Tibetan Plateau In Summer Research Articles

A Study on the Trend of the Upper Tropospheric Water Vapor over the Tibetan Plateau in Summer

Using ERA-Interim and ERA-5 reanalysis datasets combined with measurements from the Microwave Limb Sounder (MLS), we divided the Tibetan Plateau (TP) into three regions to analyse the long-term trends of water vapor in the upper troposphere and its possible mechanisms. The conclusions are as follows. The water vapor at 200 hPa in the summers from 1979 to 2016 over Region 1 (34°N-40°N, 80°E-100°E) shows a positive trend, which is caused by the rising surface temperature and the northward shift of the South Asian High (SAH). The positive trend of water vapor over Region 2 (28°N-33°N, 80°E-90°E) is attributed to the enhanced South Asian summer monsoon, which leads to increases in horizontal transport of water vapor and vertical water vapor flux. The increase in the vertical transport of water vapor is responsible for the increase in water vapor over the Region 2. The negative trend of water vapor over Region 3 (27°N-30°N, 93°E-98°E) is mainly due to the weakening of water vapor transport caused by the weakening of convective activities.

Transport of Asian surface pollutants to the global stratosphere from the Tibetan Plateau region during the Asian summer monsoon.

Due to its surrounding strong and deep Asian summer monsoon (ASM) circulation and active surface pollutant emissions, surface pollutants are transported to the stratosphere from the Tibetan Plateau region, which may have critical impacts on global climate through chemical, microphysical and radiative processes. This article reviews major recent advances in research regarding troposphere–stratosphere transport from the region of the Tibetan Plateau. Since the discovery of the total ozone valley over the Tibetan Plateau in summer from satellite observations in the early 1990s, new satellite-borne instruments have become operational and have provided significant new information on atmospheric composition. In addition, in situ measurements and model simulations are used to investigate deep convection and the ASM anticyclone, surface sources and pathways, atmospheric chemical transformations and the impact on global climate. Also challenges are discussed for further understanding critical questions on microphysics and microchemistry in clouds during the pathway to the global stratosphere over the Tibetan Plateau.

Numerical study on the climatic effect of the lake clusters over Tibetan Plateau in summer

The weather research and forecasting model including a one-dimensional thermal diffusion lake model is adopted to investigate the summer climatic effect of the lake clusters over Tibetan Plateau (TP) during 2008–2014 based on two experiments with and without the lakes. Overall, the model can reasonably reproduce the daily variations of lake surface temperature and the spatial patterns of 2 m air temperature (T2m) and precipitation over TP during summer. Sensitivity results show that the effects of TP lakes on the over-lake T2m and precipitation exhibit distinctive seasonal and diurnal features and strong space dependence. Generally, the TP lakes tend to cool the local T2m and enhance the precipitation over the lake and surrounding areas. With the summer advances, the cooling effect of TP lakes weakens while the lake-induced enhancement of precipitation becomes more evident. During daytime, the TP lakes decrease the T2m and suppress the short-duration (≤ 6 h) rainfall in afternoon. However, the TP lakes increase the T2m and strengthen the convective rainfall over the lake and surrounding areas by simultaneously enhancing both short and long-duration (> 6 h) precipitation during nighttime. The lakes over the southeastern central TP (CTP) lead to slight warming and pronounced precipitation increases, while the other lakes in CTP mainly cause significant cooling and suppressed precipitation. Such opposite effects are mainly because the lakes over the western and northeastern CTP hardly produce nighttime warming and the associated circulation changes favorable for the convective precipitation as found over the southeastern CTP, suggesting that the climate effects of TP lakes may be modulated by the lake intrinsic features, local terrain distributions, and background atmospheric circulations.

