Mean circulation ice zones and concentration 721 Mean annual circulation

While the characteristics of ice motion will be examined in detail in Section 7.3, it is useful at this point to examine the mean annual drift. Since 1979, the IABP has maintained a network of drifting buoys in the Arctic Ocean. Ice drift records are also available from the NP program. Additional data have been collected by manned US drifting camps, for example T-3. Gridded fields assembled from these Lagrangian drift records provide for assessments of the large-scale ice drift. Figure 7.3 shows the mean annual pattern based on these data.

The mean annual drift of the pack ice consists of two primary features. The first is the Beaufort Gyre, the clockwise (anticyclonic) motion of ice in the Canadian Basin. The second is the Transpolar Drift Stream (TPDS), characterizing the motion of ice away from the Siberian coast, across the pole and through Fram Strait. This mean pattern reflects roughly equal contributions by winds and surface ocean currents. Hence, the mean ice drift pattern roughly resembles the distribution of mean sea level pressure. The overlay of annual sea level pressure in Figure 7.3 is for 1979-99, the period that most of the drift data are based on.

With a mean drift speed of 1-3 cm s-1, typically, 5-10 years are required for ice to make one circuit around the Beaufort Gyre (Thorndike, 1986). Ice can circulate around

Figure 7.3 Mean annual sea ice drift in the Arctic, based on data from the IABP, the North Pole program and other sources, with overlay of sea level pressure from NCEP/NCAR (ice drift field courtesy of I. Rigor, Polar Science Center, University of Washington, Seattle, WA, sea level pressure field by the authors).

Figure 7.3 Mean annual sea ice drift in the Arctic, based on data from the IABP, the North Pole program and other sources, with overlay of sea level pressure from NCEP/NCAR (ice drift field courtesy of I. Rigor, Polar Science Center, University of Washington, Seattle, WA, sea level pressure field by the authors).

the gyre for many years. It can take up to 3 years for a parcel of ice to move along the TPDS (Thorndike, 1986). Ice velocities in the TPDS progressively increase toward Fram Strait, where mean drift speeds are 5-10 cm s-1. Still faster ice velocities are observed in the Greenland Sea. Approximately 20% of the total ice area of the Arctic Basin annually exits through Fram Strait (Thorndike, 1986), of which 80% consists of MYI (Gow and Tucker, 1987).

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