Interannual and Decadal Variations of Snow Cover overQinghai-Xizang Plateau and Their Relationships to Summer Monsoon Rainfall in China

Interannual and decadal variations of winter snow cover over the Qinghai-Xizang Plateau (QXP) are analyzed by using monthly mean snow depth data set of 60 stations over QXP for the period of 1958 through 1992. It is found that the winter snow cover over QXP bears a pronounced quasi-biennial oscillation, and it underwent an obvious decadal transition from a poor snow cover period to a rich snow cover period in the late 1970’s during the last 40 years. It is shown that the summer rainfall in the eastern China is closely associated with the winter snow cov-er over QXP not only in the interannual variation but also in the decadal variation. A clear relationship ex-ists in the quasi-biennial oscillation between the summer rainfall in the northern part of North China and the southern China and the winter snow cover over QXP. Furthermore, the summer rainfall in the four cli-mate divisions of Qinling-Daba Mountains, the Yangtze-Huaihe River Plain, the upper and lower reaches of the Yangtze River showed a remarkable transition from drought period to rainy period in the end of 1970’s, in good correspondence with the decadal transition of the winter snow cover over QXP. Key words Snow cover over Qinghai-Xizang Plateau - Summer monsoon rainfall in China - Interannual and decadal variations This study was supported by the National Key Programme for Developing Basic Sciences (G 1998040900 Part I).

Formation of the Summertime Ozone Valley over the Tibetan Plateau:The Asian Summer Monsoon and Air Column Variations

The summertime ozone valley over the Tibetan Plateau is formed by two influences,the Asian summer monsoon（ASM） and air column variations.Total ozone over the Tibetan Plateau in summer was ～33 Dobson units（DU） lower than zonal mean values over the ocean at the same latitudes during the study period 2005-2009.Satellite observations of ozone profiles show that ozone concentrations over the ASM region have lower values in the upper troposphere and lower stratosphere（UTLS） than over the non-ASM region.This is caused by frequent convective transport of low-ozone air from the lower troposphere to the UTLS region combined with trapping by the South Asian High.This offset contributes to a ～20-DU deficit in the ozone column over the ASM region.In addition,along the same latitude,total ozone changes identically with variations of the terrain height,showing a high correlation with terrain heights over the ASM region,which includes both the Tibetan and Iranian plateaus.This is confirmed by the fact that the Tibetan and Iranian plateaus have very similar vertical distributions of ozone in the UTLS,but they have different terrain heights and different total-column ozone levels.These two factors（lower UTLS ozone and higher terrain height） imply 40 DU in the lower-ozone column,but the Tibetan Plateau ozone column is only ～33 DU lower than that over the non-ASM region.This fact suggests that the lower troposphere has higher ozone concentrations over the ASM region than elsewhere at the same latitude,contributing ～7 DU of total ozone,which is consistent with ozonesonde and satellite observations.

DECOUPLED SUMMER AND WINTER MONSOON IN NORTHEAST TIBET AND NORTHWEST LOESS PLATEAU DURING THE LAST INTERGLACIATION

Modern meteorological observations have proved that climate change in the northeast Tibet plateau is characteristic of alternations of plateau summer and winter monsoons, and climate change in Chinese Loess plateau is geared by variations of East Asian summer and winter monsoon strengths. A transitional zone between regions dominated by plateau monsoon and East Asian monsoon respectively is located at around 110°E in China. The two monsoon systems are driven by different forcing aspects.Here we show the two climatic systems change during the last interglacial period (IG) by examining geological records. Two aeolian loess\|paleosol sequences,one is located in northeast Tibet plateau closed to Xining and the other one in the northwest Loess plateau closed to Huanxian, were investigated. Age frames of the paleosol and intercalated loess are achieved by Thermoluminescence dating, palaeomagnetic measurements and stratigraphy correlation. Samples taken from Huanxian section were at 5cm intervals, and samples from Xining section were taken at every 10cm. The samples were measured for magnetic susceptibility (MS), rubidium/strontium value (Rb/Sr), Calcium carbonate content (CaCO\-3) and grain\|size distribution (GS). Detail time scale is obtained by two steps. First, correlate MS curves with deep\|sea oxygen isotope time series of stage 4,5 and 6 of Martinson et al (1987) to assign ages of boundaries of stratigraphic units. Second, linearly interpolate ages between the obtained ages and therefore get age of each sampling point.

