Anomalous Snow Cover Research Articles

The Subseasonal Feedback of Extreme Anomalous Tibetan Plateau Snow Cover Events on the Atmosphere

Abstract The Tibetan Plateau snow cover exhibits notable subseasonal variability and plays a crucial role in influencing the atmosphere. This study employs numerical experiments to investigate the atmospheric feedback resulting from extreme anomalous snow cover events on the Tibetan Plateau, with a focus on both local and nonlocal atmospheric temperatures. The findings reveal that diabatic heating, directly induced by these events, leads to a local surface energy cooling response over the Tibetan Plateau, contributing to a reduction in local temperatures. This cooling effect amplifies local atmospheric temperature anomalies associated with extreme anomalous Tibetan Plateau snow cover events, constituting approximately 50% of the total final local surface air temperature anomalies. Furthermore, the Tibetan Plateau snow cover, through adiabatic processes, exerts a nonlocal influence on atmospheric temperature and circulation. The atmospheric temperature responses downstream of the Tibetan Plateau vary at different heights and regions, featuring both cold and warm anomaly responses. These variations depend on the relative contributions of horizontal advection and vertical advection in adiabatic heating. Significance Statement Snow cover is influenced by the atmosphere, and in turn, it affects the atmosphere. This study examines the feedback of extreme anomalous Tibetan Plateau snow cover events on the atmosphere at the subseasonal time scale, with a focus on atmospheric temperature. Our findings indicate that Tibetan Plateau snow cover has both local and nonlocal feedback effects on the atmosphere. In particular, extreme anomalous Tibetan Plateau snow cover amplifies local surface air temperature anomalies. The feedback contributes to approximately 50% of the total final local surface air temperature anomalies.

A skillful prediction scheme for April precipitation over central East China

Central East China (30°N–42°N, 105°E–122°E), around the lower and middle reaches of the Yangtze River valley to north China, is one of the main agricultural bases over East Asia. Spring precipitation can significantly influence the regional agricultural activity and production, such as the crop planting and winter wheat growing over the region. Here, we find that different from the March and May precipitation over central East China, the April precipitation there shows relatively weak relationships with the preceding El Niño-Southern Oscillation (ENSO) and Indian Ocean basin mode (IOB) which are usually used as the prediction signals. Therefore, in this study, from the perspective of the prediction, we identify two predictors in the preceding months for the variability in the April precipitation over central East China: (1) the sea surface temperature (SST) over the subtropical North Atlantic in January and (2) the snow cover around the region to the east of Lake Baikal in March. The anomalous SST signal shows a good persistence from January to April, and further stimulates a wave train propagating from the subtropical North Atlantic eastward to East Asia during April, which could contribute to an anomalous East Asian trough and thereby the precipitation anomalies over central East China. The anomalous snow cover signal in March also tends to persist to the following April, and the associated anomalous cooling/warming could exert an impact on the East Asian trough, consequently influencing the April precipitation over central East China. Based on the two predictors, we establish a statistical prediction scheme and further evaluate its predictive skills. The results suggest the potential of the scheme for the prediction of the April central East China precipitation.

Impacts of Autumnal Eurasian Snow Cover on Predominant Modes of Boreal Winter Surface Air Temperature Over Eurasia

AbstractIn the present paper, the linkage between the predominant modes of boreal winter surface air temperature (SAT) anomalies over Eurasia and the preceding autumnal Eurasian snow cover is investigated for the period of 1979–2014. The first leading winter Eurasian SAT mode exhibits a strong signal over the region northward from 40°N; in association with this SAT mode, a significant zonal dipole pattern in snow cover can be observed in the midlatitude Eurasia during November. Such anomalous November snow cover strengthens the wave activity flux upward propagating over the middle latitudes, which can change the stratospheric polar vortex thereafter. In the following winter, the anomalous polar vortex signal propagates downward to the surface and gives rise to the negative Arctic Oscillation‐like structure. Consequently, this leads to the winter SAT anomalies occupying the area north of 40°N. The second leading winter Eurasian SAT mode shows negative values northward from 60°N as well as positive over the middle latitudes of Eurasia; in association with the second SAT mode, the significant anomalous snow cover appears to the east of the Ural Mountains during October. Such an anomalous snow signal can persist from October until the subsequent winter and produce a wave‐like structure with a southeastward propagation from the Ural Mountains to the midlatitude Eurasia in the midtroposphere and upper troposphere, which results in the dipole anomalous temperatures lying in the middle to high latitudes of Eurasia. The preceding autumn snow cover signal is valuable for the prediction of the winter SAT over the Eurasian continent.

