Tibetan Plateau Heating Research Articles

Tibetan Plateau vortex‐associated precipitation and its link with the Tibetan Plateau heating anomaly

AbstractThe Tibetan Plateau (TP) is known as the Asian water tower that supplies fresh water for millions of people in Asia. The Tibetan Plateau vortices (TPVs) are one of the major precipitation‐producing weather systems over the TP. The characteristics of TPV‐associated precipitation from 1979 to 2017 are quantitatively analysed based on three daily precipitation datasets. The influence of TPVs on precipitation is more significant in the warm season (May–September) because the precipitation and TPV activities both peak in the warm season. The TPV‐associated precipitation (VAP) dominates the total precipitation over most of the TP in the warm season. The VAP contributes more than 50% of the total precipitation in the warm season over 30% of the area over the TP, particularly in the central TP, while the VAP accounts for more than 25% of the total precipitation in the cold season over about 20% of the area over the TP. Therefore, TPVs are the major systems to produce precipitation on the TP in the warm season, and they are also important in the cold season in certain regions. The interannual variation of VAP is mainly determined by the number, rather than the precipitation intensity, of TPVs. Furthermore, the heating anomaly caused by the TP thermodynamic effect is an influential factor of VAP. Stronger (weaker) TP heating and resulting ascending motion strengthen (attenuate) the convergence near the TP surface and the divergence in the upper troposphere. The thermal adaption‐associated circulation generates more (less) TPVs and VAP.

Seasonal Variation of the Westerly Jet over Asia in the Last Glacial Maximum: Role of the Tibetan Plateau Heating

AbstractThe westerly jet (WJ) is an important component of atmospheric circulation, which is characterized by prominent seasonal variations in intensity and position. However, the response of the WJ over Asia during the Last Glacial Maximum (LGM) is still not clear. Using general circulation model experiments, the seasonal behaviors of the WJ over central Asia and Japan are analyzed in this paper. The results show that, compared to the present day (PD), the WJ presents a complicated response during the LGM, both in intensity and position. Over central Asia, it becomes weaker in both summer and winter. But over Japan, it is enhanced in summer but becomes diminished in winter. In terms of position, the WJ over central Asia shifts southward in both summer and winter, whereas the WJ over Japan moves southward in summer but does not change obviously relative to PD in winter. Such WJ changes are well explained by meridional temperature gradients in high troposphere, which is closely linked to seasonal thermal anomalies over the Tibetan Plateau (TP). Despite cooler LGM conditions, the anomalous warming center over the TP becomes stronger in summer. Derived from the heat budget equation, the stronger heating center is mainly caused by the weaker adiabatic cooling generated from ascending motion over the area south of the TP. In winter, the cooling over the TP is also strengthened, mostly owing to the subsidence-induced weaker adiabatic heating. Due to the importance of the WJ, the potential role of TP thermal effects should be a focus when explaining the East Asian climate change during the LGM.

Precursor Effect of the Tibetan Plateau Heating Anomaly on the Seasonal March of the East Asian Summer Monsoon Precipitation

AbstractThe East Asian summer monsoon (EASM) has complex drivers and is characterized by an abrupt onset and northward jumps of the main rain band. By analyzing multisource observational data and reanalysis data sets during 1961–2016, we addressed the connection between the preceding diabatic heating of the Tibetan Plateau (TP) and the seasonal march of the EASM. We show that the positive surface sensible heating and condensation heating anomalies over the TP lead the onset, duration, and total precipitation of the rainy season in South China (the first stage of the EASM) by about 2 and 1 month, respectively. A significant positive condensation heating anomaly over the TP can be detected about 1 month before the onset of the rainy season in Southwest China (the second stage of the EASM). By contrast, the relationship between heating of the TP and the onset of the rainy season is relatively weak during the remaining stages of the EASM, including the Meiyu and North China rainy seasons. We suggest that the strong heating of the TP intensifies the low‐level southwesterly circulation to the southeast and facilitates the convergence of moisture and ascending motion, leading to an earlier onset of the rainy season. Sensitive experiments with a linear baroclinic model further confirmed the role of heating of the TP in generating circulation conditions favorable for the onset of the rainy season in both South and Southwest China. The TP heating anomaly therefore acts as a precursor of the early stage onset of the EASM.

