Atmospheric Precipitation 711 Annual Cycle of Precipitation

The amount of precipitation over any area depends on the moisture content of the air, the pattern of synoptic scale weather systems affecting the area, and the topography and the character of the underlying surface. The moisture contcnt of the air can be described using the concept of precipitable water. Precipitable water is defined as the "depth to which liquid water would stand if all the water vapour in a vertical column of uniform cross-section, extending from the earth's surface to the top of the atmosphere, were condensed" (Maxwell 1980). Serrcze et al. (1994b) published maps presenting the fields of precipitable water above the Arctic for the surface to 300-hPa layer for January and July (Figures 7.1a and b, respectively).

Precipitable water vapour in January is highest (4-6 mm) in the southernmost part of the Atlantic region, dropping to about 2 mm over the central Arctic Ocean. Serreze et al. (1994b) connected this with the spatial distribution of troposphere temperatures. In July, precipitable water reaches its annual maximum, with values ranging from about 12 mm over the northern part of Baffin Bay and 12-13 mm over the central Arctic Ocean to 15-19 mm in the southernmost parts of the Arctic. A clear zonal distribution may be observed in this month, which reflects the zonal tropospheric temperature pattern in summer. Other factors influencing the precipitation in the Arctic have been described in Chapters 1, 2, and 6.

Figure 7.1. Fields of prccipitable water for the surface to 300-hPa layer (mm) for January (a) and July (b) (after Serreze et at. 1994b).

In most climatological handbooks describing the annual cycle of precipitation in the Arctic, it is usually maintained that the precipitation is at its highest in summer and lowest in winter. This statement is in agreement with what has already been said about the moisture content in the atmosphere. However, it does not take into account other factors, which in some areas of the Arctic can significantly change the "picture". The most important of these is, of course, the synoptic scale atmospheric circulation. The inspection of the monthly totals of atmospheric precipitation in selected stations representing all climatic regions in the Arctic (Figure 7.2) reveals the existence of at least two main types of the annual courses. The first type is characterised by the highest precipitation occurring mainly in the autumn months, when the temperature is still relatively warm (particularly in September and October) and cyclonic activity is only slightly lower than in winter, and the lowest is in spring when the anticyclonic activity is greatest (Scrreze et al. 1993). Such annual cycles of precipitation occur in the Arctic areas, where atmospheric circulation is strongest (Atlantic, Pacific and Baffin Bay region). This is particularly evident in the data from .Ian Mayen (Figure 7.2b), Mys Shmidta (Figure 7.2g), and Clyde A (Figure 7.2j). The parts of the Arctic with the most continental climate (the Canadian and Siberian regions) show a maximum of precipitation in summer and minimum in winter. In these areas, precipitation depends mainly on air temperature, which determines the magnitude of evaporation and the upper limit of the air's water vapour capacity. This type of annual cycle is clearest in the stations Ostrov Kotelny (Figure 7.2f), Resolute A (Figure 7.2h), and Coral Harbour A (Figure 7.2i). In the central Arclic Ocean the highest precipitation occurs in summer, but the lowest is in spring (see Table 14 in Radionov et al. 1997). The amount of precipitation in the entire Arctic is significantly higher in the second half of the year (60-70% of the annua! total).

7.1.2 Spatial Patterns

The precipitation amounts presented for the Arctic in its entirety (except for the inner part of Greenland) come from meteorological stations located on the seacoast below 200-m a.s.l. As has already been shown from measurements carried out on Spitsbergen (e.g., Kosiba 1960; Baranowski 1968; Markin 1975; Marciniak and Przybylak 1985), summer precipitation on the glaciers (200-400 m a.s.l.) is two to three times greater than that measured on the tundra. The mean summer and annual vertical gradients were estimated to be about 35^40 mm/100 m and 80 mm/100 m, respectively (Markin 1975). For this reason, the snow accumulation measurements on the glaciers or on ice caps cannot be used to correct measurements of precipitation on the coastal stations. Some authors compare the results of snow accumulation on glaciated areas with the neighbouring meteorological stations to estimate the magnitude of the impact of wind upon gauge collection of precipitation (see e.g. Bromwich and Robasky 1993).

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Figure 7.2. Annual course of atmospheric precipitation (according to mean monthly totals) in selected Arctic stations, 1961 -1990 (after Przybylak 1996b). a - Danmarkshavn, b - Jan Mayen, c - Malye Karmakuly, d - Polar GMO E.T. lCrenkelya, e - Ostrov Dikson, f- Ostrov Kotelny, g - Mys Shmidta, h - Resolute A, i - Coral Harbour A, j - Clyde A, k - Egedesminde.

Figure 7.3. Spatial distribution of (lie annual totals of atmospheric precipitation (in mm) in the Arctic, 1951-1990 (after Przybylak 1996a).