The Effect of Boreal Late Autumn Snow Cover over Western and Central China on the Northern Hemisphere Wintertime Blocking Frequency

The impact of snow cover in western and central China during late autumn on wintertime blocking occurrence is investigated using reanalysis data. The study results show that wintertime atmospheric circulations affected by late autumn snow cover anomalies form favorable conditions for increased blocking frequency (BF), especially in the North Pacific and North Atlantic. Evidence is also presented that the stratosphere–troposphere interactions are the key mechanism of the lag response of wintertime North Pacific and North Atlantic BFs to the late autumn snow cover. That is, positive anomalous snow cover can induce a dipole anomaly in the geopotential height field over the lower stratosphere, due to the decrease of the 300–1000-hPa thickness and the concurrent variation between the East Asian plateau jet and the polar front jet. The associated positive geopotential height anomalies are located over northwestern Eurasia. Meanwhile, western and central China shows remarkably negative geopotential height anomalies. Also, the corresponding atmospheric circulation in the lower stratosphere increases the Eliassen–Palm flux that propagates into the stratosphere through the constructive interference between the forced and climatological waves. The upward wave activity fluxes collapse the polar vortex in the stratosphere, resulting in the downward propagation of the geopotential and wind anomalies from the stratosphere. Consequently, the decreased zonal wind speed in the upper layer of the blocking region forms conditions favorable for wintertime blocking.

Partial least regression approach to forecast the East Asian winter monsoon using Eurasian snow cover and sea surface temperature

Seasonal prediction of the East Asian (EA) winter monsoon (EAWM) is of great significance yet a challenging issue. In this study, three statistical seasonal prediction models for the EAWM are established using three leading modes of the Eurasian snow cover (ESC), the first leading mode of sea surface temperature (SST) and the four leading modes of the combination of the ESC and SST in preceding autumn, respectively. These leading modes are identified by the partial-least square (PLS) regression. The first PLS (PLS1) mode for the ESC features significantly anomalous snow cover in Siberia and Tibetan Plateau regions. The ESC second PLS (PLS2) mode corresponds to large areas of snow cover anomalies in the central Siberia, whereas the third PLS (PLS3) mode a meridional seesaw pattern of ESC. The SST PLS1 mode basically exhibits an El Nino-Southern Oscillation developing phase in equatorial eastern Pacific and significant SST anomalies in North Atlantic. A strong EAWM tends to emerge in a La Nina year concurrent with cold SST anomalies in the North Atlantic, and vice versa. After a 35-year training period (1967–2001), three PLS seasonal prediction models are constructed and the 11-year hindcast is performed for the period of 2002–2012, respectively. The PLS model based on combination of the autumn ESC and SST exhibits the best hindcast skill among the three models, its correlation coefficient between the observation and the hindcast reaching 0.86. This indicates that this physical-based PLS model may provide another practical tool for the EAWM. In addition, the relative contribution of the ESC and SST is also examined by assessing the hindcast skills of the other two PLS models constructed solely by the ESC or SST. Possible physical mechanisms are also discussed.

Snow cover response to temperature in observational and climate model ensembles

AbstractThe relationship between land surface temperature and snow cover extent trends is examined in three distinct types of ensembles over the 1981–2010 period: an observation‐based ensemble, a representative selection of CMIP5 coupled climate model output, and two large initial condition coupled climate model ensembles. Observation‐based estimates of snow cover sensitivity are stronger than simulated over midlatitude and alpine regions. Observed sensitivity estimates over Arctic regions are consistent with simulated values. Anomalous snow cover extent trends present in one data set, the NOAA climate record, obscure the relationship to surface temperature seen in the rest of the analyzed data. The spread in modeled snow cover trends reflects roughly equal contributions from intermodel variability and from natural variability. Together, the anomalous relationship between surface temperature and snow cover expressed in the NOAA climate record and the large influence of natural variability present in the simulations highlight the importance of ensemble‐based approaches.

