Geoscience Reference
In-Depth Information
Icelandic and Aleutian lows are maintained largely by their position
downstream of the major mid-tropospheric stationary troughs, surface
heating contrasts, and regional cyclone development (cyclogenesis). The
winter Siberian High is a low-level feature driven by radiative cooling.
The Icelandic and Aleutian lows are much weaker during summer and
the Siberian high is replaced by mean low pressure. Weak low pressure
also characterizes the central Arctic Ocean in summer, contrasting with a
pronounced high over the Beaufort Sea in spring.
Winter cyclone activity on the Pacific side of the Arctic is most
common in the vicinity of the Aleutian Low. By comparison, there is
an extension of the North Atlantic cyclone track (of which the Icelandic
Low is part of) deep into the Arctic. Cyclogenesis and large cyclone
deepening rates are common, in part because of lee-side development
along the southeast coast of Greenland and enhanced baroclinicity (strong
horizontal temperature gradients) along the sea ice boundary. Bifurcation
(splitting) of existing cyclones approaching Greenland from the south is
also common. Because of the strong eddy activity and relatively warm,
open water, the Atlantic sector is a primary pathway for transport of
atmospheric energy into the Arctic (
Chapter 3
). Winter also sees a high
frequency of mesoscale Polar Lows over open water areas of the marginal
seas where convective activity is favored. While the Atlantic cyclone track
weakens in summer, cyclone activity increases over land areas and the
central Arctic Ocean.
A relative maximum in frontal frequencies is found during summer
along northern Eurasia from about 60-70°N, best expressed over the
eastern half of the continent. A similar relative maximum is found over
Alaska, which although best expressed in summer, is present year-
round. These features arise from strong differential heating between
the Arctic Ocean and snow-free land. Orography appears to sharpen
the baroclinicity. The regions of maximum summer frontal frequency
correspond to preferred areas of cyclogenesis.
One of the most import large-scale modes of atmospheric variability
affecting the Arctic is the North Atlantic Oscillation (NAO), which
describes covariation in the strengths of the Icelandic Low and the
Azores High to the south. The NAO can be viewed as the Atlantic-side
component of the Arctic Oscillation (AO), also known as the Northern
Annular Mode (NAM), which describes a shift of atmospheric mass
between the Arctic and mid-latitudes, associated with strengthening
and weakening of the circumpolar vortex. When there is less mass than
normal in the Artic and more mass than normal in middle latitudes,
the AO is in its positive mode; in the reverse situation, the AO is in its
negative mode. The NAO and AO are present year-round but vary in their
seasonal structure. The AO and NAO have broadly had similar impacts
on Arctic climate, including pronounced influences on SAT and (as
