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    Time series of the monthly mean ice thickness at ∼79°N. The 1987–88 measurements are obtained at 75°N (Vinje 1989) and adjusted to 79°N according to recent contemporary measurements at the two locations (not published). The mean of the 1990–97 measurements is 2.87 m as indicated by the broken line, the std dev is 0.68 m, and the mode is 2.85 m. (Available online at http://www.npolar.no/ADACIT.)

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    Gray columns: Time series of the parameterized monthly ice volume flux through Fram Strait, 1950–2000. Black line: 12-month running mean

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    Time series of the parameterized annual ice volume flux as tuned with recent observations indicated by filled circles (VNK98 plus updates). The mean annual ice flux caused by wind-induced and density-driven ocean currents is, according to the model formula, equal to 1848 km3 yr−1 (=12 × 154 km3 month−1), corresponding to about 60% of the long-term mean, 2900 km3 yr−1, indicated by the horizontal line. This percentage share compares with 50% obtained from a study of buoy drifts in the Arctic Ocean by Thorndike and Colony (1982)

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    Time series of (a) the annual mean air pressure and (b) the winter mean air pressure at 80°N–10°W in FS, and at 73°N–20°E in the BS. The linear regression lines indicate a fall in the annual mean air pressure of 3.1 and 2.6 hPa in FS and BS, respectively. The corresponding falls in the air pressure for the winter months are 7.1 and 6.6 hPa

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    Time series of the anomalies of the parameterized annual ice volume flux

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    The effect of a flux change in the Fram Strait on the winter MY ice area in the AO the following year. The extreme year to year variations (which are determinative for the high correlation) correspond to the extreme variations between 1993 and 1996 given in Fig. 5. Adopted from Fig. 6 in Johannessen et al. (1999) and from VKN98 plus updates

  • View in gallery

    Time series of the annual ice area flux through the Fram Strait as tuned from the observations in 1990–96. The mean annual ice area efflux, 1.10 × 106 km2 (σ = 0.16 × 106 km2), is indicated by the horizontal broken line

  • View in gallery

    Time series of the 6-yr moving net anomaly wind-induced ice volume export through the Fram Strait (left scale) and the corresponding effect on the Arctic Ocean mean ice thickness anomaly (right scale). The relationship between the two scales is [(left scale)/(Arctic Ocean mean annual ice extent)] = −right scale. Here C indicates periods with cyclonic ice circulation and thinning of the ice, and A indicates periods with anticyclonic ice circulation and thickening of the ice in the Arctic Ocean according to Proshutinsky and Johnson (1997)

  • View in gallery

    The 6-yr running means of the winter (DJFM) air pressure anomaly in BS and FS. The trend lines indicate the 50-yr air pressure tendency

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    The 1950–2000 monthly mean air pressure difference, ΔP, between 81°N, 10°W and 73°N, 20°E as read from Die Grosswetterlagen Europas

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    (triangles) Annual cycle of the monthly mean thickness of ice floes. (Open water observations excluded). (circles) Annual cycle of the monthly mean ice thickness for 1990–96. (All observations included). (broken line) Annual cycle of the ice thickness mode. The ice thickness is estimated from the ratio between draft and ice thickness, 1.136, as observed from drillings in the Fram Strait. The Sep cross-stream mean ice draft is 1.8 m (= 2.3 m × 0.89/1.136). The thicker ice floes observed during the summer (triangles) illustrate the repeated influx of thicker ice from north of Greenland observed to take place during this season. Adapted from VNK98

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    (upper curve) The annual cycle of the cross-stream ice velocity, , for 1950–2000. (middle curve) The cross-strait current-induced ice velocity, c, 1993–95. Here is determined from V/(B), where V is given by (4), is the mean cross-strait ice thickness 1990–96 (2.56 m), and B is the mean breadth of the ice stream, 315 km. Here c is determined as a residual for the different seasons by regression analysis in analogy with relationship (2), using the data in Table 8 in VNK98. (lower line) The annual mean velocity of the 1993–95 cross-stream density-driven current as determined below

  • View in gallery

    Annual cycle of the ice volume flux through the Fram Strait. The dotted line shows the 1950–2000 parameterized mean monthly volume flux and the solid line shows the mean monthly volume flux adjusted for the annual cycle of the mean ice thickness observed in 1990–96 (cf. Fig. 11)

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Fram Strait Ice Fluxes and Atmospheric Circulation: 1950–2000

