Role of the Tibetan Plateau thermal forcing in the summer climate patterns over subtropical Asia

The mechanism of the Tibetan Plateau (TP) thermal forcing in influencing the summer climate patterns over subtropical Asia is investigated by means of NCEP/NCAR reanalysis diagnosis. Results show that since the TP is a huge elevated heating source with the strongest heating in the surface layers in summer, the thermal adaptation results in a shallow cyclonic circulation near the surface and a deep anticyclonic circulation above it. According to the steady barotropic vorticity equation for large scales, airflow must converge in the lower layers and diverge in the higher layers over the eastern side of the TP. However, the western side of the TP is characterized by a reversed structure, i.e., divergence in lower layers but convergence in higher layers. Hence, pumping and sucking processes bring in upward and downward movement over the east and west sides of the TP, respectively. Such a circulation is embedded in the large-scale circulation that is forced by the Eurasian continental heating. Because the TP together with Iran Plateau are located at the central and eastern parts of the continent, and, because the orography-induced circulation is in phase with the continental scale circulation, the role of the TP thermal forcing is to intensify the East Asian monsoon to its east and the dry and hot desert climate in mid-Asia to its west. The summertime thermal forcing of the Rockies and Andes can generate similar circulations along the two subtopics as the TP does since they are located near the western coasts. But, the lower troposphere poleward flow that is induced by orographic thermal forcing does not coincide with the poleward flows over the eastern coastal region that is induced by continental heating and the monsoon rainfall in North and South America is not as strong as in East Asia. However, the equatorward flow and the associated subsidence induced by the two mountain ranges along the western coasts of both North and South America are in phase with those induced by continental heating. These contribute to the formation of the stable low stratus clouds and strong long-wave radiative cooling over the eastern subtropical Pacific regions just off the western coast of the continent.     

Recent Progress in the Impact of the Tibetan Plateau on Climate in China

Studies of the impacts of the Tibetan Plateau （TP） on climate in China in the last four years are reviewed. It is reported that temperature and precipitation over the TP have increased during recent decades. From satellite data analysis, it is demonstrated that most of the precipitation over the TP is from deep convection clouds. Moreover, the huge TP mechanical forcing and extraordinary elevated thermal forcing impose remarkable impacts upon local circulation and global climate. In winter and spring, stream flow is deflected by a large obstacle and appears as an asymmetric dipole, making East Asia much colder than mid Asia in winter and forming persistent rainfall in late winter and early spring over South China. In late spring, TP heating contributes to the establishment and intensification of the South Asian high and the abrupt seasonal transition of the surrounding circulations. In summer, TP heating in conjunction with the TP air pump cause the deviating stream field to resemble a cyclonic spiral, converging towards and rising over the TP. Therefore, the prominent Asian monsoon climate over East Asia and the dry climate over mid Asia in summer are forced by both TP local forcing and Eurasian continental forcing.
Due to the longer memory of snow and soil moisture, the TP thermal status both in summer and in late winter and spring can influence the variation of Eastern Asian summer rainfall. A combined index using both snow cover over the TP and the ENSO index in winter shows a better seasonal forecast.
On the other hand, strong sensible heating over the Tibetan Plateau in spring contributes significantly to anchor the earliest Asian monsoon being over the eastern Bay of Bengal （BOB） and the western Indochina peninsula. Qualitative prediction of the BOB monsoon onset was attempted by using the sign of meridional temperature gradient in March in the upper troposphere, or at 400 hPa over the TP. It is also demonstrated by a numerical experiment and theoretical study that the heating over the TP lea     

青藏高原的热力和动力作用对亚洲季风区环流的影响

利用NCEP/NCAR再分析资料,研究了青藏高原热状况的季节变化、动力和热力作用对周围环流,特别是对亚洲热带季风环流的影响。高原对西风带的机械作用在冬季最强,春季次之。冬季的机械作用形成以高原为主,南侧气旋性、北侧反气旋性的"偶极子"偏差环流,它比传统认识的爬坡、绕流的影响范围大得多,遍及东亚的高、低纬度。随着西风带的北移和高原总加热在4月由负变正,南侧气旋性偏差环流增强并逐渐北移,6月形成气旋盘踞整个高原的夏季型。在高原南侧,高原冬季偶极型、夏季加热的作用导致孟加拉湾地区常年存在印缅槽,使得印度半岛的感热加热始终强于中南半岛,而中南半岛上空的潜热加热大于印度半岛。印缅槽的演变存在明显的半年周期,证明2月初和8月初的较强低压槽分别对应冬季高原最强的动力强迫和夏季高原最强的热力强迫。对低纬经向风场的分析还表明,季风爆发前高原的热力作用尤为重要,是导致江南春雨的形成,亚洲季风最早在孟加拉湾东部爆发,最后在印度半岛爆发的原因。     

