Ice exchange between the Arctic Seas and the Arctic Basin

Ice exchanges between the marginal seas and the Arctic Basin, along with thermodynamic processes, influence the ice cover structure in these seas and hence their sea ice extent (Gudkovich and Nikolayeva, 1963; Gudkovich et al., 1972; Gudkovich and Doronin, 2001).

Unfortunately, no direct, sufficiently long-term ice-drift observations are available near the boundaries between the marginal seas and the Arctic Basin, and calculation methods must be used for estimating the ice exchange (ice area or volume) and its variability in time. In the "export" seas, which contribute ice to the Arctic Basin during much of the year, the simplest calculation method is based on "isobaric drift'' ratios proposed by N.N. Zubov (1944):

where w is the projection of the ice-drift speed to axis y; k is the isobaric coefficient; dP

and —- is the projection of the atmospheric pressure gradient on axis x. dx

If axis x is directed along the "entry" section with a length l, approximately coinciding with the northern boundary of the sea, then integrating Equation 4.8 along this axis from the western to the eastern sea boundary results in:

Here, S is the area of the ice cover passing through section I at unit time. This area is determined by the dimension of coefficient k and the corresponding scale of averaging of the baric chart. When using the mean monthly charts of atmospheric pressure at sea level, the isobaric coefficient dimension is km2/hPa • month. These values were derived from observations of the total ice drift in the Arctic Basin, increased by 25%, according to the empirical ratio of corresponding mean annual values (Gudkovich and Nikolayeva, 1963).

Equation 4.9 indicates that the resulting ice exchange area does not clearly depend on section length and is proportional to the atmospheric pressure difference at its ends. It is assumed that the ice cover (regardless of ice concentration) does not disappear along the entire section length. Thus, a correct estimate of the ice exchange volume requires information on ice concentration and thickness and their changes in time.

Estimates of the area of ice exchange between the Barents, Kara, and Laptev Seas and the Arctic Basin for 1937 to 2003 were based on monthly differences in atmospheric pressure between Spitsbergen and Franz-Josef Land, between Franz-Josef Land and Severnaya Zemlya, and between Cape Arktichesky (Severnaya Zemlya) and Kotel'ny Island (Novosibirskie islands) with account for the monthly values of isobaric coefficients published in Gudkovich and Nikolayeva (1963). Using these data, seasonal changes in the corresponding mean multiyear values shown in Figure 4.15a, b, c were determined: in summer (mainly from May to August), ice is exported to these seas from the Arctic Basin, and in winter (mainly from September to March), ice is exported from these seas to the Arctic Basin. These data suggest that more than 40,000 km2 (with a standard deviation of about 70,000 km2) is exported on average from the Barents Sea to the north, about 120,000 km2 (standard deviation of about 90,000 km2) from the Kara Sea, and more than 290,000 km2 (standard deviation of more than 90,000 km2) from the Laptev Sea in winter. The ice export from the Arctic Basin in summer comprises on average about 25,000 km2 for the first two seas and about 10,000 km2 for the Laptev Sea (standard deviations are about 50,000-70,000, and more than 115,000km2, respectively). There is significant ice exchange between the Barents and Kara Seas. For much of the year (from August to June), ice is exported from the Kara Sea to the Barents Sea. The area of this ice is comparable to the ice export from the Kara Sea to the Arctic Basin for the winter period. These estimates do not account, in explicit form, for the influence of the gradient currents, which may influence the values given above.

According to Gudkovich and Nikolayeva (1963), in a year that westerly and southwesterly winds increase over the eastern Barents Sea during October-December, the setup they create in the Kara Sea increases ice export from this sea toward the north. Dominant easterly and northeasterly winds produce the opposite result. This study also shows that ice export from the eastern East Siberian Sea and

1000 km

S, 1000 km

Month

1000 km

S, 1000 km

Month

Month

Figure 4.15. Mean multiyear values of seasonal changes in the calculated ice exchange of the Barents (a), Kara (b) and Laptev (c) Seas with the Arctic Basin (thin lines characterize data that were increased or decreased by standard deviation values).

X XI XII I II III IV V VI VII VIII IX

Month

X XI XII I II III IV V VI VII VIII IX

Month

Figure 4.15. Mean multiyear values of seasonal changes in the calculated ice exchange of the Barents (a), Kara (b) and Laptev (c) Seas with the Arctic Basin (thin lines characterize data that were increased or decreased by standard deviation values).

the southwestern Chukchi Sea during the period considered is strongly influenced by wind field vorticity in the vicinity of Wrangel Island. Anticyclonic vorticity increases the ice export, and cyclonic vorticity results in additional ice flow from the north.