A Possible Impact of Cooling over the Tibetan Plateau on the Mid-Holocene East Asian Monsoon Climate

By using a 9-level global atmospheric general circulation model developed at the Institute of Atmospheric Physics （IAP9L-AGCM） under the Chinese Academy of Sciences, the authors investigated the response of the East Asian monsoon climate to changes both in orbital forcing and the snow and glaciers over the Tibetan Plateau at the mid-Holocene, about 6000 calendar years before the present （6 kyr BP）. With the Earth＇s orbital parameters appropriate for the mid-Holocene, the IAP9L-AGCM computed warmer and wetter conditions in boreal summer than for the present day. Under the precondition of continental snow and glacier cover existing over part of the Tibetan Plateau at the mid-Holocene, the authors examined the regional climate response to the Tibetan Plateau cooling. The simulations indicated that climate changes in South Asia and parts of central Asia as well as in East Asia are sensitive to the Tibetan Plateau cooling at the mid-Holocene, showing a significant decrease in precipitation in northern India, northern China and southern Mongolia and an increase in Southeast Asia during boreal summer. The latter seems to correspond to the weakening, southeastward shift of the Asian summer monsoon system resulting from reduced heat contrast between the Eurasian continent and the Pacific and Indian Oceans when a cooling over the Tibetan Plateau was imposed. The simulation results suggest that the snow and glacier environment over the Tibetan Plateau is an important factor for mid-Holocene climate change in the areas highly influenced by the Asian monsoon.

The Relationship between Precipitation and Airflow over the Tibetan Plateau in Boreal Summer

Based on the observation data and the reanalysis datasets, the variability and the circulation features influencing precipitation in the Tibetan Plateau (TP) are investigated. Taking into account the effects of topography, surface winds are deconstructed into flow-around and flow-over components relative to the TP. Climatologically, the flow-around component mainly represents cyclonic circulation in the TP during the summer. The transition zone of total precipitation in the summer parallels the convergence belt between the southerlies and the northerlies of the flow-over component. The leading mode of rainfall anomalies in the TP has a meridional dipole structure, and the first principal component (PC1) mainly depicts the variation of rainfall in the southern TP. The wet southern TP experiences strengthened flow-over, which in turn mechanistically favors intensified ascent forced by the flow-over component. In addition, variations in the Indian summer monsoon (ISM) have an important role in influencing the flow over the southern TP, and the ISM ultimately impacts the precipitation over southern TP.

Interdecadal change of snow depth in winter over the Tibetan Plateau and its effect on summer precipitation in China

This paper obtained a set of consecutive and long-recorded observational snow depth data from 51 observation stations by choosing, removing and interpolating original observation data over the Tibetan Plateau for 1961-2006. We used monthly precipitation and temperature data from 160 stations in China for 1951-2006, which was collected by the National Climate Center. Through calculating and analyzing the correlation coefficient, significance test, polynomial trend fitting, composite analysis and abrupt change test, this paper studied the interdecadal change of winter snow over the Tibetan Plateau and its relationship to summer pre- cipitation and temperature in China, and to tropospheric atmospheric temperature. This paper also studied general circulation and East Asian summer monsoon under the background of global warming.