Asymmetrical Response of the East Asian Summer Monsoon to the Quadrennial Oscillation of Global Sea Surface Temperature Associated With the Tibetan Plateau Thermal Feedback

AbstractThe leading mode of the global sea surface temperature anomaly (SSTA) on the interannual time scale is a quasi‐quadrennial oscillation mode, corresponding to the El Niño–Southern Oscillation cycle. Using observational data diagnosis and numerical simulations, we show that the global SSTA during the quadrennial cycle has significant impacts on the anomaly of diabatic heating over the Tibetan Plateau (TP) in summer. Idealized atmospheric general circulation model (AGCM) experiments forced by the global SSTA with and without the thermal forcing of the TP diabatic heating anomaly reveal the relative contributions of the SSTA direct effect and the TP feedback to the anomalous response of the East Asian summer monsoon (EASM). During the quadrennial oscillation, in the developing year summer, the diabatic heating weakens over the whole TP, inducing suppressed EASM precipitation accompanied by an anomalous cyclone over the northwestern Pacific and an anticyclone in North China. In the decaying year summer, the TP heating, characterized by an anomalous negative‐positive pattern, induces an anomalous anticyclone in South China and an anomalous cyclone in North China. However, without the anomalous TP thermal forcing, in both developing and decaying years the EASM shows similar response patterns with an anomalous anticyclone over the northwestern Pacific, an anomalous cyclone in North China, and an enhanced main rainfall band. Therefore, the TP thermal feedback plays a key role in the asymmetric responses of the EASM to the global SSTA during the different stages, characterized by suppressed precipitation in developing years and an enhanced main rainfall band in decaying years.

Modulation of springtime surface sensible heating over the Tibetan Plateau on the interannual variability of East Asian dust cycle

Abstract. Previous observational evidence and numerical simulations have revealed that the surface sensible heating in spring (March–April–May, MAM) over the Tibetan Plateau (TPSH) can affect the Asian regional hydrological cycle, surface energy balance, and climate through altering atmospheric heat source of the Tibetan Plateau (TP). This study aims to investigate the impacts of MAM TPSH on the interannual variability of East Asian dust cycle by using CAM4-BAM (version 4 of the Community Atmosphere Model coupled to a bulk aerosol model), MERRA-2 (version 2 of the Modern-Era Retrospective analysis for Research and Applications) surface dust concentration, and TPSH measurements. Our simulations show that the surface dust concentrations over the East Asian (EA) dust source region and over the northwestern Pacific (NP) in MAM are significantly positively correlated with TPSH, with regionally averaged correlation coefficients of 0.49 for EA and 0.44 for NP. Similar positive correlations are also shown between the MAM TPSH measurements averaged over the 73 observation sites and the surface dust concentration from MERRA-2. Simulation-based comparisons between strongest and weakest TPSH years reveal that, the MAM surface dust concentration in the strongest TPSH years increases with relative differences of 13.1 % over EA and 36.9 % over NP. These corresponding differences are found in MERRA-2 with 22.9 % and 13.3 % over EA and NP, respectively. Further simulated results show that the processes of whole dust cycles (e.g., dust loading, emission, and transport, as well as dust deposition) are also significantly enhanced during the strongest TPSH years over EA and NP. Through enhancing the TP heat source, stronger TPSH in MAM generates an anticyclonic anomaly in middle and upper troposphere over the TP and over the downstream Pacific region, respectively. These atmospheric circulation anomalies induced by the increased TPSH result in increasing the westerly winds over both EA and NP, which in turn increases dust emissions over the dust source, and dust transport over these two regions, as well as the regional dust cycles. These results suggest that addressing the East Asian dust changes in the future climates requires considering not only increasing greenhouse gas emissions but also the variations of the TP's heat source under global warming.

