Tibetan Plateau Snow Cover Research Articles

Double‐mode adjustment of Tibetan Plateau heating to the summer circumglobal teleconnection in the Northern Hemisphere

ABSTRACTA quasi‐stationary circumglobal teleconnection (CGT) in the Northern Hemisphere in summer is associated with summer droughts and floods and exhibits some vorticity anomalies. The CGT anomaly and its forcing sources were explored based on reanalysis data and the EI Nino 3.4 index. The term ∂v′/∂x was used to describe the multi‐scale variability of vorticity and the CGT anomaly, which had a 7‐wavenumber and a small interannual amplitude of ∂v′/∂x from 1987 to 2000 but a large amplitude from 2000 to 2010. The interannual CGT intensity was enhanced by an EI Nino sea surface temperature pattern in spring via key forcing sources that led to an enduring effect on the summer circulation; it was also a response to anomalies in Tibetan Plateau (TP) heating in spring–summer in two ways. First, increasing TP heating in the mid‐eastern TP caused an eastward shift and northerly extension of the South Asian High (SAH), which exhibited a Tibetan‐mode SAH as well as a 7‐wavenumber CGT anomaly. This enhanced the CGT over Eurasia and extended the wave pattern of the EI Nino contribution over the North Pacific and Eurasia. Second, increased snow cover in the west of the TP could have offset the TP heating effect and adjusted the position and intensity of the Tibetan‐mode SAH. This phenomenon weakened vorticity and wave amplitude and finally led to a weak CGT anomaly. This finding indicates a coupled effect of double‐mode TP heating (i.e. both heating and TP snow cover) on the CGT anomaly, which reveals the linkage of TP heating with weather over Eurasia and North America that could be predicted by TP heating and other factors.

Impacts of Spatiotemporal Anomalies of Tibetan Plateau Snow Cover on Summer Precipitation in Eastern China

Abstract Tibetan Plateau (TP) snow cover undergoes significant temporal and spatial variations during the winter and spring months. This study investigates the relationship between the spatiotemporal distribution of winter–spring snow cover (SC) over the TP and summer precipitation in eastern China (EC) using the singular value decomposition (SVD) method. Four simulation experiments are designed to validate the results of SVD analysis. Both observations and simulations show that heavier snow cover in the southern TP leads to more rainfall in the Yangtze River basin and northeastern China, and less precipitation in southern China, whereas heavier snow cover in the northern TP results in enhanced rainfall in southeastern and northern China and weakened precipitation in the Yangtze River basin. The linkage is attributed to anomalous westerly winds in the upper troposphere at around 200 hPa and to changes of the southern branch of westerlies at 500 hPa on the south side of the TP, which are caused by lasting diabatic heat anomalies over the TP. The shifts in position of the westerly jet at the exit region and negative anomalies of geopotential height at 500 hPa further result in anomalous anticyclone over the East China Sea and the corresponding 850-hPa water vapor convergence and influence the anomalous summer precipitation belt in EC.

Impacts of Tibetan Plateau Snow Cover on the Interannual Variability of the East Asian Summer Monsoon

AbstractThe relationship between Tibetan Plateau (TP) snow cover and the East Asian summer monsoon (EASM) has long been discussed, but the underlying mechanism remains controversial. In this paper, the snow–albedo and snow–hydrology feedbacks over the TP are investigated based on multiple sources of snow data for the period 1979–2011. The results indicate that winter snow cover plays an important role in cooling local air temperature through the snow–albedo effect; the TP surface net solar radiation in years with above-normal snow cover is approximately 18 W m−2 less than that in below-normal snow cover years. However, data analysis demonstrates that persistent effects of winter snow cover are limited to the period from winter to spring over most parts of the central and eastern TP. Therefore, the preceding snow cover over the central and eastern TP exerts little influence over either the in situ summer atmospheric heat source or the EASM, because of its limited persistence. In contrast, the effects of winter or spring snow cover anomalies over the western TP and the Himalayas can last until summer, and these anomalies further influence the EASM by modulating moisture transport to eastern China and favoring eastward-propagating synoptic disturbances that are generated over the TP. Generally, above-normal snow cover over the western TP and the Himalayas facilitates abundant summer precipitation between the Yangtze and Yellow River basins, which is confirmed by results from a regional Weather Research and Forecasting model simulation.

