Ice export through Fram Strait

Ice export from the Arctic Basin to the Greenland Sea through Fram Strait is a significant ice balance component for both regions and an important factor in climatic changes in the Arctic. The ice exchange through Fram Strait determines most of the ice discharge of the Arctic Basin, thus influencing the thermal and dynamic processes in the basin and its marginal areas.

Ice drift observations in Fram Strait are carried out episodically. From the 1930s through the 1960s, information on ice-drift speed in the strait was obtained from observations made by a few drifting expeditions that were carried to the Greenland Sea by wind and current. Later, the amount of information increased substantially as a result of observations from drifting buoys transmitting to satellites. This information was used to develop statistical methods for calculating the ice exchange through the strait. Quite a large number of studies have been devoted to the methodology of calculating the ice quantity passing through the strait for a particular time interval (Laushkin, 1962; Gudkovich and Nikolayeva, 1963; Vowinchel, 1964; Lebedev and Uralov, 1977; Gorbunov et al., 1985; Vinje and Finnekasa, 1986; Gudkovich and Pozdnyshev, 1995; Alekseev et al., 1997). Most of the methods developed during these studies are based on the empirical dependencies of the ice-drift speed in the strait on the baric gradient value in the area of the East Greenland Current. That is why there is satisfactory correlation of data obtained by different methods on inter-annual and multiyear changes in the area and on the volume of ice exported through the strait. More recently, dynamic-thermodynamic sea ice models were developed, allowing the calculation of the ice transport through the strait taking into account both dynamic and thermodynamic processes (Harder et al., 1998).

The differences in various investigators estimates of the area and volume of ice exchange through Fram Strait are mainly explained by differing volumes of observational data, different approaches to determining the speed of gradient currents, and different accounts of the cross-non-uniformity of the speed of ice drift and currents and/or flow width and ice cover concentration.

Long-term data series on the speed of the mean monthly ice export to the Greenland Sea through Fram Strait were calculated by the methodology developed by Gudkovich and Nikolayeva (1963). These authors devote considerable attention to calculating the gradient ice-drift component, which plays a large role in ice drift over the entire length of the East Greenland Current (Gudkovich and Pozdnyshev, 1995). For calculating the gradient current in Fram Strait, this study employed the theory on wind currents in a baroclinic sea developed by Lineikin (1955), who derived the equations that allow calculating the speed of the wind and gradient components of the current in an infinite and deep channel, based on wind distribution across the channel. In the stationary and uniform field of tangential friction stresses directed parallel to the channel axis, the value of the desired longitudinal current speed component at the surface can be sufficiently accurately expressed by

where U is the current speed (m/s); r is tangential wind stress; l is strait width; u is the vertical component of the Earth's angular rotation speed; m is the horizontal turbulent exchange coefficient; b is the vertical gradient of conventional water density; and g is gravity acceleration.

By substituting in this formula the values of l = 45 • 104m, g = 9.8 m/s2, u = 7-10"5 s"1, m = 108 kg • m"1 • s"1, b = 7-10"5 m"1 (from data taken during the expedition onboard the diesel-electric ship Ob' in 1956), we approximately derive:

In the East Greenland Current, the sea surface is not directly influenced by the wind but rather by the ice moving relative to the water under the influence of the wind. In this case, the tangential friction is determined by the quadratic law:

where C is the friction coefficient of the bottom ice surface; p is the water density; and W is the wind drift speed.

The expression of W through the atmospheric pressure gradient using linear empirical ratios allowed calculation of a family of parabolas corresponding to different values of the friction coefficient C. The results were compared with the gradient drift speed from NP-1 station records (December 1937) and the icebreaking ship G. Sedov (December 1939-January 1940). To average the speeds, the locations of these expeditions were taken into account relative to the outflow cores, which were usually situated near the continental slope. The results of dynamic processing of hydrographic observations in the strait area carried out by the Sever-7 and Ob' expeditions (1955 and 1956, respectively) were also used. The best match between the observed data and Equation (4.4) was for C = 0.004. The calculated water velocity was smaller than observed by approximately 0.025 m/s. This value (0.025 m/s) characterizes the water current which is determined predominantly by river runoff and inflow of Pacific water to the Arctic Ocean via Bering Strait (due to sea level difference between the Pacific and Atlantic Oceans) (Proshutinsky, 1993).

