Influences of elevated heating effect by the Himalaya on the changes in Asian summer monsoon
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Based on a series of topographical and thermal sensitivity experiments, the physical processes on the changes of Asian summer monsoon caused by the Himalaya elevated heating were investigated. Six different Himalaya–Iranian Plateau mountain heights were used: 0, 20, 40, 60, 80, and 100 % in the first group (called HIM). The no sensible heating experiments (called HIM_NS) were also performed with the same six mountain heights, but the surface sensible heating was not allowed to heat the atmosphere. The results indicate that the elevated heating effect of the Himalaya gradually intensified when the Himalaya uplifts. The establishment of SASM over the South Asian land which is characterized by the strong precipitation over south slope of the Tibetan Plateau and the huge warm anticyclone in the upper troposphere are in proportion to the elevated heating effect of the Himalaya. Further analysis suggests that the surface heat fluxes over the Himalaya keep almost unchanged during the uplifting, but the lifted condensation level reduces gradually over the regions where the mountain uplifts. The condensation moisturing increases correspondingly and leads to the increase of latent heating in the upper troposphere. Therefore, the positive feedback between the moist convection over the south slope of the Himalaya and monsoon circulation over Indian subcontinent forms and the successive precipitation over the South Asian land is maintained.
KeywordsTibetan Plateau Asian Summer Monsoon East Asian Summer Monsoon Diabatic Heating Monsoon Precipitation
The highlands in the world are important topographical forcings to the global climate change. The Tibetan Plateau (TP) located in the Asian continent is the largest mountain where on its south edge there lies the Himalaya, the highest peak in the world. Previous studies documented that the existence of Tibetan Plateau (TP) has close relationships with the formation and variation of the Asian summer monsoon (Manabe and Terpstra 1974; Wu and Zhang 1998; Duan and Wu 2005; Zhang et al. 2006; Kitoh et al. 2010; Liu et al. 2012). Hahn and Manabe (1975) tested mountain to no-mountain experiments in a general circulation model (GCM) and found the South Asian summer monsoon (SASM) cannot extend north to the Asian inland if the topography of TP is removed in the model. More complex experiments such as testing different topographic heights of the TP in the GCMs have been performed in later studies (Chen et al. 1999; Liu 1999; Kitoh 2004; Jiang et al. 2008), their results show an identical conclusion that the with the uplift of the TP height from 0 to the contemporary era height, the South Asian summer monsoon precipitation extends gradually from the Indian Ocean to the Asian inland with the peak monsoon precipitation occurring on the south slope of the Himalaya which is similar to the evolution of the observed Asian summer monsoon precipitation. Moreover, Liu and Yin (2002) indicated that the evolution of East Asian monsoon is more sensitive to the uplift of the Tibetan Plateau than that of the South Asia monsoon. Kitoh (2004) suggested that the increasing of precipitation over Asia reaches a threshold when the TP is uplifted to 60 % of its realistic altitude.
Recently, more and more studies revealed that the response of Asian summer monsoon is quite sensitive to thermodynamic forcing in different subregions of TP. Boos and Kuang (2010) showed the spatial pattern and strength of the SASM are almost unaffected when the plateau part of TP is removed but the Himalaya is preserved. Similar results had been achieved by Tang et al. (2013) with regional model. They suggested that the Indian summer monsoon is primarily intensified by the forcing of southern TP while the East Asian summer monsoon is mainly enhanced by the forcing of the central TP. Zhang et al. (2012) who used CAM4 to test the different subregional uplifts within the Himalaya–Tibetan Plateau on Asian summer monsoon evolution and found that the increasing precipitation over South Asia is more sensitive to Himalaya uplift. In a word, a number of recent numerical experiments show that the existence of the Himalaya is more important than the whole TP in controlling evolutions of SASM.