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。

春季青藏高原积雪年际变率的年代际转型对东亚夏季风影响的研究进展

冬春青藏高原积雪异常是东亚夏季风的重要预测因子之一。本文系统回顾了近20年关于青藏高原积雪年际变率的年代际转型影响东亚夏季风的相关研究,主要结论如下:(1)20世纪90年代初春季青藏高原积雪的年际变率从东西偶极型转变为全区一致型,这主要受北太平洋、热带大西洋海温异常变化的影响,也与南极涛动、北极涛动的变化密切相关;(2)春季青藏高原积雪年际变率的年代际转型可通过影响东亚高层的副热带西风急流和低层的水汽输送,进而影响东亚夏季风降水格局变化;(3)青藏高原积雪异常可通过“高原大气河”的机制影响梅雨雨带;(4)大西洋年代际振荡可调节春季青藏高原积雪与梅雨降水关系的年代际变化,当大西洋年代际振荡为正(负)位相时,春季青藏高原积雪与梅雨的关系加强(减弱)。最后,本文对青藏高原积雪异常影响东亚季风变化的关键科学问题进行了讨论与展望。

A multi-location joint field observation of the stratosphere and troposphere over the Tibetan Plateau

The unique geographical location and high altitude of the Tibetan Plateau can greatly influence regional weather and climate.In particular, the Asian summer monsoon(ASM) anticyclone circulation system over the Tibetan Plateau is recognized to be a significant transport pathway for water vapor and pollutants to enter the stratosphere. To improve understanding of these physical processes, a multi-location joint atmospheric experiment was performed over the Tibetan Plateau from late July to August in 2018, funded by the fiveyear(2018–2022) STEAM(stratosphere and troposphere exchange experiment during ASM) project, during which multiple platforms/instruments—including long-duration stratospheric balloons, dropsondes, unmanned aerial vehicles, special sounding systems, and ground-based and satellite-borne instruments—will be deployed. These complementary methods of data acquisition are expected to provide comprehensive atmospheric parameters(aerosol, ozone, water vapor, CO_2, CH_4, CO, temperature, pressure,turbulence, radiation, lightning and wind); the richness of this approach is expected to advance our comprehension of key mechanisms associated with thermal, dynamical, radiative, and chemical transports over the Tibetan Plateau during ASM activity.

A preliminary study of the quantitative couplingbetween loess-paleosol sequences and summermon soon changes

The Loess Plateau-Northeastern Tibetan Plateau in China is famous for its deep andcontmuous distributed loess, in which soils (developed from loess) vary gradually with differentbiological climates changing from subhumid to arid, which provide a natural proving ground forstUd}' the coupling of loess-paleosol and climatic changes quantitatively. Thirty-two sections weresampled (collected) for measuring chemical element contents, clay content (and other items) and 60sections were collected for carbonate analysis to establish transfer functions between pedogenesisand climate. Finally, we estimated the paleoclAnate (P/T) in the region with transfer functions,It isrevealed that the variation of paleocldriate reflected by transfer functions accorded closely with theclimatic pattern resulted from recent work. Specifically, it was about 140 ha BP that summermonsoon reached Lanzhou or further, paleosol S1 (equaling to MIS 5 in deep sea records) developedstrongly in a large scale, even touching upon northeanstern Tibetan Plateau. And, the estimated P/Tinformation also indicated the cold and humid period during 50-30 ka BP, mean annual Precipitationwas about 400-500 nun in Angutan recorded by mega-interstadial paleosol Sm.

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 midlatitnde air. Previous studies have also shown that Indian snmmer 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 snmmer monsoon （EASM） in the UK Met Office＇s HadGEM3-GA6 and China＇s Institute of Atmospheric Physics FGOALS-FAMIL global climate models. The models chosen featnre 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.

Impacts of meteorological parameters and emissions on decadal,interannual, and seasonal variations of atmospheric black carbon in the Tibetan Plateau

We quanti?ed the impacts of variations in meteorological parameters and emissions on decadal, interannual, and seasonal variations of atmospheric black carbon(BC) in the Tibetan Plateau for 1980-2010 using a global 3-dimensional chemical transport model driven by the Modern Era Retrospective-analysis for Research and Applications(MERRA) meteorological ?elds. From 1980 to 2010, simulated surface BC concentrations and all-sky direct radiative forcing at the top of the atmosphere due to atmospheric BC increased by 0.15 μg m^(-3)(63%) and by 0.23 W m^(-2)(62%), respectively, averaged over the Tibetan Plateau(75-105°E, 25-40°N). Simulated annual mean surface BC concentrations were in the range of 0.24-0.40 μg m^(-3) averaged over the plateau for 1980-2010, with the decadal trends of 0.13 μg m^(-3)per decade in the 1980 s and 0.08 in the 2000 s. The interannual variations were -5.4% to 7.0% for deviation from the mean, 0.0062 μg m^(-3) for mean absolute deviation, and 2.5% for absolute percent departure from the mean. Model sensitivity simulations indicated that the decadal trends of surface BC concentrations were mainly driven by changes in emissions, while the interannual variations were dependent on variations of both meteorological parameters and emissions. Meteorological parameters played a crucial role in driving the interannual variations of BC especially in the monsoon season.