To check the reliability of the above calculations, the boundary of ice with total concentration 7-10 tenths in late September in the Laptev Sea was compared with the boundary of old (second- and multiyear) ice dominance (partial concentration 5 tenths and greater) in March of the following year. The difference in the latitude of these boundaries at meridians spaced at 5° of longitude was assumed to be the value of ice motion along the corresponding meridians for the six winter months (October March). The ice exchange area of the seas with the Arctic Basin (S) for the designated period was determined by:

where A<mean is the average difference in latitude between the aforementioned ice boundaries at meridians (in degrees); AA is the difference in longitude between the meridians of the eastern and western boundaries of the sea (in degrees); <mean is the latitude of both ice boundaries averaged by meridian, and a coefficient of 12350 converts the degrees of latitude to km2.

The average annual ice exchange of the Laptev Sea with the Arctic Basin for the winters from 1954 to 2002 calculated using Equation 4.10 is 294,000 km2 or 55% of the sea area, which essentially coincides with the value presented above that was calculated from the atmospheric pressure difference. Calculations of the ice exchange area for other seasons are not expected to result in significant differences. Adding 55,000 km2 of the calculated average ice export from April to September to this winter ice exchange value results in an estimate of 349,000 km2, assuming significant interannual variability does not cause this estimation to differ strongly from average ice export from this sea to the Arctic Basin (309,000 km2) for the years 1936 to 1995, as calculated by means of a semi-empirical model described by Alexandrov et al., (2000). These authors modeled ice drift using a large-scale dynamic-thermodynamic model to determine the relationship of ice export from the Laptev Sea to the north and east with alternating cyclonic (CR) and anticyclonic (AR) circulation regimes. The authors concluded that during an AR, ice export increases to the north and decreases to the east. A CR has the opposite effect, with weaker ice export to the north and stronger export to the east. A similar phenomenon was noted during increased cyclonic activity over the Arctic Basin during the first twentieth century Arctic warming when the icebreaking vessel G. Sedov drifted during the winter of 1937-1938 to the east and onto the shelf north of the New Siberian Islands. An east current that appeared at the time was later called the "G. Sedov current." This current was probably absent during the Fram's drift, when there was a cold period similar to that of the 1960s-1970s.

Estimating ice exchange between the Arctic Basin and the East Siberian and the Chukchi Seas is more complex. During the winter period, onshore winds are often observed here, which should result in the export of a large amount of ice from the Arctic Basin to these seas. The intensity of this process increases from west to east. However, our analysis of ice area change in the eastern part of the East Siberian Sea and in the Chukchi Sea shows that throughout the winter, ice export from these seas to the Arctic Basin dominates (Gudkovich and Nikolayeva, 1963).

This conclusion is also confirmed by other data from the present study: The area of ice exported to the Arctic Basin from the East Siberian Sea during the winter period comprises only 38,000 km2; however, determining its value from the change in location of the close ice boundary (7-10 tenths) in late September, and of the prevailing old ice boundary in March, (Figure 4.16) yields more than 355,000 km2. About 100,000 km2 of this quantity is located to the north of the New Siberian Islands, and should mainly comprise additional ice exported from the Laptev Sea. However, in this case, ice export from the East Siberian Sea also contributes substantially to the movement of ice from the shelf seas to the Arctic Basin. It should be noted that all data on the position of the ice edge were obtained by means of processing AARI routine 10-day periodicity ice charts in the WMO SIGRID format for 1954-1992, i.e., for the period with the smallest number of gaps in the historical dataset (see Mahoney et al. (2008) for a full description of the dataset).

Figure 4.16. Average 1954-1991 boundaries of prevailing old ice in March (1) and close residual ice in late September of the preceding year (2). Line segments at meridians characterize corresponding standard deviations.

Figure 4.16. Average 1954-1991 boundaries of prevailing old ice in March (1) and close residual ice in late September of the preceding year (2). Line segments at meridians characterize corresponding standard deviations.

In the Chukchi Sea, onshore winds prevail on average much of the year (except for June-July). The winter ice flow from the north calculated by the atmospheric pressure difference along the Wrangel Island to Cape Lisborne transect comprises more than 300,000 km2 on average, whereas the displacement of ice boundaries indicates the dominance of ice export to the Arctic Basin (on average, 14,000 km2). These differences are attributed, as noted above, to the influence of the Pacific Ocean current, whose speed increases as it approaches the Bering Strait, and by resistance of the internal ice cover to compression processes.

It should also be noted that dynamic divergence of the ice cover near the northern sea boundary, which is often observed during prevailing cyclonic baric fields in the summer, can slightly influence the location of the boundary of prevailing old ice at the end of winter and hence cause overestimation of ice export to the north obtained when using the methodology described.

