2. Description of the climatic conditions of the Russian Arctic
The border separating the Russian Arctic from the subarctic regions extends along the southern coast of the Barents Sea, across the northern part of the Ural mountains, across the southern part of the Gulfs of the Ob and Yenisey rivers, and includes the total area of the Taimyr Peninsula. From there the boundary runs eastward along the arctic coast, roughly along 70 degrees North latitude and passes south-east across the Chukchi Peninsula to the Bering Sea [1, 49]. The area located the north of this border includes land areas including expanses of barren tundra over permafrost, and the peripheral seas of the Arctic Ocean covered by sea ice (see Fig. 1). Most of the islands and archipelagoes in the Arctic seas are covered with ground ice [1, 15].
A large part of the Arctic is an ocean covered by ice. Astronomical conditions contribute to the formation of a polar climate, whereas the presence of the ocean defines this as a marine climate at the same time. The warming influence of the ocean in winter is decreased to some extent, because the seas are covered by ice. In the polynyas and leads that form between the ice floes, however, the heat exchange during the cold part of the year is extremely intense due to large difference between the water temperature (-1.7 degrees Celsius) and the air temperature (about -30 degrees Celsius). During the warm part of the year, the cooling influence in marine areas is extremely strong because the ice absorbs such a large amount of heat during melting that it is not always able to melt completely during the summer.
A distinguishing characteristic of the radiation and illumination conditions in a region is the alternating of polar night and polar day each year each covering an equal time interval (see Table 3). Incident solar radiation, absent during polar night, increases steeply when polar night ends, and reaches a maximum in May or June [1, 8]. This is the primary driver of the seasonal change in the basic meteorological elements as well as the atmospheric phenomena including HMPs. For example, high winds, snowstorms, low air temperatures, and snowfall are observed essentially within the long cold season, whereas fog, rain, hail and thunderstorms are observed during the short warm season.
The yearly amount of incident solar heating and the characteristics of the surface are considered to be relatively stable factors. They give rise to the severity of the present climate of the regions under study, however, the basic factor that gives rise to the fluctuations in meteorological conditions and drives them up to hazardous levels is atmospheric circulation. Many natural phenomena occur due to the passage of cyclones. All of the following determine the weather conditions in the Arctic: specific properties of individual air masses, air mass trajectories, air mass transformation under the influence of the properties (e.g. water, ice, ground) and orography of the underlying surface, as well as interactions between air masses with different properties. In the Arctic, the prevalent air mass is characterized by low temperatures and small moisture content. The continental air of the subarctic in winter is significantly cooler and dryer than arctic marine air.
During the warm part of the year, arctic air masses are formed exclusively over the arctic basin. When an arctic air mass moves to the south, the lower layers heat up and the stratification of the lower atmosphere is unstable.
Midlatitude air masses, which penetrate to the Arctic in cyclones, have different properties depending on where they formed. The properties of continental winter air from midlatitudes (continental polar) are close to those of the Arctic.
Air masses from temperature latitudes that reach the Arctic are cooled at the bottom and take on inversion stratification. Cooling is accompanied by condensation processes including fogs, icing, riming, stratus cloud formation, and precipitation. These air masses have been observed to penetrate into the Arctic right up to the North Pole.
In winter (January) the polar front between masses of polar air and midlatitude air is located, on average, between 60 degrees and 70 degrees North. In summer the boundary of the polar front is parallel to that in winter, but it is located about 5-10 degrees to the north basically along the continental margin. The winter distribution of pressure fields in the Arctic is characterized by the intensive development of three climatic action centers: the Iceland and Aleutian lows, which reach maximum levels of activity in winter, and the Asian winter anticyclone, an extension of which is directed north-eastward toward the East Siberian and Chukchi seas [54, 68].