A Numerical Investigation on Microphysical Properties of Clouds and Precipitation over the Tibetan Plateau in Summer 2014

In order to improve our understanding of microphysical properties of clouds and precipitation over the Tibetan Plateau (TP), six cloud and precipitation processes with different intensities during the Third Tibetan Plateau Atmospheric Science Experiment (TIPEX-III) from 3 July to 25 July 2014 in Naqu region of the TP are investigated by using the high-resolution mesoscale Weather Research and Forecasting (WRF) model. The results show unique properties of summertime clouds and precipitation processes over the TP. The initiation process of clouds is closely associated with strong solar radiative heating in the daytime, and summertime clouds and precipitation show an obvious diurnal variation. Generally, convective clouds would transform into stratiform clouds with an obvious bright band and often produce strong rainfall in midnight. The maximum cloud top can reach more than 15 km above sea level and the velocity of updraft ranges from 10 to 40 m s−1. The simulations show high amount of supercooled water content primarily located between 0 and −20°C layer in all the six cases. Ice crystals mainly form above the level of −20°C and even appear above the level of −40°C within strong convective clouds. Rainwater mostly appears below the melting layer, indicating that its formation mainly depends on the melting process of precipitable ice particles. Snow and graupel particles have the characteristics of high content and deep vertical distribution, showing that the ice phase process is very active in the development of clouds and precipitation. The conversion and formation of hydrometeors and precipitation over the plateau exhibit obvious characteristics. Surface precipitation is mainly formed by the melting of graupel particles. Although the warm cloud microphysical process has less direct contribution to the formation of surface precipitation, it is important for the formation of supercooled raindrops, which are essential for the formation of graupel embryos through heterogeneous freezing process. The growth of graupel particles mainly relies on the riming process with supercooled cloud water and aggregation of snow particles.

Thermodynamic controls of the Western Tibetan Vortex on Tibetan air temperature

The “Western Tibetan Vortex” (WTV)—also termed the Karakoram Vortex—dominates the middle-to-lower troposphere and the near-surface air temperature variability above the western Tibetan Plateau (TP). Here, we explore the thermodynamic mechanisms through which the WTV modulates air temperature over the western TP by diagnosing the three major terms of the thermodynamic energy equation—adiabatic heating, horizontal temperature advection, and diabatic heating—that maintain the atmospheric thermal balance. We composite these major terms to examine the differences between anti-cyclonic and cyclonic WTV events. Our theoretical approach demonstrates that adiabatic sinking-compression (rising-expansion) provides the overwhelming control on both the middle-to-lower tropospheric and lower stratospheric temperature increases (decreases) under anti-cyclonic (cyclonic) WTV conditions over the western TP high mountain area in all four seasons. This also explains the mechanisms behind the anomalous temperature “dipole” found between the mid-lower troposphere and lower stratosphere when the WTV was initially identified. Spatially, adiabatic heating effects are centred on the central western TP in summer and the south slope centring at 70°–80°E of the TP in other seasons. The other two terms, horizontal temperature advection and diabatic heating, have localized importance over the edges of the western TP. In a case study over the Karakoram area, we further demonstrate that adiabatic heating (rising-expanding-cooling/sinking-compressing-warming) is the dominant thermodynamic process controlling Karakoram air temperatures under WTV variability, except for at the very near surface in autumn and winter. Our analysis methods can be applied to investigate the thermodynamic processes of other atmospheric circulation systems or climate variability modes.

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

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

Southern Hemisphere Origins for Interannual Variations of Snow Cover over the Western Tibetan Plateau in Boreal Summer

The climate response to the Tibetan Plateau (TP) snow cover (TPSC) has been receiving extensive concern. However, relatively few studies have been devoted to revealing the potential factors that can contribute to the TPSC interannual variability, especially during boreal summer. This study finds that the May Southern Hemisphere (SH) annular mode (SAM), the dominating mode of atmospheric circulation variability in the SH extratropics, exhibits a significant positive relationship with the interannual variations in western TPSC during boreal summer. Observational analysis and numerical experiments manifest that the signal of the May SAM can be “prolonged” by a meridional Indian Ocean tripole (IOT) sea surface temperature anomaly (SSTA) via atmosphere–ocean interaction. The IOT SSTA pattern persists into the following summer and excites anomalous local-scale zonal–vertical circulation. Subsequently, a tropical dipole rainfall (TDR) mode is induced with precipitation anomalies between the tropical western Indian Ocean and the eastern Indian Ocean–Maritime Continent. Rossby wave ray tracing diagnosis reveals that the wave energies, generated by the latent heat release of the TDR mode, can propagate northward into the western TP. As a response, abnormal cyclone (or anticyclone) and upward (or downward) movement are triggered over the western TP, providing favorable dynamical conditions for more (or less) TPSC. Moreover, the strong May SAM is usually followed by a cold air temperature anomaly over the western TP in summer, which is unfavorable for snow-cover melting, and vice versa. In brief, the IOT SSTA plays an “ocean bridge” role and the TDR mode plays an “atmosphere bridge” role in the process of the May SAM impacting the following summer TPSC variability. The results may provide new insight into the cross-equatorial propagation of the SAM influence.