Weather and Climate Effects of the Tibetan Plateau

Progress in observation experiments and studies concerning the effects of the Tibetan Plateau (TP) on weather and climate during the last 5 years are reviewed. The mesoscale topography over the TP plays an important role in generating and enhancing mesoscale disturbances. These disturbances increase the surface sensible heat (SH) flux over the TP and propagate eastward to enhance convection and precipitation in the valley of Yangtze River. Some new evidence from both observations and numerical simulations shows that the southwesterly flow, which lies on the southeastern flank of the TP, is highly correlated with the SH of the southeastern TP in seasonal and interannual variability. The mechanical and thermal forcing of the TP is an important climatic cause of the spring persistent rains over southeastern China. Moreover, the thermodynamic processes over the TP can influence the atmospheric circulation and climate over North America and Europe by stimulating the large-scale teleconnections such as the Asian-Pacific oscillation and can affect the atmospheric circulation over the southern Indian Ocean. Estimating the trend in the atmospheric heat source over the TP shows that, in contrast to the strong surface and troposphere warming, the SH over the TP has undergone a significant decreasing trend since the mid-1980s. Despite the fact that in situ latent heating presents a weak increasing trend, the springtime atmospheric heat source over the TP is losing its strength. This gives rise to reduced precipitation along the southern and eastern slopes of the TP and to increased rainfall over northeastern India and the Bay of Bengal.     

Interannual Variability of Late-spring Circulation and Diabatic Heating over the Tibetan Plateau Associated with Indian Ocean Forcing

The thermal forcing of the Tibetan Plateau(TP) during boreal spring,which involves surface sensible heating,latent heating released by convection and radiation flux heat,is critical for the seasonal and subseasonal variation of the East Asian summer monsoon.Distinct from the situation in March and April when the TP thermal forcing is modulated by the sea surface temperature anomaly(SSTA) in the North Atlantic,the present study shows that it is altered mainly by the SSTA in the Indian Ocean Basin Mode(IOBM) in May,according to in-situ observations over the TP and MERRA reanalysis data.In the positive phase of the IOBM,a local Hadley circulation is enhanced,with its ascending branch over the southwestern Indian Ocean and a descending one over the southeastern TP,leading to suppressed precipitation and weaker latent heat over the eastern TP.Meanwhile,stronger westerly flow and surface sensible heating emerges over much of the TP,along with slight variations in local net radiation flux due to cancellation between its components.The opposite trends occur in the negative phase of the IOBM.Moreover,the main associated physical processes can be validated by a series of sensitivity experiments based on an atmospheric general circulation model,FAMIL.Therefore,rather than influenced by the remote SSTAs of the northern Atlantic in the early spring,the thermal forcing of the TP is altered by the Indian Ocean SSTA in the late spring on an interannual timescale.     

A regional climate model simulation of summer monsoon over east asia: a case study of 1991 flood in yangtze-huai river valley

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Interannual Influences of the Surface Potential Vorticity Forcing over the Tibetan Plateau on East Asian Summer Rainfall

The influences of interannual surface potential vorticity forcing over the Tibetan Plateau (TP) on East Asian summer rainfall (EASR) and upper-level circulation are explored in this study. The results show that the interannual EASR and associated circulations are closely related to the surface potential vorticity negative uniform leading mode (PVNUM) over the TP. When the PVNUM is in the positive phase, more rainfall occurs in the Yangtze River valley, South Korea, Japan, and part of northern China, less rainfall occurs in southern China, and vice versa. A possible mechanism by which PVNUM affects EASR is proposed. Unstable air induced by the positive phase of PVNUM could stimulate significant upward motion and a lower-level anomalous cyclone over the TP. As a result, a dipole heating mode with anomalous cooling over the southwestern TP and anomalous heating over the southeastern TP is generated. Sensitivity experiment results regarding this dipole heating mode indicate that anomalous cooling over the southwestern TP leads to local and northeastern Asian negative height anomalies, while anomalous heating over the southeastern TP leads to local positive height anomalies. These results greatly resemble the realistic circulation pattern associated with EASR. Further analysis indicates that the anomalous water vapor transport associated with this anomalous circulation pattern is responsible for the anomalous EASR. Consequently, changes in surface potential vorticity forcing over the TP can induce changes in EASR.     