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Abstract

Observations reveal a strong correlation between the ice fluxes through the Fram Strait and the cross-strait air pressure difference. Using this difference, ΔP, as a parameter, the mean ice export from the Arctic Ocean through the Fram Strait is estimated to be ∼2900 km3 yr−1 over the past 50 yr. The variability of the annual efflux, which is solely determined by the variability in wind stress, is characterized by a standard deviation of 667 km3. Although the 1950s and 1990s stand out as the two decades with maximum flux variability, significant variations seem more to be the rule than the exception over the whole period considered. There is no temporal trend in the series, indicating long-term stationary conditions, and, consequently, that the mean annual ice efflux corresponds to the mean annual ice formation in the Arctic Ocean. Comparisons indicate that flux anomalies have a high predictive skill for subsequent anomalies, both in the reservoir and in the recipient. As the wind forcing is the major agent for the ice efflux, the downstream and upstream anomalies seem to be initiated in association with variations in the high-latitude atmospheric circulation. This is more the case as the correlation between the ice efflux and the remote air pressure difference between Iceland and Portugal approaches zero (0.1) when considering the whole 50-yr period. A noticeable fall in the winter air pressure of ∼7 hPa is observed in the Fram Strait and the Barents Sea during the last five decades. The regional uniformity in the fall renders, however, a temporally invariant magnitude of ΔP, indicating no clear effect of the Arctic ocillation on the efflux variability. Decadal alternating variations of ΔP indicate the existence of alternating minima and maxima in the annual wind-induced efflux. The corresponding decadal maximum change in the Arctic Ocean ice thickness is of the order of 0.8 m. These temporal wind-induced variations may help explain observed changes in portions of the Arctic Ocean ice cover over the last decades. Due to an increasing rate in the ice drainage through the Fram Strait during the 1990s, this decade is characterized by a state of decreasing ice thickness in the Arctic Ocean.

Corresponding author address: Dr. Torgny Vinje, Norwegian Polar Institute, Middelthunsgt. 29, P.O. Box 5156-Majorstua, N-0032 Oslo, Norway. Email: torgny.vinje@online.no

Abstract

Observations reveal a strong correlation between the ice fluxes through the Fram Strait and the cross-strait air pressure difference. Using this difference, ΔP, as a parameter, the mean ice export from the Arctic Ocean through the Fram Strait is estimated to be ∼2900 km3 yr−1 over the past 50 yr. The variability of the annual efflux, which is solely determined by the variability in wind stress, is characterized by a standard deviation of 667 km3. Although the 1950s and 1990s stand out as the two decades with maximum flux variability, significant variations seem more to be the rule than the exception over the whole period considered. There is no temporal trend in the series, indicating long-term stationary conditions, and, consequently, that the mean annual ice efflux corresponds to the mean annual ice formation in the Arctic Ocean. Comparisons indicate that flux anomalies have a high predictive skill for subsequent anomalies, both in the reservoir and in the recipient. As the wind forcing is the major agent for the ice efflux, the downstream and upstream anomalies seem to be initiated in association with variations in the high-latitude atmospheric circulation. This is more the case as the correlation between the ice efflux and the remote air pressure difference between Iceland and Portugal approaches zero (0.1) when considering the whole 50-yr period. A noticeable fall in the winter air pressure of ∼7 hPa is observed in the Fram Strait and the Barents Sea during the last five decades. The regional uniformity in the fall renders, however, a temporally invariant magnitude of ΔP, indicating no clear effect of the Arctic ocillation on the efflux variability. Decadal alternating variations of ΔP indicate the existence of alternating minima and maxima in the annual wind-induced efflux. The corresponding decadal maximum change in the Arctic Ocean ice thickness is of the order of 0.8 m. These temporal wind-induced variations may help explain observed changes in portions of the Arctic Ocean ice cover over the last decades. Due to an increasing rate in the ice drainage through the Fram Strait during the 1990s, this decade is characterized by a state of decreasing ice thickness in the Arctic Ocean.

Corresponding author address: Dr. Torgny Vinje, Norwegian Polar Institute, Middelthunsgt. 29, P.O. Box 5156-Majorstua, N-0032 Oslo, Norway. Email: torgny.vinje@online.no

1. Introduction

The annual variation in the air pressure distribution in the Arctic Ocean and the Nordic Seas show some characteristics that, to a large extent, determine the ice flux through the Fram Strait. One of the most pronounced features, based on observations over the period 1946–93, is the northeast Atlantic trough, extending from Iceland over the Norwegian, Barents, and Kara Seas into the Siberian marginal seas (Proshutinsky and Johnson 1997). This trough is most pronounced during the polar night, and to a large extent, the air pressure distribution in its northern flank determines the velocity and direction of the transpolar ice drift stream exiting through Fram Strait. Using the monthly mean air pressure differences across the northern flank of the northeast Atlantic trough as a parameter, and averages of recent measurements of ice thickness and ocean current velocities in the strait (Vinje et al. 1998), a model formula is developed for the study of the wind-induced variation of the ice volume efflux since 1950.