Influence of the Madden-Julian oscillation on Tibetan Plateau snow cover at the intraseasonal time-scale.

The Tibetan Plateau (TP), known as the third pole of the Earth, has snow cover with intraseasonal to decadal variability that affects weather and climate both inside and outside the TP. However, the factors that generate the TP snow cover (TPSC) anomalies at the intraseasonal time-scale are unclear. This report reveals the influence of the Madden‒Julian oscillation (MJO), which is the most dominant component of the tropical intraseasonal variability, on TPSC. We focus on wintertime snow cover over the central and eastern TP, where the intraseasonal variability is large. TPSC increases/decreases in the MJO phases 8‒1/4–5, when the eastward-propagating MJO suppressed/enhanced convection locates over the Maritime Continent. Such a change in TPSC leads to the most dominant positive/negative anomalies of TPSC in the following phases 2‒3/6‒7 due to the non-significant change of TPSC in these phases. There is anomalous moisture advection over the upstream of the TP caused by MJO-excited large-scale atmospheric circulation. The advection process generates the low-frequency eastward-propagating anomalous water vapour from upstream to the TP that influences precipitation and, eventually, TPSC.

Can the Tibetan Plateau snow cover influence the interannual variations of Eurasian heat wave frequency?

The Eurasian continent has experienced significant year-to-year variations of summer heat waves during the past decades. Several possible factors, such as ocean temperature, soil moisture, and changes in land use and greenhouse gases, have been identified in previous studies, but the mechanisms are still unclear. In this study, it is found that the Tibetan Plateau snow cover (TPSC) is closely linked to the interannual variations of summer heat waves over Eurasia. The TPSC variability explains more than 30 % of the total variances of heat wave variability in the southern Europe and northeastern Asia (SENA) region. A set of numerical experiments reveal that the reduced TPSC may induce a distinct teleconnection pattern across the Eurasian continent, with two anomalous high pressure centers in the upper troposphere over the SENA region, which may lead to a reduction of the cloud formation near the surface. The less cloud cover tends to increase the net shortwave radiation and favor a stronger surface sensible heat flux in the dry surface condition over the SENA region, resulting in a deeper, warmer and drier atmospheric boundary layer that would further inhibit the local cloud formation. Such a positive land–atmosphere feedback may dry the surface even further, heat the near-surface atmosphere and thereby intensify the local heat waves. The above dynamical processes also operate on interdecadal time scales. Given the reduction of the TPSC could become more pronounced with increasing levels of greenhouse gases in a warming climate, we infer that the TPSC may play an increasingly important role in shaping the summer heat waves over the SENA region in next decades.

The Role of Tibetan Plateau Snow Cover in the 1978 and 2001 Western North Pacific Typhoon Seasons

An inverse correlation has been identified between the annual number of landfall typhoons along East China Seaboard (ECS) and the Tibetan Plateau snow cover (TP-SC) during the preceding winter and current spring. This correlation suggests that the Tibetan Plateau snow plays a key role in the East Asia–West Pacific regional climate system, and the system further influences typhoon track pattern off ECS. In this paper, major climatic factors accounted for the startling contrast in the number of landfall typhoons along ECS during the 1978 (2) and 2001 (7) western North Pacific (WNP) typhoon seasons were investigated. Among other climate/ocean conditions associated with typhoon activity in WNP, the preceding winter and spring TP-SC plays a crucial role to modulate the ECS landfall frequency in 1978 and 2001.

