The Fram Strait outflow

As the primary region for the export of sea ice and low-salinity water out of the Arctic, the Fram Strait outflow merits special attention. A number of efforts have been made to estimate the ice area or volume flux through Fram Strait based on mass balance requirements, measurements of ice drift and thickness through the strait, models or a combination of models and observations. The total ice flux comprises a wind-driven and a current-driven component. Determining the volume flux requires accounting for the ice velocity through the strait, the ice concentration and ice thickness.

Some early estimates based on direct measurements include Wadhams (1983) and Vinje and Finnekasa (1986). Vinje and Finnekasa (1986) also used an indirect method to estimate the flux. A regression equation was developed between available time series of the observed monthly averaged drift speed profile across the strait and the monthly difference in sea level pressure between Fram Strait (81° N, 15° W) and the central area of the Norwegian Sea (73° N, 5° E). The slope of the regression equation represents the fraction of the ice velocity due to time-varying winds while the y-intercept represents the constant effect of the ocean current. The regression equation was used to compile volume flux time series by incorporating climatological monthly values of the ice thickness and concentration. The advantage of this approach is that since long time series of sea level pressure are available, long time series of the estimated flux can be obtained.

Time series of estimated ice concentration and velocity are available from satellite data since the late 1970s. Moored upward-looking sonar (ULS) sites in and around the strait have provided estimates of ice thickness since about 1990. Kwok and Rothrock (1999) took advantage of satellite and ULS data to obtain the cold-season (October-May) area flux over the period 1978-96. Ice velocities across the strait were obtained from an ice tracking procedure like that outlined earlier using sequential images from SMMR and SSM/I. The derived velocities were used in conjunction with SMMR and SSM/I ice concentration data to obtain area flux time series. They obtained monthly volume fluxes by adjusting the velocities based on profiles of mean monthly ice thickness from ULS. However, their technique was not applied to summer months because of biasing of the passive microwave data by melt effects.

A more recent study is that of Vinje (2001). A regression-based approach was used along with the best available estimates of ice velocity and ice thickness from SAR, buoy drifts and ULS to provide an ice volume record extending from 1950 to 2000. The mean annual flux is about 2900 km3. Vinje estimates that 60% of the total annual ice

Figure 7.13 Mean annual cycle of the ice volume flux through Fram Strait. The dotted line shows the 1950-2000 parameterized mean monthly flux. The solid line shows the volume flux adjusted for the mean ice thickness observed from 1990 to 1996 using upward-looking sonar (from Vinje, 2001, by permission of AMS).

Figure 7.13 Mean annual cycle of the ice volume flux through Fram Strait. The dotted line shows the 1950-2000 parameterized mean monthly flux. The solid line shows the volume flux adjusted for the mean ice thickness observed from 1990 to 1996 using upward-looking sonar (from Vinje, 2001, by permission of AMS).

Figure 7.14 Time series of the parameterized monthly ice volume flux through Fram Strait (grey columns) and the 12-month running mean (black line) (from Vinje, 2001, by permission of AMS).

volume flux is wind driven while about 40% is due to the background ocean current. The mean cross-strait ice thickness is 2.56 m.

A striking feature of the outflow is its pronounced annual cycle (Figure 7.13). The largest volume fluxes are found in autumn and winter, when there are strong northerly winds in the strait. The flux is smallest in summer when the northerly winds slacken and are replaced by weak southerlies in July and August. A comparison between the two lines in Figure 7.13 illustrates the effects of adjusting for the mean annual cycle of ice thickness. The adjustment accounts for the thicker ice observed from March through July and the thinner ice for the rest of the year.

A conspicuous feature of Vinje's (2001) reconstructed monthly time series (Figure 7.14) is the large variability. While there are no obvious long-term trends in Figure 7.14, other investigators have noted a trend in either the volume or area fluxes from the late 1970s through much of the 1990s (e.g., Kwok and Rothrock, 1999). These trends are related to a change toward the generally positive phase of the Arctic Oscillation and North Atlantic Oscillation. This issue will be examined in Chapter 11.

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