The annual average snow cover in the Northern Hemisphere during 1997 was more than 3% below the long-term (1972-96) mean, and was the third lowest value in the historical record of satellite data dating back to 1972. Only 1990 and 1988 recorded less annual mean snow cover than was observed in 1997. Northern Hemisphere snow cover has been below normal during nine of the past ten years, with only the 1996 value being above normal during this period.
Northern Hemisphere snow cover extent during 1997 was below normal in all months except August and September (Fig. 18). This contrasts with heavy snow years (such as 1996) in which above-normal snow cover is typically recorded in 8-10 months of the year. During 1997, monthly snow cover deficits exceeded 2 x 106 km2 during February and March. The March mean snow cover extent was the second lowest value in the historical record for that calendar month, with only March 1990 recording a lower value. The February mean snow cover extent was the fourth lowest value in the record for that month.
Over Eurasia, snow cover extent was below normal throughout much of the year (Fig. 19), consistent with above-normal annual mean temperatures throughout the region (Fig. 4). Snow cover deficits reached 0.8 x 106 km2 during DJF in response to a reduced southward extent of the snow pack across central Europe, most of southern Russia and northwestern China (Fig. 19a). These conditions were associated with abnormally warm surface temperatures (2°-3°C above average) across the region (see section 5, Fig. 67), and are consistent with previous studies relating snow cover extent to surface air temperatures (Groisman et al. 1994).
Even larger snow cover deficits (1.4 x 106 km2) were observed over Eurasia during MAM (Fig. 19b), in response to an early-season snow melt over central Siberia, northern Mongolia and western China. This early melt was also associated with much warmer-than-normal surface temperatures (4°-6°C above-average) throughout the region (see section 5, Fig. 69). As previously noted, this anomalous warmth was heavily linked to a persistent pattern of broad southwesterly geostrophic flow throughout the region, in association with an anomalous large-scale trough over northern Europe and Scandinavia and a persistent ridge across central Siberia (see section 5, Fig. 70). Similar circulation features contributed to significantly above-normal surface temperatures in central Siberia during January-May 1995, also resulting in a notable reduction in the areal extent of snow cover during that period (Halpert et al. 1996).
The SON 1997 season featured an abnormally late development of the cool-season snow pack and a continuation of below-normal snow cover over much of central Siberia (Fig. 19c). These conditions were associated with a continuation of abnormally warm temperatures throughout the region (see section 5, Fig. 73), in response to the reestablishment of the pattern of broad southwesterly geostrophic flow and above-normal geopotential heights throughout the region (see section 5, Fig. 74).
In North America, the DJF and MAM seasons featured above-normal snow cover across the northwestern United States and southwestern Canada. These conditions resulted from repetitive winter storms and cold-air outbreaks, in response to two prominent, and sometimes related, atmospheric phenomena: 1) recurring high-latitude blocking and block evolution in the vicinity of the Bering Sea and Alaska, and 2) substantial variability in the eastward extent of the East Asian jet stream over the eastern North Pacific. The abnormal melt of this snow pack contributed to significant flooding in the Columbia and Willamette River Gorges in the Pacific Northwest section of the United States during late 1996/early 1997 (Halpert and Bell 1997), and to massive flooding of the Red River Valley and its tributaries (located in the Northern Plains states) during April [see section 4a(1)].
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