Characteristics of the Tibetan Plateau snow cover variations based on daily data during 1997–2011

This paper studies the characteristics of snow cover variations for the Tibetan Plateau (TP) region (25°-45°N, 65°-105°E). The TP region’s daily snow cover data are a subset of the Interactive Multisensor Snow and Ice Mapping System (IMS) 24 km-by-24 km snow cover dataset for the entire Northern Hemisphere. A database of the daily snow cover for the TP region was developed from February 4, 1997–March 15, 2012. Thus, an animation of the TP snow cover was made as a data product of this research. The maximum percentage snow cover (67 %) occurred on February 6, 2008 and the minimum (0.5 %) on September 1, 2009. The average snow cover is 16 %. The seasonal cycle of the monthly TP snow cover reaches maximum in January (about 37 %) and minimum in August (2 %). The trend of the snow cover reduction is 4.0 % per decade, with a total reduction of 5.7 % from February 4, 1997 to March 15, 2012. The Hilbert–Huang Transform and Fourier spectral analyses indicate the existence of a cycle of TP snow cover having a period of 2–3 years. The histogram and higher statistics moment analyses imply that the positive skewness favors more spring snowstorms than spring droughts and the sharp peakedness at the climatology indicates the snow cover predictability by climate normals.

Possible association of the western Tibetan Plateau snow cover with the decadal to interdecadal variations of northern China heatwave frequency

Northern China has been subject to increased heatwave frequency (HWF) in recent decades, which deteriorates the local droughts and desertification. More than half a billion people face drinking water shortages and worsening ecological environment. In this study, the variability in the western Tibetan Plateau snow cover (TPSC) is observed to have an intimate linkage with the first empirical orthogonal function mode of the summer HWF across China. This distinct leading mode is dominated by the decadal to inter-decadal variability and features a mono-sign pattern with the extreme value center prevailing over northern China and high pressure anomalies at mid- and upper troposphere over Mongolia and the adjacent regions. A simplified general circulation model is utilized to examine the possible physical mechanism. A reduced TPSC anomaly can induce a positive geopotential height anomaly at the mid- and upper troposphere and subsequently enhance the climatological high pressure ridge over Mongolia and the adjacent regions. The subsidence associated with the high pressure anomalies tends to suppress the local cloud formation, which increases the net radiation budget, heats the surface, and favors more heatwaves. On the other hand, the surface heating can excite high pressure anomalies at mid- and upper troposphere. The latter further strengthens the upper troposphere high pressure anomalies over Mongolia and the adjacent regions. Through such positive feedback effect, the TPSC is tied to the interdecadal variations of the northern China HWF.

Modulation of the Tibetan Plateau Snow Cover on the ENSO Teleconnections: From the East Asian Summer Monsoon Perspective

Abstract The East Asian summer monsoon (EASM) may exhibit rather large variability between years characterized by the same ENSO phase. Such inconsistency reduces the EASM predictability based on ENSO. Results in this study show that the Tibetan Plateau snow cover (TPSC) exerts a modulating effect on ENSO teleconnections and ENSO significantly correlates with the EASM only during the reduced TPSC summers. Three-dimensional circulation structures are examined to manifest that the typical ENSO signals in reduced TPSC summers tend to be stronger than in excessive TPSC summers. Numerical and theoretical evidences indicate that the anomalously reduced TPSC can force positive geopotential height anomalies at the upper troposphere and weaken the jet streams across eastern Asia and northwestern Pacific. Governed by such basic state zonal flows, the extratropical Rossby wave response to the ENSO forcing usually has a larger amplitude and pronounced westward development. In such case, ENSO extends its influences to eastern Asia and enhances its connection with the EASM.