However, the associated physical process that the Himalaya modulating the Asian summer monsoon is still not clear. Boos and Kuang (2010, 2013) emphasized the blocking effect of the Himalaya produce strong SASM by insulating warm, moist air over continental India from the cold and dry extratropics while its thermal forcing is not important. Nevertheless, Wu et al. (2012a) indicated that the experiment in Boos and Kuang (2010) still contains the heating effect of the Himalaya mainly on its south slope which could act as a strong heating source to produce monsoon precipitation and circulation. Analogously, in the series studies of the topography uplift experiments (Chen et al. 1999; Liu 1999; Kitoh 2004; Jiang et al. 2008), the extension of Asian summer monsoon is the results of the combined effects of thermal and mechanical forcing by the uplifted TP, and the physical processes related to the Asian summer monsoon evolution are not clearly demonstrated. It is necessary to take additional thermal experiments on the uplifted Himalaya to further address the role of the Himalaya in producing strong Asian summer monsoon and the associated physical processes.
Therefore, this article attempts to clarify several specific scientific questions in studying the role of the Himalaya in Asian monsoon dynamics: whether is the elevated heating effect of the Himalaya important than the non-elevated heating in producing strong Asian summer monsoon over Asian continent? If elevated heating is crucial, what is the decisive factor of the heating effect, the increased elevation, or its thermal properties? And what is the associated physical process? To address these issues, we here carry out a series orographical and thermal experiments based on progressive idealized Himalaya–Iranian Plateau uplift and investigate the differences between heating and no heating experiments. The remainder parts of the paper are arranged as follows: Section 2 introduces the model we used and the experimental design. Section 3 presents the simulation results: the responses in monsoon precipitation and circulation to elevated heating of the Himalaya. Section 4 analyzes the changes in diabatic heating and moist process to reveal the processes of the Himalaya elevated heating effect in modulating monsoon activities. Section 5 shows the final conclusions and discussions.
2 Model, datasets, and experimental design
2.1 Model and datasets
The general circulation model used here is the version-2 Spectral Atmosphere Model developed at IAP-LASG, SAMIL2 (Bao et al. 2013). The atmospheric model SAMIL2, has the horizontal resolution R42 (2.81° longitude × 1.66° latitude) with 26 vertical layers in a σ–p hybrid coordinate, extending from the surface to 2.19 hPa. The mass flux cumulus parameterization of Tiedtke (1989) is used to calculate convective precipitation. The cloud scheme is a diagnostic method parameterized by low-layer static stability and relative humidity (Slingo 1980). A nonlocal scheme is employed to calculate the eddy-diffusivity profile and turbulent velocity scale, and the model incorporates nonlocal transport effects for heat and moisture (Holtslag and Boville 1993). The radiation scheme employed is an updated Edwards–Slingo scheme (Edwards and Slingo 1996; Sun and Rikus 1999). SAMIL2 is a spectral model which has been frequently used to study Asian monsoon dynamics (Duan et al. 2008; Wu et al. 2012b, He et al. 2013; Hu et al. 2015) and climate changes in various aspects (Ren et al. 2009; Wu and Zhou 2013; Zhang and Zhou 2014; Song and Zhou 2013).
The reanalysis dataset used in this study is from the ECMWF ERA-40 (Uppala et al. 2005) monthly mean data from 1979 to 1998 at http://apps.ecmwf.int/datasets/, and the precipitation dataset used is Global Precipitation Climatology Project (GPCP) (Adler et al. 2003) monthly mean data from the same period at http://www.esrl.noaa.gov/psd/data/gridded/data.gpcp.html.
2.2 Experimental design
T 0 + 0.2 × (T 100 − T 0)
T 0 + 0.4 × (T 100 − T 0)
T 0 + 0.6 × (T 100 − T 0)
T 0 + 0.8 × (T 100 − T 0)
The other group (referred to HIM00_NS–HIM100_NS) also contains six experiments in which the topography is the same as the first group (Table. 1), but the surface is not allowed to heat the atmosphere over 20–40° N, 60–110° E where the elevation is above 500 m in each experiment, i.e., the vertical diffusive heating term in the atmospheric thermodynamic equation was set to 0 in the regions of the Himalaya. All of the experiments are integrated with prescribed, seasonally varying climatological (1990–1999) sea surface temperature (SST) and sea ice. The experiments are integrated for 7 model years, in which the first 2 years are spin up time and the mean of the last 5 years are used for analysis. The differences between HIM and HIM_NS group experiments in the condition of progressive Himalaya uplift indicate the sensitivity of SASM to the topographical elevated heating.
