A Modeling Study of the Effects of Anomalous Snow Cover over the Tibetan Plateau upon the South Asian Summer Monsoon

The e?ect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SMMR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo e?ect, which is compensated by weaker upward sensible heat ?ux associated with colder surface temperature, whereas the e?ects of snow melting and evaporation are relatively smaller. The anomalies of surface heat ?uxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May. The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this in?uence is only obvious in early summer and almost disappears in later stages.

REORGANIZATION OF THE ASIAN MONSOON SYSTEM AT ABOUT 2.6 Ma AGO AND ITS IMPLICATIONS FOR THE RISING OF THE TIBETAN PLATEAU

More and more concentrations have been focused on the uplift of the Tibetan Plateau and its effects on regional and global environment. The development of Asian monsoon has a close relationship with the uplift of the Tibetan Plateau. The understanding for the development of Asian monsoon is the key to the understanding of the process and the mechanism of the environmental evolution of the Asian region and to the understanding of the history of the uplift of the Tibetan Plateau. It is found that a big change of the Asian monsoon system occurred at about 2 6Ma ago. The winter monsoon strengthened (Ding et al.,1992 ), the change of winter monsoon began to in the opposite phase to the summer monsoon in East Asia (An et al., 1998 ), the tropical Southwest monsoon was hard to influence the North China (Li, 1999). However, whether there were changes in direction of winter monsoon and strength of summer monsoon occurred and how to change are still not clear. In this paper, the changes of the Asian monsoon system at about 2 6Ma ago are discussed, based on the reanalysis and combination of the results of atmospheric simulations and the geological records existed. It is suggested that, at about 2 6Ma ago, both winter monsoon and summer monsoon strengthened obviously, because that the amplitude of the climatic change increased a lot—warmer during the warm periods and colder during the cold periods. The direction of winter monsoon in Northern China changed from North—West—West to Northwest or North—West—North, because that the expansion direction of eolian\|deposit\|distribution area in China changed from mainly eastward to mainly southward. The area influenced by subtropical monsoon increased, but the area influenced by tropical monsoon decreased in China, because of the rising of the Tibetan Plateau and the southwest\|ward migration of the center of the Asian Low. At some time of the Pliocene, the tropical monsoon had penetrated onto the Qinghai\|Xizang region and influenced on the North China and Northwest China. However, after 2.6Ma B.P., the effects of the tropical monsoon on Northwest China and the northern part of the Tibetan Plateau decreased. All these suggest that the Tibetan Plateau perhaps reached about 2000m elevation at about 2 6Ma B.P..

Evaluation of CORDEX regional climate models in simulating temperature and precipitation over the Tibetan Plateau

Using a regional climate model（RCM） is generally regarded as a promising approach in researching the climate of the Tibetan Plateau, due to the advantages provided by the high resolutions of these models. Whilst previous studies have focused mostly on individual RCM simulations, here, multiple RCMs from the Coordinated Regional Climate Downscaling Experiment are evaluated in simulating surface air temperature and precipitation changes over the Tibetan Plateau using station and gridded observations. The results show the following：（1） All RCMs consistently show similar spatial patterns, but a mean cold（wet） bias in the temperature（precipitation） climatology compared to station observations. The RCMs fail to reproduce the observed spatial patterns of temperature and precipitation trends, and on average produce greater trends in temperature and smaller trends in precipitation than observed results. The multi-model ensemble overall produces superior trends in both simulated temperature and precipitation relative to individual models. Meanwhile, Reg CM4 presents the most reasonable simulated trends among the five RCMs.（2） Considerable dissimilarities are shown in the simulated quantitative results from the different RCMs, which indicates a large model dependency in the simulation of climate over the Tibetan Plateau. This implies that caution may be needed when an individual RCM is used to estimate the amplitude of climate change over the Tibetan Plateau.（3） The temperature（precipitation） in 2016–35, relative to 1986–2005, is projected by the multi-model ensemble to increase by 1.38 ± 0.09 °C（0.8% ± 4.0%） and 1.77 ± 0.28 °C（7.3% ± 2.5%） under the RCP4.5 and RCP8.5 scenario, respectively. The results of this study advance our understanding of the applicability of RCMs in studies of climate change over the Tibetan Plateau from a multiple-RCM perspective.