Evaluation of Five Grid Datasets against Radiosonde Data over the Eastern and Downstream Regions of the Tibetan Plateau in Summer

In this study, horizontal wind (U and V), air temperature (T), and relative humidity (RH) modelled by the European Centre for Medium-Range Weather Forecasts Reanalysis Interim (ERA-Interim), the National Aeronautics and Space Administration (NASA) Modern Era Retrospective Analysis for Research and Applications (MERRA), the Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction (NCEP) Climate Forecast System Version 2 (CFSv2), and the NCEP Final Operational Global Analysis data and the NCEP Final Operational Global Analysis data (NCEP-FNL) products have been compared with observations at 11 radiosonde stations over the eastern and downstream regions of the Tibetan Plateau (TP) from late June until the end of July during 2011 to 2015. The mean bias of all variables for the five gridded datasets (GDs) in the Sichuan Basin (SCB) is larger than that for the TP. The mean values of U, V, and T from each grid dataset are generally consistent with the radiosonde values, whereas considerable bias in the mean RH exists at upper levels. The diurnal variation of the mean bias and root-mean-square (RMS) error in the basin are stronger than those in the TP and the negative/positive peak usually occurs at 06:00 UTC and 18:00 UTC in the basin or at 12:00 UTC in the TP. The inter-annual variations in the basin are significantly stronger, and the maximum values of the variations usually occur at upper levels or near the surface, except for V. The weather conditions have a crucial influence on the performance of the gridded datasets. The mean bias and RMS error of T in the TP on cloudy days are obviously larger than those during sunny conditions. Considerable but unsteady differences occur in the mean bias and RMS error of U and V in different weather conditions. On average, the four variables in the TP are more sensible to the weather conditions.

The link between Tibetan Plateau monsoon and Indian summer precipitation: a linear diagnostic perspective

The thermal forcing of the Tibetan Plateau (TP) is analyzed to investigate the formation and variability of Tibetan Plateau Summer Monsoon (TPSM), which affects the climates of the surrounding regions, in particular the Indian summer monsoon precipitation. Dynamic composites and statistical analyses indicate that the Indian summer monsoon precipitation is less/greater than normal during the strong/weak TPSM. Strong (weak) TPSM is associated with an anomalous near surface cyclone (anticyclone) over the western part of the Tibetan Plateau, enhancing (reducing) the westerly flow along its southern flank, suppressing (favoring) the meridional flow of warm and moist air from the Indian ocean and thus cutting (providing) moisture supply for the northern part of India and its monsoonal rainfall. These results are complemented by a dynamic and thermodynamic analysis: (i) A linear thermal vorticity forcing primarily describes the influence of the asymmetric heating of TP generating an anomalous stationary wave flux. Composite analysis of anomalous stationary wave flux activity (after Plumb in J Atmos Sci 42:217–229, 1985) strongly indicate that non-orographic effects (diabatic heating and/or interaction with transient eddies) of the Tibetan Plateau contribute to the generation of an anomalous cyclone (anti-cyclone) over the western TP. (ii) Anomalous TPSM generation shows that strong TPSM years are related to the positive surface sensible heating anomalies over the eastern TP favoring the strong diabatic heating in summer. While negative TPSM years are associated with the atmospheric circulation anomalies during the preceding spring, enhancing northerly dry-cold air intrusions into TP, which may weaken the condensational heat release in the middle and upper troposphere, leading to a weaker than normal summer monsoon over the TP in summer.