Contribution of Tibetan Plateau Snow Cover to the Extreme Winter Conditions of 2009/10

Most of the northern hemisphere experienced extreme climate conditions during winter 2009/10. This winter in Canada was characterized by the warmest and driest conditions in the past 60 years. Across much of the United States, Europe and northern Asia, persistent below-normal temperatures caused a significant adverse economic and societal impact. Dynamical seasonal forecasting systems failed to predict this winter's extreme conditions. Here we show that the snow-cover anomaly over the Tibetan Plateau and adjacent areas is significantly correlated with the atmospheric circulation pattern that contributes to these anomalous winter conditions. A statistical model using this snow-cover information and the El Niño signal in autumn 2009 was able to predict the general distribution of the anomalous conditions for this winter. This implies that an improved understanding of the Tibetan Plateau snow-cover effect and its representation in general circulation models are important for seasonal predictions, particularly of high-impact climate events. R ésumé [Traduit par la rédaction] Pendant l'hiver 2009/10, les conditions climatiques ont été extrêmes dans la majeure partie de l'hémisphère Nord. Cette année-là, les mois d'hiver au Canada ont été les plus chauds et les plus secs jamais enregistrés au cours des 60 dernières années. Un peu partout aux États-Unis, en Europe et dans le nord de l'Asie, les températures inférieures à la normale persistantes ont eu des conséquences catastrophiques sur le plan socio-économique. Or les systèmes dynamiques de prévisions saisonnières ne sont pas parvenus à prévoir les conditions extrèmes de cet hiver. Dans la présente étude, nous démontrons la corrélation étroite entre l’épaisseur anormalement faible de la couverture de neige sur le plateau Tibétain et dans les régions limitrophes, d'une part, et la configuration de la circulation atmosphérique, responsable de ces conditions hivernales anormales. Grâce au traitement des données sur la couverture de neige et du signal El Niño au moyen d'un modèle statistique, nous avons réussi à prédire la répartition générale des conditions anormales pour l'hiver. Nous en déduisons qu'il est important de mieux comprendre les effets de la couverture de neige du plateau Tibétain et sa représentation dans les modèles de circulation générale pour les prévisions saisonnières, notamment les phénomènes météorologiques lourds de conséquences.

IOD influence on the early winter tibetan plateau snow cover: diagnostic analyses and an AGCM simulation

Using diagnostic analyses and an AGCM simulation, the detailed mechanism of Indian Ocean Dipole (IOD) influence on the early winter Tibetan Plateau snow cover (EWTPSC) is clarified. In early winter of pure positive IOD years with no co-occurrence of El Nino, the anomalous dipole diabatic heating over the tropical Indian Ocean excites the baroclinic response in the tropics. Since both baroclinic and barotropic components of the basic zonal wind over the Arabian Peninsula increase dramatically in early winter due to the equatorward retreat of the westerly jet, the baroclinic mode excites the barotropic Rossby wave that propagates northeastward and induces a barotropic cyclonic anomaly north of India. This enables the moisture transport cyclonically from the northern Indian Ocean toward the Tibetan Plateau. The convergence of moisture over the plateau explains the positive influence of IOD on the EWTPSC. In contrast, the basic zonal wind over the Arabian Peninsula is weak in autumn. This is not favorable for excitation of the barotropic Rossby wave and teleconnection, even though the IOD-related diabatic heating anomaly in autumn similar to that in early winter exists. This result explains the insignificant (significant positive) partial correlation between IOD and the autumn (early winter) Tibetan Plateau snow cover after excluding the influence of ENSO. The sensitivity experiment forced by the IOD-related SST anomaly within the tropical Indian Ocean well reproduces the baroclinic response in the tropics, the teleconnection from the Arabian Peninsula, and the increased moisture supply to the Tibetan Plateau. Also, the seasonality of the atmospheric response to the IOD is simulated.