Large contrasts between the air temperature and that of the underlying surface in the northern parts of the Atlantic and Pacific oceans are caused by the close proximity of warm and cold currents, intensifying the arctic front and giving rise to the development of intensive cyclone activity in these regions. The power of the extension of the Asian pressure maximum, whose origin is caused by both thermal and dynamical processes, is amplified by orographic factors, in particular the stagnation of cold air in the valleys and basins of Siberia. As a result, a stable secondary center of high pressure forms producing a direct effect on the climatic conditions of the Arctic. The average pressure over the Arctic from October to March is characterized by the development of Icelandic lows that reach the Barents and Kara seas [1, 54]. The deepest and most extensive low is directed from Iceland toward the northeast and east to the Arctic seas. This is confirmed by the trajectories of cyclones that are directed to these areas.
The clearly defined Aleutian minimum trough is directed to the north of the Chukchi Sea and into the arctic basin. In the winter, the arctic region where pressure increases is described as a "bulkhead" and extends from eastern Siberia across the arctic basin to Canada.
The maximum frequency of winter cyclones in the Eurasian Arctic is recorded in the western region (the Barents and Kara Seas). The mean monthly number of cyclones that pass through the southern Barents Sea in winter is six, and through the southern Kara Sea is five. In the eastern region the frequency of cyclones is significantly less. Some cyclones reach there that have formed or regenerated in the northern area of the Pacific Ocean in the region of the Aleutian Low. Cyclones move across the Bering Strait to the Chukchi Sea, and from there most continue to northern Alaska. A few move westward and north-westward across the Chukchi Peninsula. Some cyclones formed in the Pacific come out through the lower course of the Kolyma River into the East Siberian Sea. On average, four cyclones pass through the southern East Siberian Sea and the lower course of Kolyma river in each of winter months. These storms sometimes cause rapid warming and cause melting even in the middle of winter. The well-known local storm wind in Pevek (called the "yuzhak") is due to such cyclones emerging from the lower reaches of Kolyma river.
The frequency of anticyclones in the western area of the Arctic during this season is quite low: one anticyclone per month. Anticyclones can cause fog and low air temperatures. These anticyclones move mainly from the circumpolar regions through the eastern parts of the Barents and Kara seas. Some of them also move into the Barents Sea from the northern coast of Greenland.
Eastern Siberia,where strong cooling occurs, is a center of frequent anticyclones in winter. This is where the lowest air temperatures in the northern hemisphere are recorded. On average, 5 to 10 anticyclones are recorded there each a month.
April and May are typically spring months. The winter circulation regime is disrupted. The pressure gradients in the Arctic decrease. The eastern trough of the Icelandic Low decreases in April and disappears completely in May, but its circumpolar branch is still clearly evident in April. The Aleutian depression also decreases and fills up to a large extent. The Siberian High disappears and in May is replaced by a wide but shallow depression. The Arctic anticyclone remains located over the eastern part of the Arctic basin.
Northern Atlantic cyclones move along trajectories close to those occurring in winter. Pacific cyclones, for the most part, circle eastern Siberia, but do not cross it. The frequency of cyclones decreases everywhere, and their trajectories approach those occurring in summer. The frequency of large anticyclones also decreases everywhere, except in the Arctic basin.
The summer distribution of air pressure is fundamentally different than the winter distribution. Source regions for cyclone circulation are filled up and shrink due to the decrease in temperature contrasts over the northern parts of the Atlantic and Pacific oceans. The number of cyclones moving to the arctic seas from the Northern Atlantic does not exceed one or two in July and August, but the frequency of cyclones moving from north of Greenland to the pole increases, and a source region of a shallow lows forms. This is a result of cyclones moving into the Arctic from midlatitudes (Siberia and Canada). However, this type of cyclone can develop and on occasion even form there.
Cyclones develop over the Arctic coast and to the south due to significant contrasts in air temperature resulting from the flow of cold air masses from the Arctic to heated continent. Cyclones move primarily toward the east and northeast to the arctic seas, especially in the east. In this case, the average number of cyclones in the Chukchi Sea in July can be as high as six, and in the East Siberian Sea as much as four to five. In the other seas the frequency of cyclones is less, typically two to three per month.