Relationship between the thermal condition of the Tibetan Plateau and precipitation over the region from eastern Ukraine to North Caucasus during summer

Remarkable climate anomalies occurred in Europe in recent years, but the reasons are not entirely disclosed. This entails further exploring the physical mechanism of anomalous climate variability over Europe on the basis of previous studies. Using NCEP-NCAR reanalysis and CMAP precipitation datasets, we investigated the relationship between the thermal condition of the Tibetan Plateau (TP) and precipitation over Europe during summer and related mechanisms behind through observational analyses and simulation experiments. The results show that the summer TP surface air temperature (SAT) is significantly correlated with the simultaneous precipitation over the region from eastern Ukraine to the North Caucasus (EUNC; 43°–51° N, 34°–46° E) during the period 1979–2017. The effect of anomalous TP heating plays, to some extent, an active role in linking the summer TP SAT with EUNC precipitation. The variability of the TP SAT is a result of the anomalous TP heating, but it can reflect the variability in temperature of a thick tropospheric air column over the TP well. Corresponding to higher (lower) TP SAT, the higher (lower) temperature anomaly appears over the TP and extends westwards through the transport of anomalous temperature fluxes from the TP to the EUNC regions in the upper troposphere, resulting in the expansion (withdrawal) of the South Asian high (SAH) and associated less (more) precipitation over the EUNC region. Numerical simulations basically reproduce the above-mentioned physical process, which confirms that, through stimulating the variability of upper-tropospheric temperatures over the TP and a larger area to its west and associated upstream large-scale atmospheric circulation, the summer TP’s thermal condition can modulate and expand the impact of the SAH to the EUNC region and therefore affect the concurrent precipitation in situ. This study implies an importance of the thermal anomaly of the TP to the variability of EUNC precipitation during summer.

Variations in the Sensible Heating of Tibetan Plateau and Related Effects on Atmospheric Circulation Over South Asia

Thermal effects of the Tibetan Plateau (TP) play an important role in the atmospheric circulation over south Asia, which may have complex implications on the development of regional climate. In the study, we utilized 86 station-based observations (1980–2018) to investigate the variations in sensible heating on the plateau surface and their possible impacts on south Asian atmospheric circulation in terms of the Indian Subtropical High. With 10-year records expansion relative to prior studies, our results suggested the significant increasing trend in regional sensible heat (SH) flux from 2001 to 2018 (0.259 W m− 2 a− 1, p < 0.05) contrary to the earlier pronounced decrease (-0.172 W m− 2 a− 1, p < 0.05; 1980–2000) especially in cold season. Most (33) of the 35 stations that decreased in cold-season SH flux before the shift year 2000, did not remain such temporal trend after 2000. Totally, there were respectively 44 and 5 stations showing increasing and decreasing trends in cold-season SH flux from 2001 to 2018, in contrast to the corresponding 6 and 35 stations during the period of 1980–2000. Our results also indicated that the later stronger sensible heating from the TP exhibited statistically close correlation (-0.574~-0.445 o/(W m− 2), p < 0.05) with the Indian Subtropical High, which tended to move southerly and shrink. Our study highlighted the possible weakening effect of the TP’s intensifying sensible heating on the atmospheric circulation over south Asia due to climate warming.

Synergy Effects of the Indian Summer Monsoon and the Tibetan Plateau Heating on Summer Rainfall over North China

An analysis based on July-August precipitation reveals that there is a tripole pattern of the precipitation distribution, that is, significantly increased rainfall over North China (NC) is related to the increased rainfall over the Indian subcontinent (IS) and the decreased rainfall over the southeastern Tibetan Plateau (TP) and vice versa, that corresponds to the Indian summer monsoon (ISM) and TP heating pattern, which are interactive. Therefore, it is necessary to investigate the effect of NC rainfall-related atmospheric circulation and the physical linkage with the two thermal forcings together. The linear baroclinic model (LBM) is applied to determine the dynamics of the process. The results show that an enhanced ISM is accompanied by reduced TP heating, favors convection and easterly anomaly over the IS, and produces a Gill-type Rossby wave that affects the vorticity over North Africa. Meanwhile, there is another Rossby wave originating in North Africa and moving eastward to the Pacific Ocean, which interferes with circulation at mid- to high-latitudes, i.e., it strengthens the cyclone over the Baikal region and stretches the western Pacific subtropical high (WPSH) northward to northeastern Asia, and results in abundant water vapor transported to NC. Furthermore, the strong convection over the IS excites the Kelvin waves over the equatorial region, which moves eastward and generates anticyclones over Philippines, consequently leading to the Pacific-Japan (PJ) pattern. The PJ pattern cooperates with the wave train at midlatitudes, resulting in abundant water vapor being transported to NC. The summer rainfall over NC is therefore modulated by synergistic effect of both the ISM and TP heating.