Influence of the Tibetan Plateau on the Summer Climate Patterns over Asia in the IAP/LASG SAMIL Model

A series of numerical experiments are carried out by using the Spectral Atmospheric Model of State Key Laboratory of Numerical Modeling Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics （SAMIL） to investigate how the Tibetan Plateau （TP） mechanical and thermal forcing affect the circulation and climate patterns over subtropical Asia. It is shown that, compared to mechanical forcing, the thermal forcing of TP plays a dominant role in determining the large-scale circulation in summer. Both the sensible heating and the latent heating over TP tend to generate a surface cyclonic circulation and a gigantic anticyclonic circulation in the mid- and upper layers, whereas the direct effect of the latter is much more significant. Following a requirement of the time-mean quasi-geostrophic vorticity equation for large-scale air motion in the subtropics, convergent flow and vigorous ascending motion must appear to the east of TP. Hence the summer monsoon in East China is reinforced efficiently by TP. In contrast, the atmosphere to the west of TP is characterized by divergent flow and downward motion, which induces the arid climate in Mid-Asia.

High-resolution peat records for Holocene monsoon history in the eastern Tibetan Plateau

The variations of summer and winter monsoons during the Holocene in theeastern Tibetan Plateau are shown to follow two basic models based on the reliable dating andhigh-resolution monsoon proxies determinations, one being a synchronous model in that both summerand winter monsoons are strengthening or decreasing, and the other to form a complementary pattern.These two different patterns evenly interact with each other on different time scales and togethercompose a complicated monsoon climatic model in this region. The climatic condition integrated bywinter and summer monsoons is synchronous to the global pattern, which also shows the instability ofthe Holocene climate on centennial-millennial timescale. The abrupt monsoon event in about 6.2 kacat. BP is much more severe than that in ca. 8.0 ka cal. BP, which indicates the regional characterof the Asian monsoon and that the Asian monsoon climate is indeed a window on the global climatesystem.

中国21kaBP气候模拟的初步试验

本文使用含有陆面过程的 9层大气环流谱模式 (AGCM+SSi B) ,在地球轨道参数和下垫面边界条件驱动下 ,对 2 1ka BP的气候进行模拟试验 .结果表明 ,2 1ka BP时中国东部干旱 ,西部和青藏高原湿润 ,全国普遍降温 .该模拟结果基本捕捉了由古湖泊资料和孢粉资料重建的气候特征 .对模式输出的大气环流场和降水场的分析揭示出 ,2 1ka BP东亚夏季风环流明显减弱 ,而青高原夏季风环流增强 ;冬季风环流较现在略有增强 .该模拟结果的意义在于 :(1)率先用具有物理机制的数值化模型再现了末次盛冰期中国的气候状况 ,结果与地质资料基本一致 ;(2 )从动力机制的角度解释了 2 1ka BP气候与现代气候存在巨大差别的原因在于辐射变化。

Modeling the climate effects of different subregional uplifts within the Himalaya-Tibetan Plateau on Asian summer monsoon evolution

Considering the different uplifting time of different subregions of the Himalaya-Tibetan Plateau(TP),a series of numerical simulations have been conducted with the Community Atmosphere Model(CAM4) developed at the National Center for Atmospheric Research to explore the effects of the phased tectonic uplift of the Himalaya-TP on the evolution of Asian summer monsoons.The results show that the uplifts of the Himalaya and northern TP significantly affect the evolutions of South Asian summer monsoon and northern East Asian summer monsoon respectively.That is,the tectonic uplift of the Himalaya intensifies the South Asian summer monsoon circulation and increases the precipitation in South Asia,whereas the uplift of the northern TP intensifies the northern East Asian summer monsoon circulation and increases the precipitation in northern East Asia.Compared with previous simulations,current comparative analyses of modeling results for different subregional uplifts within the Himalaya-TP help deepen our understanding of the evolutionary history of Asian monsoons.

