Eurasian Snow Cover Research Articles

The Role of Boundary Conditions in AMIP-2 Simulations of the NAO

Abstract The simulated North Atlantic Oscillation (NAO) teleconnection patterns and their interannual variability are evaluated from a suite of atmospheric models participating in the second phase of the Atmospheric Model Intercomparison Project (AMIP-2). In general the models simulate the observed spatial pattern well, although there are important differences among models. The NAO response to interannual variations in sea surface temperature (SST) and snow-cover boundary forcings are also evaluated. The simulated NAO indices are not correlated with the observed NAO index, despite being forced with observed SSTs, indicating that SSTs are not driving NAO variability in the models. Similarly, although a number of studies have identified a link between Eurasian snow extent and the phase of the NAO, no such link is apparent in the AMIP-2 results. It appears that, within the framework of the AMIP-2 experiments, the NAO is an internal mode of atmospheric variability and that impacts of SSTs and Eurasian snow cover on the phase of the NAO are not discernable. However, these conclusions do not necessarily apply to decadal-scale and longer variability or to coupled atmosphere–ocean models.

Influence of Eurasian snow cover in spring on the Indian Ocean Dipole

The Indian Ocean Dipole (IOD) shows a significant negative correlation with the extent of Eurasian snow cover in spring. The anomalies in the land-sea temperature contrast, which are induced by the anomalies of Eurasian snow cover in spring, reverse the patterns of convection activ- ity anomalies over the western and eastern tropical Indian Ocean in summer. Moreover, for heavier and lighter than normal Eurasian snow cover in spring, anomalies in the vertical zonal circulation over the tropical Indian Ocean (Walker circulation) and in the vertical meridional circulation between the Indian Ocean and the Eurasian continent (Hadley circulation) occur in summer. The Walker and Hadley circulation anomalies probably play an important role in the occurrence and duration of IOD events. Eurasian snow cover in spring is presumably one of the factors that trigger IOD events. These results provide a basis for further investigation of the mechanisms linking snow cover, the Indian monsoon, atmospheric circulation, and sea surface temperature.

Potential future changes in the Indian summer monsoon due to greenhouse warming: analysis of mechanisms in a global time-slice experiment

In this study the potential impact of the anticipated increase in the greenhouse gas concentrations on different aspects of the Indian summer monsoon is investigated, focusing on the role of the mechanisms leading to these changes. Both changes in the mean aspects of the Indian summer monsoon and changes in its interannual variability are considered. This is done on the basis of a global time-slice experiment being performed with the ECHAM4 AGCM at a high horizontal resolution of T106. The experiment consists of two 30-year simulations, one representing the present-day climate (period: 1970–1999) and one representing the future climate (period: 2060–2089). The time-slice experiment predicts an intensification of the mean rainfall associated with the Indian summer monsoon due to the general warming, while the future changes in the large-scale flow indicate a weakening of the monsoon circulation in the upper troposphere and only little change in the lower troposphere. The intensification of the monsoon rainfall in the Indian region is related to an intensification of the atmospheric moisture transport into this region. The weakening of the monsoon flow is caused by a pronounced warming of the sea surface temperatures in the central and eastern tropical Pacific and the associated alterations of the Walker circulation. A future increase of the temperature difference between the Indian Ocean and central India as well as a future reduction of the Eurasian snow cover in spring would, by themselves, lead to a strengthening of the monsoon flow in the future. These two mechanisms compensate for the weakening of the low-level monsoon flow induced by the warming of the tropical Pacific. The time-slice experiment also predicts a future increase of the interannual variability of both the rainfall associated with the Indian summer monsoon and of the large-scale flow. A major part of this increase is accounted for by enhanced interannual variability of the sea surface temperatures in the central and eastern tropical Pacific.

A Stratified Diagnosis of the Indian Monsoon—Eurasian Snow Cover Relationship

Abstract This study is aimed at reconciling inconsistencies between modeling studies of the Indian monsoon–Eurasian snow cover relationship, which support the Blanford hypothesis, and recent diagnostic studies, which have largely failed to show its existence. Recently released version 2 NOAA/NESDIS satellite-based retrievals of snow cover are used. A focus is given to diagnosing (a) spatial and temporal complexity in the Eurasian snow cover distribution, (b) the role of ENSO in modulating the snow cover–all-India rainfall (AIR) relationship, and (c) the spatial distribution of the rainfall association within India. With these goals in mind, the climatological distribution of snow cover and its interannual variability are first assessed. While snow cover is largest in northern Eurasia, its variability is most pronounced in southwestern (SW) Asia between the Black and Caspian Seas, and across the northern Indian, Himalayan, and Tibetan Plateau (NIHT) regions. In regions of large variability, and for Eurasia...

