Net Radiation and Other Elements of the Heat Balance

3.5.1 Net Radiation

The net radiation balance of the surface is a result of the subtraction of its long-wave component from its short-wave component. The net short-wave radiation in the Arctic is always positive or equal to zero (polar night). The effective radiation exists throughout the whole year and it is mainly positive in the sense given in the previous section. For the mean monthly and annual averages which we have analysed, it is always positive, as was shown in the previous section (see Figure 3.6). Vowinckel and Orvig (1970) distinguished two types of radiation regime in the Arctic: the Norwegian Sea and the packice types (Figure 3.7). The first type occurs over open ocean areas north of the Arctic Circle. The characteristic feature of this type is the occurrence of a large negative balance (lower than one can expcct) during winter, which almost completely reduces the positive balance during summer. In comparison with the pack-ice type, the radiation balance of the first type shows significantly greater changes in the annual march from -2 cal/cm2/day to 3 cal/ cm2/day (-8.4 J/cm2/day to 12.5 J/cm2/day) versus -1 cal/cm2/day to 2 ca!/ cm2/day (-4.2 J/cm2/day to 8.4 J/cnr7day). From Figure 3.7 it may be seen that the net radiation balance is a very small residual of large components of incoming and outgoing radiation fluxes. Thus, as Vowinckel and Orvig (1970) notice, the balance will be highly sensitive to slight inaccuracies in the estimated incoming and outgoing radiation. For more details see Vowinckel and Orvig (1970).

b) 75*N-180"W T5°N-60*W 90'H

Figmv 3.7. Radiation regimes in tlic Arctic: (a) Norwegian Sea lype and (b) pack ice type after Vbwincket and Orvig (1970). a total incoming radiation, cloudless sky; b - actual total incoming radiation; c - actual total radiation absorbed on the ground; d long-wave radiation from the ground; e - long-wave incoming radiation, ovcrcast sky; f- actual long-wave incoming radiation: g - long-wave incoming radiation, cloudless sky; h - actual radiation balance; i - long-wave radiation by C03.

in January (Figure 3.8), solar radiation is not present in the greater part of the Arctic, so in this month the radiation balance is caused by effective radiation. The net balance is equal to -8 kJ/cm2 almost over the whole Arctic, except the open water in the Greenland, Norwegian, and Barents seas and Baffin Bay. The highest negative values, up to -25 kJ/cm2, occur in the eastern part of the Greenland Sea, especially near the western coast of Spitsbergen. Some polynyas in the Kara and Laptev seas also have values (-13 kJ/cm2) which are lower than normal.

In spring (April), the radiation balance is still slightly negative in most of the Arctic. Small positive values occur only in the southern parts of the Canadian Arctic and Pacific regions (up to about 3 kJ/cm2). Significantly greater values of the net radiation balance (up to 13 14 kJ/cm2) are noted in the southern part of the Atlantic region. However, the highest values (up to 21 kJ/cm2) are recorded in Baffin Bay (Figure 3.8).

In July (Figure 3.9), the radiation balance reaches its highest positive values. In the central part of the Arctic it varies between 15 kJ/cm2 and 17 kJ/cm2, near the sea-ice edge it is equal to 34 kJ/cm2, and in the open water of the Arctic seas it is at its highest, reaching as much as 42 kJ/cm2. Continental parts of the Arctic receive 30-35 kJ/cm2.

In October (Figure 3.9), the radiation balance of the surface becomes negative again over the entire Arctic. In the Arctic Ocean and the Arctic seas covered by sea ice, the values of the balance oscillate mainly from -5 kJ/cm2 to 6 kJ/cm2. Similar values are also observable in the continental Arctic. Open water near the sea-ice edge (Greenland, Norwegian, Barents, and Chukchi seas, and Baffin Bay) has the highest negative radiation balance (from -8 kJ/cm2 to -13 kJ/cm2).

The annual values of the net radiation balance (Figure 3.3b) are negative in the central part of the Arctic, mainly above 77-82°N reaching -12 kJ/cnr at the North Pole. The lowest observable values have been noted, however, in the centre of the northern part of the Greenland Ice Sheet (-16 kJ/cm2) and in the Greenland Sea near the coast of Spitsbergen (-17 kJ/cm2). In the continental parts of the Arctic (< 70°N), especially in the Canadian Arctic, the net radiation values exceed 70 kJ/cnr and probably reach more than 100 kJ/cm2 in the southernmost fragments. High values of radiation balance (100-110 kJ/cm2) also occur in the southern parts of the Barents Sea, the Denmark Strait, and Baffin Bay.

Comparison of the distribution of the net radiation balance in the Arctic (presented here after Khrol (1992)) with other sources reveals significant differences in many cases. These differences are greatest in the warm half-year and are probably caused by the different methods used for net radiation balance calculations.

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Figure 3.8. Average monthly (January and April) totals (in kJ/crrr) of the net radiation in the Arctic (after Khrol 1992).
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Figure 3.9. Average monthly (July and October) totals (in kJ/cm2) of the net radiation in the Arctic (after Khrol 1992).