Impact of the winter North Atlantic Oscillation (NAO) on the Western Pacific (WP) pattern in the following winter through Arctic sea ice and ENSO: part I—observational evidence

On the basis of a 51-year statistical analysis of reanalysis data, we propose for the first time that the positive phase of the Western Pacific (WP) pattern in the winter is linked to the negative phase of the North Atlantic Oscillation (NAO) in the previous winter, and vice versa. We show that there are two possible mechanisms responsible for this interannual remote linkage. One is an Arctic mechanism. Extensive Arctic sea ice in the summer after a negative NAO acts as a bridge to the positive phase of the WP in the next winter. The other mechanism involves the tropics. An El Nino occurrence after a negative winter NAO acts as another bridge to the positive phase of the WP in the following winter. The timescale of the Arctic route is nearly decadal, whereas that of the tropical route is about 3–5 years. The tropical mechanism indicates that the NAO remotely excites an El Nino in the second half of the following year. A process perhaps responsible for the El Nino occurrence was investigated statistically. A negative NAO in the winter increases Eurasian snow cover. This anomalous snow cover then intensifies the cold air outbreak from Asia to the western tropical Pacific. This outbreak can intensify the westerly wind burst and excite El Nino in the following year. We suggest that the phase of the NAO in the winter could be a predictor of the WP in the following year.

Eurasian Subarctic Summer Climate in Response to Anomalous Snow Cover

Abstract The summer climate in northern Eurasia is examined as a function of anomalous snow cover and processes associated with land–atmosphere coupling, based on a composite analysis using observational and reanalysis data. The analysis confirms that the snow–hydrological effect, which is enhanced soil moisture persisting later into the summer and contributing to cooling and precipitation recycling, is active in eastern Siberia and contributes to the formation of the subpolar jet through the thermal wind relationship in early snowmelt years. Strong anticyclonic differences (early − late snowmelt years) with a baroclinic structure form over eastern Siberia as a result of surface heating through the snow–hydrological effect in early snowmelt years. Surface heating contributes to the development of thermally generated stationary Rossby waves that propagate eastward to eastern Siberia. Rossby wave activity is maintained into early autumn, together with cyclonic differences over far eastern Siberia, in conjunction with the early appearance of snow cover in this region. The anticyclonic differences over eastern Siberia act like a blocking anticyclone, thereby strengthening upstream storm track activity. Furthermore, it is possible that surface anticyclonic differences over the Arctic contribute to year-to-year variability of summer Arctic sea ice concentration along the Siberian coast. The results suggest that variations in northern Eurasian snow cover and associated land–atmosphere coupling processes have important implications for the predictability of Eurasian subarctic summer climate.

Regions of autumn Eurasian snow cover and associations with North American winter temperatures

AbstractThe extent of snow cover over Eurasia during autumn has been shown to be influential in shaping atmospheric circulation over the Northern Hemisphere the following winter via the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and the Pacific/North American (PNA) teleconnections. Regions of Eurasian snow cover were derived from Principal Component Analysis and compared to winter temperatures across North America for 1967/1968–2007/2008, excluding 1969/1970 and 1971/1972. The score time series of each principal component was then compared to winter averages of the AO, NAO, and PNA indices in order to identify possible links in the snow‐temperature relationship. Results showed that autumn snow cover from northern Scandinavia to the West Siberian Plain is most significantly associated with winter temperatures over the interior of North America. More (less) frequent snow cover over this region is related to lower (higher) winter temperatures over the interior of North America in January, extending to the eastern and southern United States in February. The greatest temperature response to anomalous snow cover occurred near the geographic centre of North America where winter temperature differences exceeded 5 °C. More (less) frequent autumn snow cover across the eastern Tibetan Plateau was associated with higher (lower) temperatures in the Great Basin and eastern Canada. Copyright © 2011 Royal Meteorological Society