Low air temperature and low atmospheric moisture content in the Arctic significantly limit the values of precipitation. Mean annual totals of precipitation from the period 1951-1990 (Figure 7.3) over almost the entire Arctic (exccpt the southernmost fragments of the Atlantic and Baffin Bay regions) do not exceed 400 mm. The lowest precipitation amounts occur in the coldest part of the Arctic (the northern part of the Canadian Arctic Archipelago above 77°N and the Arctic Ocean from the Canadian side), where they are less than 100 mm. The rest of the Arctic Ocean, the central part of the Siberian region, and the northern part of the Canadian Arctic (70-77°N) also have low precipitation (< 200 mm). Anticyclonic activity prevails over the above areas throughout the entire year (Serreze et al. 1993). The highest annual precipitation totals (> 500 mm), on the other hand, occur in the warmest areas of the Arctic, which are also characterised by the most intense cyclonic activity (the southernmost fragments of the Atlantic and Baffin Bay regions, and southern coastal parts of Greenland). Particularly high totals (> 2000 mm) are observed on the south headland of Greenland in the vicinity of the Prins Christian Sund station. The mean annual precipitation sum in this station computed from the period 1951-1980 is 2451.4 mm (Przybylak 1996a). Details aboul the great precipitation in this region have been omitted from very well known Russian atlases (Gorshkov 1980; Atlas Arktiki 1985), which give the annual sum as about 1200 mm. Only Ohmura and Reeh (1991) and Ohmura et al. (1999) show in detail on their maps this area of high precipitation (see Figure 7.4). The reason for such great sums of precipitation is twofold: 1) the high frequency of cyclonic activity, and 2) the orographically forced ascent of air masses crossing the Greenland Ice Sheet, which reach an altitude above 2000 m a.s.l. very near the coast (170-180 km from Prins Christian Sund) (Przybylak 1996a). Ohmura and Reeh (1991) note that the south-eastern coast of Greenland is directly hit by the onshore flow from the northern part of the Icelandic low, with a relatively high water-vapour content of 2.1 g/m\

100 90 80 TO 60 50 40 30 2010 0 10 100 90 80 70 6D 50 40 30 20 10 0 10
Figure 7.5. Monthly resultant wind streamlines at 850 hPa for (a) January and (b) July (after Ohmura and Reeh 1991).

In comparison with earlier maps of precipitation in Greenland (Diamond 1958, 1960; Bader 1961; Benson 1962; Mock 1967; Barry and Kiladis 1982), more recent maps (Ohmura and Reeh 1991, Ohmura et al. 1999) were constructed using not only glaciologica! data, but also meteorological data. The merging of these two data sets significantly improved our knowledge about precipitation in this part of the world. There is good agreement between Ohmura and Reeh's, and Przybylak's results on precipitation in the coastal parts of Greenland (compare Figure 7.3 and Figure 7,4), The lowest annual totals of precipitation in Greenland occur in its north-eastern part (< 100 mm), which topographically represents the north-eastern slope of the ice-sheet. Computing the monthly resultant wind for January and July for the level of 850 hPa over Greenland (Figure 7.5), Ohmura and Reeh (1991) found that the area in question in both seasons remains in a precipitation shadow, both with respect to the southwesterlies and the westerlies. Generally. there is higher precipitation {> 500 mm) on the slopes of the Greenland Ice Sheet, which are well exposed to the main wind streamlines (south-east ern, southern, and south-western). As was previously mentioned, the greatest annual totals (> 2000 mm) are observed in the south headland of Greenland (Figure 7.4).

Figure 7.6. Spatial distribution of the winter (DJF), spring (MAM), summer (JJA), and autumn (SON) atmospheric precipitation (in mm) in the Arctic, 1951 1990 (after Przybylak 1996a).

The amounts of seasonal precipitation are presented in Figure 7.6. The lowest sums of precipitation occur in spring. This minimum should be rather connected with the maximum frequency of anticyclones occurring clearly in this season than with the air temperature, the lowest values of which are reached in winter. In addition, the fact that sea ice in this season is near its maximum extent is also important. As a result, the available moisture is at a minimum. Winter precipitation sums are slightly higher than those of spring, but their spatial patterns are very similar. Spring totals of lower than 50 mm occur in about 70% of the Arctic (the Arctic Ocean, the Siberian region, almost the entire Pacific region, and the northern part of the Canadian Arctic). Precipitation of above 100 mm falls only in the south-western part of the Atlantic region and in the southern part of the Baffin Bay region. The highest seasonal mean totals of precipitation occur in summer (excluding the western and southern fragments of the Atlantic region). This maximum one can relate to the highest temperature, water vapour content and cloudiness in this season. Summer totals of precipitation below 50 mm arc observed only in the area spreading from the central part of the Siberian region to the north-eastern part of the Canadian Arctic, and on the north-eastern coast of Greenland and the surrounding Greenland Sea. Amounts of precipitation exceeding 100 mm fall only in southernmost parts of the Atlantic and Canadian regions, with highest values (> 400 mm) occurring on the southern headland of Greenland (Figure 7.6). For the inner part of Greenland, no maps exist which present seasonal precipitation totals. However, data for January and July (Atlas Arktiki 1985) arc available. The spatial patterns of precipitation in both months are similar to those based on the annual totals (Figure 7.4), i.e., the lowest values occur in the north-eastern part, and the highest in the southern and western parts of Greenland. In January, the amounts of precipitation are generally lower than in July in the northern and central parts, and are higher in the southern part of Greenland. As a result, spatial differentiation in January is greater and ranges from 5 mm (north-eastern part) to about 100 mm (southern part), while in July it ranges from 10 mm to only 75 mm, respectively.

