Synopticscale Circulation

Similarly to the mid-latitudes, synoptic-scale disturbances control the daily weather events in the Arctic. Vangengeim (1952, 1961) showed that changes of synoptic processes in the Arctic are about 1.5 times faster than in moderate latitudes. Working from this and other facts given earlier, it can be concluded that the climate of the Arctic is significantly more sensitive to atmospheric circulation variability than the climates in both moderate and low latitudes.

The first analysis of the frequency of cyclones and anticyclones, as well as their tracks, based on measurements made on the drifting station North Pole-1 (11 May 1937 - 19 Feb. 1938) was given by Dzerdzeevskii (19411945, 1945). These publications are very important for our understanding of the pressure pattern in the central Arctic, and were translated into English by the University of California, Department of Meteorology, appearing in Scientific Report No. 3, 1954. Dzerdzeevskii was the first to draw a map presenting the frequency of the number of days with cyclonic activity for each month from the period May-October. He also distinguished six types of cyclonic activity in the Arctic.

However, the first real cyclone and anticyclone climatologies could not be made until the turn of the 1950s and 1960s, when synoptic charts were more reliable due to a denser network of stations (Keegan 1958; Ragozin and Chukanin 1959; Reed and Kunkel 1960; Gaigerov 1962, 1964). From the more recent works, one can mention McKay el al. (1970), Gorshkov (1980), LeDrew (1983, 1984, and 1985), Whittaker and Horn (1984), Atlas Arktiki (1985), Scrreze and Barry (1988) and Scrrcze et a/. (1993). The last two works present an updated climatology of the synoptic systems in the Arctic, taking into account a new data set of air pressure available since 1979 within the Arctic Ocean Buoy Program.

The various sources presented here generally show similar patterns of sea-level cyclonic activity in winter in the Arctic (see Figure 2.3a). The cyclones are most frequent in the Atlantic region of the Arctic and in the Baffin Bay region. On the maps by Ragozin and Chukanin (1959) and Gaigerov (1964), the local maximum of high frequency of cyclones also occurs over the East Siberian Sea. The mean number of passing cyclones oscillates be tween 4-6 over the Norwegian-Barents-Kara seas and about 4 over the East Siberian Sea (Ragozin and Chukanin 1959). The lowest occurrence of cyclone frequency is noted in the northern part of the Canadian Arctic and in the neighbouring Arctic Ocean. A more detailed analysis made by Stepanova (1965) has shown that the highest frequency of cyclones in the central Arctic is observed with a meridional or eastern type of circulation. A low number of cyclones is noted when the western type of circulation is well developed. The majority of winter cyclones enter the Arctic from the North Atlantic and the Barents Sea. and then track north-eastwards, rarely reaching the Western Arctic. Cyclones from other regions arc not often observed.

Winter Anticyclone

Figure 2.3. Cyclone (a, b) and anticyclone (c, d) % frequencies for winter (a. c), 1952/53 -19B8/89 and summer (b, d), 1952 1989 in squares of 306.000 km-, for systems with central pressure < 1012 hPa (cyclones) and >1012 hPa (anticyclones). A cutoff contour of 3% frequency is chosen to accentuate areas of frequent synoptic activity (after Serreze et al. 1993).

Winter (January) anticyclones are almost entirely restricted to the Canadian Basin on the map presented by Serreze and Barry (1988). On Gatgerov's (1964) map this maximum is shifted to the western part of the Canadian Arc-

tic Archipelago. In addition, another maximum occurs over the Barents Sea. In turn, according to Ragozin and Chukanin (1959), the frequency of winter anticyclones is clearly highest in the northern part of Greenland and in the southern part of the Arctic Ocean neighbouring the Beaufort and Chukchi seas. Recent calculations of winter anticyclone frequencies (Serreze et al., 1993) for the period 1952/53 - 1988/89 showed a similar pattern to that presented by Ragozin and Chukanin (1959) (Figure 2.3c). In any case, the highest frequency is clearly seen in the Western Arctic.

In summer, a general decrease in atmospheric pressure occurs. As a result, the mean occurrence frequency of cyclones in this season is similar to that of winter. However, the pattern of cyclone frequency is quite different (compare Figure 2.3b with Figure 2.3a). Moreover, there is a significant divergence between different sources (compare maps in Ragozin and Chukanin 1959; Reed and Kunkel 1960; Gaigerov 1964; Stepanova 1965; Gorshkov 1980; Serreze and Barry 1988; Serreze et al. 1993). According to the most recent results (Serreze et al, 1993), summertime cyclone frequencies show maxima centred in the Barents and Kara seas as well as over the southern part of the Canadian Arctic (Figure 2,3b). Secondary maxima occur over the central Arctic in the vicinity of the North Pole and over the Laptev and East Siberian seas. Serreze and Barry (1988) note the lack of lows in the Baffin Bay region. It is a surprising result, because the other cited sources (also Serreze et al. 1993) show a high frequency of lows here. This means the period 1979-1985 analysed by Serreze and Barry (1988) does not provide a good representation of mean synoptic conditions in the Arctic, at least in the Baffin Bay region. Cyclones move into the Arctic Ocean from various directions. However, they usually arrive from the Siberian coast (Kara, Laptev, and Chukchi seas), the North Atlantic, and the Baffin Bay regions (Figure 2.4). Few cyclones enter the Arctic from the Bering Strait or the Canadian Arctic Archipelago. From the presented map it can be seen that the cyclone tracks meet in the central Arctic, especially over the Canadian Basin. This is contrary to what we observe in winter (see Ragozin and Chukanin 1959).

The summer frequency of anticyclones (Figure 2.3d) shows that they are most common over the following three regions: 1) the western part of the Canadian Arctic and the Beaufort Sea; 2) the East Siberian and Laptev seas; and 3) the Kara and Barents seas. A smaller centre also occurs over northern Greenland. The results of research by Reed and Kunkel (1960) and Ragozin and Chukanin (1959) show a similar pattern. The mean speed of both cyclones and anticyclones is significantly higher in the cold half-year, reaching a maximum in March (Ragozin and Chukanin 1959). The lowest speed of cyclones and anticyclones occurs in August and September, respectively. The mean speed of anticyclones is greater than that of cyclones and oscillates from 43 km/h in March to 35 km/h in September. Analogous values for cyclones are equal to 40 km/h (March) and 34 km/h (August).

Mean Winter Circulation

Figure 2.4. Mean cyclone motion vectors for winter (DJF) and summer (JJA), 1975- 1989. The length of each arrow is the mean vector magnitude, with the width proportional to the index of motion constancy. Vectors are only plotted for grid cells with at least a 3% frequency of cyclones (after Serreze et at. 1993).

Figure 2.4. Mean cyclone motion vectors for winter (DJF) and summer (JJA), 1975- 1989. The length of each arrow is the mean vector magnitude, with the width proportional to the index of motion constancy. Vectors are only plotted for grid cells with at least a 3% frequency of cyclones (after Serreze et at. 1993).

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