青藏高原感热气泵影响亚洲夏季风的机制

本文回顾了二十年来关于青藏高原感热驱动气泵(TP-SHAP)及其影响亚洲夏季风的研究进展,并从能量(θ)、位涡—加热(PV–Q)、和角动量守恒(AMC)的不同角度阐述其影响机制。指出高原斜坡上的表面感热加热改变了移向高原的大气质块的能量从而出现垂直抽吸的重要性。强调了高原加热产生的位涡强迫在近地层制造了强度大范围广的、环绕高原的气旋式环流,把丰沛的水汽从海洋输运到大陆,为季风对流降水提供充足的水汽条件。证明高原加热还通过改变其上空的温、压场的结构从而制造出高原上空近对流层顶的绝对涡度和位涡的最小值,在角动量平衡约束下,在亚洲季风区激发出与Hadley环流反向的季风经圈环流,从而为季风发生发展提供了大范围上升运动的背景。文中还对近年来有关青藏高原影响亚洲夏季风机制的讨论进行概述,并展望了未来的研究方向。     

The Summer Snow Cover Anomaly over the Tibetan Plateau and Its Association with Simultaneous Precipitation over the Mei-yu–Baiu region

The summer snow anomalies over the Tibetan Plateau （TP） and their effects on climate variability are often overlooked,possibly due to the fact that some datasets cannot properly capture summer snow cover over high terrain.The satellite-derived Equal-Area Scalable Earth grid （EASE-grid） dataset shows that snow still exists in summer in the western part and along the southem flank of the TP.Analysis demonstrates that the summer snow cover area proportion （SCAP） over the TP has a significant positive correlation with simultaneous precipitation over the mei-yu-baiu （MB） region on the interannual time scale.The close relationship between the summer SCAP and summer precipitation over the MB region could not be simply considered as a simultaneous response to the Silk Road pattern and the SST anomalies in the tropical Indian Ocean and tropical central-eastern Pacific.The SCAP anomaly has an independent effect and may directly modulate the land surface heating and,consequently,vertical motion over the western TP,and concurrently induce anomalous vertical motion over the North Indian Ocean via a meridional vertical circulation.Through a zonal vertical circulation over the tropics and a Kelvin wave-type response,anomalous vertical motion over the North Indian Ocean may result in an anomalous high over the western North Pacific and modulate the convective activity in the western Pacific warm pool,which stimulates the East Asia-Pacific （EAP） pattern and eventually affects summer precipitation over the MB region.     

Progress in the study on the formation of the summertime subtropical anticyclone

The studies in China on the formation of the summertime subtropical anticyclone on the climate timescale are reviewed. New insights in resent studies are introduced. It is stressed that either in the free atmosphere or in the planetary boundary, the descending arm of the Hadley cell cannot be considered as a mechanism for the formation of the subtropical anticyclone. Then the theories of thermal adaptation of the atmosphere to external thermal forcing and the potential vorticity forcing are developed to understand the formation of the subtropical anticyclone in the three-dimensional domain. Numerical experiments are designed to verify these theories. Results show that in the boreal summer, the formation of the strong South Asian High in the upper troposphere and the subtropical anticyclone over the western Pacific in the middle and lower troposphere is, to a great extent, due to the convective latent heating associated with the Asian monsoon, but affected by orography and the surface sensible heating over the continents.On the other hand, the formation of the subtropical anticyclone at the surface over the northern Pacific and in the upper troposphere over North America is mainly due to the strong surface sensible heating over North America, but affected by radiation cooling over the eastern North Pacific. Moreover, in the real atmosphere such individual thermal forcing is well organized. By considering the different diabatic heating in synthesis, a quadruple heating pattern is found over each subtropical continent and its adjacent oceans in summer. A distinct circulation pattern accompanies this heating pattern. The global summer subtropical heating and circulation may be viewed as “mosaics” of such quadruplet heating and circulationpatterns respectively. At last, some important issues for further research in understanding and predicting the variations of the subtropical anticyclone are raised.     

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon(ISM),perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air.Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau.However,given the large biases in simulating the monsoon in CMIP5 models,such results may be model dependent.This study investigates the impact of orographic forcing from the Tibetan Plateau,Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon(EASM) in the UK Met Office's Had GEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL global climate models.The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies.However,considerable differences exist in the rainfall responses over India and China,and in the detailed aspects such as onset and retreat dates.In particular,the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed.Our results show that a multi-model approach,as suggested in the forthcoming Global Monsoon Model Intercomparison Project(GMMIP) associated with CMIP6,is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.     

