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.

Thermal contrast between the Tibetan Plateau and tropical Indian Ocean and its relationship to the South Asian summer monsoon

The land-sea thermal contrast is an important driver for monsoon interannual variability and the monsoon onset.The thermal contrast between the Tibetan Plateau and the tropical Indian Ocean at the mid-upper troposphere is proposed as a thermal contrast index(TCI)for South Asian monsoon.The authors investigate the TCI associated with South Asian summer monsoon(SASM)intensity and SASM onset.It is observed that the TCI considering the Tibetan Plateau and tropical Indian Ocean demonstrates a stronger and closer correlation with SASM intensity(0.87)than either the Tibetan Plateau(0.42)or tropical Indian Ocean(-0.60)singly.It is implied that the TCI could preferably represent the impact of land-sea thermal condition on SASM activity.Further analysis reveals that the evolution of TCI is related to the SASM onset.The TCI is almost always larger in early onset years than it is in late onset years during the period before SASM onset.In addition,the change of the pentad-by-pentad increment of TCI leads the SASM variation.The correlation coefficient between the TCI increment and SASM index reaches a maximum when the TCI increment leads by 15 pentads.The results of this study show that the TCI plays an important role in SASM activities and is a potential indicator for SASM onset forecasting.

The Warming and Wetting Ecological Environment Changes over the Qinghai-Tibetan Plateau and the Driving Effect of the Asian Summer Monsoon

The impact of warming and wetting on the ecological environment of the Qinghai-Tibet Plateau(TP)under the background of climate change has been a concern of the global scientific community.In this paper,the optimized interpolation variational correction approach is adopted for the analysis of monthly high-resolution satellite precipitation products and observations from meteorological stations during the past 20 years.As a result,the corrected precipitation products can not only supplement the“blank area”of precipitation observation stations on the TP,but also improve the accuracy of the original satellite precipitation products.The precipitation over the TP shows different spatial changes in the vegetation growing season,known as the time from May to September.The precipitation in the vegetation growing season and leaf area index(LAI)in the following month show a similar change pattern,indicating a“one-month lag”response of LAI to precipitation on the TP.Further analysis illustrates the influence of water vapor transport driven by the Asian summer monsoon.Water vapor derived from trans-equatorial air flows across the Indian Ocean and Arabian Sea is strengthened,leading to the increase of precipitation in the central and northern TP,where the trend of warming and wetting and the increase of vegetation tend to be more obvious.By contrast,as a result of the weakening trend of water vapor transport in the middle and low levels in southern TP,the precipitation decreases,and the LAI shows a downtrend,which inhibits the warming and wetting ecological environment in this area.

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.

The Dynamic Plateau Monsoon Index and Its Association with General Circulation Anomalies

Based on monthly ECMWF reanalysis-Interim （ERA-Interim） reanalysis data, along with monthly precipitation and temperature data, the Dynamic Plateau Monsoon Index （DPMI） is defined. The results of a contrast analysis of the DPMI versus the Traditional Plateau Monsoon Index （TPMI） are described. The response of general circulation to northern Qinghai-Xizang Plateau summer monsoon anomalies and the correlation of the DPMI with general circulation anomalies are investigated. The results show that, the DPMI reflected meteorological elements better and depicted climate variation more accurately than the TPMI. In years when the plateau summer monsoon is strong, the low over the plateau and the trough near the eastern coast of Asia are deeper and higher than normal over South China. This correlation corresponds to two anomalous cyclones over the plateau and the eastern coast of Asia and an anomalous anticyclone in South China. The plateau and its adjacent regions are affected by anomalous southwesterly winds that transport more moisture to South China and cause more precipitation. The lower reaches of the Yangtze River appear to receive more precipitation by means of the strong westerly water vapor flow transported from the ＂large triangle affecting the region＂. In years when the plateau summer monsoon is weak, these are opposite. The plateau monsoon is closely related to the intensity and position of the South Asian high, and the existence of a teleconnection pattern in the mid-upper levels suggests a possible linkage of the East Asian monsoon and the Indian monsoon to the plateau summer monsoon.

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.

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.

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.

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.

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%

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.

Characteristics of land-atmosphere energy and turbulentfluxes over the plateau steppe in central Tibetan Plateau

The land-atmosphere energy and turbulence exchange is key to understanding land surface processes on the Tibetan Plateau(TP). Using observed data for Aug. 4 to Dec. 3, 2012 from the Bujiao observation point(BJ) of the Nagqu Plateau Climate and Environment Station(NPCE-BJ), different characteristics of the energy flux during the Asian summer monsoon(ASM) season and post-monsoon period were analyzed. This study outlines the impact of the ASM on energy fluxes in the central TP. It also demonstrates that the surface energy closure rate during the ASM season is higher than that of the post-monsoon period. Footprint modeling shows the distribution of data quality assessments(QA) and quality controls(QC) surrounding the observation point. The measured turbulent flux data at the NPCE-BJ site were highly representative of the target land-use type. The target surface contributed more to the fluxes under unstable conditions than under stable conditions. The main wind directions(180° and 210°) with the highest data density showed flux contributions reaching 100%, even under stable conditions. The lowest flux contributions were found in sectors with low data density, e.g., 90.4% in the 360° sector under stable conditions during the ASM season. Lastly, a surface energy water balance(SEWAB) model was used to gap-fill any absent or corrected turbulence data. The potential simulation error was also explored in this study. The Nash-Sutcliffe model efficiency coefficients(NSEs) of the observed fluxes with the SEWAB model runs were 0.78 for sensible heat flux and 0.63 for latent heat flux during the ASM season, but unrealistic values of-0.9 for latent heat flux during the post-monsoon period.

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.

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.

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.

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。

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.

Monsoon Variations of the Qinghai-Xizang Plateau During the Last 12,000 Years——Geochemical Evidence From the Sediments in the Siling Lake

Although the monsoon circulation of the Qinghai-Xizang (Tibetan) Plateau, in response to the seasonal variation of the atmospheric heat contrast between the plateau and the surrounding free atmosphere, is important in influencing the atmospheric circulation of the Northern Hemisphere, there are only a few continuous paleoclimatic records available, and these are limited to the plateau’s margin. Here we present a 12,000-a climatic record from Siling Lake in the central Qinghai-Xizang Plateau, constructed from the geochemical and stable isotopic studies on the sediment core CH8803, which shows that

Oscillation propagation features of the atmosphere around the Qinghai-Xizang Plateau during the spring season of typical strong and weak monsoon years

The interaction between the low-frequency atmospheric oscillation(Madden-Julian Oscillation,MJO) and the diabatic heating over the Qinghai-Xizang Plateau(QXP) from March to June is analyzed.The results show that there are respectively two and one wave trains around the QXP during the onset of the South China Sea monsoon in strong and weak monsoon years.The locations and strength of the wave train propagation differ between the strong and weak monsoon years.Because diabatic heating of the QXP prevents the low-frequency oscillation,the wave train of interaction between the diabatic heating and the zonal wind MJO propagates along the west and east of the QXP in the strong monsoon years.The distribution of the wave train interaction between the diabatic heating and the zonal wind MJO traverses the QXP and coincides with the location of the southern and northern upper-level jet streams,showing that they are remarkably correlated.An interesting and notable phenomenon is that the interaction between diabatic heating and the zonal wind MJO over the QXP suddenly disappears during the monsoon onset in weak monsoon years.