The number of anticyclones in the western regions is less in summer (one per month) than in spring. Anticyclones are observed most frequently in the Barents Sea in summer (approximately 3 anticyclones per month). A second source region of more frequent anticyclone activity is located to the north of the East Siberian Sea, but the average air pressure in this region does not exceed 1016 millibars.
In September, the summer character of barometric pressure field in the Arctic breaks down and by October the pattern is close to the winter configuration. The isolated summer depression in the Arctic basin disappears, and increasing pressure is expected. In the north Atlantic, the Icelandic Low begins to develop. In September over the Barents Sea and in October over the Kara Sea, smoothly developing minima extend out and ultimately connect to the vigorously developing Icelandic Low. Over the Chukchi Sea, the Aleutian Low begins to develop, and in October a fully formed trough of low pressure has already formed. The pressure increases over the continent. That pattern of pressure fields in autumn is caused by the increase in cyclone activity in the Northern Atlantic and in the northern Pacific Ocean and also by the prevalence of anticyclones in Siberia and Canada.
The number of cyclones in autumn in the south eastern Barents Sea reaches five in October, compared with four in the southern Kara Sea. The frequency of anticyclones in the western seas decreases to one per month. In October, cyclones seldom form over eastern Siberia (two is typical), but the number of anticyclones increase to three per month. In the eastern seas, where cyclones primarily come from across western Siberia and from the east through the Bering Strait, the frequency increases to four or five per month, while the number of anticyclones decreases to one per month. The maximum frequency of anticyclones is observed in October in the eastern part of the arctic basin (four per month). However, the intensity of the pressure system is less in autumn than in winter.
Cyclones and anticyclones propagate quickly, increasing the influence they can have as HMP. Of particular concern is their duration and intensity. Cyclone and anticyclone speeds have a clear annual pattern. Maximum speeds are observed in late winter and minimum speeds occur in the latter half of summer. Displacement speeds of mobile anticyclones throughout the year are slightly higher than cyclones speeds. The average speed of cyclones in March is 40 kilometers per hour, and 35 kilometers per hour in summer. The average speed of mobile anticyclones (excluding stationary anticyclones) is 43 kilometers per hour in March, decreasing to 36 kilometers per hour in summer. The maximum speeds of winter cyclones are 120-125 kilometers per hour versus 130-135 kilometers per hour for anticyclones. The smaller temperature contrasts, weaker pressure gradients and slower displacing speed of the pressure systems show that the intensity of summer cyclones is significantly weaker than that of winter systems.
Arctic climatic conditions are briefly characterized on the basis of observational data mostly for the period 1936 to 1980, and for some elements up to 1990. To describe the Arctic climate further we use the following names of the seasons: winter, spring, summer and autumn. Note that the duration of the seasons are not equal to those commonly used at midlatitudes. Moreover, even in the context of the Arctic the lengths of the seasons can be defined only approximately. The dates and duration periods of the seasons appropriate for the Arctic are presented in Table 4, and are based on our observations of the annual changes of various meteorological parameters. They are consistent for the entire region under consideration. May and June are considered to be spring, July and August constitute the summer, September and October the autumn, and November through April the winter.
Winter:
The duration of winter is more than half the year. The salient characteristics for this period are low air temperatures and high winds with snowstorms. Over the coastal arctic seas (except for the Chukchi Sea) the prevailing airflow is caused by greater pressure over the continent than over the adjacent sea, and the direction of flow is from the northern areas of the continent to the arctic seas.
During the winter months changes in the character of the thermal fields are very slight. Values vary in phase with one another. At the beginning of winter, the temperature decreases and then stabilizes until the end of the season. It increases somewhat in April.
