Largescale Atmospheric Circulation

The Arctic must import heat from southerly latitudes due to the net radiation loss to space from the top of the atmosphere. Investigations have also shown that almost the entire deficit of energy is supplemented by atmospheric circulation (see previous chapter). This fact ascribes considerable importance to atmospheric circulation as a climatic factor.

Schemes of general atmospheric circulation which are frequently published in handbooks show the occurrence of the so-called 'polar cell' in the Arctic. In this cell cold dense air flows out from a polar high pressure centre towards a belt of low pressure located about 60 65 N. As a result, easterly and north-easterly winds should dominate in the Arctic. However, in reality, as we know from the previous section, the Arctic high is by no means a quasipermanent feature of the arctic circulation. Thus, easterly winds are a characteristic phenomenon only for the Atlantic and Pacific sectors of the Arctic. Calculations of the average wind directions for the whole Arctic have not shown any coherence system of polar latitudes either (Barry and Chorley 1992).

Clearly simpler is the circulation in the middle and upper troposphere between about 3 km and 10 kin, where a circumpolar cyclonic vortex occurs (Barry and Hare 1974). In these layers the general westerly air circulation is present as a result of the large-scale equator-to-pole temperature gradient and the Earth's rotation (Figure 2.1).

90°

Figure 2.1. Mean height (gpm) of the 300 hPa surface for (a) January and (b) July (after Crutcher and Meserve 1970).

ED Elevations over 1000 metres □ Elevations over 2000 metres

Figure 2.1. Mean height (gpm) of the 300 hPa surface for (a) January and (b) July (after Crutcher and Meserve 1970).

Investigations of the atmospheric pressure field in the Arctic were, and still are, limited by an observation network which is generally too sparse. As has been mentioned earlier, the earliest views presented were based on theoretical considerations. Mohn (1905) published probably the first maps of atmospheric pressure distribution in the Arctic. The next, improved analysis, was provided by Baur (1929). He used all the available data from different expeditions (e.g. the Fram expedition) and data collected during the First International Polar Year of 1882/1883. According to Rodcwald (1950), Baur's mean annual chart can be accepted as representative for the years 1874-1933. Later maps became increasingly detailed (e.g. Svcrdrup 1935; Dzerdzeevskii 1941-1945; Dorsey 1949 (unpublished) presented by Petterssen et al. 1956; Prik 1959; Baird 1964; Crutcher (unpublished) presented by Barry and Hare 1974; Colony and Thomdike 1984; Gorshkov 1980; Atlas Arktiki 1985; Serreze et al. 1993; Rigor and Heiberg 1997). The authors of the first two maps assumed the existence of a permanent Arctic anticyclone in the central Arctic. As Baur (1929) wrote "...In all months of the year the pressure at the North Pole is higher than at 70° north latitude..." Similar conclusions were presented by Sverdatp (1935), whose research resulted in corrections and supplementations to Baur's maps. Since the second half of the 1930s Soviet scientists have intensified the investigation of the Arctic climate, mainly using data obtained from drifting stations NP-I (1937) - NP-31 (1991) floating on the Arctic Ocean. As has been mentioned earlier, the results of meteorological observations carried out during the drift of the first station (Dzerdzeevskii 1941-1945 ) helped to change the view of the distribution of atmospheric pressure in the Arctic. Later on, these investigations were conducted mainly by Prik, who in 1959 published her very well known work (where air temperature was also presented) which has been cited by many authors (Stepanova 1965; Vowinckel and Orvig 1970; Barry and Hare 1974; Sugden 1982). Generally, Prik (1959) confirmed the results presented by Dzerdzeevskii, but, of course, also introduced some changes. She showed that the Arctic anticyclone could only be found as a bridge of high pressure connecting the Siberian high with the Canadian high in the winter months, while in some cases it appeared in the form of a small anticyclone over the Canadian Arctic Archipelago. In the 1960s (Baird 1964) and 1970s (Crutcher, unpublished, after Barry and Hare 1974), maps presenting the mean sea-level pressure were published. However, they did not give any information about the data used in the process of map construction. In comparison with Prik's maps, they are less detailed. Generally, the January pressure distribution in the Arctic is similar in all maps, except those of Greenland. Over Greenland, we may notice the occurrence of anticyclones (Prik 1959) or at least wedges of high pressure (Baird 1964; Barry and I lare 1974), On the other hand, large differences occur in the summer distribution of air pressure. Baird's map (1964) shows the presence of high pressure in the vicinity of the North Pole. Crutcher presents quite a similar pattern. On his map, instead of a high-pressure centre, a wedge of high-pressure covers the Pole, while on Prik's map a low-pressure centre surrounds the North Pole. In the 1980s two atlases were published in Russia, in which syntheses of the Arctic climate (among others) are presented (Gorshkov 1980; Atlas Arktiki 1985). The charts of the distribution of atmospheric air pressure in both of these atlases were prepared by Prik. In the first atlas, the mean air pressure distribution for the period 1881-1970 for each month is presented. In the second one, only the mean air pressure for January and July 1881-1965 is shown. It seems to me that, at present, these atlases are the best sources of information about the mean sea-level air-pressure distribution in the Arctic. Therefore, they have been used to describe in detail the patterns of this element in the area studied. However, I must mention here a new possibility which supplements and improves our knowledge concerning pressure distribution in the central Arctic, Since 1979, a network of Arctic drifting buoys has been operated through the University of Washington Polar Science Center (Thorndike and Colony 1980; and subsequent reports through Rigor and Heiberg 1997). The pressure analysis published by Thorndike and Colony for the period 1979-1985 uses data from a dozen buoys and approximately 70 coastal and island stations around the Arctic Ocean. According to McLaren et al. (1988) they are more accurate than earlier Arctic pressure fields. This conclusion, it seems to me, is rather untrue, because Prik (for example) used data from 290 stations and 20 drifting stations in constructing the maps published in Atlas Arktiki (1985). In any case, data from buoys should significantly improve our knowledge concerning the distribution of air pressure in the Arctic. Maps presenting the mean sca-level pressure for January and July (1979-1996) published by Rigor and Heiberg (1997) show a generally similar pattern to those published in the Atlas Okeanov and Atlas Arktiki (Figures 2.2a-d). Finally, the results from a recently published work by Serreze et al. (1993), examining climatological patterns of Arctic synoptic activity for the period January 1952 - June 1989, should be briefly summarised. The authors have used National Meteorological Center sea-level pressure data set, which, since 1979, have incorporated data from arrays of drifting buoys from the Arctic Ocean Buoy Program. For winter Serreze et al. (1993) received results similar to those from most of the other works cited here, including Atlas Arktiki (1985). In summer, a much greater difference between these sources exists, although the general pattern is also quite similar. This concerns, in particular, the location of anticyclone pressure centres. In contrast to the map published in Atlas Arktiki (Figure 2.2c), Serreze et al. (1993) found the lack of a mean summer low both in the central Arctic and in the eastern part of the Canadian Arctic, although a decrease in sea-level pressure is evident in their map.

