Figure 4.10 depicts by season the frequency of extratropical cyclones north of 60° N over the period 1970-99. Figure 4.11 gives total counts of cyclogenesis (cyclone formation) events by season over the same period. Results are based on an algorithm applied to six-hourly 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-hourly 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 12-hourly pressure fields are described by Serreze (1995) and Serreze et al. (1997a). Zhang et al. (2004) describe a broadly similar algorithm.
For cyclone frequency, cyclone occurrences in grid cells of 250 km over the 30-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 30 years. All maps have been heavily smoothed to improve clarity. Values over the
Greenland Ice Sheet should be viewed with extreme caution due 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. Cyclones in this area take a northerly to easterly track. Activity peaks in the vicinity of the Icelandic Low. Another area of high cyclone frequencies is Baffin Bay. While cyclones are of course very common in the vicinity of the Aleutian Low, relatively few of these Pacific-side systems migrate into the Arctic. Relatively few closed lows are found over the central Greenland Ice Sheet. Zhang et al. (2004) provide similar results. In comparison to the early view of Pettersen (1950), based on limited data sources, it is clear that while winter cyclones over the central Arctic Ocean are infrequent in comparison to the Atlantic sector, they are by no means rare.
The winter Icelandic Low and the region extending into the Barents Sea is also a region of frequent cyclogenesis and system deepening (the deepening rate is a useful measure of cyclone intensification). Development and intensification in the Icelandic Low region appear to be influenced by a number of processes. These include bifurcation (or "splitting") of lows moving in from the south and southwest by the high orograhic barrier of Greenland, distortion of temperature and wind fields, and lee-side vorticity production off the southeast coast of Greenland (e.g., Doyle and Shapiro, 1999; Kristjansson and McInnes, 1999; Petersen et al., 2003). The entire area, however, is characterized by enhanced baroclinicity (strong temperature gradients) along the sea ice margins. Rapid deepening of lows along the ice edge is frequently observed (Serreze, 1995; Serreze etal., 1997a). Redevelopment of extratropical systems migrating from the south is also common between Greenland and Iceland (UK Meteorological Office, 1964; Whittaker and Horn, 1984). Serreze et al. (1997a) find that roughly half of all cyclones associated with the Icelandic Low form north of 55° N. The favorable synoptic environment of the Atlantic sector is consistent with the large moist static energy transports into the Arctic basin through this sector (Overland and Turet, 1994; Overland et al., 1996, see Figure 3.10), which contribute, along with open water, to the relatively high winter surface air temperatures in this region (see Figure 2.20).
Table 4.2 summarizes mean maximum 12-hour deepening rates (hPa) of winter Arctic cyclones for different regions over the period 1973-92. These were compiled by finding the maximum 12-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 12 hours, compared to only -6.1 hPa for the Central Arctic Ocean (not shown).
While 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. Cyclogenesis increases over east/central Eurasia and northwestern Canada. The North Atlantic track is weakest during summer. Referring back to Table 4.2, this is associated with much smaller maximum deepening rates, compared to winter. However, from Figures 4.10 and 4.11, we see: (1) a strong increase in cyclone activity over land areas, especially over eastern Eurasia (see also Zhang et al., 2004); (2) an attendant seasonal maximum in cyclogenesis over land areas; (3) a summer cyclone maximum over the central Arctic Ocean. Autumn illustrates the transition back toward winter conditions, with dominance of the North Atlantic track.
The summer cyclone maximum over the central Arctic Ocean arises primarily from the migration of systems generated over Eurasia and along the weakened North Atlantic track (Reed and Kunkel, 1960; Serreze, 1995). Whittaker and Horn (1984) consider the cyclogenesis maxima over eastern Eurasia and Alaska-Canada to manifest orographic cyclogenesis. However, recent work points to a strong role of coastal baroclinicity, to be addressed shortly. There have been a number of case studies of cyclone development processes over the Arctic Ocean. The series of papers by LeDrew (1984; 1988; 1989) is recommended. These studies made use of forms of the omega equation to diagnose
Table 4.2 Regional 12-h mean maximum deepening rates (hPa) of Arctic cyclones for different regions
Max deepening winter (summer)
Barents-Kara-Norwegian Seas -3.6 (-2.3)
Greenland Sea-North Atlantic -6.8 (-3.3)
Source: Based on Serreze, 1995
the contributions of different mechanisms to the vertical motions in cyclones. As with mid-latitude systems, the effects of differential vorticity advection and temperature advection (see Holton, 1992) tend to dominate. Of particular interest, however, is the sometimes significant role of local heat sources in the Arctic basin (LeDrew, 1984).
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