青藏高原三江源和河湾区夏季降水变化特征及对高原夏季风的响应

本文利用1981~2020年观测数据和ERA5再分析资料,将青藏高原腹地三江源和东南重要水汽通道河湾区作为典型研究区域,分析了降水不同时间尺度变化特征及其典型强弱年对高原季风环流系统的响应,结果表明:(1)三江源和河湾区降水的季节变化均呈双峰型分布,峰值出现在7月初和8月下旬。夏季降水在21世纪初发生年代际转折,尤其是三江源降水量在近20年增加明显。两个高原季风指数DPMI(Dynamic Plateau Monsoon Index)和ZPMI(Zhou Plateau Monsoon Index)的夏季风爆发时间均超前于河湾区和三江源降水的明显增加期。三江源夏季降水年际变化与两个高原夏季风指数有较好的相关性。三江源与河湾区虽然相邻很近,但三江源夏季降水受高原季风影响程度远大于河湾区。当高原夏季风增强(减弱)时,三江源降水量偏多(少)。(2)三江源降水偏多年,南亚高压偏东偏强,低层高原主体低压异常,有利于西南风和东南风在三江源区域交汇,南方暖湿空气能够深入高原腹地导致水汽辐合偏强。河湾区降水偏多年,河湾区及整个高原主体附近高度场并没有明显异常,河湾区的水汽输送主要有两条路径,一条来自孟加拉湾沿高原南坡的西南路径,另一条来自中亚地区穿过高原上空的西北路径,两条路径在高原东侧汇合继续向东输送。

A very strong summer monsoon event during 30-40 kaBP in the Qinghai-Xizang (Tibet) Plateau and its relation to precessional cycle

Guliya ice core records, high lake-level records in the Qinghai-Xizang Plateau and at its north side as well as vegetation succession records indicated that during the period of 30-40 kaBP, namely the later age of the megainterstadial of last glacial period, or the marine oxygen isotope stage 3, the climate of the Qinghai-Xizang Plateau was exceptionally warm and humid, the temperature was 2-4℃ higher than today and the precipitation was 40% to over 100%

Asian summer monsoon precipitation recorded by stalagmite oxygen isotopic composition in the western Loess Plateau during AD1875―2003 and its linkage with ocean-atmosphere system

Based on 5 high-precision 230Th dates and 103 stable oxygen isotope ratios (δ18O) obtained from the top 16 mm of a stalagmite collected from Wanxiang Cave,Wudu,Gansu,variation of monsoonal precipita-tion in the modern Asian Monsoon (AM) marginal zone over the past 100 years was reconstructed. Comparison of the speleothem δ18O record with instrumental precipitation data at Wudu in the past 50 years indicates a high parallelism between the two curves,suggesting that the speleothem δ18O is a good proxy for the AM strength and associated precipitation,controlled by "amount effect" of the pre-cipitation. Variation of the monsoonal precipitation during the past 100 years can be divided into three stages,increasing from AD 1875 to 1900,then decreasing from AD 1901 to 1946,and increasing again thereafter. This variation is quite similar to that of the Drought/Flooding index archived from Chinese historical documents. This speleothem-derived AM record shows a close association with the Pacific Decadal Oscillation (PDO) between AD 1875 and 1977,with higher monsoonal precipitation corre-sponding to cold PDO phase and vice versa at decadal timescale. The monsoonal precipitation varia-tion is out of phase with the PDO after AD 1977,probably resulting from the decadal climate jump in the north Pacific occurring at around AD 1976/77. These results demonstrate a strong linkage between the AM and associated precipitation and the Pacific Ocean via ocean/atmosphere interaction. This rela-tionship will aid to forecast future hydrological cycle for the AM monsoon region,and to improve forecasting potential of climatic model with observation data from cave.