3.1 Model and experimental design validations
On the other hand, the scope of this study is to investigate the elevated heating effect of the Himalaya by isolating the Himalaya from the full topography. Therefore, whether the experimental design effectively distinguishes the heating effect on progressive uplift mountain is the key foundation. We show the model grids as the blue cross lines in Fig. 1. It can be found that there are about three to four model grids in the region where the Himalaya uplifts. From this perspective, the experimental design in this study can discern the sensible heating effect on different altitude on the slope of the Himalaya. However, because there are few grid points on the south slope of TP, these grid points can hardly stand for the drastic altitude changes of the topography which could contribute to the positive precipitation bias over the south slope of TP (Fig. 2a) in the model compared to the OBS (Fig. 2b).
3.2 Changes in precipitation and 850 hPa wind
On the other hand, we also compare the monsoon responses within the no sensible heating runs, i.e., HIM00_NS to HIM100_NS (Fig. 2b, d, f, h, j, l). The results show that the SASM cannot extend to the Asian inland and no significant increases in the 850 hPa wind field. These results are consistent with the theory proposed in Wu et al. (2007) that, if the south slope of the Himalaya is not heated, the air particle must move along the isentropic surface and cannot climb up on the huge mountain. Therefore, the no sensible heating experiments suggest that the blocking effect of the Himalaya is not the driven force of the strong SASM on the Asian inland.
3.3 Changes in the South Asian High and its thermal structure
Therefore, the changes in Asian summer monsoon in the two groups of Himalaya uplift experiments indicate that the elevated heating effect of the Himalaya is intensified when mountain is uplifted. It also suggests the heating effect of the Himalaya rather than its blocking effect in producing monsoon precipitation over south slope of TP, which is consistent with the conclusions in Wu et al. (2012a). Moreover, why the elevated heating effect is intensified by the uplift of the Himalaya is still not clear and needs to be further addressed. Therefore, we analyze the changes in diabatic heating and moist process in the following section to discuss the related physical processes.
4 The processes of elevated heating
4.1 The analysis in atmospheric diabatic heating
4.2 The analysis in moist process
For the climate mean state we discussed here, the changes in the column water vapor, the surface evaporation, and the precipitation are the most three important terms in the hydrological cycle. The responses of the precipitation due to the elevated heating effect of the Himalaya are already shown in Fig. 4. Thus, we show the responses of atmospheric water vapor and surface evaporation in the following paragraph to have a complete image on the moist process over the South Asia when Himalaya uplifts.
5 Conclusions and discussions
The influences of the elevated heating effect of the Himalaya on the South Asian summer monsoon are investigated based on a series of numerical experiments in this study. We isolate the topography of the Himalaya from the whole Tibetan Plateau in the experiments and consider its different altitudes by 0 % (HIM00), 20 % (HIM20), 40 % (HIM40), 60 % (HIM60), 80 % (HIM80), and 100 % (HIM100) in the first group (HIM). The other group of experiments (HIM_NS) have been performed by preventing the surface sensible heating to heat the atmosphere (HIM00_NS, HIM20_NS, HIM40_NS, HIM60_NS, HIM80_NS, HIM100_NS) but with the same elevations as in the first group. The differences between group HIM and HIM_NS are regarded as the responses to the elevated heating effect of the Himalaya. The major conclusions obtained from the sensitivity experiments are summarized as follows.
The results of HIM group indicate that the SASM precipitation gradually intensified over the South Asian land when the Himalaya uplifts. The simulated precipitation spatial pattern in HIM100 (100 % uplift) is similar to the observed one over the South Asian region. However, due to the missed topography of the plateau part of TP, the precipitation over Central Asia disappeared in the HIM100. Furthermore, the extension and intensity of the East Asian summer monsoon have been reduced when the Himalaya uplifts from 0 to 100 %. The evolutions of the SASM from HIM00 to HIM100 are consistent with the simulation results of Kitoh (2004) who used a coupled model to test the influences of different mountain heights on the Asian summer monsoon. The north branch of SASM is gradually established on the south slope of the Himalaya.