Changes in the sub‐decadal covariability between Northern Hemisphere snow cover and the general circulation of the atmosphere

AbstractDetails of the sub‐decadal covariability relationship between continental snow cover extent anomalies and the dominant mode of atmospheric variability, referred to as the Arctic oscillation (AO) or North Atlantic oscillation (NAO), for the period 1971–2001 are explored. On the seasonal time scale, the winter AO is found to be significantly correlated with the preceding autumn Eurasian snow cover (SNCEUR) throughout the period observed. Consistent with this finding, SNCEUR variability led the AO variability on the sub‐decadal time scale in the early half of the record. However, starting in the mid 1980s, the AO and SNCEUR vary in phase. Analyses of the seasonal relationship and persistence of snow and atmospheric variables illustrate a phase shift in the sub‐decadal variability between the AO and SNCEUR due to the loss of autumn–winter SNCEUR autocorrelation replaced by a significant winter–spring persistence and the emergence of a concurrent SNCEUR–AO connection in winter and spring. Similar analysis shows that the sub‐decadal NAO variation is mostly described by the fluctuation in summer North American snow cover. Copyright © 2004 Royal Meteorological Society

Eurasian snow cover, more skillful in predicting U.S. winter climate than the NAO/AO?

Successfully predicting the phase and strength of the dominant winter mode of variability for the Northern Hemisphere (NAO/AO) is considered the most important future breakthrough in winter climate prediction; however skillful prediction of the index has been elusive. In this Letter we present a snow index constructed from observed summer and fall anomalies that is more highly correlated than the observed value of the winter AO with winter surface temperatures in the eastern United States. Ease of use and the potential for greater predictive skill could potentially render forecasting the phase and strength of the NAO/AO irrelevant as a tool for prediction of U. S. winter surface temperatures.

Summer U.K. Temperature and Its Links to Preceding Eurasian Snow Cover, North Atlantic SSTs, and the NAO

Motivated by an attempt to predict summer (June‐August) U.K. temperatures, the time-lagged correlations between summer U.K. and European temperatures and prior snow cover, North Atlantic sea surface temperatures (SSTs), and the North Atlantic Oscillation (NAO) are examined. The analysis centers on the 30-yr period 1972‐ 2001 corresponding to the interval of reliable satellite-derived land snow cover data. A significant association is found between late winter Eurasian snow cover and upcoming summer temperatures over the British Isles and adjacent areas, this link being strongest with January‐March snow cover. Significant links are also observed between summer temperatures and the preceding late winter NAO index and with a leading principal component of North Atlantic SST variability. The physical mechanisms underlying these time-lagged correlations are investigated by studying the associated variability in large-scale atmospheric circulation over the Euro‐Atlantic sector. Seasonal expansion in the Azores high pressure system may play an important role in the time-lagged relationships. The potential seasonal predictability of summer U.K. temperatures during the period 1972‐2001 is assessed by cross-validated hindcasts and usable predictive skill is found. However, the presence and cause of temporal instability in the time-lagged relationships over longer periods of time requires further investigation.

Predictability and numerical modelling of the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the dominant pattern of atmospheric circulation variability in the extratropical Northern Hemisphere and it is a major controlling factor in basic meteorological variables such as surface wind, temperature and precipitation which have large socioeconomic impacts on energy, agriculture, industry, traffic and human health throughout the whole of Europe and eastern North America. Because of this dominant impact on the weather and climate of the wealthiest areas of the planet, there is a growing interest in quantifying the possible limits of predictability of the phenomenon and the ability of the climate numerical models of simulating it. This paper reviews recent work on predictability and methods of numerical modelling of the North Atlantic Oscillation used to simulate the phenomenon. Atmospheric models with no orography or land–sea contrasts are able to capture the main feature of the NAO; however, to capture any interannual or interdecadal variability of the NAO, atmospheric general circulation models (AGCM) with seasonally varying sea surface temperature (SSTs) forcing are required. Still, no model reproduces the recent observed upward trend in the NAO index, suggesting that either the models are deficient or external forcing such as man-made effects are responsible for this feature. Predictive patterns have been identified in the Atlantic SSTs preceding specific phases of the NAO by up to 6 months, in the atmospheric temperatures anomalies in the previous November, in the Eurasian snow cover and in the sea-ice extent over Arctic. The use of simulations based on ensemble prediction to estimate potential predictability shows the possibility of capturing the upward trend of the NAO and suggests that multiannual to multidecadal variations in the NAO are more predictable than interannual fluctuations.

