The most obvious influence of snow and ice cover on climate is its reflectivity (albedo). The albedo of the snow-ice surface is known to change over a wide range: from 0.98 for freshly fallen snow to 0.10-0.30 for deep puddles, heavily polluted ice, and open water leads among sea ice floes. Instrumental data (Budyko, 1969) shows that the average albedo of the Earth-atmosphere system with ice cover equals 0.62, while for the ice-free areas it is about 0.30. A relatively high albedo value strongly decreases solar radiation absorption by the snow-ice surface. According to Brooks (1952), it decreases air temperature in the Arctic by several tens of degrees Celsius. Fluctuations in the ice cover area also change the average albedo of the Earth-atmosphere system, which affects the state of the global climatic system.
In addition to the effect of albedo on Arctic air temperature, the heat insulating effect of the ice cover has a large influence. The ocean-to-atmosphere heat flux through ice, including the latent heat of ice formation, is mainly determined by the vertical temperature gradient between the water surface and the air. This flux decreases with increasing thickness of ice and snow. An ocean covered by several years' accumulation of ice releases only a small amount of heat to the atmosphere in winter. Cracks and fractures resulting from dynamic processes in the ice play a significant role in this release of heat (Buzuyev et al., 1999); although insignificant in area (a few percent of the ice cover), these features account for about 50% of the heat flux from the ocean to the atmosphere (Makshtas, 1984).
Budyko (1962, 1966, 1968, 1969) employed several schemes for estimating the influence of polar ice on Arctic thermal conditions. His calculations showed that under ice-free conditions the mean annual air temperature in the Central Arctic would have increased by approximately 15°C compared to current conditions. The highest air temperature increase would have occurred at the coldest time of the year, while in the summer months it would not have increased more than several degrees Celsius.
Thus, the Arctic ice cover significantly decreases the air temperature above it and contributes to the increased horizontal gradient of the air temperature between the low and high latitudes of Earth's Northern Hemisphere. The atmospheric heat influx to the Arctic, which should increase with increasing ice cover area, depends on this gradient (a negative feedback). The role of the meridional gradient of the air temperature in forming the general planetary air flow from east to west in temperate latitudes is equally important for understanding climate change.
Air masses are transformed as they pass over various surfaces, and ice distribution plays an especially important role in these transformations. The available theoretical studies provide a mathematical description of air temperature transformation in a simplified formulation (Doronin, 1959; Nikolayev, 1963). The empirical data presented by Nikolayeva and Shesterikov (1970) are quite accurately approximated by hyperbolas, with parameters given by Appel and Gudkovich (1992).
Heat exchange between the atmosphere and the ocean changes especially sharply at ice edges (Vize, 1944a). As Budyko (1969) shows, its decrease at the ice edge extends in a slightly weaker form to temperate and even tropical latitudes due to the air temperature transformation over the open ocean. According to his calculations, the mean planetary temperature decreases more than two degrees near the earth's surface. The temperature decrease in the zone from the equator to 60°N compared to ice-free conditions ranges from 1.5° to 2.7° C, and in higher latitudes it increases sharply to more than 12°C.
The influence of changes in air temperature, which depends on the position of the ice edge, has not only a global but also a regional and even a local character. This is indicated in Teitelbaum (1977, 1979), where the problem of the effect of sea ice extent in the Arctic Seas on air temperature is solved using statistical methods. It is convincingly shown that at the beginning of the ice-melt period (May-June), the air temperature controls further decay of the ice cover, because the albedo value depends on its anomaly (see also Gudkovich et al., 1972). However, with the appearance of open water, the air temperature, which depends on the ratio of ice-covered/ ice-free water, gradually becomes predominantly a result of sea ice extent.
The intensity of cyclonic (anticyclonic) activity in the atmosphere depends on energy drawn from the horizontal gradients of heat fluxes across the underlying surface and the air temperature above it (Pogosyan, 1972; Nikolayev, 1981; Nikiforov, 2006). These conditions usually occur near the ice edge. As shown by Treshnikov et al. (1967) and Bulgakov (1975), the ice edges in the Antarctic and the Pacific Ocean at the end of winter are usually located near sharp changes in convection depth. Abramov and Frolov (1987) employed a numerical model to calculate the heat loss from the surface of the Barents Sea in the fall-winter period. They showed that the mesoscale variability of sea-air heat exchange during fall-winter depends on water stratification at the ice edge and influences the location of average trajectories of extra-tropical cyclones that cross the sea in a zonal direction. The data obtained by Popov (2002) indicate that even such mesoscale phenomena as flaw polynyas can significantly influence the transformation of a thermobaric field over the northern polar area.
The influence of the ice cover on atmospheric circulation is manifested in such phenomena as oscillations in the ocean-ice cover-atmosphere system (Gudkovich and Kovalev, 2002a) (see Chapter 4 for more details). According to Zakharov (1996, 1997), the relationship between the sea ice extent of the Arctic Seas and the strength of the Arctic High also results from ice-cover influence on atmospheric circulation (Vize, 1940). This influence also extends to the prevailing trajectories of cyclones, which move southward with an increase in the Arctic High and northward at its decrease.
An extensive zone of decreased ice thickness observed in the Arctic Seas in winter is dependent on summer melting and subsequent ice export to the Arctic Basin. The heat flux to the atmosphere across this ice is slightly greater than that from the cold continents to the south and from thick multiyear ice to the north. Using a scheme of the average distribution of the calculated ice zones with a different time of formation from that of Gudkovich et al. (1972) and Gudkovich and Doronin (2001), Nikiforov (2006) calculated the heat flux to the atmosphere through first-year and younger ice. The average value of this flow was 500 • 103 kJ/m2 for a season or 63 • 103 kJ/m2 for each winter month. "Therefore it is not surprising that the area of the Arctic Seas is a 'highway' for the Atlantic cyclones frequently penetrating the East-Siberian Sea" (Nikiforov, 2006, p. 98). These cyclones form the Atlantic-Arctic pressure depression, which contributes to heat advection to the Arctic. Its development depends on processes in the North European basin and the ice state in the Arctic Seas (positive feedback).
The influence of ice cover on the exchange of gases between the atmosphere and the ocean is less evident. It is known that the concentration of greenhouse gases in the atmosphere, on which the intensity of long-wave heat emissions from the Earth-atmosphere system to space depends, is regulated by the processes of gas exchange between the atmosphere and the ocean. Gas exchange between atmosphere and ocean in ice-covered ocean areas is very limited. Therefore, an increase or a decrease in the ice cover area and ice concentration should be reflected in the concentration of greenhouse gases in the atmosphere resulting in further climatic changes (Golubev et al., 2004). However, taking into account that the solubility of a gas in water decreases with an increase in water temperature and that these changes are in the opposite phase to changes in sea ice extent, the influence of anomalies in the ice cover area and water temperature in the ice-free region act in opposite directions. Corresponding changes in the biosphere also play a role in these processes.
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