2. Measurements used as basis for the parameterized ice volume efflux

Ice draft was measured during 1990–96 from 1 to 4 upward looking sonars (ULSs) in Fram Strait. The ULSs were sitting at a nominal depth of 50 m on top of moorings, anchored at depths of between 500 and 2000 m at 79°N across the main ice stream. The observation interval is 4 min, and the observed ice velocity suggests that monthly ice thickness data are collected at intervals of about 30 m from about 10 000 locations over an upstream distance of about 150 km in the Arctic Ocean (Vinje et al. 1998, hereafter referred to as VNK98).

The sonar aperture angle is 2°, indicating a diameter of the footprint of the sonar beam on the ice canopy of nearly 2 m. Accordingly, the above sampling interval indicates that the individual measurements are not biased by overlapping with footprint areas of adjacent measurements.

The jetlike ocean current in Fram Strait (Foldvik et al. 1988) is reflected in a jetlike ice flow through the strait. The ice efflux, therefore, is studied at three intervals: 0°–5°W, 5°–10°W, and 10°–13°W at 79°N. The normalized cross stream ice velocity profile, derived from synthetic aperture radar (SAR) imageries and buoy drifts, reads:
UU0e0.08L
where L = −2.5, −7.5, and −12.5 render the mean meridional ice velocity component, U (m s−1), in the above intervals. Here U0 (m s−1) is the meridional (extrapolated) velocity at L = 0°. Monthly mean ice velocities in the interval 0°–5°W are correlated with the monthly mean air pressure difference, ΔP, over the northern flank of the Northeast Atlantic Trough. The regression equation reads:
U0 to −5P
where the explained variance is R2 = 0.89. When ΔP is zero, the residual velocity is 0.11 m s−1, which is the annual mean velocity of the wind-induced and density-driven current for the period 1993–95. The residual velocity compares with an expected lower value farther north, 0.095 m s−1, obtained by a similar correlation method at 81°N in Fram Strait for the period 1976–84 from buoy drifts (Vinje and Finnekåsa 1986). The cross-stream distribution of the observed ice thickness, h (m), renders the following relationships: h0 to −5 = 0.68h−5, h−5 to −10 = h−5, and h−10 to −15 = h−5, and consequently
h0 to −15h−5
The above relationships have been used to parameterize the monthly wind-induced ice volume flux, V (km3 month−1) expressed with the model formula
VP
where ΔP is the monthly mean air pressure difference between 80°N, 10°W and 73°N, 20°E read from Die Grosswetterlagen Europas (monthly mean weather maps of the German Weather Service). The constant, 154 km3 month−1, represents the monthly mean ice volume flux induced by the ocean currents, representing the 1993–95 annual cross-stream mean of the thermohaline effect plus the transfer of momentum to the upper-ocean surface via the wind-driven ice circulation. The effect of the thermohaline circulation on the ice drift has been held constant, and equal to 40% of the total annual efflux, as observed in 1993–95 (cf. section 7). The rest, 60%, is accordingly due to wind stress. As the mean of h−5 = 2.87 m for the 1990–96 ULS observations, the mean cross-stream ice thickness becomes 2.56 m for this period. Ice thickness measurements for an extended period are given in Fig. 1.

3. Time series of the monthly ice volume export

The time series of the parameterized monthly ice volume flux, reveals a highly variable annual amplitude (Fig. 2). While the average winter maximum is 300–400 km3 month−1 with several extremes of 5–600 km3 month−1, the average summer minimum is 5–10 times less. The 12-month running mean curve indicates that extreme annual effluxes on average recur at a period of 4–5 yr.

The time series of the monthly fluxes indicates that it is only during the recent decade that the Arctic Ocean has served as a recipient of ice, blown northward from the Greenland Sea. This influx of thinner ice is particularly large during November 1993, and the corresponding deviation from the long-term mean monthly winter efflux (−400 to −600 km3 month−1) indicates anomalous drift and compacting in the Arctic Ocean during this month. The special drift conditions occurred in connection with the prevalence of a trough, extending from Iceland over Greenland and the Arctic Ocean to the Bering Sea (Die Grosswetterlagen Europas), indicating the existence of only two centres of action in the circumpolar circulation pattern during this month. Presumably, the November 1993 conditions contributed considerably to the mean cyclonic ice drift observed in the Arctic Ocean for the year 1993 (Proshutinsky and Johnson 1997).