As a result, the empirical expression for calculating the average speed of the export current in Fram Strait (in m/s) for each month had the form:

where G is the average baric gradient (hPa/km) calculated for two sections: from Cape North-East (Greenland) to Amsterdam Island (Spitsbergen), and from Clavering Island (73°30'N, 21°30'W) to a point at the intersection of the parallel 70°N with the Greenwich meridian. The value of G was averaged for three preceding months, including the month for which the current speed was calculated. This accounted for the assumed time for establishment of baroclinic currents.

The speed of the wind component of the ice exchange through the strait was calculated using mean monthly baric gradients in Fram Strait, perpendicular to the strait axis, using the known values of isobaric coefficients (Gudkovich and Nikolayeva, 1963). The speed values derived were added to the corresponding gradient drift speed values. The average width of the ice flow in the strait was assumed to be 340 km.

The use of this methodology made it possible to calculate a data series for the area of ice exported through Fram Strait for each month of the twentieth century. An archive of mean monthly atmospheric pressure charts for 1900-2000, available at the AARI, was used to calculate baric gradients.

A 100-year analysis of mean monthly ice exchange between the Arctic Basin and the Greenland Sea showed that the most reliable data became available in the late 1920s to the early 1930s, when atmospheric pressure information in the area adjoining Fram Strait became more or less accurate. Due to a gap in observations during

World War II, from 1941 to 1945, analysis of the calculated data was performed for a number of years from 1946/1947 to 2002/2003. Based on these data, the mean annual area of ice exported to the Greenland Sea comprises 650,000 km2. Based on satellite passive microwave data, Kwok, Cunningham and Pang (2004) estimated the mean annual ice export for 1978-2002 as 866,000 km2. Both values are much lower than the estimates published by Gordienko and Karelin (1945) and Vinje (1992): 1.04 and 1.08 million km2, respectively.

A possible cause of overestimation of the ice exchange values from observations of the drift of radio buoys near Fram Strait might be a rapid increase in drift speed moving southward, which allows incorporating only the resulting vectors for comparatively short time intervals (up to a week). Note that Gudkovich and Doronin (2001) found that the average ice-drift speed increases with a decreasing period of averaging. On the other hand, due to a large cross non-uniformity of ice export speed, most observations characterize conditions near the current core, where drift speeds are much higher than they are on average along the transect.

An analysis of the data using the methodology described above shows that the intensity of ice exchange through Fram Strait changes throughout the year, increasing in winter and decreasing in summer (Figure 4.13). On average, two-thirds of the annual ice export occurs from November to April and only one-third from May to October. There are significant interannual fluctuations in the ice exchange area. A spectral analysis reveals cycles lasting 8-10 years and about 2-3 years.

Vinje and Finnekasa (1986) calculated the average annual speed of ice export through Fram Strait using Arctic buoy drift data for 1976-1984 obtained during the ICEX (Ice Experiment), AOBP (Arctic Ocean Buoy Program), and MIZEX (Marginal Ice Zone Experiment) programs. They showed that the ice transport ranges from 125 • 103 to 173 • 103 m3/s (with a mean of 153 • 103 m3/s). These authors derived the regression equation relating the ice export speed for weekly time intervals (Q, m3/s) to the atmospheric pressure difference (AP, hPa) between 81°N, 15°W and 73°N, 5°E:

Figure 4.13. Average annual variations of the area of mean monthly ice export from the

Arctic Basin to the Greenland x XI I

Sea through Fram Strait.

II III IV V Vt VII VIII IX

month

Figure 4.14. (a) Interannual fluctuations of the total ice area of the Siberian shelf seas in August, and (b) areas of ice exported from the Arctic Basin through Fram Strait. The values of the bold curves are smoothed by a polynomial to the power of 6.

The correlation coefficient of this relationship for winter is 0.95. However, the values of Q strongly depend on the estimate of average ice thickness in the strait, which varies from 2.66 to 4.06 m in the calculations of different authors.

Based on the methodology of Gudkovich and Nikolayeva (1963), with a correlation coefficient of 0.91, Alekseev et al. (1997) derived a regression equation relating the value of Q calculated by Equation 4.6 with the total ice exchange area (X, km2) for the winter:

Figure 4.14b shows long-period changes in the total area of ice export through Fram Strait from October of one year to August of the next year for 1931-2000. An

Table 4.4. Correlation coefficients between the long-period fluctuations of the area of ice exported through Fram Strait (October-August) and total ice area of the Arctic Seas Asian shelf in August for the period 1931-2000 at different time lags.