3.5.2 Sensible Heat and Latent Heat

The net radiation balance presented in the previous section is the most important component of the heat balance of the surface. Yet, as we know, the energy is not only transported by radiation. It can also be transported from the surface to the atmosphere by evaporation and sensible heat and from the atmosphere to the surface by condensation and sensible heat. However, our knowledge concerning these two fluxes is still limited. As may be seen from the previous section, radiation balance computations can be compared with observations. This permits us to check the correctness of the formulas used for the radiation balance computations. Such a possibility does not exist in the case of the sensible and latent heat fluxes because no accurate direct measurement techniques exist. Thus, our knowledge about this part of the heat balance comes only from computations, which use for this purpose both different climatic data (mainly air and sea/land temperature, air humidity, and wind speed) and different characteristics of land and sea surface. Shuleykin (1935), Budyko (1956), Untersteiner (1964), Vowinckel and Taylor (1965), Ariel et al (1973), Khrol (1976), and Murashova (1986) developed methods of computing these fluxes. Calculating geographical distributions of the elements of the heat balance of the Arctic surface is a difficult and time-consuming task. Therefore, the existing literature is very meagre. Vowinckel and Taylor (1965) computed evaporation and sensible heat fluxes separately for the following areas: the central Polar Ocean, Kara-Laptev Sea, East Siberian Sea, Beaufort Sea, and the 5° latitude belts in the Norwegian-Barents Sea. For more details see this paper or Vowinckel and Orvig (1970). Only Russian climatologists have presented results of the distribution of the heat balance elements in the Arctic in the form of maps (Budyko 1963; Gorshkov 1980; Atlas Arktiki 1985; Khrol 1992). Budyko's maps concern the whole Earth and therefore they include only the southernmost parts of the Arctic. The maps presented here come from Khrol (1992). The values of sensible heat and latent heat fluxes were calculating using methodology proposed by Ariel et al. (1973), Khrol (1976) and Murashova (1986).

3.5.2.1 Sensible Heat

In January (Figure 3.10), the sensible heat flux is positive over almost the entire Arctic, except the open water areas in the Greenland, Norwegian, and Barents seas, and in the Denmark Strait and Baffin Bay (including the polynya known as North Water). The highest positive values occur in the central Arctic, Greenland, the northern continental parts of the Russian Arc tic, and the northern part of the Canadian Arctic Archipelago (4 5 kJ/cm2). The greatest loss of energy (up to -60 kJ/cm2), may be observed in the areas occupied by warm sea currents (West Spitsbergen Current, Norwegian Current, Murmansk Current and West Greenland Current),

In April (Figure 3.10), the pattern of distribution of sensible heat is very similar to that in January. The main observable differences concern the magnitude of the fluxes. In April both positive and negative sensible heat fluxes are lower. The highest values oscillate between 2 kJ/cm2and 3 kJ/cm2. while the lowest are between -25 kJ/cm2 and -34 kJ/cm2. The average decrease of the sensible heat in comparison with January is equal to 1-2 kJ/cm2 in most of the Arctic (except in open water, where this decrease is greater).

In July (Figure 3.11), the sensible heat is negative in the central Arctic, with highest values near the North Pole (-2 kJ/cm2). These negative values are spread more to the south (up to 70 N) from the Pacific region side. Greater negative values occur in the interior of Greenland (-8 kJ/cm2) and most of all in the continental part of the Arctic (up to -15 kJ/cm2). Between these two regions with negative values of sensible heat, there is a belt with positive values reaching as high as 15-17 kJ/cm2. Even higher values may be noted locally in the south-western part of the Canadian Arctic. This is connected with the advection of warm continental air from the South.

In October (Figure 3.11), the sensitive heat fluxes again became positive in the Arctic Ocean (up to 3 kJ/cm2) and in Greenland (up to 5 kJ/cm2). In the Arctic seas covered by sea ice, the fluxes are mainly slightly negative (except the Beaufort Sea and possibly the Laptev Sea) and are influenced by the advection of cold air from the continent. The negative values arc stronger (up to -25 kJ/cm2) in the seas with open water (the Norwegian, Greenland, Barents, and Chukchi seas, the Danish Strait, Baffin Bay and the Bering Strait). The northern part of the continental Arctic (including the Canadian Arctic Archipelago) has slightly positive sensible heat (rarely exceeding 2 kJ/cm2).

Annual values of sensible heat (Figure 3.12a) are positive in the Arctic Occan covered by perennial sea ice (up to 21 kJ/cm2) from the Canadian side, in Greenland, and in the Greenland Sea occupied by the cold East Greenland Current (up to 63 kJ/cm2). Very high positive values arc also observed locally in sea water areas between the islands of the Canadian Arctic Archi-pclago. Moderate negative sensible heat fluxes are noticeable in the continental part of the Arctic (up to about -22 kJ/cm2). On the other hand, very great losses of energy occur mainly in the eastern part of the Greenland Sea, where the warm West Spitsbergen Current reaches the sea-ice edge (-368 kj/ cm2). A significant loss of energy also occurs in the polynyas and leads areas (up to -42 kJ/cm2).

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Figure 3,10. Average monthly (January and April) totals (in kJ/cm') of the sensible heat in the Arctic (after Khrol 1992).
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