青藏高原和亚洲夏季风动力学研究的新进展

亚洲夏季风环流受海陆和伊朗高原—青藏高原大地形的热力作用调控.亚洲季风所释放的巨大潜热又对大气环流形成反馈.这种相互反馈过程十分复杂,揭示其物理过程对理解气候变化格局的形成和变化以及提高天气预报及气候预测的准确率十分重要.夏季北半球副热带对流层上层环流的主要特征是存在庞大的南亚高压(SAH)以及强大的对流层上层温度暖中心(UTTM).本文介绍了温度—加热垂直梯度(T-Q_Z)理论的发展,并用以揭示SAH和UTTM的形成机制.指出沿副热带欧亚大陆东部的季风对流潜热加热及其中西部的表面感热加热和高层长波辐射冷却是导致SAH和UTTM在南亚上空发展的原因.文中还介绍了Gill模型用于上部对流层研究的局限性及解决的办法.     

Some dynamical properties of idealized thermally-forced meridional circulations in the tropics

Summary Through the use of a zonal balance model we investigate the properties of the tropical meridional circulation to a range of specified diabatic forcing fields for climatologically observed zonal winds. As in earlier studies, the solutions show that latent heat release away from the equator forces an asymmetric meridional circulation in response the anisotropy in the inertial stability parameter with respect to the meridional location of the forcing. The presence of strong zonal flows appears to play a relatively minor role in determining the magnitude and asymmetry of the meridional circulation, whereas the structure of the diabatic heating, particularly the meridional breadth, proves to be of much greater importance.A dynamic efficiency factor, which provides an analytic measure of the efficacy of diabatic heating at generating zonal kinetic energy, generally exhibits a meridionally symmetric structure except during Northern Hemisphere summer. This asymmetry gives rise to a pronounced sensitivity of zonal kinetic energy generation to the meridional location of ITCZ convection. Further examination of the flow pattern suggests that for zonal flows representative of those over the Indian Ocean during the Northern Hemisphere summer months, meridional displacements of the heating of less than 20° latitude can result in as much as an order of magnitude difference in the rate of kinetic energy generation. Solution of the balance system also implies the existence of a feedback mechanism, between zonally-organized convection and the energetics properties of the large-scale flow, that is highly sensitive to the meridional location of the convection.With 11 FiguresThe National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Formation and Variation of the Atmospheric Heat Source over the Tibetan Plateau and Its Climate Effects

To cherish the memory of the late Professor Duzheng YE on what would have been his 100 th birthday, and to celebrate his great accomplishment in opening a new era of Tibetan Plateau(TP) meteorology, this review paper provides an assessment of the atmospheric heat source(AHS) over the TP from different data resources, including observations from local meteorological stations, satellite remote sensing data, and various reanalysis datasets. The uncertainty and applicability of these heat source data are evaluated. Analysis regarding the formation of the AHS over the TP demonstrates that it is not only the cause of the atmospheric circulation, but is also a result of that circulation. Based on numerical experiments, the review further demonstrates that land–sea thermal contrast is only one part of the monsoon story. The thermal forcing of the Tibetan–Iranian Plateau plays a significant role in generating the Asian summer monsoon(ASM), i.e., in addition to pumping water vapor from sea to land and from the lower to the upper troposphere, it also generates a subtropical monsoon–type meridional circulation subject to the angular momentum conservation, providing an ascending-air large-scale background for the development of the ASM.     

夏季青藏高原上空热力异常与其上下游大气环流联系的研究进展

围绕夏季青藏高原热力异常与其上、下游大气环流在年际尺度变化上的联系,对最新的研究成果做了简要介绍。通过观测资料分析与数值试验,指出在年际尺度上夏季青藏高原热力异常与同期亚洲-太平洋涛动（APO）具有显著且稳定的联系,前者可能通过调节亚洲和中东太平洋热带外大尺度垂直环流异常影响后者。另外,夏季青藏高原热力异常对高原上空及更大范围上对流层温度的年际变化也有一定贡献,进而通过对上游大尺度环流的调节作用影响到同期西非萨赫勒地区的降水。夏季青藏高原热力异常只是导致其上、下游大气环流年际变化的一个原因,其他影响效应尚需进一步探讨。      

青藏高原-孟加拉湾大气热力差异与夏季暴雨

利用1979-2019年多年平均5-8月的逐日气象资料,采用EOF,MV-EOF、相关分析和合成分析等方法,对夏季青藏高原-孟加拉湾的大气热源与中国东部暴雨的时空演变特征及两者之间的联系进行探讨.研究结果表明:MV-EOF能够很好地表现不同要素的空间分布特征及其时间演变之间的联系.结果显示:在气候平均状态下,强降水事件...     

