Geoscience Reference
In-Depth Information
Table 4.1.
Mean and extreme monthly values (hPa) of the Siberian Anticyclone
central pressure for November, January, and March (1873-1988)
November
January
March
Maximum
1042.3
1048.5
1038.8
Mean
1032.7
1037.5
1031.1
Minimum
1027.2
1029.8
1024.9
Source
: Based on Sahsamanoglou et al. (
1991
).
(Ding,
1990
).
Table 4.1
summarizes mean and extreme pressures under the Siberian
High for November, January, and March. On average, the Siberian High is strongest
in January and February. D. Walland and I. Simmonds (
1997
) suggest a negative
(self-regulating) snow cover feedback in the vicinity of the winter Siberian High.
They argue that cooling by an anomalously extensive snow cover strengthens the
Siberian High, increasing the static stability of the lower troposphere. This results in
decreased precipitation, limiting further advance of the snow field. Similarly, anom-
alously reduced snow cover weakens the Siberian High and decreases the static
stability of the lower troposphere, encouraging increased precipitation and advance
of the snow cover. Significant correlations have been found between anomalies of
snow cover and the sea level pressure and height fields over the Arctic and North
Atlantic (Cohen and Entekhabi,
1990
; Cohen, Saito, and Entekhabi,
2001
).
The winter Icelandic and Aleutian lows are complex features. In part, they man-
ifest low-level thermal effects of the comparatively warm ocean bordering the ice
margin and cold land. Surface pressures tend to be comparatively low over a warm
surface as the overlying air is warmer, and hence less dense than over a cold sur-
face. One of the observations that supports this idea is that isobars are roughly
parallel to and more tightly crowded along the coastlines of Greenland and eastern
Asia - Alaska, respectively (Wallace,
1983
). Furthermore, both lows are located
downstream of the major mid-tropospheric stationary troughs (
Figure 4.7
) where
cyclogenesis is favored by upper-level divergence (see Barry and Carleton,
2001
,
pp. 459-461). They are hence part of the primary North Atlantic and North Pacific
cyclone tracks, respectively. Finally, there are strong regional cyclone develop-
ment processes associated with enhanced baroclinicity (strong horizontal tempera-
ture gradients along the sea ice margin) and the orographic effects of the southern
Greenland ice sheet (discussed later). The winter Icelandic Low is part of a broad
area of low pressure extending into the Barents and Kara seas. This feature reflects
the thermal influences mentioned earlier and the penetration of cyclones northeast-
ward into the Arctic Ocean.
As the latitudinal gradient of solar heating weakens in spring and summer, the
general circulation also weakens. This is manifested in a weakening of the primary
storm tracks and the low pressure centers of action. Springtime weakening of the
Siberian High represents a general response to the increasing solar heating and loss
of the snow cover. Development of a closed anticyclone off the Canadian Arctic
