Geoscience Reference
In-Depth Information
Archipelago in spring appears to reflect the infrequency of transient cyclones and
the shift from eastern Siberia of the locus of lowest tropospheric temperatures.
The mean annual cycle of SLP varies across the Arctic. Based on a harmonic
analysis using NCEP/NCAR data, R. Cullather and A. Lynch (
2003
) show that
within the Atlantic sector, pressures tend to be highest in July and lowest in January.
The area of the Siberian high is characterized by a February maximum and August
minimum. By comparison, the Canada Basin/Laptev Sea region is dominated by
a March maximum and September minimum. In an earlier effort, J. Walsh (1978)
found that SLP averaged from 70-90°N exhibits a semi-annual cycle, with maxima
in April and November, and minima in July and February. Cullather and Lynch
(
2003
) examined this semi-annual cycle more closely and find it to be best expressed
along the periphery of northern Greenland and extending to the pole. Their har-
monic analysis depicts pressure maxima in May and November. Dynamically, this
semiannual cycle can be related to seasonal variations in mass convergence as mass
moves from Eurasia and into the Canadian Arctic Archipelago in spring and the
reverse condition in autumn. The semiannual cycle exhibits pronounced interannual
variability, associated with mass exchanges with the primary storm tracks in the
north Atlantic and Pacific.
4.5.2
Cyclone Activity
Figure 4.9
depicts by season the frequency of extratropical cyclones north of 60°N
over the 1970-1999 period.
Figure 4.10
gives total counts of cyclogenesis (cyclone
formation) events by season over the same period. Results are based on an algo-
rithm applied to six-hour NCEP/NCAR SLP fields. Cyclones are identified using a
series of search patterns, testing whether a grid-point SLP value is surrounded by
grid point values at least 1 hPa higher than the central point being tested. Cyclones
are tracked by comparing system positions on subsequent six-hour charts using
a nearest-neighbor approach. Cyclogenesis is defined as the first appearance of a
closed (1 hPa) isobar. Earlier versions of the algorithm based on twelve-hour pres-
sure fields are described by Serreze (
1995
) and Serreze et al. (
1997
). X. Zhang et al.
(
2004
) describe a broadly similar algorithm.
For cyclone frequency, cyclone occurrences in grid cells of 250 km over the
thirty-year period were summed by season. The cell counts were then divided by
the number of years. Hence, the plotted values represent system centers per season.
For example, in any grid cell enclosed by the 3.0 contour, there is an average of at
least three cyclone centers per season. For cyclogenesis, we simply summed events
by grid cell over the thirty years. All maps have been heavily smoothed to improve
clarity. Values over the Greenland Ice Sheet should be viewed with extreme caution
owing to the problems of reducing pressures to sea level.
Building from previous discussion, winter cyclone activity is most prominent
over the Atlantic side of the Arctic. This is consistent with the large atmospheric
energy transports into the Arctic basin through this sector (Overland and Turet,
1994
;
Overland, Turet, and Oort,
1996
; Serreze et al.,
2007
, see
Plate 4
). These cyclones