New evidences on the climatic causes of the formation of the spring persistent rains over southeastern China

The spring persistent rains (SPR) over southeastern China (SEC) are a unique synoptic and climatic phenomenon in East Asia. A former study has found that the southwesterly flow which lies on the southeastern flank of the Tibetan Plateau (TP) is one of the deflected westerly flows of the TP, and it is suggested to be the direct climatic cause of SPR. This study found that the southwesterly flow is also highly correlated with the sensible heating of the southeastern TP in interannual variability, in addition to having a high correlation in seasonal variability. These facts suggest that the thermal forcing of the TP is another important climatic cause of SPR. Numerical sensitivity experiments further prove that the mechanical and thermal forcings of the TP are the climatic causes of the formation of the SPR. On the other hand, the Nanling Mountains and Wuyi Mountains (NWM) over southeastern China not only increase the SPR precipitation amount evidently, but also make the SPR rain belt move to the south by blocking the strong southwesterly flow.     

NUMERICAL STUDY OF TROPICS-EXTRATROPICS INTERACTION

A nonlinear steady-state baroclinic primitive-equation numerical model of atmospheric forced stationarywaves is used to investigate the tropics-extratropics interactions.Newtonian cooling,Rayleigh friction andbiharmonic horizontal diffusion are included in the model.The Eliassen-Palm (EP) cross-section and three-dimensional wave activity flux,which was derived by Plumb (1985) for linear quasi-geostrophic stationarywaves on a zonal flow,are used as diagnostics for the vertical and horizontal propagation of the waves.Results of the numerical experiments and diagnostics analyses suggest that the extratropical influenceon the tropical large-scale motion is important.The mid-latitude orographic forcing,especially of the Qing-hai-Xizang Plateau,and the extratropical thermal forcing make substantial contribution to the main-tenance of the cyclonic circulation over the eastern tropical and subtropical Pacific as well as the inversecirculation over the western Pacific in the upper troposphere.In addition,the longitudinal variation ofdiabatic heating in tropics has a significant influence on the wintertime stationary waves at higher latitudes.     

青藏高原大气水分循环特征

青藏高原对亚洲季风环流的形成有重要作用,同时作为"世界屋脊"拥有丰富的冰川、积雪、河流、湖泊和地下蓄水层。青藏高原特殊大地形动力和热力作用深刻地影响着亚洲与全球大气水分循环,也对全球气候与环境产生深远的影响。基于青藏高原在亚洲夏季风系统大气水分循环过程的重要地位,从青藏高原对全球大气水分循环重要作用的视角,综述了青藏高原大气水分循环过程中青藏高原局地热力对流、高原的"阶梯式"水汽流爬升"第二类条件不稳定(CISK)"物理模型、青藏高原视热源结构影响及多尺度水汽汇流通道、海洋-青藏高原"水汽源-汇"结构、青藏高原跨半球垂直环流圈水分循环结构、青藏高原大气水分循环综合模型等的相关研究进展,剖析了青藏高原大气水分循环综合模型的研究背景,探讨了青藏高原特殊大地形热力驱动机制及其云水效应,描述出与青藏高原热力驱动的亚洲区域和跨半球垂直环流圈水分循环结构,揭示了青藏高原热力强迫与海洋-大气-陆地水文过程特殊的相互反馈作用。青藏高原发源的亚洲河流水系是为人口众多的亚洲区域供给生活、农业和工业用水的重要水资源之一。因此,认识在全球变暖背景下青藏高原的水分循环及其对水资源变化影响至关重要,仍需深入地探讨青藏高原大气水分循环机制及其全球影响效应。     

7月青藏高原大气热源空间型及其与东亚大气环流和降水的相关研究

对1958～1999年的7月份NCEP/NCAR再分析资料中青藏高原区域大气热源强度(整层气柱的总非绝热加热率)做旋转经验正交函数分析,结果表明该区域内大气热源强度的空间分布特征复杂,各地差异显著。前4个REOF型的加热中心位于高原东北部、高原西南部、克什米尔地区以及高原东南部地区上空。小波分析还表明各空间型都有2～4a的变化周期。文中计算了前4个RPC与东亚中、低空纬向风(U)、经向风(V)、纬向水汽通量(Q_u)、经向水汽通量(Q_v)的相关系数,并用这些相关系数构造矢量,进而分析其流场和水汽通量散度场,发现高原不同区域的大气加热异常所对应的东亚大气环流形势及降水也大不相同,由此表明,在研究高原加热对中国气候的影响时,应注意加热的空间分布特征。