In January, the air temperature is -20 degrees Celsius in the northern Barents Sea and is -14 to -16 degrees Celsius on the coast of Novaya Zemlya. Air temperatures decrease over the Kara Sea in January to -18 to -20 degrees Celsius in the southwestern area, down to -30 to -32 degrees Celsius in the northeastern area.
To a much lesser extent, though clearly evident nonetheless, the heating effects of Pacific cyclones and the warm sea current penetrating into the Chukchi Sea are observed in this area. In this area the January air temperature is about -22 to -24 degrees Celsius. The lowest winter temperatures are observed over the Laptev Sea, where the average monthly air temperature stays nearly constant at about -32 to -33 degrees Celsius.
In the Arctic, clouds are present for almost the entire year. The skies are overcast about four to six times more often than they are clear. Most often, the sky is covered by low clouds (Stratus and Stratocumulus).
In winter the maximum precipitation occurs in the Barents Sea. In the southwestern Barents Sea during the winter months, the mean amount of precipitation is 40 millimeters per month. In the northeastern Barents Sea and the southwestern Kara Sea the amount of precipitation decreases to 20 millimeters per month. In the northern Kara Sea the winter monthly totals for precipitation range from 12 to 15 millimeters, in the Laptev Sea they are 10 millimeters, and in the eastern area of the East Siberian Sea and in the Chukchi Sea precipitation is 30 millimeters per month.
In winter there is very little snowfall. Accumulation rates are for the most part less than 0.03 millimeters per hour. Occasionally snowfall rates of 0.2 millimeters per hour are observed, but heavy snowfall (0.7 millimeters per hour) is very rare. Partial pressure of water vapor is small throughout the year, especially in winter. From December through March, the average monthly value ranges from 0.4 to 0.8 millibar in the northeastern Kara Sea and in the Laptev and East Siberian seas. In the warmer seas (the Barents Sea, the southwestern Kara Sea, and the Chukchi Sea) the humidity increases to 1.5 or 2.0 millibars. Changes in the atmospheric circulation in late winter cause a sudden change in the wind patterns from the winter regime to the spring regime. In April in the western seas, the prevailing winds are from the south, characteristic of winter. In the northern parts of the Laptev and East Siberian seas, easterly winds tend to prevail.
Spring:
In May, the wind regime is close to the summer pattern everywhere. Along the coasts, just as in winter, there is a significant tendency of the wind to deviate from geostrophic flow due to the influence of local orographic conditions. In May, the degree of cloudiness increases strongly over the entire Arctic.
In spring, the air temperature starts to increase rapidly, most quickly in the continental regions, because the continent is heated more intensely than the seas are. The highest temperature (about 0 degrees Celsius) is observed in the southwestern Barents Sea, and the lowest values (-24 to -25 degrees Celsius) are observed in the center of the Arctic basin.
In May air temperature remains negative, but the character of the spatial temperature field approaches that of summer. The lowest temperature (-12, -13 degrees Celsius) is observed in the area of the pole, and the highest temperature is recorded in the southern Arctic regions. In the southeastern Barents Sea and in the southeastern Kara Sea, the air temperature is -3 to -6degreesC. Along the coast of the Laptev and East Siberian Seas, the temperature is lower, -7 to -8 degrees Celsius, but in the Chukchi Sea it is higher at -3 to -4 degrees Celsius.
The air temperature rises above 0 degrees Celsius on the Arctic islands and in the coastal zones in the southern parts of the peripheral seas every year at the beginning of June. In the northern areas, this transition occurs near the end of June.
summer:
The location of the features of the pressure field - a depression over the continent and near the pole, high pressure in the eastern part of the Arctic basin and over the Barents Sea -give rise to prevalent wind patterns that are opposite to winter conditions. Over the entire continental area of the Eurasian Arctic, the prevailing winds blow from the seas primarily to the northeast. In the western half of the Arctic the winds are primarily northerly and northeasterly, and in the eastern half they are primarily northeasterly and easterly. So, along almost the entire extent of the Arctic seas a monsoon wind type is clearly observed.