Land-atmosphere-ocean coupling associated with the Tibetan Plateau and its climate impacts.

This paper reviews recent advances regarding land–atmosphere–ocean coupling associated with the Tibetan Plateau (TP) and its climatic impacts. Thermal forcing over the TP interacts strongly with that over the Iranian Plateau, forming a coupled heating system that elevates the tropopause, generates a monsoonal meridional circulation over South Asia and creates conditions of large-scale ascent favorable for Asian summer monsoon development. TP heating leads to intensification and westward extension (northward movement) of the South Asian High (Atlantic Intertropical Convergence Zone), and exerts strong impacts on upstream climate variations from North Atlantic to West Asia. It also affects oceanic circulation and buoyancy fields via atmospheric stationary wave trains and air–sea interaction processes, contributing to formation of the Atlantic Meridional Overturning Circulation. The TP thermal state and atmospheric–oceanic conditions are highly interactive and Asian summer monsoon variability is controlled synergistically by internal TP variability and external forcing factors.

Climate modulation of summer rainstorm activity in eastern China based on the Tibetan Plateau spring heating

Rainstorm intensity and frequency in eastern China are closely associated with the Asian summer monsoon, one of the largest and most pronounced circulation systems on Earth. Changes in monsoon strength are linked to mechanical and thermal forcing by the Tibetan Plateau (TP), which, as an extensive elevated landmass, forms a vast heat source that protrudes into the free atmosphere. However, the impact of TP heating variability on rainstorm behavior in eastern China remains unclear. By analyzing 50 years of climate data, significant interannual variability was identified in the occurrence of rainstorms (precipitation amounts of > 50 mm), with decreased trend in northern China and increased variability in southern China. Results show that these changes are closely related with the springtime TP heating, indicating that recent changes in rainstorm distributions in eastern China are consistent with interdecadal variability in TP thermal forcing.

Response of the Indian Ocean to the Tibetan Plateau Thermal Forcing in Late Spring

Abstract The thermal effect of the Tibetan Plateau (TP) is known to exert substantial impacts on the atmospheric general circulation, suggesting that it may also influence the wind-driven circulation in the ocean through air–sea interactions. Here, several coupled general circulation model experiments are performed in order to investigate the short-term response of the Indian Ocean to the TP surface heat source in late spring (May). The results indicate that positive TP heating anomalies can induce significant atmospheric circulation responses over the northern Indian Ocean, characterized by easterly anomalies in the upper troposphere due to the enhanced South Asian high and lower-level southwesterly anomalies from the heat pumping effect. As a result, the surface wind speed over the northern Indian Ocean is reinforced, leading to intensified oceanic evaporation and subsequently cooler potential temperatures in the mixed layer. Wind-driven currents in the mixed layer are also affected. In the Bay of Bengal, Ekman transport facilitates water volume movement from west to east. In the Arabian Sea, water movement is weaker and the southward component is relatively more important. Both these areas show local meridional circulations with offshore upwelling in the northwest. Moreover, the cross-equatorial current is also enhanced in the eastern part of the tropical Indian Ocean. Overall, the upper layer in the northern Indian Ocean is efficiently modulated by the TP thermal forcing within one month.

The Tibetan Plateau Surface-Atmosphere Coupling System and Its Weather and Climate Effects: The Third Tibetan Plateau Atmospheric Science Experiment