Regression analysis shows no trend in the wind-induced ice efflux over the period considered. The recent increase in the strength of the westerlies, indexed by the air pressure difference between Iceland and Portugal, the North Atlantic oscillation (NAO) winter index (Hurrell 1995), is accordingly not reflected in an increasing departure from the 50-yr mean wind-induced ice efflux. This is illustrated by the very low, or nonexisting correlation (0.1) between the NAO winter index and the ice efflux during the corresponding months. A similarly low correlation (0.1) is also found by Hilmer and Jung (2000), for nearly the same period, 1958–97. Over shorter periods, a positive correlation of 0.6 is observed for 1978–97. This compares with the correlation of 0.66 observed for the area flux by Kwok and Rothrock (1999), for the same period, and with 0.77 obtained by Alekseev et al. (1997) from a parameterized ice volume flux series for 1976–96. However, the present time series renders a negative correlation of −0.32 for the preceding period, 1962–78, indicating that it is only for certain periods that the ice flux from the Arctic Ocean is significantly associated with the NAO winter index in a positive way. The period-dependent correlation is discussed by Dickson et al. (2000), partly on the basis of the present series, by Hilmer and Jung (2000), and by Häkkinen and Geiger (2000), and the authors offer comprehensive observational and modeling evidence for an unstable link between the NAO and the ice export from the Arctic Ocean.

4. Time series of annual ice volume flux and air pressure

a. Annual ice volume flux

Due to the stronger atmospheric circulation during the polar night, the main part of the annual volume flux takes place during the cold season (cf. Fig. 10). The intensity of a certain circulation mode, developing during the autumn and winter, is not observed to persist past the subsequent summer. The below annual means, therefore, refer to the 12-month period August–July.

The time series of annual sums of the parameterized monthly ice volume flux, expressed by the model formula (4), has been tuned with the observed annual volume fluxes for the period 1990–97 (Fig. 3). The tuning comprises an optimizing of the correspondence between the parameterized flux and the observed one for the overlapping period by inclusion of the observed ice thickness for the individual years. The difference between the parameterized annual flux and the observed one, is maximum 13% and generally less than about 5% for the period in question (cf. Fig. 3). The mean annual flux is ∼2900 km3 yr−1, and the standard deviation corresponds to 23% of the mean. This compares with respective, 2870 km3 yr−1 and 20%, estimated from the numeric model by Hilmer et al. (1998) using January–December annual means.

The most conspicuous features in the time series of the annual efflux are the significant fluctuations, which seem to be more the rule than the exception. The 1950s and the 1990s stand out as the two decades with the largest variability over the period considered.

To put the present series into a broader context it can be mentioned that the extreme efflux observed in 1962 (Fig. 3) corresponds in time with an extreme simulated by Walsh et al. (1985) for the period 1951–80, by Häkkinen and Geiger (2000) for the period 1951–93, and by Polyakov and Johnson (2000) for the period 1946–97. The extreme maximum efflux in 1995 corresponds in time with a maximum simulated by Hilmer et al. (1998) for the period 1958–97 and with the above mentioned simulation by Polyakov and Johnson. A certain correspondence should be expected as all the investigations rely upon the observed atmospheric circulation, though with different refinement and resolution.

The annual ice volume export is fluctuating around a mean of 2900 km3 yr−1, with an insignificant linear trend of −10 km3 yr−2, suggesting close to stationary conditions for the period 1950–2000. The variability in the ice efflux is moreover so large that a far longer series is necessary to obtain statistical significance regarding temporal changes. As the majority of the ice that leaves the Arctic Ocean passes through Fram Strait (VNK98), stationary conditions imply that the annual ice formation in the reservoir is of the same order as the mean annual ice efflux.

b. The air pressure distribution in Fram Strait (FS) and Barents Sea (BS)

The ice efflux is parameterized by a single parameter, ΔP, representing the air pressure difference across the northern flank of the northeast Atlantic trough, between 80°N–10°W in FS and 73°N–20°E in the BS. The time series of the annual mean air pressure at the two locations (Fig. 4a), shows a highly variable temporal course. There is, however, a clear indication of a parallel reduction at the two locations in the annual mean air pressure of ∼3 hPa. An analogue parallelism is also observed for the reduction in the mean air pressure for the winter months: December–January–February–March (DJFM, Fig. 4b). The variability is, however, far higher for the winter season. We note also the significant fall in the air pressure of ∼7 hPa, indicating that an intensification of the high-latitude winter circulation has taken place over the past 50 years. This concurs with the observation of a significant increase in the winter and spring circulation in the central Arctic, particularly after the late 1980s (Walsh et al. 1996). In addition, Thompson and Wallace (1998) conclude that a noticeable strengthening of the polar vortex has taken place during wintertime over the past 30 years. They observe a variability in this increase denoted as the Arctic oscillation (AO), reflected in a variation of the sea level pressure (SLP).

The observed increase in the high-latitude atmospheric circulation is, however, not reflected in a corresponding increase in the ice efflux through Fram Strait. It is assumed that the similarity in the temporal reduction rate of the air pressure at the two locations, reflects the observed long-term stationary conditions in the ice efflux; or that the air pressure fall is so uniform over the area considered that it does not affect the wind stress gradient across the Fram Strait. This is also reflected in the very low correlation of −0.15 between the SLP expansion coefficient of the AO (Thompson and Wallace 1998) and ΔP. Based on simulations from a coupled ice–ocean model, Häkkinen and Geiger (2000) also conclude that the AO does not have much influence on the sea ice export through the Fram Strait.

