Characteristics Of Sea Ice Data In The Arctic Seas

In general, the geographical terminology used in this book follows the Russian definitions published in Anon. (O). Treshnikov et al. (1967) define the Arctic Basin as a "near-pole abyssal basin, restricted by the continental slope." The Beaufort and Lincoln Seas are the marginal zones of the Arctic Basin. The North European Basin encompasses the Greenland, Norwegian, Barents, and White Seas as well as the Arctic Seas of Siberia (the Kara, Laptev, East Siberian, and Chukchi Seas). Baffin Bay, Davis and Smith Straits, Hudson Bay, and the straits of the Canadian Arctic archipelago compose the East Canadian region of the Arctic Ocean (e.g., Zakharov, 1996; Smirnov, 1974). Based on the major characteristics of the area's ice regime, the Greenland, Iceland, Norwegian, Barents, and Kara Seas are collectively known as the Nordic Seas, as proposed by Vinje (1998).

This study focuses on climatic changes in the region along the Northern Sea Route: the North European Basin, the Arctic Seas of Siberia, and the adjoining areas of the Arctic Basin (see Figure 2.1). The total area of this region is about 5 million km2; it includes approximately 75% of the Arctic Ocean (Alekseev et al., 2004). The sea ice edge very rarely extends beyond the marginal seas; thus, it can be assumed that the variability of the ice extent within this region is determined by the variability of ice extent in the North European Basin and the Arctic Seas of Siberia. Zakharov (1997) estimates that the North European Basin contributes 53% of Arctic Ocean ice extent variability for June-October, and the Arctic Seas of Siberia contribute 47%. Corresponding contributions in August are 26% and 74%, respectively. In winter (November-May), the main contribution to ice extent variability is almost equally shared by the Greenland and Barents Seas (48% and 52%, respectively).

Investigation of long-term changes in ice extent influenced by climatic variability requires observation series covering at least a century. This study draws mainly on data from observations in the Arctic Ocean east of Greenland, that is, from the

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Figure 2.1. Boundaries of the Arctic Ocean and its seas: the Greenland (1), Barents (2), Kara (3), Laptev (4), East Siberian (5), Chukchi (6), and Beaufort Seas (7), Baffin Bay (8), Hudson Bay (9), and the Norwegian Sea (10).

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Figure 2.1. Boundaries of the Arctic Ocean and its seas: the Greenland (1), Barents (2), Kara (3), Laptev (4), East Siberian (5), Chukchi (6), and Beaufort Seas (7), Baffin Bay (8), Hudson Bay (9), and the Norwegian Sea (10).

Greenland Sea in the west to the Chukchi Sea in the east. Systematic data on the Greenland and Barents Seas cover the last third of the nineteenth century and almost the entire twentieth century (excluding the period of the Second World War). Regular observations in the Siberian shelf seas (Kara to Chukchi Seas) began in 1938 (Vize, 1940). The most reliable data on ice extent in the Arctic Seas are AARI airborne and satellite observations covering the period since 1940. The data on ice extent in the Arctic Seas in August for the period 1924-1939 were collected by Vize (1944a), who compiled all shipborne and airborne observations available at that time on ice edge positions during the initial period of active development of the Northern Sea Route. Several experimental "through" voyages from the Barents Sea to the Bering Sea along the Northern Sea Route (Sibiryakov in 1932, Cheluskin in 1933, Litke in 1934) were carried out. In 1935, the first through voyages of four cargo motor ships were made from west to east and from east to west. Episodic airborne ice reconnaissance in the Arctic began in 1924. Airplanes regularly carried out ice reconnaissance over the Kara Sea beginning in 1929, and over the Laptev Sea beginning in 1935. In 1938, this pioneering period of airborne ice reconnaissance ended. Since then, airborne ice observations have been regularly carried out along the entire Northern Sea Route (Vize, 1948).

Based on 1924-1939 sea ice and other data, Vize (1944a) investigated the correlation between the variability of the total ice extent of the Arctic Seas and indicators of the intensity of atmospheric circulation, such as atmospheric pressure and the total area influenced by the Arctic High. Vize's results were corroborated by later studies of Gudkovich et al. (1972), whose more extensive data covered a much longer time period. Findings from these investigations were used for development of methods for long-term forecasts of sea ice conditions along the Northern Sea Route (Gudkovich et al., 1972), indirectly confirming sufficient reliability of Vize's data for use in the study of long-term (climate) variability in the ice extent of the Arctic Seas. From 1940 to 1979, ice charts were constructed from data collected by regular visual airborne sea ice reconnaissance; the charts for the period 1980-1992 were based on airborne and satellite observation data, and beginning in 1993, only satellite observations have been used in making the charts (Borodachev and Shilnikov, 2002).

