Geoscience Reference
In-Depth Information
Table 4.2.
Regional 12-h mean maximum deepening rates (hPa) of
Arctic cyclones for different regions
Region
Max Deepening
Winter (Summer)
Central Arctic Ocean
−1.5(−1.9)
Barents/Kara/Norw. Seas
−3.6(−2.3)
Greenland Sea/N. Atlantic
−6.8(−3.3)
Baffin Bay/Davis Strait
−3.2(−2.2)
Alaska/Canada
−0.8(−1.7)
Eurasia
−1.1(−1.8)
Source
: Based on Serreze (
1995
).
advection increasing with height, promoting vertical motion and divergence. This
region also lies just to the south of a strong gradient in sea surface temperature,
associated with the contrast between the cold Labrador Current and the warmer
North Atlantic Current. Given that winter cyclone counts are highest near the locus
of the mean Icelandic Low, it follows naturally that this region appears as a max-
imum in the frequency of strong deepening. Lee-side cyclogenesis is common in
this region (Kristjánsson and McInnes,
1999
). The area of preferred deepening just
south of Svalbard corresponds to the most northward penetration of open water in
the Atlantic, in close proximity to very cold air over the Arctic Ocean.
Table 4.2
summarizes mean maximum twelve-hour deepening rates (hPa) of
winter Arctic cyclones for several regions over the 1973-1992 period. These were
compiled by finding the maximum twelve-hour deepening rate of all cyclones
through their lifecycle, and then averaging those cases falling in each region. The
much larger maximum deepening rates in the Greenland Sea/North Atlantic region
(which includes the Icelandic Low) compared to land areas is readily apparent.
In the Greenland Sea/North Atlantic, the lowest decile maximum deepening rate
is −16.8 hPa over twelve hours, compared to only −6.1 hPa for the Central Arctic
Ocean (not shown).
Whereas spring shows decreased vigor of the North Atlantic track both in terms
of cyclone frequency and cyclogenesis, there is a deeper penetration of the track
into Eurasia. It is important to note, however, how cyclogenesis increases over east/
central Eurasia and northwestern Canada. The North Atlantic track is weakest dur-
ing summer. Referring to
Table 4.2
, this is associated with much smaller maximum
deepening rates, compared to winter. However, from
Figures 4.9
and
4.10
, we see: (1)
a strong increase in cyclone activity over land areas, especially over eastern Eurasia;
(2) an attendant seasonal maximum in cyclogenesis over land areas; and (3) a sum-
mer cyclone maximum over the central Arctic Ocean. Autumn illustrates the transi-
tion back toward winter conditions, with dominance of the North Atlantic track.
The summer cyclone maximum over the central Arctic Ocean was first noted by
Dzerdzeevskii (
1945
), then by Reed and Kunkel (
1960
), and much more recently