While the factors discussed above introduce large errors in calculations of ice exchange between the East Siberian Sea and the Arctic Basin in winter, springsummer (April-September) calculations are significantly more reliable, because offshore winds tend to dominate during this period. The total calculated area of ice exported to the Arctic Basin for in the spring-summer season comprises 105,000 km2, on average, with a standard deviation of 127,000 km2.

It is impossible to estimate ice exchange between the Chukchi Sea and the Arctic Basin in summer; an intensified Pacific Ocean current in the Bering Strait at this time and heat advection in the atmosphere usually cause a significant part of the ice cover to melt within the sea. In some years, when an extended baric depression is established in the adjoining areas of the Arctic Basin, rapid movement of a large amount of ice from the Arctic Basin to the eastern East Siberian Sea and the Chukchi Sea can occur.

The estimates above show that, on average, about 1 million km2 of the ice cover is transported annually from the Arctic Seas to the Arctic Basin, which is comparable to current estimates of the area of ice exported annually from the Arctic Basin to the Greenland Sea. (e.g., Koesner, 1973; Mironov and Uralov, 1991; Vinje, 1986). Given a typical ice thickness value, we can estimate the volume of ice exported to the Arctic Basin during a winter to be approximately 1500-2000 km3. This value is about half as large as the available estimates of ice export to the Greenland Sea in winter (Vinje and Finnekasa, 1986; Alekseev et al., 1997), which can be accounted for by ice growth, ice ridging, and other processes that occur during transport of the ice to Fram Strait.

As the thickness of the ice cover involved in ice exchange constantly changes, the best approach for estimating corresponding ice volume should be based on the dynamic-thermodynamic models of ice cover evolution. This requires performing model calculations for long time intervals (years), during which the resulting drift speed becomes comparable to the systematic error of calculation (Gudkovich and Doronin, 2001). The models should be improved, especially to account for gradient currents and rheological properties of the ice cover.

As shown in Frolov et al. (2005), Gudkovich et al. (1972), and Gudkovich and Doronin (2001), the ice exchange of the marginal seas with the Arctic Basin in winter influences the formation of macro-scale ice structures, expressed in ice zones of different age, and hence of different thickness. The ice melting rate and the disappearance of ice in the seas the following summer depends on the latter. Due to various hydrometeorological conditions typical of different seas, there are differences in the thicknesses of ice zones of the same age. The areas of the corresponding zones determined by the ice exchange intensity during a particular period also differ significantly. As a result, non-deformed ice formed during the autumn-winter period in the Barents Sea typically is not thicker than 100 cm by the beginning of spring melting. Average ice thickness in the Kara Sea is 130-170 cm, in the western Laptev Sea 180-190 cm, in the eastern Laptev Sea and the western East Siberian Sea 200215 cm, and in the Chukchi Sea 150-170 cm.

When spring-summer melting occurs at average intensity, the anomalies of the area of ice exchange and ice growth in winter almost completely determine the sea ice extent of the Barents Sea the next summer. In the other Arctic Seas the influence of ice exchange with the Arctic Basin on the subsequent disappearance of ice is shifted to the end of winter as ice of earlier formation will not have time to melt during a short Arctic summer. The closest connection between sea ice extent and ice exchange is observed during April-June (or May-July), when favorable (or unfavorable) anomalies in ice exchange with the Arctic Basin are accompanied by corresponding air temperature anomalies that initiate melting and seasonal changes in ice cover reflectivity (albedo).

Examining mean multiyear data on the ice exchange of the seas with the Arctic Basin in the summer may lead to an incorrect conclusion that ice exchange during this time interval plays a small role in the ice balance of the seas: the area of ice brought to

the sea or exported to the Arctic Basin from June to September comprises on average only 3%-8% of the area of the seas. However, the interannual variability of these values is quite significant. The amplitude of ice exchange fluctuations compared to the area of the sea for June-July is 22% for the Kara Sea, 42% for the Laptev Sea, and 24% for the East Siberian Sea; for the June-to-August period, the fluctuations in the three seas are 23%, 48%, and 40%, respectively, and for the June-to-September period, they are 37%, 70%, and 64%, respectively (Gudkovich and Doronin, 2001).

So, the direct role of summer ice exchange with the Arctic Basin in the ice balance of the seas during anomalous years is significant, especially for the Laptev Sea and the East Siberian Sea. In these seas, its absolute value for June-September in 30% of cases exceeds 20% of the area of the seas, while in the Kara Sea, this occurs in only 15% of cases. This component plays an even smaller role in the summer ice balance of the Barents Sea.

Figure 4.17. (a) Changes in the average latitude of prevailing old ice boundaries at the end of winter. (b) Ice export to the Arctic Basin for the winter period. (c) Average latitude of the boundaries of residual ice at the end of summer (in the preceding year) in the Laptev (on the left), the East Siberian (at center), and the Chukchi (on the right) Seas.

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