Time lag (years)

0

1

2

3

4

5

6

7

8

9

10

11

12

Correlation coefficients

0.43

0.56

0.67

0.75

0.80

0.81

0.80

0.77

0.72

0.66

0.58

0.49

0.39

approximation of data by a polynomial to the power of 6 (bold curve) indicates the cyclic character of these changes, with the cycle lasting about 60 years. Figure 4.14a shows that the fluctuations of total sea ice extent of the Arctic Seas of the Siberian shelf (from the Kara to the Chukchi Seas) have a similar character.

It is remarkable that increased ice export through Fram Strait is accompanied by increased sea ice extent in the Arctic Seas, contrary to the opinions of those who assume that ice export to the Greenland Sea increases during climate warming, accompanied by a decrease in sea ice extent in the Arctic Seas (Rigor et al., 2002; Makshtas et al., 2002; Hassol, 2004).

As shown in Figure 4.14, ice export fluctuations slightly precede corresponding sea ice extent changes in the Arctic Seas. The cross-correlation function between the smoothed values of ice export and total sea ice extent exhibits the highest correlation coefficients at time lags (sea ice extent after export) of 4, 5, and 6 years (Table 4.4). Following decreased ice export through Fram Strait in the early 1990s, a tendency for its increase was observed. Based on the time lags shown in Table 4.4, a transition to the phase of increased sea ice extent in the Arctic Seas would be expected at the beginning of the twenty-first century, as confirmed by Figure 4.14a.

So, fluctuations of ice export through Fram Strait occur approximately 4-6 years ahead of total sea ice extent fluctuations in the Arctic Seas. However, ice export to the Greenland Sea has a different influence on the sea ice extent of various seas. In this regard, it is of interest to compare the mean annual area of ice export from the Arctic Basin with the difference of sea ice extent in the Severozemelsky region (northeastern Kara Sea and western Laptev Sea) and the Wrangelevsky region (eastern East Siberian Sea and southwestern Chukchi Seas) in August. This difference characterizes the general distribution of the ice cover along the Northern Sea Route. It turns out that the correlation coefficient for this synchronous relationship is +0.40 (or 0.23 at P = 95% confidence level). With a 7-year shift (the difference in sea ice extent after ice export), the correlation coefficient increases to 0.62. So, 7 years after an increase in ice export from the Arctic Basin, the sea ice extent of the Severozemelsky region increases and that of the Wrangelevsky region decreases, and vice versa. This repeated pattern confirms the results mentioned above, in particular the opposing ice conditions in the western and eastern Arctic Seas.

A phenomenon termed "speed leveling along the general drift of the export flow" by Volkov and Gudkovich (1967) causes ice export through Fram Strait to affect ice cover dynamics in the Arctic Basin and its marginal seas. It is also possible that the inflow of Pacific Ocean water through Bering Strait increases when there is an increase in water and ice export through Fram Strait and decreases at its attenuation (Gudkovich, 1961).

The average drift and current speed in Fram Strait for the preceding year influences the ice exchange between the Arctic Basin and the Laptev, East Siberian, and Chukchi Seas in winter (October-March). The increased ice export to the Greenland Sea contributes to the increased ice export from these seas to the Arctic Basin, and its decrease results in the opposite effect (Gudkovich and Nikolayeva, 1963).

A 20-year observation series by Gudkovich and Kovalev (1967) shows that the latitude of the multiyear ice boundary in spring and the ice edge in the subsequent autumn in the northern Chukchi Sea (at 185-190°E) depends on the anomalies of ice export through Fram Strait. The correlation coefficient achieves its maximum value (0.86) if the ice export value is averaged for two preceding years. In the same study, a correlation of ice export to the Greenland Sea with the area of anticyclonic water circulation in the sub-Pacific Ocean sector of the Arctic Basin and the location of the core of the Transarctic current was revealed: upon increased ice export, the circulation expands and its core moves to the west; decreased export has the opposite effect. A cycle of 8-10 years was noted for these changes.

The influence of ice flow from the Arctic Basin to the Greenland Sea on the sea ice extent of the East Greenland ice belt was included in ice balance calculations by Lebedev and Uralov (1977). They concluded that in addition to ice exchange, the ice area in this region depends on the thermal processes (ice formation and melting) taking place. Similar results were obtained by Moritz (1988).

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