d. Northern Hemisphere snow cover

The areal extent of Northern Hemisphere snow cover during 1998 was below the 1972-97 mean in every month except October (Fig. 19a ). During February the snow cover extent was the second lowest in the historical record for that calendar month, eclipsed only by February 1995 conditions.

With the exception of 1996, the 12-month running mean of snow cover extent has been below normal over the hemisphere since 1987. This contrasts with the above-average areal extent of snow cover observed throughout the 1970s. This interdecadal variability in snow cover extent is evident over both Eurasia (Fig. 19b) and North America (Fig. 19c).

Over Eurasia the 1998 snow cover extent (Fig. 19b) was generally below average during January-June and above-average during SON. The DJF 1997/98 season featured a reduced southward extent of the wintertime snow pack across both eastern Asia and western Russia/ eastern Europe (Fig. 20a) and increased snow cover in the region east of the Caspian Sea. The dipole pattern of anomalies in these regions was linked to a large-amplitude atmospheric circulation pattern at upper levels (see section 5, Fig. 64), characterized by a strong ridge over Europe and a very strong trough extending southward from western Siberia to the Caspian Sea. This circulation contributed to anomalously high temperatures across Europe and western Russia (see section 5, Fig. 63) and to reduced precipitation over portions of western Russia (see section 5, Fig. 63), thus leading to reduced snow cover in these regions. Farther east, it contributed to lower-than-normal temperatures and above-normal precipitation, which led to increased snow cover east of the Caspian Sea.

The MAM season featured an early springtime retreat of the snowpack over eastern Asia and a delayed snow melt across western and central Russia (Fig. 20b), while the SON season featured an expanded region
of snow cover across most of Asia and eastern Europe (Fig. 20c). In both seasons, these conditions are consistent with the locations of the upper-level ridges and troughs, and with the attendant temperature and precipitation patterns.

In North America the areal extent of snow cover was below normal throughout the year (Fig. 19c), with record low values observed in February, May, and June and near-record low values observed in January, April, and November. In mid-1998 the 12-month running mean of anomalous snow cover extent dropped a to record low value of -1.0 x 106 km2.

In Canada, snow cover extent was below-normal in all seasons (Fig. 20), as record warmth dominated the country during much of the year (Figs. 4, 5a). In the United States, snow cover was below-normal during DJF across the Midwest, the Ohio Valley, and much of the mid-Atlantic region (Fig. 20a), and above-normal in the West. This above-normal snow cover then continued in the West during MAM and was also evident in portions of the central Plains states (Fig. 20b).

In both Canada and the United States these conditions were linked to an anomalous upper-level atmospheric circulation that extended across the North Pacific and North America in association with ongoing strong El Niņo conditions (see section 3c). Prominent aspects of this circulation included 1) increased zonal flow across the continent, along with significantly reduced northwesterly flow, 2) a very powerful wintertime jet stream and enhanced storm activity over the eastern North Pacific and west-central U. S., and 3) an overall shift of the wintertime jet stream and storm track toward the southern tier of the U. S..

These conditions contributed to a widespread flow of mild, marine air throughout the continent and to record warmth across Canada. In the United States the increased storminess contributed to abundant snowfall and above-normal snow cover over the mountainous regions of the West. Also, many of these storms and their attendant cold-frontal boundaries were completely de-coupled from the cooler Canadian air masses to the north. This combination of increased marine air throughout the northern U. S., and a nearly complete absence of cold-air outbreaks from Canada, contributed to extremely mild conditions and below-normal snow cover extent across the central and eastern U. S.. During SON, a snow cover deficit of 0.7 x 106 km2 and a delayed development of the snow pack were again observed across Canada (Fig. 20c), as extremely warm temperatures persisted across the country.

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