Improvement of snowpack simulations in a regional climate model

Improvement of snowpack simulations in a regional climate model Jiming Jin 1 and Norman L. Miller 2 * Departments of Watershed Sciences and Plants, Soils, and Climate and Utah Climate Center, Utah State University, Logan, UT, USA Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. *Miller’s work is partially supported by the U.S. Department of Energy and LBNL under Contract No. DE- AC02-05CH11231. Abstract: To improve simulations of regional-scale snow processes and related cold-season hydroclimate, the Community Land Model version 3 (CLM3), developed by the National Center for Atmospheric Research (NCAR), was coupled with the Pennsylvania State University/NCAR fifth-generation Mesoscale Model (MM5). CLM3 physically describes the mass and heat transfer within the snowpack using five snow layers that include liquid water and solid ice. The coupled MM5–CLM3 model performance was evaluated for the snowmelt season in the Columbia River Basin in the Pacific Northwestern United States using gridded temperature and precipitation observations, along with station observations. The results from MM5–CLM3 show a significant improvement in the SWE simulation, which has been underestimated in the original version of MM5 coupled with the Noah land-surface model. One important cause for the underestimated SWE in Noah is its unrealistic land-surface structure configuration where vegetation, snow and the topsoil layer are blended when snow is present. This study demonstrates the importance of the sheltering effects of the forest canopy on snow surface energy budgets, which is included in CLM3. Such effects are further seen in the simulations of surface air temperature and precipitation in regional weather and climate models such as MM5. In addition, the snow-season surface albedo overestimated by MM5–Noah is now more accurately predicted by MM5–CLM3 using a more realistic albedo algorithm that intensifies the solar radiation absorption on the land surface, reducing the strong near-surface cold bias in MM5–Noah. The cold bias is further alleviated due to a slower snowmelt rate in MM5–CLM3 during the early snowmelt stage, which is closer to observations than the comparable components of MM5–Noah. In addition, the over-predicted precipitation in the Pacific Northwest as shown in MM5–Noah is significantly decreased in MM5 CLM3 due to the lower evaporation resulting from the longer snow duration. KEY WORDS: land-surface model; regional climate model; snow; vegetation INTRODUCTION In the western United States (WUS), seasonally accumulated snow mass has become increasingly important to water resources, because of the rapidly increasing demand for water to supply the expanding economy and population. Over the last decade, scientists have made a significant effort to understand and forecast snowmass variability in the WUS (Miller et al., 1999; Serreze et al., 1999; Knowles and Cayan, 2002; Jin and Miller, 2007). Observational studies have found a downward trend in snowpack over the last five decades, which is consistent with the smaller ratio of snowfall to rainfall and earlier snowmelt in the WUS, a finding consistent with global warming observations (Mote, 2003; Stewart et al., 2004). Snowmelt timing is also related to winter and spring temperature variability (Dettinger and Cayan, 1995; Cayan et al., 2001). McCabe and Clark (2005) examined eighty-four rivers in the WUS to better understand snowmelt runoff variability and associated climate processes. They found that earlier snowmelt has a high correlation with increased spring and early summer atmospheric pressure and temperature over the WUS, indicating that snowmelt trend and variability is tightly linked to atmospheric forcings. On the other hand, snow mass in the WUS also affects atmospheric processes. Lo and Clark (2002) found that an inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall in the WUS; a similar relationship was previously found in Eurasia by Bamzai and Shukla (1999). This correlation indicates that the land surface often warms up more quickly with a lower snow depth and smaller snow cover area (or conversely, it warms up more slowly with a higher snow depth and larger snow cover area) when compared with normal conditions, suggesting that anomalous snow depth and cover can significantly affect atmospheric circulation patterns. Thus, accurate forecasting of accumulated snow mass and snowmelt timing is essential to both understanding climate variability and managing water resources in the WUS. However, it is very difficult to obtain the detailed observations throughout the WUS, which would allow a full understanding of snow mass and cover variations, as well as related processes. Numerical models are important tools for investigating and quantifying interactions between snow and climate, as well as for the advance of

A study of the sensitivity of the surface temperature in Eurasia in winter to snow-cover anomalies: The role of the stratosphere

This study is devoted to the influence that anomalous snow cover in the Siberian autumn has on the surface temperature in the winter months using the intermediate complexity model of a climate system. The possible mechanisms of the effect that a thermal forcing anomaly on surfaces attributed to the troposphere-stratosphere interaction has on the temperature regime in winter are considered.

Time‐lagged effects of spring Tibetan Plateau soil moisture on the monsoon over China in early summer

AbstractThe objective of the present study is to investigate the effect of initial soil moisture in the Tibetan Plateau (ISMTP) during the spring period on the local and regional climate in subsequent months. Numerical experiments with different ISMTP are made from 1 April to 30 June for the year 1998 using a regional climate model. This was a wet year with anomalously high summer precipitation over China. The same experiments have also been done for the year 1995, which was a normal year with respect to the summer rainfall over China. In both years, the snow amount over the Tibetan Plateau (TP) was higher than normal in the preceding winter/spring. The results of the experiments indicated that an increase in the ISMTP may lead to decreases in the air temperature, vertical velocity, and surface heating over the TP in June. In addition to the local effects, an increase in the ISMTP may also lead to modifications of the climate in some remote regions in early summer, such as an increase in the amount of monsoon precipitation over the mid to lower reaches of the Yangtze River region in eastern China and a decrease over the south China region. These precipitation differences due to the changes in ISMTP are consistent with those due to anomalous snow cover over the TP in winter or spring. Physically, the Tibetan High is weakened with an increase in ISMTP, which leads to a change in the strength of the Asian summer monsoon. Copyright © 2007 Royal Meteorological Society

