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626 H. UEDA, M. SHINODA AND H. KAMAHORI pentad 1.5 0.0 -1.5 Snow-disappearance Pentads 66 67 68 69 70 71 72 73 74 75 76 77 78 79 year anomaly (45-60N, 30-60E) 80 81 82 83 84 85 86 87 88 89 90 Figure 8. Year-to-year variations of snow-disappearance pentads anomalies for the EEP (45–60 °N, 30–60 °E) Figure 9. Composite difference of snow boundary between late-disappearance years (1979, 1980, 1987) minus early disappearance years (1975, 1983, 1990). Positive (negative) values mean slow (rapid) northward retreat of snow-cover extent interesting feature in Figure 10 is that differences of surface temperature between the extreme years are absent in February and early March, whereas they are much enhanced in mid winter and in late March and April. This might suggest a periodicity of atmospheric circulation patterns across the continent that may be associated with seasonal change of the so-called North Atlantic oscillation (Portis et al., 2001) or other atmospheric teleconnection patterns. In order to confirm the atmospheric conditions relevant to early snow-disappearance, we present the composite differences of vertically integrated meridional advection during March and April (Figure 11). Larger heating anomalies can be found to the northeast of the Black sea. This pattern resembles those found in the horizontal (meridional plus zonal) advection (not shown) and wind fields (Figure 3(a) and (b)). These results Copyright © 2003 Royal Meteorological Society Int. J. Climatol. 23: 615–629 (2003)
EURASIAN SNOW COVER AND ATMOSPHERIC CIRCULATION 627 Figure 10. Latitude–time section of composite anomalies of the surface temperature between late years (1979, 1980, 1987) years minus early years (1975, 1983, 1990). Solid (dashed) line denotes poleward retreat of snow boundary for late (early) years. Light (dark) shading indicates temperature anomaly below −4 °C (−8 °C) Figure 11. Composite anomaly of vertical integrated meridional advection (W m −2 ) for March and April between late years (1979, 1980, 1987) minus early years (1975, 1983, 1990) suggest that the enhanced advection of warm southwesterly air in the lower troposphere during the snowdecreasing phase of March and April is an important factor for the regulation of snow-disappearance timing. 6. CONCLUSIONS AND DISCUSSIONS The seasonal and interannual features of snow-disappearance timing over the Eurasian continent and its relation to the surface temperature were investigated by using station data for the FSU between 1966 and Copyright © 2003 Royal Meteorological Society Int. J. Climatol. 23: 615–629 (2003)
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