Practically no data are available on Siberian shelf seas ice extent for the beginning of the twentieth century (1900-1923). In order to obtain a full 100-year data series on the ice cover of the Arctic Seas, an attempt was made to reconstruct them from a variety of sources containing descriptions of Arctic voyages at the beginning of the twentieth century. At the end of the nineteenth century and the beginning of the twentieth century, commercial, trade, and expedition vessels sailed in the Kara Sea, the eastern East Siberian Sea, and the Chukchi Sea. At the same time, the western East Siberian Sea and the Laptev Sea were visited more rarely. Observations from these voyages provided a basis for characterization of ice conditions in the studies of some Arctic investigators. Lesgaft (1913) compiled shipborne observations of Kara Sea ice conditions through 1911, and Nansen (1915) recorded annual descriptions of ice navigation conditions in the same sea from 1870 to 1913.

Substantial information on ice conditions in the Kara Sea and the eastern Arctic from Kolyma to the Bering Strait is contained in Itin (1933) and Sibirtsev and Itin (1936) for each year from 1900 to 1934. These authors analyzed multiyear variability of ice navigation conditions using a 5-point scale from ice-free years (1 point) to very severe years (5 points). Detailed data on the ice situation in the Arctic Seas during the period of the Arctic expeditions at the beginning of the century is presented in Vize (1948). Descriptions of ice conditions for separate years are contained in volumes on Sailing Directions for the Kara, Laptev, East Siberian, and Chukchi Seas published during the period 1935-1939 (Anon. (I, J, K, L)).

Charts showing ice conditions with the routes of ships and ice-edge positions are most valuable in these publications. In most cases, along with a description of the character of ice conditions (favorable, unfavorable), these studies include coordinates or orientation marks (islands, capes, and distances to them) and ice-edge positions along the ship routes. These mostly fragmentary data were plotted on the charts and then supplemented by expert assessment of the ice-edge position within the sea. Expert assessment was based on the characteristics of ice-edge positions under the main types of ice conditions—heavy, average, or light, determined on the basis of multiyear studies of the Arctic Seas ice regime using data for the period covered by reliable observations.

For 24 years at the beginning of the last century, the ice edge positions in August were reconstructed (and then the ice extents were calculated) in the Kara Sea for 10 years, in the Laptev Sea for 7 years, in the East Siberian Sea for 6 years, and, most importantly, in the Chukchi Sea for 18 years. For all other cases, the ice extent was calculated using mean monthly atmospheric pressure at regular grid points by a physical-statistical model developed on the basis of a simplified discriminant analysis. The model satisfactorily performed ice forecasts and calculations (Kovalev and Yulin, 1998). The correlation coefficient of the calculated data to the data reconstructed from published sources is 0.72. Thus, the combination of reconstructed data and observational materials made it possible to perform a comparative analysis of the changes in ice extent over the seas of a vast region (about 5 • 106 km2) throughout the entire twentieth century.

The seas under consideration can be divided into two significantly differing groups. The first group, including the Greenland and the Barents Seas, is characterized by the fact that part of this area remains ice free even in winter (Anon. (F)). The Norwegian Sea, which is also included in this group, is not considered here, since ice appears at its northern and western boundaries only in some years. Much of the White Sea is ice-covered in the second half of winter, and it is usually ice-free in summer; systematic data on the ice extent of this sea in February and March are available from 1951. For these months the average ice extent of the White Sea comprises only 10% of the ice extent of the Barents Sea. The correlation coefficient that characterizes the interannual changes in the ice extent of these seas from 1951 to 1994 is 0.50 (its significant value at P = 95% is not greater than 0.29). The changes in the ice extent of these seas are also quite similar: the ice extent in both seas slightly increases from the beginning of the 1950s to the end of the 1960s, after which it decreases until the middle of the 1990s. Therefore, we did not consider the state of the White Sea ice cover separately in this study.

The characteristics of the Arctic Seas ice regime for the first group indicate that the interannual variability of their ice extent is similar in all seasons. To determine seasonal differences, changes in the ice extent were examined in April-May when the ice extent is at a maximum and in August when it is close to the annual minimum.

The second group includes the Siberian shelf seas from Kara to Chukchi. Over much of the year they are mostly covered by very close ice (see sea ice term definitions under "Sea-Ice Nomenclature" in Anon. (M)), so the interannual variability of ice extent of these seas is observed only in the summer. For its characterization, the data for August were used, which closely correlate with the ice extent changes in July and September (Gudkovich et al., 1972).

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