Spring northward retreat of Eurasian snow cover relevant to seasonal and interannual variations of atmospheric circulation

AbstractAn observational study is made of the seasonal and interannual variations of spring snow‐disappearance over the Eurasian continent and the circulation mechanisms causing those variations. The spring northward retreat of the snow boundary over the East European Plain (EEP) between 30 and 60° E is faster (0.4° per day) than to the east of the Ural Mountain range (0.3° per day). These migrations of the snow boundary lag behind the appearance of the surface air temperature 0 °C by about 1 to 5 pentads.The analyses of the atmospheric heat and moisture budgets showed that the seasonal intrusion of warm air associated with southwesterly winds is primarily responsible for the rapid snowmelt in March and April over the EEP. In addition, the adiabatic heating of descending air plays a secondary role in the snowmelt in mid‐March. On an interannual time scale, horizontal warm advection also plays an essential role in the spring northward retreat of snow cover extent.The present study confirms the previous finding that the surface air temperature anomalies, produced during the seasonal snow‐disappearance period, diminished in May, suggesting a weak dynamical linkage between the EEP snow cover and Asian summer monsoon. Copyright © 2003 Royal Meteorological Society

The role of snow cover fluctuations in multiannual NAO persistence

In this article authors explore the relation in the observed data between the Eurasian snow cover anomalies and the NAO variability on interannual to decadal time scales. Results reached in this study suggest that in winter and early spring, NAO type atmospheric circulation influences the extent of snow cover and the snow cover affects the atmosphere in the late spring, summer and early autumn leading to a mechanism that seems to be responsible for the multiannual NAO persistence in the last half century.

Increase in the Asian southwest monsoon during the past four centuries.

Climate reconstructions reveal unprecedented warming in the past century; however, little is known about trends in aspects such as the monsoon. We reconstructed the monsoon winds for the past 1000 years using fossil Globigerina bulloides abundance in box cores from the Arabian Sea and found that monsoon wind strength increased during the past four centuries as the Northern Hemisphere warmed. We infer that the observed link between Eurasian snow cover and the southwest monsoon persists on a centennial scale. Alternatively, the forcing implicated in the warming trend (volcanic aerosols, solar output, and greenhouse gases) may directly affect the monsoon. Either interpretation is consistent with the hypothesis that the southwest monsoon strength will increase during the coming century as greenhouse gas concentrations continue to rise and northern latitudes continue to warm.

Influence of Eurasian spring snow cover on Asian summer rainfall

AbstractThe Eurasian snow cover anomaly in spring has been considered as one of the important factors affecting Asian summer monsoon variability. Using the long time series (1922–98) of Eurasian spring (March–April) snow cover (ESSC) reconstructed by Brown (2000. Journal of Climate 13: 2339) and snow cover (1973–98) and depth (1979–87) data from satellite observation, the influences of ESSC on the all‐India monsoon (June–September) rainfall (AIMR) and the summer rainfall over all parts of Asia are examined. It is found that the statistical relation between AIMR and ESSC changes over a multi‐decadal time scale. The negative correlation between them has increased markedly since the mid 1970s. The region where the summer rainfall has the strongest and most stable negative correlation with the preceding ESSC is located in northern Mongolia, south of Lake Baikal. The correlation between the summer rainfall and ESSC increases after the data are treated with a low‐pass filter, showing that the impact of snow cover may be seen more clearly with the removal of the effect of El Niño–southern oscillation. Comparative analyses for contrasting years with excessive and deficient snow cover show that the anomalies of ESSC occur mainly in northwestern Eurasia. In the years of excessive ESSC anomalies, cooling and a cyclonic circulation anomaly in the lower troposphere appear over the northern part of Eurasia, leading to a Rossby‐wave‐train‐like circulation response, then a weakened East Asia summer monsoon and deficient rainfall with an anticyclonic circulation anomaly south of Lake Baikal. Anomalies with opposite signs occur in the years of deficient snow cover. Copyright © 2002 Royal Meteorological Society.