Current Progresses in Study of Impacts of the Tibetan Plateau on Asian Summer Climate

The current progresses in the study of impacts of the Tibetan Plateau on Asian summer climate in the last decade are reviewed. By analyzing evolution of the transitional zone between westerly to the north and easterly to the south （WEB）, it is shown that due to the strong heating over the Tibetan Plateau in spring, the overturning in the prevailing wind direction from easterly in winter to westerly in summer occurs firstly over the eastern Bay of Bengal （BOB）, accompanied with vigorous convective precipitation to its east. The area between eastern BOB and western Indo-China Peninsula thus becomes the area with the earliest onset of Asian monsoon, which may be referred as BOB monsoon in short. It is shown that the summertime circulations triggered by the thermal forcing of the Iranian Plateau and the Tibetan Plateau are embedded in phase with the continental-scale circulation forced by the diabatic heating over the Eurasian Continent. As a result, the East Asian summer monsoon is intensified and the drought climate over the western and central Asian areas is enhanced. Together with perturbations triggered by the Tibetan Plateau, the above scenarios and the associated heating have important influences on the climate patterns over Asia. Furthermore, the characteristics of the Tibetan mode of the summertime South Asian high are compared with those of Iranian mode. Results demonstrate that corresponding to each of the bimodality of the South Asian high, the rainfall anomaly distributions over Asia exhibit different patterns.

Continental drift, plateau uplift, and the evolutions of monsoon and arid regions in Asia, Africa, and Australia during the Cenozoic

Monsoon and arid regions in the Asia-Africa-Australia(A-A-A) realm occupy more than 60% of the total area of these continents. Geological evidence showed that significant changes occurred to the A-A-A environments of the monsoon and arid regions, the land-ocean configuration in the Eastern Hemisphere, and the topography of the Tibetan Plateau(TP) in the Cenozoic. Motivated by this background, numerical experiments for 5 typical geological periods during the Cenozoic were conducted using a coupled ocean-atmosphere general circulation model to systemically explore the formations and evolutionary histories of the Cenozoic A-A-A monsoon and arid regions under the influences of continental drift and plateau uplift. Results of the numerical experiments indicate that the timings and causes of the formations of monsoon and arid regions in the A-A-A realm were very different. The northern and southern African monsoons existed during the mid-Paleocene, while the South Asian monsoon appeared in the Eocene after the Indian Subcontinent moved into the tropical Northern Hemisphere. In contrast, the East Asian monsoon and northern Australian monsoon were established much later in the Miocene. The establishment of the tropical monsoons in northern and southern Africa, South Asia, and Australia were determined by both the continental drift and seasonal migration of the Inter-Tropical Convergence Zone(ITCZ), while the position and height of the TP were the key factor for the establishment of the East Asian monsoon. The presence of the subtropical arid regions in northern and southern Africa,Asia, and Australia depended on the positions of the continents and the control of the planetary scale subtropical high pressure zones, while the arid regions in the Arabian Peninsula and West Asia were closely related to the retreat of the Paratethys Sea. The formation of the mid-latitude arid region in the Asian interior, on the other hand, was the consequence of the uplift of the TP.These results from this study provide insight to the important roles played by the earth's tectonic boundary conditions in the formations and evolutions of regional climates during geological times.