A modeling study of the effects of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon

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

Land surface conditions over Eurasia and Indian summer monsoon rainfall

Using observations of snow cover, soil moisture, surface air temperature, atmospheric circulation, and Indian summer monsoon precipitation from 1870 to 2000, we examine the relationship between interannual variations of the strength of the monsoon and land surface conditions over Eurasia. For the periods 1870–1895 and 1950–1995, strong Indian summer monsoon precipitation was preceded by warmer than normal temperatures over Europe and North America in the previous winter and over western Asia in the previous spring but colder temperatures over Tibet. The European temperature anomalies were related to the positive phase of the North Atlantic Oscillation (NAO). Related negative snow cover anomalies in Europe in winter and central Asia in spring were produced by circulation and temperature anomalies. The snow‐albedo feedback is always operating, but the snow by itself did not physically control the monsoon. Anomalous snow cover impacts on temperature were not prolonged by soil moisture feedbacks because of its short time memory, and there was no obvious relationship between soil moisture and the monsoon. Strong Indian summer monsoon precipitation was actually preceded by higher than normal Tibetan snow cover in winter and spring in contrast to the suggestion of Blanford [1884] more than a century ago. The correlation between Indian summer monsoon rainfall and winter land temperatures and snow cover only exists when interannual variation of the NAO is very strong, and therefore NAO is not a robust predictor of the monsoon. Climate models show that the relationship between NAO and monsoon is random.

The effects of anomalous snow cover of the Tibetan Plateau on the surface heating

On the basis of snow data and AWS (Automatic Weather Station) data obtained from the Tibetan Plateau in recent years (1993 to 1999), the features of sensible heat, latent heat and net long-wave radiations are estimated, and their variations in more-snow year (1997/ 1998) and less-snow year (1996/ 1997) are analyzed comparatively. The relationships between snow cover of the Tibetan Plateau and plateau’s surface heating to the atmospheric heating are also discussed. The difference between more-snow and less-snow year in spring is remarkably larger than that in winter. Therefore, the effect of anomalous snow cover of the Tibetan Plateau in winter on the plateau heating appears more clearly in the following spring of anomalous snow cover.

Effect of western U.S. snow cover on climate

AbstractThe purpose of this study is to identify characterize and quantify local, regional and remote effects of snow cover on western U. S. climate and water resources. An ensemble of predictability and sensitivity studies was made with the U.S. National Center for Atmospheric Research (NCAR) Community Climate Model, version 3 (CCM3) to investigate the relative roles of snow-cover anomalies and initial atmospheric states in the subsequent accumulation and ablation seasons. The suite of model experiments focuses on the direct effect of snow on regional climate anomalies and ultimately will be used to examine the lagged effect of anomalous snow cover on the climate. The set of ensemble simulations presented here looks at the climate-system response to anomalously high and low snow cover at the start of the ablation season over the western U.S.A. These current results suggest that the initial state of snow cover is more important than the initial state of the atmosphere or of sea-surface temperatures because of direct thermal effects on the surface and subsequent indirect, dynamical effects on the atmospheric circulation.

Sensitivity of the Asian summer monsoon to an anomalous Eurasian snow cover within the Météo-France GCM

Sensitivity of the Asian summer monsoon to an anomalous Eurasian snow cover within the Météo-France GCM