In summer over the open seas, the prevailing wind is weak as a result of the diffusing away of the barometric field resulting in small horizontal pressure gradients. At the same time, the spatial distribution of the air temperature distribution is exhibits very specific characteristics. The lowest air temperature is observed in the in the central Arctic near the pole. In July the average value is slightly below 0 degrees Celsius, and in August it decreases to about -2 degrees Celsius. This temperature stability is related to the conditions of the underlying surface. The ocean is covered with melting ice, and the temperature of upper layers of the water is equal to about -1.5 degrees Celsius. Air temperature changes over the arctic seas are closely correlated with sea ice conditions. In June, when the seas are almost completely ice covered, the correlation is weak, but in July and especially in August, when ice edge moves to the north, it is clearly evident. Isotherms in summer months are a result of the ice cover distribution.
Air temperature is higher over the coasts of large islands and archipelagoes than over the surrounding seas as a result of stronger heating of the land. The temperature over glaciers, however, is lower than over the coastal areas. On Franz Josef Land and the Severnaya Zemlya archipelago for example the difference is approximately 3degreesC, and it is as much as 7 degrees Celsius lower on Novaya Zemlya. The period of stable temperature above 0 degrees Celsius is the longest in the southwestern Barents Sea (170-180 days), and it decreases to the north and east. The duration of this period is 80 to 100 days in the center of the Kara and Chukchi seas, on the coast of Taimyr Peninsula and in the southern parts of the Laptev and East Siberian seas. In more northern areas of these seas it decreases further to only 20 to 30 days. In central part of the Arctic basin there is no warm period at all, except in occasional years when its duration is 15 days or less.
On the Eurasian arctic coast air temperature can occasionally exceed 10 degrees Celsius in some years. This situation can play an important role for the growth of vegetation. The average duration during episodes when the temperature is 10 degrees Celsius or higher for the arctic coast region is about 10 hours in June, 85 hours in July, 45 hours in August and 7 hours in September or about 5 % of warm period from June to September. Continental arctic areas (Khatanga, Chokurdakh, Cherskiy) experience more favorable temperature conditions. There the cumulative duration of episodes with air temperatures of 10 degrees Celsius or more is approximately 30% of warm season, and in July and August it increases to between 50 and 60%.
Temperature increases up to 20 degreesC and above are unusual for the arctic coastal areas. The average duration of such periods is at most a few hours per year (Table 5). In arctic areas that are furthest from the coast, the mean annual duration of air temperature of 20 degrees Celsius or higher can exceed 100 hours. At a few stations, July air temperature of 30 degrees Celsius or higher have been recorded, but the maximum duration never exceeded 10 to 15 hours. In warm months the atmospheric humidity increases up to 5-11 mb.
In the summer, a high degree of cloudiness is observed over the entire Arctic region. Due to constant temperatures over the water adjacent to ice at its melting temperature, frequent temperature inversions, and the frequent occurrence of fog, continuous low level stratus clouds form and persist over the entire warm period. The region with maximum cloudiness is the Barents Sea with 26 to 28 overcast days per month. A decrease in the cloudiness is observed in the southern areas of the arctic seas. Monthly precipitation increases and reaches maximum in August (25-50 millimeters) over most of the Arctic. On the arctic islands and in the northern continental areas the amount of precipitation changes significantly depending on the surface topography. For example, more precipitation falls on the windward sides of mountainous islands such as Novaya Zemlya, Severnaya Zemlya, Wrangel Island than near the base of the mountains and glaciers. The vertical gradient of precipitation depends on geographical location. For example, the gradient in annual precipitation is equal to 80 millimeters per 100 meter rise on Spitsbergen and 40 millimeters per 100 meter on Wrangel Island.