Bridging the knowledge gap on the evolution of the Asian monsoon during 26–16 Ma

The evolution of the Asian monsoon from the Late Oligocene to the Early Miocene is poorly understood.Here,we first reconstruct the precipitation data of central Tibet during 26–16 million years ago(Ma),applying the coexistence approach to sedimentary pollen data,and detect an intensified Asian monsoon with
1.35 Ma and
0.33 Ma cycles.Paleoclimate modeling is used to show the importance of paleogeographic location in the development of the paleomonsoon.In addition,the results of spectral analysis suggest that the fluctuations in the Asian monsoon during 26–16 Ma can be attributed to the long-period cyclicities in obliquity(
1.2 Ma).These findings provide climate data that can be used to understand the Asian monsoon evolution during the Late Oligocene to Early Miocene and highlight the effects of paleogeographic patterns and long-period orbital forcings on the tectonic-scale evolution of the Asian monsoon.

基于青藏高原和印度洋区域热力状况的气候预测先兆信号

围绕青藏高原和印度洋区域热力状况对南亚季风、水汽输送和区域气候的影响,本文梳理总结了相关研究进展,重点聚焦在青藏高原和印度洋区域热力状况和热力差异与季风活动、水汽输送等重要气候指标的联系,并据此提出了印度洋和高原区域在次季节、季节尺度的气候预测指标和方法。对认识青藏高原和印度洋区域热力状况对气候的影响,改善该区域气候预测的能力具有参考意义。

距今2000年青藏高原湖泊水位下降的区域特征及机理

基于亚洲夏季风与西风的影响范围将青藏高原划分为3个研究区,通过对比湖泊沉积物中多代用指标与晚全新世火山活动、北半球温度和亚洲季风指数,探讨了2 kaBP前后高原湖泊水位下降的原因,并分析了不同区域湖泊对气候波动响应的空间差异。结果表明,青藏高原西南部湖面水位下降幅度大于西北部,更甚于高原东北部。这可能是因印度夏季风(Indian Summer Monsoon,简称ISM)强度减弱,高原西南部的湖泊更依赖于ISM降水的补给,因此对该季风所带来的水汽通量的减少更加敏感。而且,该时期的北大西洋涛动(North Atlantic Oscillation,简称NAO)的位相由负转正,使得青藏高原北部水汽辐合增强、降水偏多而南部降水偏少,进而导致高原南部湖面水位下降幅度普遍大于北部湖泊。导致青藏高原气候趋于冷干的主要原因,本文归因于该阶段厄尔尼诺(EI Niño)的加强。除此之外,该时期南半球环状模(Southern Annular Mode,简称SAM)冬夏季的不同位相也通过复杂的海气耦合过程,跨越赤道对青藏高原气候起到了降温减湿的作用。

未来情景羌塘高原野牦牛栖息地分布及其影响因素

近30年来,羌塘高原野牦牛(Bos mutus)种群数量虽缓慢恢复,但仍为野生有蹄类中仅有的易危物种。由于其对人为活动规避明显且具有极强的攻击性,野牦牛栖息地分布和质量数据仍很匮乏。把野外调查与最大熵(Maxent)、土地利用模拟模型(FLUS)、InVEST三种模型相结合,系统分析羌塘高原野牦牛栖息地分布及其影响因素,并通过未来气候、未来土地覆被和未来食物情景构建2050年不同温室气体排放浓度(RCPs)情景下羌塘高原生境状况,预测栖息地变化状况,以期为青藏高原生物多样性维护提供数据支撑。结果发现:2020年前后野牦牛栖息地总面积为25.1万km^(2),集中分布在那曲市北部,阿里地区分布零散。栖息地以草原和荒漠为主,部分位于冰川区,野牦牛对气候条件反应敏感,偏好生活在暖季降雨量约在200mm,冷季降雨量约10mm,年最低温度-30℃的区域,坡度耐受性高。约92%的野牦牛栖息地位于羌塘国家自然保护区内,仅有南部约1.2万km^(2)栖息地与人类活动交叠明显。2050年前后羌塘高原暖湿化明显,草原面积增加,野牦牛栖息地将向西北部无人区扩张,目前栖息地分布较零散的阿里地区也将出现大面积高质量栖息地,三种RCPs情境下栖息地面积分别为28.2万km^(2)(RCP2.6)、28.4万km^(2)(RCP4.5)和28.0万km^(2)(RCP8.5),新增栖息地以极重要栖息地为主,边界与羌塘自然保护区范围更加吻合,自然保护地体系建设将有力支撑野牦牛的保护。

