Longterm Means and Extreme Values

Low T and low water vapour content in the Arctic air result in low P. Its mean annual totals between 1951 and 1990 throughout the whole of the Arctic (except the southern parts of ATLR and BAFR) do not exceed 400 mm (Table 5.1, Figure 6.1). They are lowest in the coolest part of the Arctic, which is the northeastern part of CANR (< 100 mm). They are also low (< 200 mm) over the Arctic Ocean, in the central part of SIBR, and in the north of CANSRn - regions with clearly dominating anticyclone systems (Serreze et al. 1993). The highest annual totals are in the warmest areas, which are dominated by intensive cyclone activity. They are particularly high (> 2000 mm) in the small area of the south headland of Greenland near the Prins Christian Sund station. This is caused both by frequent cyclones in the area, as well as by the Greenland ice sheet, which has an elevation of more than 2000 m ca. 170-180 km north of the station and forces the air mass to rise. Similar results and explanations for spatial distribution in the Arctic have been presented by Burova (1983), the authors of Atlas Arktiki (1985), and Przybylak (1996a).

It is interesting to compare seasonal P totals (Figure 6.2). In most of the Arctic area the lowest P occurs in spring, which should be connected more with a clear annual maximum of anticyclone frequency in this season of the year than with T, which is lowest in winter. The winter P is slightly higher than the spring P, and their distributions are similar (Figure 6.2). In spring P < 50 mm occurs in the north of ATLR, in SIBR, PACR, CANSRn and IARCR (ca. 70% of the Arctic). P higher than 100 mm occurs only in the southwestern part of ATLSRs and in the south of BAFR. The highest seasonal P totals in the Arctic occur in summer, except in one region dominated by intensive cyclone activity, which is ATLR, excluding its northern and eastern parts. Obviously, this should be connected with the highest values of water vapour content in the air, cloud cover, and T which were observed at that time. In summer P < 50 mm is observed only in the small area connecting the central part of SIBR with the northeastern part of CANR. It falls below 100 mm in the northern parts of ATLR and CANR as well as in SIBR, PACR, and IARCR (ca. 70% of the Arctic). The highest P (> 200 mm) occurs on the south and southeastern coasts of Greenland, with a maximum exceeding 400 mm (Figure 6.2).

Figure 6.1. The spatial distribution of annual P (in mm) in the Arctic, 1951-1990.

Figure 6.1. The spatial distribution of annual P (in mm) in the Arctic, 1951-1990.

According to an analysis carried out by Przybylak (1996a) of the annual course of P between 1961 and 1990 based on mean monthly totals, in the areas exposed to the strong influence of atmospheric circulation (ATLR, PACR, and BAFR) the maximum P falls in one of the autumn months, when the intensity of circulation is slightly lower than in winter, while T is much higher. However, the minimum occurs in spring as a result of strong anticyclones (Serreze et al. 1993). In the rest of the Arctic, characterised by the most continental climate, P has a typical annual course, reaching a maximum in the summer months and a minimum in the winter months. According to Okolowicz's typology (1969), this type of climate is called 'polar', whereas the former is called 'oceanic-advectional'.

Figure 6.2. The spatial distribution of winter, spring, summer, and autumn P (in mm) in the Arctic, 1951-1990.

The range of the spatial variability of P is broadest in winter and narrowest in summer. It is also much wider in ATLR and BAFR (which are exposed to intensive cyclone activity throughout the year) than in the remaining area of the Arctic.

Spatial distributions of both seasonal and annual P are roughly connected with zones, i.e. they usually get smaller as the latitude gets higher. The most substantial exceptions to the rule occur in the areas whose climate is shaped by advections of warm and humid masses of air from the south.

Figure 6.3 presents the changes of mean 10-year P total anomalies in the period 1951-1990. In the warmest decade in the Arctic (1951-1960) negative anomalies occurred in around half of the area. They occurred mainly in ATLR, CANR, and BAFR, with a maximum exceeding 100 mm on the east coast of Greenland. A continuous area ofpositive anomalies extends from Novaya Zemlya to Alaska and covers ATLSRe, SIBR, PACR, and most of IARCR. Two smaller areas are located in the Greenland Sea from Jan Mayen to the northeastern part of Greenland, and in the central part of BAFR. In the coolest decade (1961-

1970) a substantial continuous area of positive anomalies (except for a small part around the Ostrov Dikson station) covers a larger part of the Arctic than in the previous decade. The border of the anomalies was shifted more westwards in the Russian Arctic and in the IARCR. Positive P anomalies in this decade were also observed in the south of BAFR and in the southeast of ATLSRs. The highest values (> +60 mm) were observed at the west and east ends of the Russian Arctic and on the southwestern coast of Greenland (Figure 6.3). The greatest negative anomalies (< -80 mm) were observed in the area between Bear Island and Jan Mayen. In the period 1971-1980, which was characterised by average thermal conditions, P was below the norm throughout most of the Arctic area. Positive anomalies were observed only in the central part of ATLR, in the west of CANSRn, and in BAFR (Figure 6.3). However, it should be noted that in this decade the spatial variability of the anomalies is the lowest. Annual distribution of anomalies in the 1980s is clearly the opposite of their distribution in between 1951 and 1960, in spite of the fact that in terms of T the decades were

Figure 6.3. The spatial distribution of the anomalies of mean annual 10-year P (in mm) with reference to the 1951-1990 mean in the Arctic. Negative anomalies are hatched.

not significantly different. It is difficult to explain such behaviour of P. The reason might be the fact that the warming in the period 1951-1960 was definitely caused to a greater extent by natural factors of climate change, whereas the warming of the last decade was caused more by human activity.

P below the norm was observed in most of the Arctic in winter and summer (Figure 6.4). In winter, positive anomalies were observed only in the central part of ATLR and in small parts of CANR and BAFR, whereas in summer they occurred in bigger parts of CANR and BAFR and in a small part of southwestern ATLSRs. Negative P anomalies also dominated in the Arctic in the other seasons of the year. Their lowest values and spatial distribution are observed in spring (Figure 6.4).

Figure 6.4. The spatial distribution of the anomalies of mean winter, spring, summer, and autumn P in the 1980s with reference to the 1951-1990 means in the Arctic. Negative anomalies are hatched.

The results are quite surprising, as it is commonly assumed that P should increase along with the warming of the Arctic. Such forecasts were also obtained when using climatic models (IPCC 1990, 1992). However, Table 5.1

and Figures 6.3 and 6.4 present a clearly contrary relationship. It is worth remembering, however, that T variability in the Arctic during the period ofob-servations was not considerable. For that reason the decision was made to check the relationship between P and T for the period characterised by the greatest warming in the Arctic during the last 100-150 years. As has been mentioned in Chapter 5, such a period occurred from the 1920s to the 1940s. However, such a check can be conducted for a much smaller number of stations (Table 6.1). Having analysed Tables 6.1 and 5.5 we can state that in the majority of the stations which were characterised by positive T anomalies, negative P anomalies were observed at the same time. P was above the norm in the only station where a negative T anomaly was recorded in the 1930s (Coppermine).

Table 6.1. Seasonal and annual anomalies of 10-year P (in mm) in selected Arctic stations with the longest data series

Station

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