5. Ice flux anomalies and subsequent down- and upstream anomalies

a. Corresponding downstream anomalies

Some of the ice efflux anomalies given in Fig. 5 are associated with observed anomalies in the recipient area. For example, the repeated positive anomalies for the period 1965–68, adding up to ∼2300 km3, is contemporary with the influx of low upper-water salinity and ice advected from the east Greenland Current into the Iceland Sea during the extreme ice years of 1965 and 1968 (Malmberg 1972). This event also precedes the marked negative salinity anomaly observed in 1970–71 in the Labrador Sea (Lazier 1995). More recently, the 1981–84 positive anomalies, also adding up to ∼2300 km3, correspond again in time with a second significant negative salinity anomaly in 1984 in the Labrador Sea (cf. Fig. 7 in Lazier 1995). A correlation study indicates a clear maximum correlation for a 3-yr propagation time of anomalies between the Fram Strait and the Labrador Sea.

Gammelsrød et al. (1992) and Blindheim et al. (2000) report on a negative temperature anomaly in the Norwegian Sea with a minimum temperature centered on 1980–82. Blindheim et al. argue that this change is caused mainly by influx to the Norwegian Sea of colder water from the east Greenland Current via the east Icelandic Current. This cooling could possibly also be enhanced by the return to the Norwegian Sea of the 1965–68 anomalies via the North Atlantic circulation system (Mysak and Venegas 1998). The cooling in the Norwegian Sea since 1980 is also subsequently reflected in a noticeably reduced oceanic (thermal) effect on the April ice extent in the Barents Sea since 1984 (Vinje 2001). A large positive ice efflux anomaly of close to 1500 km3 is observed over only 1 yr, 1994–95. This anomaly is, however, carried so rapidly through the Denmark Strait by strong NE winds (Hilmer et al. 1998), that it should not affect the conditions in the eastern part of the Nordic Seas, but rather the area southwest of the Denmark Strait during subsequent years.

b. Corresponding upstream anomalies

Ice efflux anomalies should also reflect anomalies in the Arctic Ocean. A markedly increased, or reduced export of ice should indicate an extra subsequent thinning, or thickening of the ice upstream of the Fram Strait.

Using microwave imageries, Johannessen et al. (1999) estimate a reduction of 14% of the area of multiyear (MY) ice during winter in the Arctic Ocean over the period 1978–98. The significant year to year variations, which are observed, particularly after 1987, can be compared with the significant variations observed in the ice efflux through Fram Strait for the overlapping period, 1990–98.

Comparing the year to year change of the ice area efflux through Fram Strait with the 1-yr-lagged change in MY ice area in the Arctic Ocean, a strong negative correlation of −0.95 is obtained (Fig. 6). This indicates that positive/negative anomalies in the Fram Strait efflux correspond optimally with 1-yr-lagged negative/positive anomalies in the winter MY ice area in the Arctic Ocean.

As the ice has a much higher velocity than the ocean mixed layer, buoyancy anomalies will be distributed relatively rapidly in the circulation system. It is therefore possible that freezing processes under the considerably increased efflux during the winter of 1994/95 (Fig. 5) may have added salt to the maximum divergent exit route along the Eurasian Basin (Hilmer et al. 1998), where increased salinity was observed in the summer of 1995 (Steele and Boyd 1998).

The significant variability in the ice efflux during the 1990s is similar in magnitude to the variability observed during previous decades. It may therefore be assumed that the ice thickness distribution and salt release in the Arctic Ocean have varied markedly from period to period during the last 50 yr (cf. section 6).

c. Correspondence between ice thickness in Fram Strait and the Arctic Ocean

A previous comparison between all ice thickness measurements in the Arctic Ocean, 1958–92, and the 6-yr-long series from Fram Strait (VNK98), indicates that there is a good correspondence between the ice thickness in the Arctic Ocean and the Fram Strait. Recent submarine data for the period 1993–97 (Rothrock et al. 1999) indicates a mean ice draft at the end of the melt season of 1.8 m for an area upstream of the Fram Strait, covering about 40% of the Arctic Ocean. Rothrock et al.'s draft compares with the September cross-stream mean ice draft of 1.8 m measured in Fram Strait 1990–96 (cf. Fig. 11 and caption). For a more careful comparison, we have to consider the footprint error (FPE), which is dependent upon the operational depth and the aperture angle of the sonar (Wadhams 1981). Provided an aperture angle of 2°, the FPE causes a systematic overestimation of the mean ice draft, ranging from about 0.2 to 0.4 m for sonar depths of 50 and 100 m, respectively (VNK98). Assuming an operational depth of the submarines of 100 m, the above September mean ice draft, corrected for the FPE, would be about 1.4 and 1.6 m, in the Arctic Ocean and in the Fram Strait, respectively. Although the ice passing through the Fram Strait is affected to a larger extent by the thicker ice north of Greenland than is the ice in the central Arctic Ocean, the similarity in the two figures indicates a close correspondence between the ice thickness in the reservoir and in the exit area at the end of the melting season.