Contribution of the Autumn Tibetan Plateau Snow Cover to Seasonal Prediction of North American Winter Temperature

Abstract Predicting surface air temperature (T) is a major task of North American (NA) winter seasonal prediction. It has been recognized that variations of the NA winter T’s can be associated with El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). This study presents observed evidence that variability in snow cover over the Tibetan Plateau (TP) and its adjacent areas in prior autumn (September–November) is significantly correlated with the first principal component (PC1) of the NA winter T’s, which features a meridional seesaw pattern over the NA continent. The autumn TP snow cover anomaly can persist into the following winter through a positive feedback between snow cover and the atmosphere. A positive TP snow cover anomaly may induce a negative sea level pressure and geopotential height anomaly over the eastern North Pacific, a positive geopotential height anomaly over Canada, and a negative anomaly over the southeastern United States—a structure very similar to the positive phase of the Pacific–North America (PNA) pattern. This pattern usually favors the occurrence of a warm–north, cold–south winter over the NA continent. When a negative snow cover anomaly occurs, the situation tends to be opposite. Since the autumn TP snow cover shows a weak correlation with ENSO, it provides a new predictability source for NA winter T’s. Based on the above results, an empirical model is constructed to predict PC1 using a combination of autumn TP snow cover and other sea surface temperature anomalies related to ENSO and the NAO. Hindcasts and real forecasts are performed for the 1972–2003 and 2004–09 periods, respectively. Both show a promising prediction skill. As far as PC1 is concerned, the empirical model hindcast performs better than the ensemble mean of four dynamical models from the Canadian Meteorological Centre. Particularly, the real forecast of the empirical model exhibits a better performance in predicting the extreme phases of PC1—that is, the extremely warm winter over Canada in 2009/10—should the model include the autumn TP snow cover impacts. Since all these predictors can be readily monitored in real time, this empirical model provides a real-time forecast tool for NA winter climate.

Yangtze runoff, precipitation, and the East Asian monsoon in a 2800 years climate control simulation

Variability of the Yangtze catchment hydrology is closely linked with the Tibetan Plateau snow cover and the large scale atmospheric circulation in East Asia. These connections are analyzed in a control simulation (2800 years) of the atmosphere–ocean general circulation model (ECHAM5/MPIOM) coupled to vegetation and ocean biogeochemistry modules. (i) Up to decades in time scale, runoff, soil wetness, and temperature show long term memory (LTM) with an intradecadal scaling of the power spectrum with the exponent β ≈ 0.6, while precipitation, snow depth, and snow melt reveal no memory. (ii) Multi-decadal variability of runoff, soil wetness, and temperature shows weak LTM similar to snow depth and melt on the Tibetan Plateau. (iii) On the annual time scale, the atmospheric circulation impact of Hadley cell and ENSO on precipitation, temperature and snow melt are weak but significant; on decadal time scales the subtropical monsoon modulates the catchment hydrology. Temperature is anticorrelated with precipitation and soil wetness in the Yangtze catchment. Singular spectrum analysis highlights the 3–4 year ENSO mode in the monsoon indices, precipitation, and snow depth, explaining their high correlations. Projecting snow depth on ENSO shows long term variations explaining conflicting results obtained from correlations in short data sets.

An Interdecadal Change in Southern China Summer Rainfall around 1992/93

Abstract The present study documents a pronounced interdecadal change in summer rainfall over southern China around 1992/93 and explores the plausible reasons for this change. The summer rainfall is persistently below normal during 1980–92 and above normal during 1993–2002. Coherent changes in atmospheric circulation are identified over East Asia and the South China Sea (SCS)–western North Pacific (WNP). The increase in rainfall is accompanied by an increase in lower-level convergence, midtropospheric ascent, and upper-level divergence over southern China. The changes in lower-level winds feature two anomalous anticyclones: one over the SCS–subtropical WNP, and the other over north China–Mongolia. The outflows from the two anomalous anticyclones converge over southern China, leading to anomalous moisture convergence, enhanced ascent, and increased rainfall. The development of the northern anticyclone is related to an increase in the Tibetan Plateau snow cover in the preceding winter–spring that leads to a contrast in temperature change between the plateau and the surrounding regions. The relatively small temperature change over the plateau, coupled with increases in temperature to the west and the east, leads to an increase in surface pressure extending northward from the plateau. The development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial Indian Ocean that enhances lower-level convergence and ascent. The accompanying upper-level divergent flows from the tropical Indian Ocean to the SCS–WNP lead to the development of anomalous descent and lower-level anomalous anticyclone over the SCS–WNP.