The Tibetan Plateau (TP) is a key area affecting forecasts of weather and climate in China and occurrences of extreme weather and climate events over the world. The China Meteorological Administration, the National Natural Science Foundation of China, and the Chinese Academy of Sciences jointly initiated the Third Tibetan Plateau Atmospheric Science Experiment (TIPEX-III) in 2013, with an 8–10-yr implementation plan. Since its preliminary field measurements conducted in 2013, routine automatic sounding systems have been deployed at Shiquanhe, Gaize, and Shenzha stations in western TP, where no routine sounding observations were available previously. The observational networks for soil temperature and soil moisture in the central and western TP have also been established. Meanwhile, the plateau-scale and regional-scale boundary layer observations, cloud-precipitation microphysical observations with multiple radars and aircraft campaigns, and tropospheric-stratospheric air composition observations at multiple sites, were performed. The results so far show that the turbulent heat exchange coefficient and sensible heat flux are remarkably lower than the earlier estimations at grassland, meadow, and bare soil surfaces of the central and western TP. Climatologically, cumulus clouds over the main body of the TP might develop locally instead of originating from the cumulus clouds that propagate northward from South Asia. The TIPEX-III observations up to now also reveal diurnal variations, macro- and microphysical characteristics, and water-phase transition mechanisms, of cumulus clouds at Naqu station. Moreover, TIPEX-III related studies have proposed a maintenance mechanism responsible for the Asian “atmospheric water tower” and demonstrated the effects of the TP heating anomalies on African, Asian, and North American climates. Additionally, numerical modeling studies show that the Γ distribution of raindrop size is more suitable for depicting the TP raindrop characteristics compared to the M-P distribution, the overestimation of sensible heat flux can be reduced via modifying the heat transfer parameterization over the TP, and considering climatic signals in some key areas of the TP can improve the skill for rainfall forecast in the central and eastern parts of China. Furthermore, the TIPEX-III has been promoting the technology in processing surface observations, soundings, and radar observations, improving the quality of satellite retrieved soil moisture and atmospheric water vapor content products as well as high-resolution gauge-radar-satellite merged rainfall products, and facilitating the meteorological monitoring, forecasting, and data sharing operations.

Quasi-biweekly impact of the atmospheric heat source over the Tibetan Plateau on summer rainfall in Eastern China

We investigated the intraseasonal relationship between the atmospheric heat source over Tibetan Plateau (TP) and summer rainfall in eastern China. A quasi-biweekly oscillation (QBWO) is the common dominant periodicity over both the TP and eastern China. Crucially, the TP heat source QBWO leads rainfall QBWO over its downstream region by 2–8 days. The QBWO over the TP shows a significant eastward propagation. Phase composite further identified the specific propagation pathway: TP QBWO first propagates eastward to central China, southward to southern China, and then northeastward to the lower reaches of the Yangtze River with a zigzag propagating pathway. Mechanistically, the eastward propagation of the upper level divergence caused by the movement of the 200 hPa jet core and south Asian high contributes to the eastward propagation of the rainfall QBWO. Simultaneously, the eastward (westward) shift in the western north Pacific subtropical high causes deepening (flattening) of the monsoon trough and the occurrence of an anomalous cyclone (anticyclone) over the western north Pacific. An anomalous anticyclonic (cyclonic) circulation around the TP appears with the wet-to-dry (dry-to-wet) phase transition of the TP QBWO. Together, they play a crucial part in the southward (northeastward) movement of the QBWO. Further dynamic analysis indicates that the horizontal moisture advection acts on the whole eastward-propagating process of the QBWO, whereas the horizontal vorticity advection is only significant when QBWO is over the TP. This study shows that the fluctuations in TP heat source are important indicators in extended forecasts of summer rainfall in eastern China.

Linking quasi-biweekly variability of the South Asian high to atmospheric heating over Tibetan Plateau in summer

This study investigates the summertime day-to-day variability of the South Asian high (SAH) driven by atmospheric heating over Tibetan Plateau (TP) using the NCEP/NCAR reanalysis dataset. We first isolate the day-to-day variability of SAH in summertime based on the intensity of the summertime Tibetan Plateau upper-atmospheric heat source (TPUHS). It shows that anomalously stronger TPUHS days are accompanied with SAH center over Iranian Plateau (IP or the IP phase of the SAH) and the SAH center moves to TP (or the TP phase) during weaker TPUHS days, which contrasts with the corresponding relationship between SAH IP/TP phase and weaker/stronger TP heating in monthly and longer timescale. We further demonstrate that the SAH IP/TP phase coincides with stronger/weaker low surface pressure anomalies over TP. The stronger ascending and upper-tropospheric divergence anomalies above TP during stronger TPUHS days connect to compensatory stronger descending and upper-tropospheric convergence anomalies over IP. Such day-to-day SAH variability associated with TPUHS exhibits a quasi-biweekly time scale with a pronounced westward propagating signal.