AN AGCM +SSiB MODEL SIMULATION ON CHANGES IN PALAEOMONSOON CLIMATE AT 21 KA BP IN CHINA

The numerical simulation experiment of climate at Last Glacial Maximum (LGM.21 ka BP) in China is made by using an atmospheric general circulation model (AGCM) coupled with land surface processes (AGCM+SSiB) and earth orbital parameters and boundary forcing conditions at 21 ka.The modeled climate features are compared with reconstructed conditions at 21 ka from paleo-lake data and pollen data.The results show that the simulated climate conditions at 21 ka in China are fairly comparable with paleo-climatological data.The climate features at 21 ka in China from the experiment are characterized by a drier in the east and a wetter in the west and in the Tibetan Plateau as well.According to the analysis of distribution of pressure and precipitation,as well as the intensity of atmospheric circulation at 21 ka,monsoon circulation in eastern Asia was significantly weak comparing with the present.In the Tibetan Plateau,the intensity of summer monsoon circulation was strengthened,and winter monsoon was a little stronger than the present. The simulation with given forcing boundary conditions,especially the different vegetation coverage,can reproduce the climate condition at the LGM in China,and therefore provides dynamical mechanisms on the climate changes at 21 ka.

青藏高原积雪对中国夏季风气候的影响

利用ＳＶＤ等方法对青藏高原积雪与中国区域降水的关系作了诊断分析。并用区域气候模式（ＲｅｇＣＭ２）对高原积雪的气候效应进行了模拟。结果表明：青藏高原积雪对中国夏季风气候的影响是显著的。积雪的增加会明显减弱亚洲夏季风的强度，使华南的降水减少，江淮流域的降水增多。高原冬季积雪深度的增加，比积雪面积的扩大和春季积雪深度的增加对后期气候的影响更大。

2.5Ma以来柴达木盆地的气候干湿变化特征及其原因

长期以来 ,一直认为柴达木盆地第四纪气候在波动中向干旱方向发展。原因是青藏高原的隆升阻挡了来自印度洋、孟加拉湾的水汽。然而 ,来自柴达木盆地新的证据表明 ,柴达木盆地第四纪气候演变的总体趋势是冰期越来越干燥 ,间冰期干燥程度却存在减弱的趋势。并且这种变化是呈阶段性的。最明显的阶段划分应在 0 .8～ 0 .6Ma前后。这种现象可以用青藏高原的隆升做出比较合理的解释 :青藏高原的隆升 ,不仅通过增强冬季亚洲高压 (西伯利亚高压 )使冬季风增强 ,使东亚季风区冰期气候更加干燥寒冷 ,而且还可以通过增强夏季亚洲低压 (印度低压 ) ,使夏季风增强、间冰期气候更加温暖湿润 ,从而使得中国季风区冰期 -间冰期的气候变差增大。可是 ,柴达木盆地位于青藏高原北缘 ,我国西北内陆区受夏季风影响较弱 ,间冰期或者湿润期湿润程度的增大如何与高原隆升和季风系统的调整相联系 ,还有待于进一步的研究。

青藏高原积雪对亚洲夏季风影响的诊断及数值研究

通过对青藏高原多、少雪年的合成分析及数值试验，研究了青藏高原积雪对亚洲夏季风和我国东部气候异常的影响。结果表明：青藏高原积雪造成亚洲大气环流较大的年际变化。高原积雪改变了高原陆面春、夏季的热状况，使亚洲夏季风爆发推迟２０天左右。高原积雪通过以下物理过程影响亚洲夏季风和我国东部气候：高原积雪多（少）→高原春、夏季的感热弱（强）→感热加热引起的上升运动弱（强），高原强（弱）环境风场→不利（有利）于高原感热通量向上输送→高原上空对流层加热弱（强）→高原对流层温度低（高）→高原南侧温度对比弱（强）→造成亚洲夏季风弱（强）→我国长江流域易涝（旱）。