6. Ice volume efflux variations and the effect on the Arctic Ocean ice thickness

The area of the Arctic Ocean, ∼6.7 × 106 km2, divided by the annual mean ice area efflux, 1.10 × 106 km2 (Fig. 7), indicates a mean renewal time of the ice in the reservoir of ∼6 yr.

Gordienko and Karelin (1945) gave the first ice efflux figure of 1.04 × 106 km2 yr−1 for the period 1933–44. Later on, a number of authors have by various methods estimated the annual ice area efflux to be of the order of 1 000 000 km2 yr−1 [Volkov and Gudkovichš (1967) for 1954–64, Vinje (1982) for 1966–75, Vinje and Finnekåsa (1986) for 1976–84, World Climate Research Programme (1994) for 1979–92, and VNK98 for 1990–96]. The estimated mean renewal time seems accordingly to be valid also for the extended period ∼1930–2000.

A similar mean renewal time is also obtained for the ice volume in the Arctic Ocean: {[(6.7 × 106 km2) × (0.00256 km)]/2900 km3 yr−1} = 6 yr. When distributing the 6-yr net ice volume export anomaly over the entire Arctic Ocean area, the oceanwide mean 6-yr ice thickness anomaly may be obtained as a balance for the volume drainage through the Fram Strait (Fig. 8).

A comparison between the left and right scales in Fig. 8, indicates that the variation of the ice volume flux through the Fram Strait causes an alternating thinning and thickening of the Arctic Ocean ice fields over a period of 7–12 yr, within a full cycle length of 15–19 yr. Provided a distribution of the volume flux anomalies over the entire Arctic Ocean, the maximum decadal thinning, or thickening is of the order of 0.8 m. As the effect of the thermohaline circulation has been held constant in the parameterized efflux formula, this decadal variability in ice thickness is solely due to a cyclic variation in the atmospheric circulation, indexed by ΔP.

The period of 7–12 yr observed in the efflux (Fig. 8) and in ΔP (Fig. 9), compares with the period of ∼9 yr simulated by Häkkinen and Geiger (2000). The authors also conclude that the dominant decadal period in the high-latitude ocean is produced by local (wintertime) wind forcing linked to the Barents Sea throughflow. The period also corresponds with a period of 10–15 yr in ice-thinning and ice-thickening processes due to alternating cyclonic and anticyclonic ice circulation in the Arctic Ocean (Proshutinsky and Johnson 1997; Polyakov et al. 1999). Their modeled cyclonic and anticyclonic ice circulation periods are noted in (Fig. 8). Apart from the 1950s and the 1960s, there seems to be a fair correspondence between the variation in the ice circulation mode in the Arctic Ocean and the variation in the ice volume efflux through the Fram Strait.

7. Annual cycles

The 50-yr mean annual cycle of the two main parameters (P and the mean ice thickness are given in Figs. 10 and 11, respectively). Relatively strong northerly winds are observed over the Fram Strait from October to April with considerably weaker winds for the rest of the year, May–September. Southerly weak winds prevail during July and August only. The wind-driven component of the ice efflux is therefore on average close to zero for the summer months, when the ice is moving mainly with density-driven ocean currents (cf. Fig. 12).

The cross-stream ice velocity, , attains its maximum of ∼0.15 m s−1 for the winter months and an average minimum of ∼0.05 m s−1 during the summer months when the ice is mainly carried by the surface currents (Fig. 12). The annual mean cross-stream ice velocity is ∼0.11 m s−1. Correlation analysis shows that c = 0.27 + 0.045, (R2 = 0.77), suggesting an annual mean cross-stream, density-driven current, of 0.045 m s−1. This compares with 0.025–0.05 m s−1 obtained from a three-dimensional model on the basis of averaging 40 yr of data (Polyakov and Timokhov 1995). I. V. Polyakov (2000, personal communication) states also that the winter currents are slightly stronger by 0.01 m s−1 than the summer currents. The annual mean of c amounts to ∼60% of , in accordance with the above partition between wind- and current-induced fluxes (cf. Fig. 3 caption). Of this 60%, about 40% is induced by density-driven currents and 20% by the wind-induced current, caused by the frictional transfer of momentum between ice and water. Summing up, this indicates that in the Fram Strait 60% of the ice flux is related to ΔP, and 40% is related to the density-driven background current, or the thermohaline circulation.