Observed Relationship of Spring and Summer East Asian Rainfall with Winter and Spring Eurasian Snow

AbstractThis study investigates the relationship between spring and summer rainfall in East Asia and the preceding winter and spring snow cover/depth over Eurasia, using station rainfall observations, satellite-observed snow cover, satellite-derived snow water equivalent, and station observations of the number of days of snow cover and snow depth. Correlation analysis shows that snow-depth anomalies can persist from winter to spring whereas snow cover anomalies cannot in most regions of Eurasia. Locally, snow cover and snow-depth anomalies in February are not related in most regions to the north of 50°N, but those anomalies in April display consistent year-to-year variations. The results suggest that the winter snow cover cannot properly represent all the effects of snow and it is necessary to separate the winter and spring snow cover in addressing the snow–monsoon relationship.Spring snow cover in western Siberia is positively correlated with spring rainfall in southern China. The circulation anomalies associated with the western Siberian spring snow cover variations show an apparent wave pattern over the eastern Atlantic through Europe and midlatitude Asia. Spring snow cover over the Tibetan Plateau shows a moderate positive correlation with spring rainfall in southern China. Analysis shows that this correlation includes El Niño–Southern Oscillation (ENSO) effects. In contrast to the Indian summer monsoon rainfall for which the ENSO interferes with the snow effects, the Tibetan Plateau snow cover and ENSO work cooperatively to enhance spring rainfall anomalies in southern China. In comparison, ENSO has larger impacts than the snow on spring rainfall in southern China.

West North Pacific typhoon track patterns and their potential connection to Tibetan Plateau snow cover

The spatial characteristics and temporal variability of the West North Pacific (WNP) typhoon tracks are studied by analyzing the spatial pattern and temporal variability of the empirical orthogonal functions (EOFs) of the WNP typhoon track density function (TTDF) from 1945 to 2004. The results show that WNP typhoon tracks exhibit three principal EOF Modes. The first EOF Mode represents the contrasting “active” versus “inactive” typhoons defined by the overall frequency and life span of the typhoons that develop in the WNP basin. The second EOF shows a north–south dipole Mode in the TTDF depicting a seesaw pattern in typhoon frequency between Northeast Asia and Southeast Asia. The third EOF describes an east–west dipole Mode in TTDF depicting a zonal seesaw pattern between typhoons that tend to make landfalls in East Asia and typhoons that tend to stay away from the East Asia landmasses. Further analysis of the EOF time series of the WNP TTDF indicates that an important climatic factor associated with the WNP typhoon activity is the snow cover over the Tibetan Plateau (TP), which is also correlated with the East Asia summer monsoon (EASM). Thus, a mechanism linking the TP snow cover and the WNP typhoon activity is the response of the EASM in the WNP region to the TP snow cover, and the subsequent effect of EASM on the development and steering of the WNP typhoons.

Relationship between western North Pacific typhoon activity and Tibetan Plateau winter and spring snow cover

The annual frequency of western North Pacific (WNP) typhoons and the number of landfall typhoons in China are shown to negatively correlate with the Tibetan Plateau snow cover (TP‐SC) during the preceding winter and spring. When TP‐SC was above normal, fewer typhoons formed in the WNP and made landfall in China, and vice versa. The observed seasonal lag correlation between the TP‐SC and the WNP typhoon activity suggests the existence of a seasonal memory of the East Asia – West Pacific regional climate system which provides a means for the seasonal prediction of WNP typhoon activity and typhoon landfall frequency in China.