Influence of Dynamic and Thermal Forcing on the Meridional Transport of Taklimakan Desert Dust in Spring and Summer

The Weather Research and Forecasting Model coupled with chemistry (WRF-Chem) associated with in situ measurements and satellite retrievals was used to investigate the meridional transport of Taklimakan Desert (TD) dust, especially in summer. Both satellite observations and simulations reveal that TD dust particles accumulate over the Tibetan Plateau (TP) and the Tianshan Mountains in summer, resulting in higher dust concentration up to 85 μg m−3 here. The proportions of meridional transport of TD dust in summer increase up to 30% of the total output dust over the TD. Further, the impacts of thermal and dynamic forcing on the meridional transport of TD dust to the TP and Tianshan Mountains are investigated based on composite analysis and numerical modeling. It is found that the weakness of the westerly jet over East Asia significantly decreases the eastward transport of TD dust. More TD dust particles lifted to higher altitude reach up to 8 km induced by the enhanced sensible heating in summer. Under the influence of the northerly airflow over the TD regions, the TD dust particles are strengthened southward and transported to the northern slope of the TP through topographic forcing. Moreover, the cyclonic circulation raises dust particles to higher altitude over the TP. It can further intensify the TP heat source by direct radiative forcing of dust aerosols, which may have a positive feedback to the southward transport of TD dust. This research provides confidence for the investigation of the role of TP dust with regard to the radiation balance and hydrological cycle over East Asia.

Summer Atmospheric Heat Sources over the Western–Central Tibetan Plateau: An Integrated Analysis of Multiple Reanalysis and Satellite Datasets

Multiple bias-corrected top-quality reanalysis datasets, gauge-based observations, and selected satellite products are synthetically employed to revisit the climatology and variability of the summer atmospheric heat sources over the Tibetan Plateau (TP). Verification-based selection and ensemble-mean methods are utilized to combine various datasets. Different from previous works, this study pays special attention to estimating the total heat source (TH) and its components over the data-void western plateau (70°–85°E), including the surface sensible heat (SH), latent heat released by precipitation (LH), and net radiation flux (RD). Consistent with previous studies, the climatology of summer SH (LH) typically increases (decreases) from southeast to northwest. Generally, LH dominates TH over most of the TP. A notable new finding is a minimum TH area over the high-altitude region of the northwestern TP, where the Karakoram mountain range is located. We find that during the period of 1984–2006, TH shows insignificant trends over the eastern and central TP, whereas it exhibits an evident increasing trend over the western TP that is attributed to the rising tendency of LH before 1996 and to that of RD after 1996. The year-to-year variation of TH over the central–eastern TP is highly correlated with that of LH, but that is not the case over the western TP. It is also worth noting that the variations of TH in each summer month are not significantly correlated with each other, and hence study of the interannual variation of the TP heat sources should consider the remarkable subseasonal variations.

Role of Atlantic air\u2013sea interaction in modulating the effect of Tibetan Plateau heating on the upstream climate over Afro-Eurasia\u2013Atlantic regions

Previous studies have demonstrated that atmospheric diabatic heating over the Tibetan Plateau (TP) exerts significant influences on the “upstream” climate of the Atlantic-African-European sector in boreal summer. Using the NCAR Community Earth System Model, this study demonstrates that the TP-induced change in North Atlantic sea surface temperature (SST) including evident warming over the mid-latitude North Atlantic and cooling over the south can in turn modulate the above TP impact. Compared with the TP heating experiment without Atlantic SST variation, anomalous wave train pattern appears with north-northeastward downstream influences when the change in Atlantic SST is considered. The wave train pattern is characterized by three positive centers over the North Atlantic, the Arctic Ocean, and east of Japan, and four negative centers over northeastern North America, North Europe, the mid-Atlantic, and the northwestern Pacific. It intensifies the northern portions of the TP-induced tropospheric anticyclones over the extratropical North Atlantic and the cyclones over northeastern North America and North Europe. Correspondingly, precipitation decreases over the northwestern Atlantic but increases over northeastern North America and North Europe. Due to the easterly anomalies on the southern side of the weakened thermal low over subtropical Africa, precipitation over the Sahel decreases, indicating a weakening of TP-induced precipitation dipole over the tropical eastern Atlantic and West Africa when the Atlantic SST influence is considered. Overall, the modulation of Atlantic SST variation accounts for above 20 percent of the upstream climate signals induced by the TP thermal effect.