チベット、東ヨーロッパ、シベリアの積雪偏差が大気に及ぼす影響の比較実験

The atmospheric response to the regional snow mass anomalies in early spring over Tibet, Eastern Europe, and Siberia is compared by general circulation model ensemble experiments. The positive snow mass anomalies over Tibet produce the largest cooling anomalies in the atmosphere from spring to early summer. Almost no significant forcing anomalies are systematically formed by the snow mass anomalies over Eastern Europe and Siberia.The model experiments show that the cooling source over Tibet works significantly to delay the seasonal transition from spring to summer in the Northern Hemisphere. It is confirmed by the simulated weak Asian monsoon; a weak lower tropospheric monsoon jet in South Asia, a weak large-scale divergence center at the upper troposphere in Southeast Asia, negative lower-layer geopotential height anomalies in the North Pacific and the North Atlantic, and the weak Walker circulation in the equatorial Pacific are simulated.The above noticeable response of the atmosphere to the snow mass anomalies over Tibet is associated with the following characteristics of Tibet as compared with Eastern Europe and Siberia;(1) The small snow melt speed over Tibet, probably due to its high elevation, maintains the early spring additional snow mass until almost the end of the climatic snow-melt season.(2) The snow albedo is effective because of large solar incidence and relatively small cloudiness over Tibet.(3) The existence of the anomalous snow cover works to cut the upward sensible heat flux rather than the evaporation over Tibet because of its dry ground condition.(4) The dry ground over Tibet makes it possible that all the anomalous snow-melt water is not drained as runoff, but a part of it is stored in the ground.(5) As pointed out by many researchers, the Tibetan Plateau plays an important rôle in the establishment of the Asian monsoon.The above conditions (1)-(5) for the effective snow mass anomalies are found in the model Tibet, but these conditions would be satisfied by the real regions where the snow mass anomalies affect the atmosphere efficiently.The extended atmospheric responses are found in May for the East European case and in August for the Siberian case. These are accompanied by significant ground condition anomalies in northern Eurasia. Those ground condition anomalies are not directly related to the successive ground condition anomalies followed by the anomalous snow mass melt, and seem to be created after the water and heat of the initial snow mass anomalies are supplied into the atmosphere.

ユーラシア大陸の積雪が北半球暖候季の大気に与える影響について

The effect of excessive snow mass over the Eurasian continent on the spring and summer climate is investigated by using the MRI⋅GCM. The ensemble mean of the four runs (SNOW runs) with the excessive snow mass of 5 cm (water equivalent) at the beginning of March over the snow cover area of the continent is compared with that of the control runs, to deduce the effect of the snow mass on the climatic parameters in the later seasons.The main results are summarized as follows:(1) In spring, the albedo effect is dominated in the lower latitudes particularly over the Tibetan Plateau. The reduced net radiation by the anomalous snow cover balances the reduced surface sensible and latent heat fluxes, which account for the significant decrease of surface temperature, cloudiness and total diabatic heating over there in the SNOW runs.(2) In summer, in contrast, the snow-hydrological effect is significant, particularly in the mid-latitudes. The increase of ground wetness in the SNOW runs causes anomalous cooling and higher pressure near the surface. A moderate signal of the weakened Asian summer monsoon is also obtained. However, the increase of evaporation activates cumulus convection, which partly compensates for the decrease of total diabatic heating by the cooling near the surface. This evaporation/convection feedback seems to work, on the other hand, to sustain the increased ground wetness throughout the summer.(3) The atmospheric teleconnection patterns induced by the anomalous snow cover over the Tibetan Plateau and east Asia significantly appear over the north Pacific and the North American continent in spring through late summer. These anomalous circulations cause the considerable decrease of surface temperature over the northeastern part of North America.(4) The implication of these results for the Ice Age issue is also briefly discussed.

Effect Of Eurasian Snow Cover On Summer Climate Of The Northern Hemisphere: A GCM Study

Observational studies have shown that Eurasian snow-cover anomalies during winter-through-spring seasons have a great effect on anomalies in atmospheric circulation and climate in the following summer season through snow albedo feedback (Hahn and Shukla, 1976; Dey and Bhanu Kumar, 1987). Morinaga and Yasunari (1987) have revealed that large-scale snow-cover extent over central Asia in late winter, which particularly has a great effect on the circulation over Eurasia in the following season, is closely related to the Eurasian pattern circulation (Wallace and Gutzler, 1981) in the beginning of winter.Some atmospheric general circulation models (GCM) have suggested that not only the albedo effect of the snow cover but also the snow-hydrological process are important in producing the atmospheric anomalies in the following seasons (Yeh and others, 1984; Barnett and others, 1988).However, more quantitative evaluations of these effects have not yet been examined. For example, it is not clear to what extent atmospheric anomalies are explained solely by snow-cover anomalies. Spatial and seasonal dependencies of these effects are supposed to be very large. Relative importance of snow cover over Tibetan Plateau should also be examined, particularly relevant to Asian summer monsoon anomalies. Moreover, these effects seem to be very sensitive to parameterizations of these physical processes (Yamazaki, 1988).This study focuses on these problems by using some versions of GCMs of the Meteorological Research Institute. The results include the evaluation of total snow-cover feedbacks as part of internal dynamics of climatic change from 12-year GCM integration, and of the effect of anomalous snow cover over Eurasia in late winter on land surface conditions and atmospheric circulations in the succeeding seasons.