The differences between HIM and HIM_NS indicate that the formation of SASM over the South Asian land is quite sensitive to the elevated heating effect of the Himalaya. The monsoon precipitation on the south slope of the Himalaya and lower troposphere cyclonic circulation gradually intensified when surface sensible heating elevated as the Himalaya uplifts. Moreover, the increased latent heating in the free troposphere act as an important heating source in the formation of the SASM structure: The upper troposphere air becomes warmer and the South Asian High is strengthened and extends west over the Asian continent.
The analysis of the diabatic heating and moist process suggested that the surface sensible heating directly heats the air in the lower troposphere on its south slope, and the moist air is easier to be lifted up to the LCL when the Himalaya uplifts. Therefore, the condensation moisturing increases correspondingly and leads to the increases of precipitation and latent heating in the free troposphere. Because the latent heating strengthened mainly from 300 to 700 hPa, the vertical inhomogeneous diabatic heating force an anticyclone in the upper troposphere and moist convergence in the lower troposphere which forms the positive feedback between the moist convection and large-scale circulations.
This study highlights the elevated heating effect of the Himalaya is crucial in triggering moist convections and modulating Asian summer monsoon over the South Asian inland. The numerical simulation results also testify the assumption raised in Ye and Gao (1979) and Wu et al. (1997) that the sensible heating of TP is so important because it directly heats the air in the middle and lower troposphere. However, currently, there are still shortages on the accurate estimation of the sensible heating fluxes over the Tibetan Plateau. For example, Ma et al. (2011) emphasized that the parameterization methodology on estimating heat fluxes over TP is not an easy issue, and more field observations, more accurate radiation transfer models, and more satellites data have to be used to reduce the uncertainty in the calculation of heat fluxes over TP. Yang et al. (2009) developed a new scheme on estimating the heat fluxes over TP based on the statistical relationships derived from high-resolution experimental data and found it is much better than previous schemes. Chen et al. (2013) developed a DEM (digital elevation model)-based radiation model to estimate instantaneous clear sky solar radiation for obtaining accurate energy absorbed by the mountain surface. Therefore, the new schemes and datasets should be used in parameterizing the heat fluxes over TP in the GCMs to get more realistic simulation on the TP climatic effect studies.
Another challenge in studying TP dynamics is the development of the high-resolution global models. The Himalaya is very steep in the reality and its influence to Asian climate calls for a finer model with 10–25 km resolution to distinguish the mountain with the whole TP. Bacmeister et al. (2014) compared the simulation results between the high-resolution (0.23° latitude × 0.31° longitude) model, Community Atmosphere Model versions 5 and 4 (CAM5 and CAM4), and a typical resolution of 0.9° latitude × 1.25° longitude. Overall, the simulated climate of the high-resolution experiments is not dramatically better than that of their low-resolution counterparts, but improvements appear primarily where topographic effects may be playing important role, such as the summertime Indian monsoon. Furthermore, to improve the simulation ability of high-resolution model, the improvements in the model physics and introduction of nonhydrostatic framework are both necessary (Pope et al. 2007; Satoh 2008).
Additionally, air–sea interaction need to be considered in the climate simulation of the TP elevated heating impactions. Many studies (Kitoh 1997; Okajima and Xie 2007; Koseki et al. 2008) claimed that the role of air–sea interaction is very important to the orgraphic effects on the monsoons and global climate change. Kitoh (1997) shows that the global mean sea surface temperature (SST) dropped 1.4 °C with the mountain uplift, mainly due to increased lower tropospheric clouds in the subtropical eastern Pacific. Okajima and Xie (2007) indicates that the response is opposite between the atmospheric and coupled models during spring and early summer when the global orography is removed, while the SST cooling is responsible for all these differences. Therefore, the responses of SASM and global climate to the elevated heating effect of TP could be very different when considering the air–sea coupling. Further numerical experiments and analysis are suggested to be carried out to get a more comprehensive understanding on the roles of the TP on climate change.
This study was jointly funded by the China Meteorological Administration (No. GYHY201406001), the National Key Basic Research Program of China (Grant 2014CB953904), the National Natural Science Foundation of China (Grants 41405091, 91337110), the State Key Laboratory of Loess and Quaternary Geology (Grant SKLLQG1216), and Strategic Leading Science Projects of the Chinese Academy of Sciences (Grant XDA11010402).
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