青藏高原影响亚洲夏季气候研究的最新进展

文中回顾了近 10a来吴国雄等在青藏高原影响亚洲夏季气候研究方面的最新进展。通过分析东西风交界面的演变证明 ,由于青藏高原的春季加热 ,亚洲季风区对流层低层冬季盛行偏东风转变为夏季偏西南风最早发生在孟加拉湾东部 ,与其相伴随的激烈对流降水出现在其东面。因此孟加拉湾东部至中印半岛西部是亚洲季风最早爆发的地区。同时也指出盛夏伊朗高原和青藏高原加热所激发的同相环流嵌套在欧亚大陆尺度的热力环流中 ,从而加强了东亚的夏季风 ,加剧了中西亚的干旱 ;并通过其所激发的波动对夏季东亚的气候格局产生重要影响。文中还比较了夏季南亚高压的伊朗模态和青藏模态性质的异同及其对亚洲夏季降水异常分布的不同影响。

青藏高原大气热源和冬春积雪与中国东部降水的年代际变化关系

利用NCEP 1950—2004年逐日再分析资料,采用倒算法,对青藏高原大气热源的长期变化进行了计算,结果发现,青藏高原及附近地区上空大气春夏季热源在过去50年里,尤其是最近20年,表现为持续减弱的趋势。而1960—2004年青藏高原50站的冬春雪深却出现了增加,尤其是春季雪深在1977年出现了由少到多的突变。用SVD方法对高原积雪和高原大气热源关系的分析表明,二者存在非常显著的反相关关系,即高原冬春积雪偏多,高原大气春夏季热源偏弱。高原大气春夏季热源和中国160站降水的SVD分析表明,高原大气春夏季热源和夏季长江中下游降水呈反相关,与华南和华北降水呈正相关;而高原冬春积雪和中国160站降水的SVD分析显示,高原冬春积雪和夏季长江流域降水呈显著正相关,与华南和华北降水呈反相关。在年代际尺度上,青藏高原大气热源和冬春积雪与中国东部降水型的年代际变化(南涝北旱)有很好的相关。最后讨论了青藏高原大气热源影响中国东部降水的机制。青藏高原春夏季热源减弱,使得海陆热力差异减小,致使东亚夏季风强度减弱,输送到华北的水汽减少,而到达长江流域的水汽却增加;同时,高原热源减弱,使得副热带高压偏西,夏季雨带在长江流域维持更长时间。导致近20年来长江流域降水偏多,华北偏少,形成"南涝北旱"雨型。高原冬春积雪的增加,降低了地表温度,减弱了地面热源,并进而使得青藏高原及附近地区大气热源减弱。

青藏高原：全球气候变化的驱动机与放大器──Ⅲ．青藏高原隆起对气候变化的影响

新生代出现的几次重大气候变化事件与青藏高原形成过程中经历的几次强烈构造运动在发生年代上的良好对应关系表明两者存在紧密的联系．或许正是青藏高原的隆起导致了气候的巨大变化和现代东亚季风的形成及加强．高大的高原对大气系统有两个重要影响，热力作用和动力作用．它们在亚洲冬季形成蒙古高压、夏季形成印度低压，从而导致东亚季风出现．青藏高原越高，其对大气的作用越显著，形成的蒙古高压、印度低压和东亚季风越强大．因此，东亚季风的形成和加强是青藏高原隆起的结果．东亚季风的加强和高原动力作用使西风带波动增加，造成冰期极地冷空气不断南侵、气候波动变大．青藏高原抬升和季风造成的化学风化增强还使大气ＣＯ２含量降低、气候变冷．因此，青藏高原隆起是控制新生代气候变化的重要因素．