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.

Pollen evidence from Baode of the northern Loess Plateau of China and strong East Asian summer monsoons during the Early Pliocene

The red clay eolian sequence on the Chinese Loess Plateau (CLP) is an important archive for paleoclimate change from the late Miocene to Pliocene,and can provide significant information for the controversial problems of East Asian monsoon evolution and its forcing mechanism.In this study,we present a pollen record from Baode,northern CLP.The record shows four stages of paleoecological evolution.From 5.6-4.4 Ma,a forest steppe ecosystem developed under an extremely warm period with high seasonal precipitation.Since 4.4 Ma,a drier episode occurred,which prompted parkland landscapes to develop.During 3.5-3.05 Ma,the environment changed to a rather open steppe ecosystem with a much cooler and drier climate.After 3.05 Ma,the vegetation evolved to forest steppe.Using the percentages of arboreal plants to mirror precipitation,and comparing with other published pollen data from the CLP,we find the existence of S-N directionality of the precipitation change and high percentage of arboreal plants in the entire CLP during the Early Pliocene,which suggest the CLP was characterized by a strong East Asian summer monsoon.The strong summer monsoon corresponds well to the low global ice volume,which may illuminate global climate mechanism for the summer monsoon evolution in the early Pliocene.

亚洲夏季风对渠江流域夏季降水的影响

基于1971—2020年NCEP大气环流再分析资料、渠江流域14个国家气象站点逐日降水资料,利用相关分析、合成分析等方法,对渠江流域夏季降水与亚洲夏季风的响应机制进行研究。结果表明:东亚夏季风与渠江流域夏季降水存在显著负相关,而高原夏季风与渠江流域夏季降水表现为显著正相关,但东亚夏季风与流域夏季降水相关性更加显著;东亚夏季风主要影响流域水汽条件,高原夏季风主要影响流域垂直运动条件。

亚洲地区夏季风期间氨气的时空分布特征

利用ENVISAT卫星搭载的迈克尔逊干涉仪和Aqua卫星搭载的AIRS探测仪观测到的大气NH3浓度数据以及全球大气化学—气候模式EMAC模拟的NH3浓度结果,分析了2008~2011年6~9月亚洲地区大气NH3的空间分布特征。结果显示,夏季时近地面NH3浓度最高值出现在印度北部,同时紧邻印度北部的孟加拉湾存在深对流,凭借青藏高原的高海拔地势,此深对流可以将寿命较短的NH3输送到上对流层和下平流层(Upper Troposphere and Lower Stratosphere,UTLS),所以在青藏高原上空出现了NH3的向上输送柱,即青藏高原是NH3向上输送的主要通道。亚洲夏季风反气旋的位置主导着NH3在UTLS区域的空间分布,反气旋内持续存在NH3高浓度中心,NH3高浓度中心位置与反气旋中心位置对应良好,会出现一个或两个NH3高浓度中心,说明反气旋内环流形式的变化对反气旋内NH3分布特征有重要影响。

An 8.1Ma calcite record of Asian summer monsoon evolution on the Chinese central Loess Plateau

Carbonates in loess-red clay sequences consist mainly of calcite and dolomite. The EDTA analysis of carbonates in different size fractions and magnetic susceptibility reveal that calcite is a sensitive index of summer monsoon. The chemical analysis of carbonates and calcite from an 8.1 Ma loess-red clay sequence at Chaona on the Chinese central Loess Plateau shows that the evolution of the Asian summer monsoon experienced four stages, namely 8.1―5.5 Ma, 5.5―2.8 Ma, 2.8―1.5 Ma and 1.5―0 Ma, with increasing intensification and fluctuation, suggesting a possible combining impacts of uplift of the Tibetan Plateau and global changes on the Asian summer monsoon.