may be associated with a non-linear response to the TP. However, this is beyond the scope of this study because the one-way nesting RCM approach does not allow feedback between the large-scale forcings. 5. Impacts of the Tibetan Plateau on the East Asian summer monsoon 5.1. The FLAT experiment Changes in the upper-atmospheric circulation near the TP region also significantly affect the EASM. Compared to the FLAT experiment, the presence of TP in the TP10 causes a significant change in the upper-atmospheric circulation because of the Tibetan High developed over the plateau (Figure 10(a)). Without the TP, planetaryscale mid-latitude troughs appear, one west of the TP at about 80 E and the other in far East Asia near Japan, at about 140 E (not shown). The two troughs are separated by a well-developed ridge, and both the troughs and ridges are excessively well developed because of the

11 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 753 Figure 8. Differences between the TP06 and TP10 (TP06 minus TP10; left panel) and between the TP14 and TP10 (TP14 minus TP10; right panel) for (a, b) the simulated precipitation (shaded; mm month 1 ) and t-values with the 90% confidence level (contour), and (c, d) the 850 hpa moisture flux (vectors; 10 2 ms 1 ) and convergence (10 5 s 1 ). weakened Tibetan High in the absence of mountain uplift. Obviously, this feature in the upper-atmospheric circulation simulated by the FLAT experiment deviates substantially from the actual summer climatology, in which there are two upper level anti-cyclonic centres over the Asian continent, one over the TP and the other over the Iran Afghanistan region (Park and Schubert, 1997). A dipole pattern with the opposite sign is apparent in the difference of 200 hpa GPH between the TP10 and FLAT experiments (TP10 FLAT), which shows positive values over the plateau and negative values to the east. The positive difference near the TP region plays a major role in weakening the trough west of the TP and in strengthening the ridge over the TP, whereas the negative difference

12 754 J-H. SONG ET AL. Figure 9. Differences in vertical structure between the TP06 and TP10 (TP06 minus TP10; left panel) and between the TP14 and TP10 (TP14 minus TP10; right panel) averaged over E for (a, d) the geopotential height (contour; gpm) and zonal wind (shaded; m s 1 ), (b, e) the relative vorticity (10 5 s 1 ) and temperature (shaded; C), and (c, f) the precipitation (mm month 1 ). east of the TP region plays a role in weakening the ridge over northeastern China. The changes in the simulated precipitation and lower level atmospheric circulation over East Asia between the FLAT and TP10 experiments (TP10 FLAT) reveal that the TP10 experiment produces more precipitation in most of the EASM region. These features are associated with the relatively low pressure system and the corresponding cyclonic circulation that develops in this region (Figure 10(b)), also consistent with the results from Zhao and Chen (2001a). The changes in atmospheric circulation due to the presence of TP are better seen in the vertical structure of the changes in eddy GPH and meridional wind components along 35 N (Figure 11). In the upper level, the relative high is distinctive and prevails over the plateau region between E, whereas a strong

13 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 755 Figure 11. Differences in vertical structure between the TP10 and FLAT experiments (TP10 minus FLAT) for geopotential height (contour; gpm) and meridional wind (shaded; m s 1 ) along 35 N. Figure 10. Differences between the TP10 and FLAT experiments (TP10 minus FLAT) for (a) the 200 hpa geopotential height (gpm) and (b) the 850 hpa geopotential height (contour; gpm), wind vectors, and precipitation (shaded; mm month 1 ). relative low appears to the east. This negative anomaly reaches downward with a slight eastward tilting, resulting in enhanced cyclonic circulation over East Asia with a maximum anomaly at about 850 hpa near 120 E. Such a low-level cyclonic circulation anomaly over East Asia tends to increase the monsoonal circulation and associated rainfall Sensitivity to TP height Figure 12 shows the 200 hpa GPH departures from the TP10 experiment for the lower-tp (TP06) and higher-tp (TP14) experiments, in which there is a clear opposite sign over the TP. That is, because the TP plays a role in forming the upper-atmosphere anti-cyclone, the TP06 experiment produces significantly negative GPH difference values over the TP region. Conversely, TP14 produces positive values. Meanwhile, impacts of the TP height on the upper-air atmospheric circulation over northeastern China are quite marginal but statistically significant, with 95% confidence in a somewhat limited area. In addition, the 200 hpa GPH differences are positive over northeastern China in both the TP06 and TP14 experiments, even though the intensity seems to be quite weak in the TP06 experiment. This marginal significance of the TP impacts over northeastern China should be investigated further to lead to a robust conclusion, because the atmospheric circulation in this region is quite important to govern the EASM circulation. In the EASM region, including southern and central China as well as the Korean peninsula, the differences of the upper-level GPH in the lower- and higher-tp experiments have opposite signs as compared to the TP10 experiment. Changes in the lower-atmospheric circulation, temperature and precipitation are shown in Figure 13. Marginal changes in temperature around ±0.5 C appear in the eastern and northern flanks of the TP at 850 hpa; however, these changes are not significant compared to those in the ISM region, i.e. northern India (Figure 13(a) and (c)). Therefore, thermal effects of the TP tend not to have a direct impact on the EASM circulation and precipitation. However, it does not mean that Tibetan thermal effects are not important on the EASM, because it is possible that changes due to thermal effects are more pronounced in other meteorological fields (e.g. precipitation, winds) than on air temperature. The EASM is mainly governed by three major flows: the southwesterlies from the Indian Ocean, a cross-equatorial southerly from near the Philippines, and an easterly trade wind from the western Pacific. Therefore, the lower-atmospheric circulation is crucial to EASM development and its migration. The lower-atmospheric circulation over southern China, particularly in the eastern flank of the TP, shows distortion

