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628 H. UEDA, M. SHINODA AND H. KAMAHORI 1990. In addition to this, the heat and moisture budgets were computed by use of twice-daily ECMWF reanalysis data for the period of 1980–90. The major results of the present study are summarized as follows: 1. The springtime northward retreat of the snow boundary is about 0.4° in latitude per day over the EEP (30–60 °E), whereas the migrating speed is slower east of the Ural mountain range, at about 0.3° per day. The northward retreat of the snow boundary takes place following the appearance of surface air temperature 0 °C. In particular, the surface air temperature increases abruptly over the EEP during March. The northward shift of the 0 °C isotherm is very rapid in late March and early April, and is approximately 1.0° in latitude per day. 2. A pronounced northward intrusion of southwesterly winds in the lower troposphere occurs during March and April over the EEP region. Later in spring, the northwesterly winds in the lower troposphere replaced the southwesterlies and dominated north of 60 °N. The diabatic heating Ql over the northern part of the EEP (north of 60 °N) is characterized by a cooling concentrated in the lower layer. This is mainly compensated by the zonal warm advection due to westerlies and the adiabatic warming relevant to descending air. On the other hand, the southern part of the EEP is under the strong influence of meridional warm air advection. This warm advection nearly balances with the negative Ql, having a peak value of −0.8 K day −1 in the lower troposphere. 3. The atmospheric moisture budget analyses showed that the evaporation began to exceed the condensation from early May, following the snow disappearance. 4. The precursory signal of surface temperature anomalies can be found from December and persists until April, whereas the surface temperature anomalies diminish abruptly in May. It is worth noting that there are no significant anomalies in the subsequent summer. As for the atmospheric circulation for the early snow-disappearance years, the low-level warm air advection is enhanced during the snow-melting phase of March and April. The potentially important land–air interaction in the Eurasian continent has not been properly monitored and simulated by a coupled atmosphere–land model. In particular, the processes regulating the interannual variation of snow-cover extent and its connection with the previous tropospheric circulation have not been studied in detail. With this background in mind, the present study revealed that the winter–spring circulation (including the northward warm advection) anomalies control the spring snow-disappearance timing, and surface air temperature anomalies were attenuated as soon as snow-cover anomalies disappeared. This fact supports the previous findings that the land-surface anomalies over the study area might not be dynamically linked with the Indian monsoon activity (Shinoda, 2001; Shinoda et al., 2001). El Niño–southern oscillation (ENSO) has a vigorous impact on the change of mid-latitude westerly regimes (Yasunari and Seki, 1992). If the interannual variation of the snow-cover extent is closely linked to the tropospheric circulation, the modulation of the westerly jet due to the ENSO phenomenon may change the summer monsoon activity through land–atmosphere interaction. However, this snow–monsoon connection was not confirmed in the present study. On the other hand, many studies have shown that ocean–atmosphere interactions over the Indian Ocean play a significant role in the interannual variations of the Asian summer monsoon activities (e.g. Meehl, 1997; Kawamura, 1998). Recently, Kawamura et al. (2001) have revealed that the baroclinic Rossby wave response to tropical convective heating over the Indian Ocean, associated with ENSO, is found in the changes of tropospheric temperature over central Asia. This finding implies another mechanism regulating the Asian summer monsoon activity. Finally, it should be noted here that the station data for the FSU in the 1990s are not available at present. This decade exhibited some of the earliest snow disappearance of snow cover over the past century (Folland et al. 2001). Much research is needed in this area to further our understanding of the dynamics of snow-related thermal and hydrological processes and their relations to the westerly regimes and monsoon variability. ACKNOWLEDGEMENTS We would like to thank Masatake E. Hori for helpful discussions, and thank the reviewers for constructive comments and suggestions. Most of the figures are made using the GMT System (Wessel and Smith, 1991). Copyright © 2003 Royal Meteorological Society Int. J. Climatol. 23: 615–629 (2003)