The monthly mean ice volume fluxes calculated by using the parameterized model formula (with a constant ice thickness) is indicated by a dotted line in Fig. 13. By introduction of the mean annual cycle of ice thickness observed 1990–96, we obtain a somewhat different curve, indicated by the solid line in Fig. 13. The difference between the two lines indicates the adjustment for the larger mean ice thickness observed during March–July and the smaller one observed during the rest of the year (cf. Fig. 11). The reduced volume flux during January and February, when the ice velocity attains its annual maximum, is mainly due to annually repeated influx of thinner ice from the margins north of Spitsbergen, Norway, by relatively strong, easterly winds (VNK98).

8. Summary and conclusions

Stationary conditions are observed for the parameterized ice efflux, in the sense that there is no linear trend over the 50-yr period considered. Stationary conditions imply that the mean annual ice efflux, 2900 km3 yr−1 (σ = 667 km3 yr−1), also represents the long-term mean annual net production of ice in the Arctic Ocean caused by the radiative heat loss.

The effect of the thermohaline circulation on the ice drift, and also the ice thickness have been held constant in the efflux formula. This implies that a variation in the ice efflux is solely determined by a variation in the atmospheric circulation, or, more specifically, the air pressure difference, ΔP, across the northern flank of the Northeast Atlantic Trough. The high variability in the annual atmospheric forcing is therefore characterized by the above high standard deviation in the efflux.

An air pressure fall of ∼7 hPa takes place in the area during the winter months over the period considered. However, the pressure fall is so regionally uniform that it does not affect the magnitude of the wind-driven efflux. This implies that the cross-strait air pressure difference, ΔP, is invariant with regard to increase in high-latitude atmospheric circulation as characterized by the Arctic oscillation (Thompson and Wallace 1998).

Although the 1950s and the 1990s stand out as the two decades with extra large variability, significant variations seems to be more the rule than the exception during the whole period. The effect of ice efflux anomalies in the Fram Strait can be traced upstream the following year, and downstream within a couple of years. The overall impression is that ice flux anomalies in the Fram Strait have a high predictive skill for subsequent anomaly occurrence in salt, temperature and ice thickness in the adjacent seas.

The mean annual ice efflux indicates a mean renewal time of the ice in the Arctic Ocean of 6 yr. The 6-yr running mean flux anomaly reveals a systematic alternating thinning and thickening of the ice in the Arctic Ocean over periods of 7–12 yr within a full cycle of 15–20 yr. On condition that the anomaly in the ice volume drainage through the Fram Strait is distributed over the entire Arctic Ocean, continuity requires that the thickening, or thinning of the oceanwide ice cover amounts to 0.8 m over periods of 7–12 yr.

The latter period compares with a period of ∼9 yr observed in the high-latitude ocean (Häkkinen and Geiger 2000). The authors ascribed this period to a variation in high-latitude wintertime wind forcing affecting the variability of the flow through the Barents Sea. This concurs with the observation of a period of ∼8 yr in the ocean temperature at the Ocean Weather station M in the Norwegian Sea (Gammelsrød et al. 1992) and the subsequent decadal variability of the oceanic thermal effect on the April ice extent in the Barents Sea (Vinje 2001). A link to the North Atlantic circulation may accordingly be effected by a decadal variation in the ocean temperature anomalies advected to the Barents Sea causing a corresponding variable effect on the local atmospheric circulation, indexed by ΔP.

The observed decadal period in the efflux anomaly compares also with the period of 10–15 yr for the alternating cyclonic and anticyclonic ice circulation in the Arctic Ocean with a respective thinning and thickening of the ice cover (Proshutinsky and Johnson 1997). A decadal thinning and thickening of the ice cover may help explain the significant thinning observed in 40% of the Arctic Ocean over the past two–three decades (Rothrock et al. 1999), as well as the variability observed in the extent of the winter multiyear ice cover in 60% of the Arctic Ocean over the last two decades (Johannessen et al. 1999).

The significant increase of ∼40% in the ice efflux from 1990 to 1997 indicates a contemporary decreasing ice thickness in the Arctic Ocean during the recent decade. However, provided a continuation of the observed cycling in the ice efflux, or the atmospheric circulation, we should expect a reduction in the ice efflux, and, a thickening of the ice in the Arctic Ocean during the next decade. A future thickening of the ice is also predicted by Polyakov et al. (1999) in connection with an observed reversal to an anticyclonic ice circulation in the reservoir toward the end of 1990.

Acknowledgments

This is a publication under the project Arctic Climate System Study (ACSYS) of the World Climate Research Programme. The Norwegian Polar Institute and the International Arctic Research Center, University of Alaska, Fairbanks, have sponsored the work. T. B. Løyning at the ACSYS ULS-data repository has evaluated the ice thickness monitoring series given online at http://npolar.no/ADACIT/. Thanks are also extended to two reviewers for supportive comments and suggestions.