The precipitation distribution pattern varies in the different seasons. In summer, fine drizzle falls. For the days with measurable precipitation, more than 1.0 millimeters within 24 hours is observed less than a half the time. In all regions, precipitation levels of 10 millimeters per day are not observed every year. Typically there are 3 to 5 days with snow in July and 6 to 8 days in June and August. In summer the snowfall is usually moist and accompanied by rain.
Autumn:
The winds in the prevailing directions that are characteristic of summer are less frequent. Wind directions that are usual for winter, are observed more often, and the frequency of intermediate wind patterns increases. In October, wind directions typical of winter begin to prevail, but their frequency is less than in winter.
In autumn the monthly precipitation characteristics are close to those of winter, in good agreement with atmosphere circulation conditions. The amount of precipitation decreases everywhere, but number of days with precipitation is even greater than in summer.
Autumn air temperatures can decrease to negative values even in late August. In September, the area with the lowest temperatures (-10, -11 degrees Celsius) is located near the north pole. Positive average monthly temperatures are still observed over most of the Barents Sea, the Chukchi Sea, and the southern parts of the Laptev and Kara seas. Continental cooling is more intense than that of the peripheral seas. In the central Arctic and mountainous parts of the Arctic, the average monthly air temperature is negative in September. The air temperature drops below 0 degrees Celsius in the southern regions in early October. At the northernmost island stations, there is no period without sub zero temperatures. At most of the continental stations, the average interval without frost is 60 days.
The cloud amount frequency increases in autumn in the eastern and western regions but decreases in the central region. For example, during October in the east and west its frequency is 85 percent to 90 percent, and in the central region it is 75 percent to 80 percent.
The regional natural and climatic conditions described above for the various regions of the Arctic define the background in which HMPs develop.
Table 3Dates and duration of polar day and polar night (taking refraction into account) | |||||||
North latitude, | Polar day (24 hour daylight) | Polar night (24 hour dark) | Days with alternating dark & Light | ||||
Degrees | Start | End | Duration | Start | End | Duration | |
70 | 17 May | 27 July | 71d | 26 Nov | 17 Jan | 53d | 241d |
75 | 29 Apr | 14 Aug | 108d | 06 Nov | 03 Feb | 89d | 168d |
80 | 14 Apr | 30 Aug | 138d | 22 Oct | 21 Feb | 120d | 107d |
85 | 08 Apr | 10 Sep | 163d | 08 Oct | 05 Mar | 148d | 54d |
90 | 19 Mar | 25 Sep | 190d | 25 Sep | 19 Mar | 165d | 10d |
Duration of the seasons in the Arctic | ||
Season | Date of start and end | Duration, days |
Spring | 1 May - 15 June | 46 |
summer | 16 June - 31 August | 77 |
Autumn | 1 Sept - 15 October | 45 |
Winter | 16 October - 30 April | 197 |
Description of continuous intervals of high air temperature | |||||
Month | TemperaturedegreesC | Average number of observation periods per month | Duration, hours | ||
Average | Maximum Continuous | ||||
Total | Continuous | ||||
Dixon Island | |||||
July | 20 | 0.4 | 2.0 | 5.0 | 9 |
Khatanga | |||||
June | 20 | 1.3 | 15.0 | 11.8 | 42 |
July | 20 | 9.1 | 109.4 | 12.0 | 87 |
30 | 0.3 | 1.6 | 6.0 | 12 | |
35 | 0.1 | 0.2 | - | 3 | |
August | 20 | 3.6 | 27.2 | 7.6 | 15 |
September | 20 | 0.1 | 0.8 | - | 12 |
Wrangel Island | |||||
July | 15 | 0.1 | 0.4 | 6 | |
Cape Schmidt | |||||
June | 20 | 0.3 | 1.8 | 5.4 | 12 |
July | 20 | 1.3 | 6.8 | 5.1 | 9 |
August | 20 | 0.7 | 4.6 | 6.9 | 15 |
Üan observation period is three hours in duration |