EURASIAN SNOW COVER AND ATMOSPHERIC CIRCULATION 629 REFERENCES Aizen. 2000. Heat exchange during snow ablation in plains and mountains of Eurasia. Journal of Geographical Research 105: 27 013–27 022. Banzai AS, Shukla J. 1999. Relation between Eurasian snow cover, snow depth, and the Indian summer monsoon: an observational study. Journal of Climate 12: 3117–3132. Barnett TP, Schlese L, Dümenil U, Roeckner E, Latif M. 1989. The effect of Eurasian snow cover on regional and global climate variations. Journal of the Atmospheric Sciences 46: 661–685. Blanford HF. 1884. On the connexion of Himalayan snowfall and seasons of drought in India. Proceedings of the Royal Society of London 37: 3–22. Dickson RR. 1984. Eurasian snow cover versus Indian monsoon rainfall — an extension of the Hahn–Shukla results. Journal of Climate and Applied Meteorology 23: 171–173. Douville H, Royer J-F. 1996. Sensitivity of Asian summer monsoon to an anomalous Eurasian snow cover within the Météo-France GCM. Climate Dynamics 12: 449–466. Hahn DG, Shukla J. 1976. An apparent relationship between Eurasian snow cover and Indian monsoon rainfall. Journal of the Atmospheric Sciences 33: 2461–2462. Folland CK, Karl TR, Christy JR, Clarke RA, Gruza GV, Jouzel J, Mann ME, Oerlemans J, Salinges MJ, Wang S-W. 2001. Observed climate variability and change. In Climate Change 2001: The Scientific Basis, IPCC Third assessment Report, Houghton JT, Ding Y, Griggs DJ, Nogues M, van der Linden PJ, Dai x, Maskell K, Johnson CA (eds). Cambridge University Press: Cambridge; chapter 2. Kawamura R. 1998. A possible mechanism of the Asian summer monsoon–ENSO coupling. Journal of the Meteorological Society of Japan 76: 1009–1027. Kawamura R, Matsuura T, Iizuka S. 2001. Interannual atmosphere–ocean variations in the tropical western North Pacific relevant to the Asian summer monsoon–ENSO coupling. Journal of the Meteorological Society of Japan 79: 883–898. Kripalani RH, Kulkarni A. 1999. Climatology and variability of historical Soviet snow depth data: some new perspectives in snow–Indian monsoon teleconnection. Climate Dynamics 15: 475–489. Liu X, Yanai M. 2001. Relationships between the Indian monsoon rainfall and tropospheric temperature over the Eurasian continent. Quarterly Journal of the Royal Meteorological Society 127: 909–937. Meehl GA. 1997. The south Asian monsoon and tropospheric biennial oscillation. Journal of Climate 10: 1921–1943. Portis DH, Walsh JE, Hamly ME, Lamb PJ. 2001. Seasonality of the North Atlantic oscillation. Journal of Climate 14: 2069–2078. Shen X, Kimoto M, Sumi A. 1998. Role of land surface processes associated with interannual variability of broad-scale Asian summer monsoon as simulated by the CCSR/NIES AGCM. Journal of the Meteorological Society of Japan 76: 217–236. Shinoda M. 2001. Climate memory of snow mass as soil moisture over central Eurasia. Journal of Geographical Research 106: 33 393–33 403. Shinoda M, Utsugi H, Morishima W. 2001. Spring-snow disappearance timing and its possible influence on temperature fields over central Eurasia. Journal of the Meteorological Society of Japan 79: 37–59. Suzuki R, Yatagai A, Yasunari T. 1998. Satellite-derived vegetation index and evapotranspiration estimated by using assimilated atmospheric data over Asia. Journal of the Meteorological Society of Japan 76: 1663–1671. Vernekar AD, Zhou J, Shukla J. 1995. The effect of Eurasian snow cover on Indian monsoon. Journal of Climate 8: 248–266. Wessel P, Smith WHF. 1991. Free software helps map and display data. EOS Transactions AGU, bf72 441: 445–446. Yanai M, Esbensen S, Chu J-H. 1973. Determination of bulk properties of tropical cloud clusters from large-scale heat and moisture budgets. Journal of the Atmospheric Sciences 30: 611–627. Yasunari T, Seki Y. 1992. Role of the Asian monsoon on the interannual variability of the global climate system. Journal of the Meteorological Society of Japan 70: 131–166. Yasunari T, Kitoh A, Tokioka T. 1991. Local and remote responses to excessive snow mass over Eurasia appearing in the northern spring and summer climate — a study with the MRI GCM. Journal of the Meteorological Society of Japan 69: 473–487. Zwiers FW. 1993. Simulation of the Asian summer monsoon with the CCC GCM-1. Journal of Climate 6: 470–486. Copyright © 2003 Royal Meteorological Society Int. J. Climatol. 23: 615–629 (2003)
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