Generally speaking, the spatial patterns of precipitation in the Arctic, both seasonal and annual, have a zonal course, i.e. a poleward decrease in precipitation is noted. The greatest disagreements with this rule occur in the areas where climate is mainly formed by very intense atmospheric circulation.

Przybylak (1996a) found, analysing the highest and lowest seasonal and annual totals of precipitation during the period 1951-1990, that in the majority of stations (64%) the maximal annual sums occurred in the coldest decades (1961-1970 and 1971-1980). In the case of the seasonal totals the results were the same, but only for the occurrence of the lowest seasonal sums of precipitation. The highest ones (except for autumn) with a similar frequency occurred in all four decades. Only the highest totals of autumn precipitation occurred more often during the warmest decadcs. It is a rather surprising result in the contcxt of the predictions presented by the climatic models (see Section 11.2). The highest annual total of precipitation in the Arctic occurred in Prins Christian Sund in 1965 (3299 mm). On the other hand, the lowest were noted in Ostrov Chetyrekhstolbovoy (25 mm, 1988) and Eureka (31 mm, 1956). The lowest seasonal sums of precipitation during the period studied ranged from 1 mm (Resolute A, winler) to 4 mm (Danmarkshavn, autumn).

Figure 7.7 presents 6 regions of coherent annual totals of precipitation in the Arctic. The Wroclaw dendrite method was used to delimit these regions. One can see that regions 1, 2, and 3 consist of two separated parts.

This means that teleeonnections of precipitation occur in the Arctic. Przybylak (1997c) also found that these are more common than teleconnections of air temperature.

Figure 7.7. Regions of coherent annual totals of precipitation in the Arctic, I95i -1990 (after Przybylak 1997c).

Figure 7.7. Regions of coherent annual totals of precipitation in the Arctic, I95i -1990 (after Przybylak 1997c).

The range of variability of both seasonal and annual totals of precipitation is very high in the Arctic. The ratio of the highest to the lowest sums of precipitation is greatest in the coldest areas of the Arctic, where anticyclones dominate (the northern part of the Canadian Arctic and Siberian region). Przybylak (1996a) computed the variability coefficient (v) for all Arctic stations from the formula v = <j / m, where o is a standard deviation and m is a mean value. These coefficients were computed for the annual and seasonal sums of precipitation over the period 1951-1990 (Figures 7.8 and 7.9). The highest variability of the annual sums (> 30%) occurs in the Arctic Ocean from the Pacific side, in the Pacific region, the eastern part of the Siberian region, the northern part of the Baffin Bay region, and the north-eastern coast of Greenland. A large variability (about 30%) is also characteristic of the area lying between Zemlya Frantsa Josifa and Severnaya Zemlya. The above-mentioned areas have the lowest amounts of precipitation in the Arctic. The annual totals of precipitation have the lowest variability (< 20%) in the southernmost parts of the Atlantic region, where the highest precipitation and the greatest occurrence of cyclones are noted.

Figure 7.8. Spatial distribution of the coefficient of variability in annual precipitation in the Arctic, 1951-1990 (after Przybylak 1996a).

The dispersion of seasonal precipitation sums is significantly greater than in the case of the annual ones. The highest coefficients of variability occur in the seasons with the lowest precipitation, i.e. in winter and spring. Their values exceed 50% in almost the entire Arctic (Figure 7.9). The spatial differentiation of variability is higher in spring, ranging from about 100% in Alaska to 30-35% in the southernmost areas of the Atlantic region. The highest values of v of winter precipitation totals do not exceed 70%, and occurred in isolated areas located in different parts of the Arctic having a continental climate. On the other hand, their lowest values are observed in the same areas as in the case of spring, but are slightly higher. The variability of summer and autumn precipitation totals is significantly lower than in previous seasons (Figure 7.9), the v ranging mainly between 30 and 50%. Similar to the case of the annual sums, (lie variability of seasonal precipitation is highest in the areas with the lowest precipitation. This means that areas with low amounts of precipitation are more sensitive to changes in factors determining the precipitation.

Figure 7.9. Spatial distribution of the coefficient of variability in winter (DJF), spring (MAM), summer (J.IA), and autumn (SON) precipitation in the Arctic, 1951 1990 (after Przybylak 1996a).

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