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Fig. 1.
Fig. 1.

Time series of the monthly mean ice thickness at ∼79°N. The 1987–88 measurements are obtained at 75°N (Vinje 1989) and adjusted to 79°N according to recent contemporary measurements at the two locations (not published). The mean of the 1990–97 measurements is 2.87 m as indicated by the broken line, the std dev is 0.68 m, and the mode is 2.85 m. (Available online at http://www.npolar.no/ADACIT.)

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 2.
Fig. 2.

Gray columns: Time series of the parameterized monthly ice volume flux through Fram Strait, 1950–2000. Black line: 12-month running mean

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 3.
Fig. 3.

Time series of the parameterized annual ice volume flux as tuned with recent observations indicated by filled circles (VNK98 plus updates). The mean annual ice flux caused by wind-induced and density-driven ocean currents is, according to the model formula, equal to 1848 km3 yr−1 (=12 × 154 km3 month−1), corresponding to about 60% of the long-term mean, 2900 km3 yr−1, indicated by the horizontal line. This percentage share compares with 50% obtained from a study of buoy drifts in the Arctic Ocean by Thorndike and Colony (1982)

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 4.
Fig. 4.

Time series of (a) the annual mean air pressure and (b) the winter mean air pressure at 80°N–10°W in FS, and at 73°N–20°E in the BS. The linear regression lines indicate a fall in the annual mean air pressure of 3.1 and 2.6 hPa in FS and BS, respectively. The corresponding falls in the air pressure for the winter months are 7.1 and 6.6 hPa

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 5.
Fig. 5.

Time series of the anomalies of the parameterized annual ice volume flux

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 6.
Fig. 6.

The effect of a flux change in the Fram Strait on the winter MY ice area in the AO the following year. The extreme year to year variations (which are determinative for the high correlation) correspond to the extreme variations between 1993 and 1996 given in Fig. 5. Adopted from Fig. 6 in Johannessen et al. (1999) and from VKN98 plus updates

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 7.
Fig. 7.

Time series of the annual ice area flux through the Fram Strait as tuned from the observations in 1990–96. The mean annual ice area efflux, 1.10 × 106 km2 (σ = 0.16 × 106 km2), is indicated by the horizontal broken line

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 8.
Fig. 8.

Time series of the 6-yr moving net anomaly wind-induced ice volume export through the Fram Strait (left scale) and the corresponding effect on the Arctic Ocean mean ice thickness anomaly (right scale). The relationship between the two scales is [(left scale)/(Arctic Ocean mean annual ice extent)] = −right scale. Here C indicates periods with cyclonic ice circulation and thinning of the ice, and A indicates periods with anticyclonic ice circulation and thickening of the ice in the Arctic Ocean according to Proshutinsky and Johnson (1997)

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 9.
Fig. 9.

The 6-yr running means of the winter (DJFM) air pressure anomaly in BS and FS. The trend lines indicate the 50-yr air pressure tendency

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 10.
Fig. 10.

The 1950–2000 monthly mean air pressure difference, ΔP, between 81°N, 10°W and 73°N, 20°E as read from Die Grosswetterlagen Europas

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 11.
Fig. 11.

(triangles) Annual cycle of the monthly mean thickness of ice floes. (Open water observations excluded). (circles) Annual cycle of the monthly mean ice thickness for 1990–96. (All observations included). (broken line) Annual cycle of the ice thickness mode. The ice thickness is estimated from the ratio between draft and ice thickness, 1.136, as observed from drillings in the Fram Strait. The Sep cross-stream mean ice draft is 1.8 m (= 2.3 m × 0.89/1.136). The thicker ice floes observed during the summer (triangles) illustrate the repeated influx of thicker ice from north of Greenland observed to take place during this season. Adapted from VNK98

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 12.
Fig. 12.

(upper curve) The annual cycle of the cross-stream ice velocity, , for 1950–2000. (middle curve) The cross-strait current-induced ice velocity, c, 1993–95. Here is determined from V/(B), where V is given by (4), is the mean cross-strait ice thickness 1990–96 (2.56 m), and B is the mean breadth of the ice stream, 315 km. Here c is determined as a residual for the different seasons by regression analysis in analogy with relationship (2), using the data in Table 8 in VNK98. (lower line) The annual mean velocity of the 1993–95 cross-stream density-driven current as determined below

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

Fig. 13.
Fig. 13.

Annual cycle of the ice volume flux through the Fram Strait. The dotted line shows the 1950–2000 parameterized mean monthly volume flux and the solid line shows the mean monthly volume flux adjusted for the annual cycle of the mean ice thickness observed in 1990–96 (cf. Fig. 11)

Citation: Journal of Climate 14, 16; 10.1175/1520-0442(2001)014<3508:FSIFAA>2.0.CO;2

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