Effect of Tibetan Plateau heating on summer extreme precipitation in eastern China

The relationship between the Tibetan Plateau (TP) heating and summer extreme precipitation in eastern China is still unclear. This study shows that when the TP heating is stronger (weaker) than normal, summer extreme precipitation tends to be more (less) over the middle and lower reaches of the Yangtze River valley. The possible mechanisms responsible for the relationship between the TP heating and summer extreme precipitation in eastern China are revealed by investigating the effects of the upper-level South Asian High (SAH). It is found that positive and negative vorticity anomalies accompanied by an ascending motion anomaly occur over the TP regions in the lower and upper troposphere, respectively, when the TP heating is stronger, resulting in the strengthening and eastward extension of the SAH. As a result, an anomalous high-level convergence and downward movement occur over the southeastern portion of the SAH, which then diverge in the mid-lower troposphere, resulting in the formation of an anomalous negative vorticity from South China to the northern part of the South China Sea. Subsequently, the western Pacific subtropical high (WPSH) intensifies and stretches westward. Consequently, the southwestern moisture transport and moisture convergence increase over the middle and lower reaches of the Yangtze River valley, resulting in more extreme precipitation. The eastward extension of the SAH and westward stretching of the WPSH are maintained by the vertically inhomogeneous distribution of precipitation heating.

Tibetan Plateau heating as a driver of monsoon rainfall variability in Pakistan

Pakistan summer monsoon rainfall consists of a large portion of the local annual total rainfall, and in the recent monsoon seasons, prolonged periods of anomalous rainfall and excessive flooding have appeared in Pakistan. A full understanding of the monsoon rainfall variability is important for the sustainable development of the country. Based on multiple data analyses and the weather research and forecasting model, the potential impact of Tibetan Plateau (TP) heating on the interannual variability of Pakistan monsoon rainfall is investigated. It is observed that a significant negative relationship exists between the thermal forcing over the southeastern TP and Pakistan monsoon rainfall in July–August. Both the data analyses and model sensitivity experiments identify that the TP heating drives a Rossby wave response in the upper atmosphere characterized with an anticyclonic anomaly over the southern TP but a cyclonic anomaly to the north. This dipole pattern of anomalous circulation induces an evident upper-level convergence over Pakistan, corresponding with remarkable vertical sinking motion. Meanwhile, in the lower troposphere, the TP heating causes anomalous westerly wind along the Himalayas over the northern India continent. Such westerly anomaly further induces less water vapor transport into Pakistan from the Bay of Bengal. Therefore, both the dynamic and thermodynamic processes regulated by positive TP heating are not beneficial for the occurrence of monsoon rainfall in Pakistan. This study proposes a new potential mechanism in which TP heating acts as a driver of Pakistan monsoon rainfall variability on interannual time scales.

Teleconnection between Summer NAO and East China Rainfall Variations: A Bridge Effect of the Tibetan Plateau

AbstractAlthough the impact of the North Atlantic Oscillation (NAO), especially the antecedent NAO in winter and spring, on East Asian summer climate has been studied extensively, the possible connection from the summer NAO (SNAO) and then the Tibetan Plateau (TP) to East China summer rainfall remains unclear. This study reveals that on interannual time scales the SNAO is significantly correlated with the variations of East China summer rainfall and the thermal forcing of the TP provides an intermediate bridge effect in this Eurasian teleconnection. The SNAO primarily regulates the rainfall variability over the TP through large-scale wave trains and the TP rainfall anomalies in turn lead to a change in local diabatic heating, which excites Rossby waves to the downstream regions. To the northeast of the TP, an anomalous barotropic cyclone is formed in the nearly entire troposphere, generating low-level northerly flow anomalies over northern China. Meanwhile, the TP heating also induces low-level southerly flow anomalies over southern China. The anomalous northerly and southerly winds converge in the lower troposphere, enhancing the summer rainfall over central East China. Compared to the SNAO, the TP thermal forcing exerts a more direct impact on the variations of East China summer rainfall in the Eurasian teleconnection discussed.