Evolution of Atmospheric Response to Early-Season Eurasian Snow Cover Anomalies

Abstract Recently it has been shown that the area extent of Eurasian snow cover during September–October–November (SON) and the principal component of the leading mode of extratropical Northern Hemisphere (NH) climate variability in the following winter are statistically correlated. In this paper, physical linkages between SON Eurasian snow cover and the wintertime climate variability in the NH atmosphere are postulated. And in order to test the proposed hypotheses, comprehensive analyses of satellite-based observations for snow cover and reanalysis data for geopotential heights and sea level pressure are used. The magnitude of the correlation between snow cover and climate variability is found to be inversely proportional to the height suggesting that snow cover may act as a lower boundary forcing to the tropospheric circulation. Conversely, however, an index constructed to capture the downward propagating circulation anomaly from the lower stratosphere to the middle troposphere is shown to be as highly ...

An apparent atmospheric teleconnection between snow cover and the North American Monsoon

Snow cover has been shown to reduce lower‐atmospheric temperatures thereby modifying atmospheric circulation patterns over time scales of days to months. Over the past three decades an inverse relationship between Eurasian winter snow cover extent and subsequent summer rainfall across India has been studied. Very little research has addressed the potential relationship between North American snow cover and the North American monsoon. The work presented here identifies an apparent inverse relationship between July and August snow cover across western North America and August precipitation across a portion of the southwestern U.S. that is susceptible to an annual monsoon circulation. Results suggest that during a summer season characterized by low snow cover across western North America, the typical subtropical ridge across the southern U.S. intensifies. The product appears to be an atmosphere that is conducive to advection of low‐level moisture northward across areas of the southwestern U.S.

Monsoonal variability in the northeastern Arabian Sea during the past 5000 years: geochemical evidence from laminated sediments

A high-resolution laminated sediment record from the oxygen-minimum zone in the northeastern Arabian Sea off Pakistan was studied to reconstruct climatic variations. To infer the monsoon-driven “moisture history” in the northeastern Arabian Sea during the past 5000 years, we have used three types of independant proxies: (1) varve thickness, (2) stable isotope ratios of planktic foraminifera; and (3) inorganic geochemical composition. On the basis of these proxies we have subdivided the sedimentary record into five intervals. The first time interval from 5000 to 3900 yr BP (Period I) is dominated by summer-monsoon activity. Deduced from Ti/Al, Zr/Al, K/Al and Sr/Ca ratios and oxygen-isotope data of planktonic foraminifera slightly enhanced winter-monsoon conditions can be inferred for the period from 3900 to about 3000 yr BP (Period II). Maximum precipitation during enhanced winter-monsoon activity at about 3100 yr BP was followed by an onset of aridification at 3000 yr BP which continued until about 2000–2200 yr BP (Period III). The textural, mineralogical and geochemical data suggest that this aridification was a gradual, century-scale process rather than an abrupt event. Comparison to late Holocene records in NW- and SW-India, the Near East and tropical Africa supports our interpretation that Period III from 3000 to 2000 yr BP is characterized by a gradual change in the atmospherical moisture budget over much of this area. Reduced monsoonal rainfall may be linked to variations of Eurasian snow cover. As deduced from varve thickness and element to Al-ratios, Period IV from 2000 to about 1500 yr BP is characterized by a summer-monsoon dominated sequence with higher precipitation than during Period III. The δ 18O record shows an abrupt shift at 2000 yr BP coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern Arabian Sea. Present-day conditions in the Makran area were established at 1500 yr BP (Period V). Based on inorganic geochemical data and meteorological records at several stations in the Makran we conclude that Period V is dominated by precipitation during the winter season. Here, declined rainfall associated with the summer-monsoon may be related to a less northward position of the intertropical convergence zone (ITCZ) during summer and a stronger influence of winter-monsoon linked phenomenona.

Potential predictability of Eurasian snow cover

Potential predictability and skill of simulated Eurasian snow cover are explored using a suite of seasonal ensemble hindcasts (i.e. retrospective forecasts), an ensemble climate simulation (spanning the years 1982–1998) and observations. Using remotely sensed observations of snow cover, we find significant point-wise correlation over the North Atlantic and North Pacific between winter and spring averaged sea-surface temperatures and Eurasian snow cover area. The observed correlation shows no discernible pattern related to the El Niño-Southern Oscillation (ENSO). The hindcasts show correlation patterns similar to the observations. However, the climate simulation shows an exaggerated ENSO pattern. The results underscore the importance of initialization in seasonal climate forecasts, and that the observed potential predictability of Eurasian snowcover cannot be solely attributed to ENSO.

