Monitoring ice thickness in Fram Strait

Time series of ice thickness in Fram Strait are evaluated together with ice drift for estimation of the ice flux from the Arctic Ocean. Ice draft was measured from moored sonars every 4 min, suggesting a nominal sampling interval of 30 m. On the basis of these data the annual cycle of many statistics can be resolved. The modal ice thickness shows a maximum of 3.15 m in May and a minimum in September of 2.43 m, indicating an annual variation range of 0.72 m. This compares with model results, comprehensive drillings, and most of the submarine observations during the last 2-3 decades, indicating no significant long-term ice thickness change. This is also concurrent with the lack of significant temperature changes in the Arctic Ocean during the same period. The mean ice thickness in the central core of the ice stream shows an annual amplitude of 1 m with a minimum in September of 2.25 m and a maximum in April-May of 3.25 m. The mean thickness of ice only (open water excluded) is 3.27 m and compares with 3.29 m which is the mean thickness of ice only from 17 submarine tracks in the Arctic Ocean 1960-1982. The mean annual ice export, which is equal to the mean annual net production of ice in the Arctic Ocean, is similar to 2850 km(3) yr(-1) for the period 1990-1996. This efflux shows significant variations from year to year with a minimum in 1990-1991 of similar to 2050 km(3) yr(-1) and a maximum in 1994-1995 of similar to 4700 km(3) yr(-1). This significant variation of nearly 130% is mainly caused by an increased atmospheric forcing and to a lesser degree by an increase in the annual mean ice thickness of similar to 20% over the same period.