The uncertainties of residual diagnosis of atmospheric diabatic heating from ERA40 and NCEP reanalysis over Tibetan Plateau

The Tibetan Plateau (TP), a giant elevated landscape, plays a key role in modulating the Asian summer monsoon via dynamic, thermodynamic, and mechanical lifting mechanisms. The spatial and temporal distribution of atmospheric diabatic heating (DH) is vitally important to understand the coupling of energy and moisture between the land surface and the atmosphere over TP. In this study, the residual diagnosis of DH from ERA40 and NCEP reanalysis, in combination with the precipitation and latent heating (LH) results from Tropical Rainfall Measuring Mission Precipitation Radar (TRMM PR) are investigated to reveal the uncertainties in DH over TP. Significant discrepancies of column integrated DH (CDH) between ERA40 and NCEP are found in the southern slope area of TP in summer. The CDH from ERA40 is much stronger than NCEP at high altitudes on the southern slope of TP. The area with strong ERA40 atmospheric heating extends from lower altitudes to higher altitudes exceeding 4000 m isohypse. However, in the heating diagnosis of NCEP, strong atmospheric heating is only located at altitudes lower than 4000 m isohypse. These differences are evident in DH at both the ground layer and 400–500 hPa layer, revealing a significant contribution from LH because sensible heating (SH) peaks at near-surface atmosphere and LH peaks at low to middle atmosphere. As an indicator of LH, the precipitation in southern TP estimated by ERA40 is strong (>10 mm/d) in the 25°–30°N area from April to October. The precipitation observations from TRMM PR and the diagnosis from NCEP diagnosis do not agree about this phenomenon. Instead, the precipitation detected by TRMM PR is mainly located in the area to the south of 25°N. The spatial pattern of overestimated precipitation by ERA40 mostly matches the DH overestimation. Based on the vertical cross-section of DH and the atmospheric circulation along 92.5°E, strong positive ERA40 DH (>4 K/d) over the high altitudes of southern TP (~25°–30°N) extends from surface to high troposphere (~300 hPa) vertically in summer. This fact indicates there is strong latent heating released from deep convective rains in those areas which makes a contribution to DH that is comparable with SH. The associated NCEP DH in the same area can reach only the low troposphere (~700 hPa) and the maximum heating generally is located in the near-surface atmosphere. This conclusion implies that in NCEP diagnosis, only shallow and weak rains can occur in this area and the surface SH dominates DH here. Even at the south edge of main body of TP (to north of 30°N), ERA40 DH in summer can be as high as 6–9 K/d and extend from the surface to ~350 hPa isobaric surface, while the associated NCEP DH is only less than 4 K/d. The diagnosis of vertical velocity and horizontal divergence at the 200 hPa layer from ERA40 is also stronger that from NCEP. The mean vertical profiles of ERA40 DH in summer show positive heating through the surface to 200 hPa and maximum value at 700 (500) hPa on the south (southwest) slope of TP. The shape agrees well with that of the TRMM LH profile, but the value of ERA40 DH is about 2–3 times that of TRMM LH. The NCEP DH profiles show a different shape with peaks at the near-surface layer indicating more contribution from surface SH than from LH, but its amplitude is relatively closer to TRMM LH. We hypothesize that the uncertainties of the vertical structure of precipitation and the associated LH in the southern slope area of TP are the main obstacles preventing us from understanding the real mechanisms of TP-atmosphere interaction.

Spatial and Temporal Features of the Frequency of Cloud Occurrence over China Based on CALIOP

Spatial and temporal distribution of cloud vertical structure are key components of global climate change. The occurrence of clouds over China and its surrounding areas has been calculated based on cloud layer products from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) at 1 km resolution. Results show significant regional differences in the frequency of cloud occurrence. Fewer clouds are found over the Mongolian Plateau and northern Indian Peninsula, with more clouds apparent over tropical seas and southern China. Cloud cover at night is slightly higher than during the day. Single-layer clouds are more common than multilayer clouds in most areas. In most areas, high-level cloud accounts for the largest proportion of single-layer clouds. The occurrence of clouds in summer and autumn is generally greater than in spring and winter. Single-layer clouds over the Mongolian Plateau and northern Indian Peninsula occur less frequently than multilayer clouds, especially in winter. Furthermore, single-layer clouds are common over the eastern part of southwest China all year round. Over parts of the Tibetan Plateau in summer, high clouds account for the largest proportion (>35%) of annual single-layer clouds, as a result of topography and enhanced summer convection.