Paleoclimate of Southwestern China for the Past 50,000 yr Inferred from Lake Sediment Records

Long sediment cores (12.5 and 13.5 m) from two lakes in Yunnan Province were used to infer the paleoclimate of southwest China over the past 50,000 yr. During the Holocene and marine isotope stage (MIS 3), bio-induced carbonate precipitation and organic matter (OM) production was high, suggesting warm temperatures and high primary productivity. In contrast, sediment inorganic carbon (IC) and organic carbon (OC) concentrations were low in last glacial deposits from 38,000 to 12,000 cal yr B.P., indicating cool temperatures and low productivity. The 50,000-yr record has alternating peaks of carbonate and coarse-grain (>38 μm) quartz that reflect warm, moist interglacial or interstadial conditions alternating with cold, dry glacial or stadial conditions, respectively. Spectral analysis of the carbonate and quartz signals reveals power concentrated at periods of 7200 and 8900 cal yr, respectively, that may reflect a nonlinear climate response to precessional forcing at a time of reduced eccentricity modulation (McIntyre and Molfino, 1996). Oxygen isotope values of calcite from Yunnan lake cores indicate the summer monsoon was weak during the last glaciation from 50,000 to 12,000 cal yr B.P. The summer monsoon intensified between 12,000 and 8000 cal yr B.P., but weakened gradually in response to insolation forcing during the mid-to-late Holocene. Our results support the Overpeck et al. (1996) model that posits a weak summer monsoon during the last glaciation that responded nonlinearly to insolation forcing when its intensity was affected by Eurasian snow cover and ice-sheet extent. The summer monsoon intensified and responded linearly to seasonal insolation forcing in the Holocene when ice volume diminished.

Relation between Eurasian Snow Cover, Snow Depth, and the Indian Summer Monsoon: An Observational Study

Satellite-derived snow cover data for 22 yr and snow depth data for 9 yr over Eurasia have been analyzed to reexamine the possible relation of snow with the Indian summer monsoon. In contrast to the previous studies that use snow cover averaged over all of Eurasia as a single number, the frequency of occurrence of snow at each grid point over Eurasia is correlated with the Indian summer monsoon rainfall. Thus specific geographical regions over Eurasia that are responsible for the well-known inverse relationship between Eurasian snow cover and Indian monsoon rainfall are delineated. It is found, somewhat surprisingly, that western Eurasia is the only geographical region for which a significant inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall. However, composites for high and low snow cover over Eurasia show spatially homogeneous large-scale patterns of snow cover and surface temperature anomalies. Winters of high and low snow cover for Eurasia are found to be associated with colder and warmer than normal temperatures, respectively, for large regions of the Eurasian continent. The inverse snow–monsoon relationship holds especially in those years when snow is anomalously high or low for both the winter as well as the consecutive spring season. Contrary to previous findings, no significant relation is found between the Himalayan seasonal snow cover and subsequent monsoon rainfall.

Corrections to “Eurasian snow cover variability and northern hemisphere climate predictability”

Table 1. Eurasian snow cover and SSTs as predictors (SON) and concurrent (DJF) of hemispheric and regional climate indices. Observed climate indices are shown in rows all climate indices are for DJF. Climate indices are, ZMW, NAO, Scandinavia or Eurasian One Pattern (SCA), West Pacific Pattern (WPP), Pacific/North American Pattern (PNA). NAO and SCA are dominant modes of variability in North Atlantic and PNA and WPP are dominant modes of variability in North Pacific in winter /Barnston and Livezey, 1987]; also shown in square brackets are the autocorrelations of each DJF climate index with its own preceding SON index. Statistical significance greater than 95% confidence level in italics and greater than 99% confidence level in bold. The correlation coefficient included in parentheses corresponds to the statistic excluding E1-Nifio years 1972, 1982, 1986, 1991. We also computed the correlations between the detrended time-series of snow cover and the zonal mean wind, NAO and SCA indices and no changes were found in the statistical significance of the correlations from those presented in the table.

Eurasian snow cover variability and northern hemisphere climate predictability

We present observational evidence demonstrating dynamic interactions and feedbacks between multi‐seasonal snow cover and winter‐time circulation anomalies over mid‐high latitudes. The cooling effect of snow cover is associated with a strengthened and more expansive Siberian high with more frequent, topographically constrained intrusions west and north. Early‐season snow cover variability leads to altered general circulation patterns consistent with the dominant mode of winter variability observed in the Northern Hemisphere troposphere. The implications of the surface‐atmosphere coupling for seasonal to interannual predictability are also discussed.