14 756 J-H. SONG ET AL. 6. Summary and concluding remarks Figure 12. Differences in the 200 hpa geopotential height (gpm) between (a) the TP06 and TP10 experiments, and (b) the TP14 and TP 10 experiments. Shaded areas indicate t-values greater than 95% confidence level. compared to the TP10 experiment. The TP06 experiment (Figure 13(a) and (b)) produces little change in the lower-atmospheric circulation, whereas the TP14 experiment (Figure 13(c) and (d)) has somewhat large differences compared to the TP10 experiment. For example, the relative anti-cyclonic centre appears over East China Sea in the TP14 experiment because of the cyclonic anomaly centre in the eastern TP associated with its higher orography. Consequently, the simulated precipitation is also consistent with the lower-atmospheric circulation (Figure 13(b) and (d)). That is, the lower-tp experiment (TP06) leads to reduction in precipitation over southern China because of the relative anti-cyclonic circulation, while the higher-tp (TP14) experiment leads to northwestward displacement of EASM precipitation because of the enhanced anti-cyclonic circulation over East China Sea. Our results confirm previous coupled GCM study results that concluded that the precipitation area moves inland with the stronger subtropical Pacific High and its associated trade winds as mountain heights increase (Kitoh, 2004). In this study, we investigated the impact of the height of the TP on the seasonal climate of the ASM using the NCEP RSM with a grid distance of 60 km. Four experiments with various TP heights (flat ogrography, 60, 100 and 140% of the real orography) were driven by the R-2 reanalysis and OISST forcing for the summer of 2004, and each experiment consisted of three ensemble members to remove internal variability of the model. In summary, a greater-tp height leads to more precipitation in northern India and less precipitation in southern India, and with a lower-tp height, the opposite precipitation patterns are simulated. The mechanism of sensitivity of ISM precipitation to the TP height obtained in this study is consistent with previous studies (e.g. Hoskins, 1991; Yanai and Wu, 2006). That is, surface heating over the plateau plays a role in producing cyclonic vorticity in the shallow lower-layer but negative vorticity in the deep upper layers aloft through atmospheric thermal adaptation. Such changes of thermal structure and corresponding vorticity in the vertical are more profound in the northern slope of the TP than in its south; however, the compensating negative relative vorticity is still apparent in south to the TP in the low TP experiment, resulting in changes of the lower atmospheric circulation over the ISM region and decreased monsoon simulated precipitation, and conversely positive relative vorticity and increased precipitation in the higher-tp. Even with the same 40% change in the height of the TP, different magnitude responses were observed between the lowerand higher- TP experiments. That is, the magnitude of change in GPH, zonal winds, and temperature are smaller in the high TP run than in the low TP run; however, changes in relative vorticity and precipitation are larger in the higher-tp experiment because of the slightly more squeezed width of the TP height difference. This feature may be associated with non-linear response to the TP. However, this is beyond the scope of this study because feedback with large-scale forcing is not allowed in the RCM approach, which is a limitation of this study. In the EASM region, thermal effects of the TP are not as significant as in the ISM region. Near surface temperature changes are limited over the eastern and northern flanks of the TP to about 0.5 C. Instead, changes in the lower-atmospheric circulation are found to be critical. Nevertheless, this does not mean unimportant Tibetan thermal effects on the EASM, because the changes caused by thermal effects may lead more pronounced changes in other meteorological fields (e.g. precipitation, winds) rather than in atmospheric temperature field. With the lower-tp height, little change appears in the loweratmospheric circulation, while with the higher-tp height, the EASM is more affected by the northwestern Pacific High, so that the monsoon precipitation moves northwestward compared to that in the control experiment. These results are consistent with those from a CGCM study by Kitoh (2004), in which the simulated precipitation area moves inland with the stronger subtropical Pacific

15 EFFECTS OF THE TIBETAN PLATEAU ON THE ASIAN SUMMER MONSOON 757 Figure 13. Differences in the 850 hpa between the TP06 and TP10 experiments (TP06 minus TP10; left panel) and between the TP14 and TP10 experiments (TP14 minus TP10; right panel) for (a, c) the geopotential height (contour; gpm) and temperature (shaded; C) and (b, d) the wind vector difference (vector) and precipitation (shaded; mm month 1 ). In (a, c), red colour arrows indicate the 850 hpa wind from the TP06 and TP14 experiment, respectively, and black colour arrows that from the TP10 experiment. High and its associated trade winds as mountain heights increase. More significant sensitivity in the higher-tp experiment seems to be associated with linkage of the relative anti-cyclonic circulation between the upper- and lower-atmosphere. That is, the enhanced northwestern Pacific High at 850 hpa and upper-level anti-cyclonic circulation over northeastern China have a northwestward tilted structure in the vertical, which is a more favourable condition to be maintained. The results of this study can be debated, as changes in the planetary scale system due to the presence of TP are not taken into account, as they would be in a GCM approach. However, the GCM approach has its own set of limitations, such as the rapid decrease in predictability of the integration time and the coarse-resolution. The RCM approach used in our study has the advantages of both high resolution and the ability to maintain realistic boundary conditions and large-scale forcing, which make it possible to isolate regional feedback from the model solution (Hong and Kalnay, 2002; Park and Hong, 2004). Therefore, although precise quantitative measures of the changes in the monsoonal circulation may not be possible because of the lack of interaction with large-scale forcing, a firm conclusion as to the overall major role of the TP in forming the ASM can be deduced. Another limitation of our study is that it examines only the single year of 2004, even though it is a nearnormal year in terms of ASM activity. In order to develop robust and quantitative measures of the TP impacts, further investigations for the long-term period using a