Impact of large-scale circulation on the water vapour balance of the Tibetan Plateau in summer

Using the monthly Interim Reanalysis (ERA-Interim) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), water vapour transported by the large-scale circulation over the Tibetan Plateau (TP hereafter) for the period of 1979–2014 has been investigated. The TP is divided into five boundaries by a reference to the 3000 m contour, and the flux of horizontally and vertically integrated atmospheric water vapour over the TP with its surrounding has been calculated. The time-averaged vertically integrated water vapour fluxes enter the TP through the southern, southwestern, western and northern boundaries with the southern boundary contributing 414.8 × 106 kg s−1. And water vapour transport is not simply input or output through one boundary. The time-averaged fluxes of water vapour on each layer show that water vapour transport to the outside/inside TP comes from different level (above/below 600 Pa) of the eastern boundary and the main exchange of water vapour occurred below the level of 500 Pa, which cannot reach the higher layer of the southern and northern boundary. There is an outflow region from 76°E to 83°E on the vertical section of northern boundary. It may be related to the topography blocking effect of northwestern TP. Then, we chose Regional Westerly and Webster and Yang monsoon indices to describe the mid-latitude westerly and the South Asian Monsoon, respectively. Through the correlation coefficient between the vertically integrated water vapour transport flux and two circulation indices, it is found that in the same area, the influence of mid-latitude westerly shows the opposite effect of the South Asian Monsoon. In years of strong/weak mid-latitude westerly or weak/strong South Asian Monsoon, the water vapour transport from northern boundary becomes stronger/weaker and from southern boundary becomes weaker/stronger. Here, the water vapour transport processes are revealed to the influence of two large-scale circulation systems.

Estimation of turbulent fluxes using the flux-variance method over an alpine meadow surface in the eastern Tibetan Plateau

The flux-variance similarity relation and the vertical transfer of scalars exhibit dissimilarity over different types of surfaces, resulting in different parameterization approaches of relative transport efficiency among scalars to estimate turbulent fluxes using the flux-variance method. We investigated these issues using eddycovariance measurements over an open, homogeneous and flat grassland in the eastern Tibetan Plateau in summer under intermediate hydrological conditions during rainy season. In unstable conditions, the temperature, water vapor, and CO2 followed the flux-variance similarity relation, but did not show in precisely the same way due to different roles (active or passive) of these scalars. Similarity constants of temperature, water vapor and CO2 were found to be 1.12, 1.19 and 1.17, respectively. Heat transportation was more efficient than water vapor and CO2. Based on the estimated sensible heat flux, five parameterization methods of relative transport efficiency of heat to water vapor and CO2 were examined to estimate latent heat and CO2 fluxes. The strategy of local determination of flux-variance similarity relation is recommended for the estimation of latent heat and CO2 fluxes. This approach is better for representing the averaged relative transport efficiency, and technically easier to apply, compared to other more complex ones.

The footprint of Asian monsoon dynamics in the mass and energy balance of a Tibetan glacier

Abstract. Determinations of glacier-wide mass and energy balance are still scarce for the remote mountains of the Tibetan Plateau, where field measurements are challenging. Here we run and evaluate a physical, distributed mass balance model for Zhadang Glacier (central Tibet, 30° N) based on in-situ measurements over 2009–2011 and an uncertainty estimate by Monte Carlo and ensemble strategies. The model application aims to provide the first quantification of how the Indian Summer Monsoon (ISM) impacts an entire glacier over the various stages of the monsoon's annual cycle. We find a strong and systematic ISM footprint on the interannual scale. Early (late) monsoon onset causes higher (lower) accumulation, and reduces (increases) the available energy for ablation primarily through changes in absorbed shortwave radiation. By contrast, only a weak footprint exists in the ISM cessation phase. Most striking though is the core monsoon season: local mass and energy balance variability is fully decoupled from the active/break cycle that defines large-scale atmospheric variability during the ISM. Our results demonstrate quantitatively that monsoon onset strongly affects the ablation season of glaciers in Tibet. However, we find no direct ISM impact on the glacier in the main monsoon season, which has not been acknowledged so far. This result also adds cryospheric evidence that, once the monsoon is in full swing, regional atmospheric variability prevails on the Tibetan Plateau in summer.