Figure 2.2. Spatial distribution of average air pressure for .lanuary (a), April (b), July (c). and October (d) in the Arctic (after Alias Arktiki 1985 (January and July) and Gorshkov 1980 (April and October)).

APRIL

GREENLAND

BanW.V

'ELLESMERE1

ICELAND

OSTROV\/l

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iEVERNAYA ZEMIYA, \

ioio—Isobars (hPa)

L - cenlte ot low pressure

Artsnsngel sk

1Qp ttO" 120" (30* HO" 150" ISO' 170* 1«0"

1Qp ttO" 120" (30* HO" 150" ISO' 170* 1«0"

The winter pressure field consists of two main belts. The first one, with low air pressure, encompasses the entire Atlantic sector of the Arctic up to the North Pole (Figure 2.2a), and is mainly controlled by the dynamic of the Icelandic low. Near Iceland on the Arctic front cyclones are generated, which then move into the Arctic, reaching as far as the Kara Sea. As a consequence of this process, in the mean pressure field, the so-called Iceland-Kara Sea trough can be seen. Another extensive trough covers the Baffin Bay region. The second belt with high air pressure encompasses almost all other parts of the Arctic, excluding the Bering Sea and Bering Strait regions. In this part of the Arctic, halfway between the Siberian high and the Canadian high, a small centre of high pressure (> 1021 hPa) is present (Figure 2.2a),

In spring (Figure 2.2b), represented by the mean pressure field for April, high pressure may be found to dominate in the whole Arctic, with the maximum (> 1020 hPa) in the northern parts of the Canadian Arctic and Greenland, in the Beaufort Sea, and in the part of the Arctic Ocean neighbouring these areas. The lowest air pressure (< 1012 hPa) is only over the Norwegian and Barents seas. One can agree with the assertion made by Vowinckel and Orvig (1970) that in spring the anticyclonic activity occurs most often over the central Arctic and that in this season the old concept of "Arctic anticyclone" is closest to being fulfilled.

The summer pressure field (Figure 2.2c) shows two centres of high pressure: the first one covers the central part of the Atlantic region (from Novaya Zemlya to Jan Mayen Island), and the second is located over the Beaufort Sea, Alaska, and the MacKenzie Basin. The data for Greenland arc not presented on this chart. However, there is some evidence that in the northern part of this island a third centre of high pressure exists (see Serreze et at. 1993, their Figure 9) A small low-pressure centre (< 1010 hPa) in the vicinity of the North Pole separates the first two high-pressure centres. A trough of low pressure spreads from here to the eastern part of (he Canadian Arctic (where a clear low-pressure centre, < 1008 hPa, exists) on the one hand, and to the central part of the continental Russian Arctic on the other. It is worth noting that the differentiation of air pressure in summer in the Arctic is significantly lower than in other seasons, especially winter. According to Serreze et at. (1993), this may be due to "a more even distribution of cyclonic activity than observed in winter, the general lack of spatial variations in mean cyclone and anticyclonc pressures, as well as the tendency in other regions for alternation between cyclonic and anticyclonic regimes."

In autumn (Figure 2.2d), the pattern of air-pressure distribution in the Arctic is quite similar to that of winter. However, neither high- nor low-pressure centres are as strong as in winter. On the other hand, the area covered by the Iceland-Kara Sea trough in autumn is greater than in winter and reaches the vicinity of Severnaya Zemlya. The same is true of the low pressure occurring in the region of the Baffin Bay and Pacific sectors of the Arctic.

One can see that the charts do not present the distribution of atmospheric pressure over Greenland. However, from other sources (e.g. Prik 1959; Rigor and Heiberg 1997) wc can say that over the year as a whole there is the occurrence of a semi-permanent high-pressure centre (or at least a wedge of high pressure).

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