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70,000

with low ice exports around 1920/1930 and high exports in the 1950s and 1960s. Kwok et al. (2004) used ULS data from 1991 to 1998 and found a mean ice export through Fram Strait of 70,000 m3/s with a standard deviation of approximately 15,000 m3/s. The authors indicated that 50% of the ice export takes place during December and March while the summer export is weak. It has to be noted that both the mean export and the standard deviation is substantially smaller in Kwok et al. (2004) than in Vinje et al. (1998) although the same ULS data and almost the same time period has been used. Kwok et al. (2004) derived the ice motion from satellite passive microwave data while Vinje et al. (1998) used the SLP gradient across Fram Strait to estimate the ice velocity.

Table 8.2 summarizes model simulations of the Fram Strait ice export. Häkkinen (1993) used an ocean-sea ice model for the Arctic and the northern North Atlantic, forced with monthly means of NCEP/NCAR-reanalysis data (Kalnay et al. 1996). The ice export is relatively small with 63,000 m3/s because ice thickness is slightly underestimated in the model. Simulations with sea ice models (Hilmer et al. 1998; Arfeuille et al. 2000) and ocean-sea ice models (Koeberle and Gerdes 2003; Haak et al. 2003), forced by 40- or 50-year reanalysis data, all indicate a high interannual to decadal variability. All model simulations show pronounced ice export events in 1967/68 and in 1994/95. The mean exports are similar (83,000-104,000 m3/s), except for the model of Arfeuille et al. (2000) that simulated an average ice export of 160,000 m3/s. Nevertheless, their ice export anomalies compare well with the other model simulations.

In this study, a global coupled atmosphere-ocean-sea ice model is used. Hence, only statistics of the time series (Fig. 8.1a) can be compared to observations and other studies. The mean export amounts to 97,000 m3/s, which is in the upper range of observation-based estimates and model simulations. The ice export is highly variable on interannual time scales with a standard deviation of 21,000 m3/s for annual mean exports. The monthly mean ice export through Fram Strait, averaged over the 500-year control integration (Fig. 8.1b), shows a pronounced seasonal cycle. The maximum occurs in March with an average of 147,000 m3/s and the minimum in August with 35,000 m3/s. This agrees with observation-based estimates by Vinje et al. (1998) and parameterizations by Vinje (2001). The standard deviation has been calculated for each month.

Table 8.2 Model simulations of the mean sea ice export through Fram Strait in m3/s

Author Time Ice export (m3/s)

Häkkinen (1993) 1955-1975 63,000

Hilmer et al. (1998) 1958-1997 91,000

Arfeuille et al. (2000) 1958-1998 160,000

Koeberle and Gerdes (2003) 1948-1998 83,000

Haak et al. (2003) 1948-2001 104,000

Koenigk et al. (2006) 500-year ctrl-run 97,000

Fig. 8.1 (a) Annual mean ice export through Fram Strait in m3/s. (b) Monthly mean ice export (solid) and ice export ± 1 standard deviation (dashed) in m3/s, averaged over the 500-year control integration for each month
Table 8.3 Correlation between seasonal mean ice exports through Fram Strait. Seasons, written in the horizontal, lead seasons, written in the vertical. The last row indicates the correlation between the annual mean ice export (averaged from September to August) and the single seasons

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