Characterizations of particle-bound trace metals and polycyclic aromatic hydrocarbons (PAHs) within Tibetan tents of south Tibetan Plateau, China

Exposure to trace metals and polycyclic aromatic hydrocarbons (PAHs) adsorbed on particulates is of a serious health concern. Levels of some trace metals in total suspended particulate and 13 PAHs of fine particulate matter were measured from nomadic tents in the southern Tibetan Plateau in summer 2010. The indoor air within the tents was seriously polluted, mainly due to yak dung combustion. Average trace metal concentrations were much higher (range of indoor/outdoor ratio 61-291) than those of the outdoor air. Additionally, enrichment factors of most trace metals of indoor air were similar to those of outdoor air, indicating outdoor air quality of the studied area was possibly influenced by pollutants emitted from local tents. Mean concentrations of total PAHs and BaP within tents was 5372.45 and 364.79 ng/m(3), hundred times higher than that of outdoor air of the Tibetan Plateau. Three- and four-ring PAHs were the predominant components. The diagnostic ratio of BaA/(BaA + Chr) was 0.33. Since Tibetan women typically spend longer time within the tents, they were exposed to PAHs (BaP exposure = 1.81 μg/m(3)) about two times of other family members. Among all the PAHs, Bap contributed the most (82.6%) of the total carcinogenicity. Similarly, the excess lifetime cancer risk for women and other family members were 2.75 × 10(-4) and 1.27 × 10(-4), respectively, indicating Tibetan herdsmen, especially women who are in charge of most house chores were at risk for adverse health effects.

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.

Deuterium excess record in a southern Tibetan ice core and its potential climatic implications

A 55-m long ice core, drilled close to bedrock from Mt. Noijin Kangsang on the southern Tibetan Plateau in summer 2007, was annually dated covering the period of 1864–2006 AD. The stable isotope ratios (δ 18O and δD) of the ice core were measured and thereby the deuterium excess (d) was calculated by d = δD − 8*δ 18O for the individual ice samples. Results show that the d values of the ice samples were predominantly controlled by the moisture sources. The significant increasing trend of annual mean d values along the ice core is mainly related to the rapid warming of the tropical Indian Ocean, although the tendency is subjected to the modulation by the western-derived moisture. The decreasing Indian monsoon precipitation on the southern Tibetan Plateau, physically linked with the increasing tropical Indian Ocean SST, reduced the share of monsoon precipitation in the annual total accumulation, making an additional contribution to the significant increase of annual mean d in the Noijin Kangsang ice core with high values during the past 143 years.

Modeling of the impacts of China's anthropogenic pollutants on the surface ozone summer maximum on the northern Tibetan Plateau

The current debate about the role of anthropogenic pollutants in a unique broad summer maximum pattern at a remote site (Waliguan, 3810 asl) on the northern edge of the Tibetan Plateau is continuing. This study quantifies the impacts of anthropogenic emissions and stratosphere‐to‐troposphere exchange (STE) on surface ozone on the Tibetan Plateau using a regional chemical transport model with an on‐line tracer tagging module. The model is shown to be well suited for simulating the unique seasonal variation at Waliguan. Model results clearly show that the regional transport of photochemically produced ozone in China is responsible for the summer maximum on the northern Tibetan Plateau with a contribution of ∼10–25 parts per billion by volume (ppbv) in summer, which is more than ∼5 ppbv of the contribution to STE. The weak local ozone photochemical production (−0.5–0.1 ppbv/day) suggests that this transport to the Tibetan Plateau mostly concentrates on photochemical products. The unique strong near‐surface convergence and upper layer divergence driven by elevated surface heating and low air density over the Tibetan Plateau in summer is its dynamic cause. These transported anthropogenic pollutants are likely to rise to the upper troposphere and circulate globally. This study also demonstrates that the effect of convection on surface ozone over the northern Tibetan Plateau is compensated by the advective updrafts in summer.

The large‐scale circulations and summer drought and wetness on the Tibetan plateau

AbstractEffects of large‐scale atmospheric circulation and surface temperatures on extreme dryness and wetness on the Tibetan plateau in summer are analysed using ERA‐40 reanalysis and observed precipitation. The extreme cases of drought and wetness can be associated with circulation anomalies in the North Atlantic/European sector and wave trains bridging the Eurasian continent. Drought in Tibet reveals an intense high pressure anomaly over Scandinavia supported by a more south‐west to north‐east orientated North Atlantic stormtrack. This creates wave trains crossing Eurasia which, on their southward ‘great circle route’, reach south‐eastern Asia where they modulate the flow north and east of the Tibetan plateau by an anticyclone–cyclone dipole suppressing moisture supply from the Bay of Bengal.Wetness in Tibet is characterised by a more zonally oriented cross Atlantic stormtrack creating a low pressure anomaly over central Europe and, associated with it, a northward shift of the sub‐tropical westerly and tropical easterly jet; wave trains emerging from the North Atlantic on their equatorward route have now a higher chance to reach the sub‐tropical jet entrance (instead of propagating further south). Then the wave trains are re‐intensified and, passing the Mediterranean–Arabian Sea route to India, interact with the monsoon's western branch to lead to ample moisture supply for Tibet.Surface temperatures give indications for positive (negative) El Niño/Southern Oscillation and Indian Ocean Dipole episodes occurring in years of extreme and severe dryness (wetness) on the Tibetan plateau. A pronounced cold surface temperature anomaly in the tropical North Atlantic precedes and accompanies drought on the plateau. Copyright © 2009 Royal Meteorological Society

Inter‐decadal variation of the summer precipitation in China and its association with decreasing Asian summer monsoon Part II: Possible causes

AbstractThe present article is the second part of a study on the inter‐decadal variability of the summer precipitation in East China, which mainly addresses the possible cause of this change. Firstly, an updated analysis of the long‐term variations of snow cover, snow days and snow depth in the preceding winter and spring over the Tibetan Plateau (TP) was done by using station and satellite data. The abrupt increase in the winter and spring snow over the TP since around 1977 has been well documented. At that time, the inter‐decadal variation of the atmospheric heating over the TP in spring and summer had been estimated. It has been revealed that the atmospheric heating fields in subsequent spring and summer over the TP assumed a significant weakening after the late 1970s. This weakening is closely related to the significantly reduced surface sensible heat flux into the atmosphere and subsequent cooling over the TP and its surrounding atmosphere. The latter was produced by the increase of surface albedo and soil hydrological effect of melting snow under the condition of abrupt increase in the preceding winter and spring snow over the TP. On the other hand, three phases of significant inter‐decadal warming of the sea surface temperature (SST) in the tropical central and eastern Pacific, which occurred in the mid‐1960s, the late 1970s and the early 1990s, respectively, have been found. The above inter‐decadal variability of heating fields over the land area in the Asian region and neighbouring oceanic region of the West Pacific has consistently reduced the land–sea thermal contrast in summer in the Asian monsoon region based on the estimate of atmospheric heating fields. This cause is likely to lead to weakening of the Asian summer monsoon. In such case, the northward moisture transport in East Asia is greatly weakened and cannot reach North China, thus causing the condition of less precipitation or droughts. In contrast, the Yangtze River basin and South China receive a large amount of moisture supply and have strong upward motion, creating favourable conditions for frequent occurrence of heavy rainfall. In the process of the southward shift of the high‐precipitation zone, two abrupt or rapid regime shifts observed in the late 1970s and the early 1990s were possibly in response to the increase in the winter and spring snow over the TP, and two major rapid warming events of the SST in the tropical central and eastern Pacific in the late 1970s and the early 1990s.Correlative analysis has further confirmed that high TP snow and oceanic forcing factors have a positive correlation with the subsequent summer precipitation in the Yangtze River basin and most of South China, and a negative correlation with the summer precipitation in North China. This correlative relationship implies that if the TP has excessive (deficient) snow in the preceding winter and spring and the tropical central and eastern Pacific anomalously warms up (cools down), North China will have decreasing (increasing) summer precipitation, whereas the Yangtze River basin and South China will have increasing (decreasing) summer precipitation. Copyright © 2009 Royal Meteorological Society