AIDJEX Bulletin #40 - Polar Science Center - University of Washington

From Figure 7 it is seen that for 27 January, when the ice was still
although fairly strong winds were blowing, the effect of this error had a
negligible effect on the ice response. In all cases the ice was predicted
to remain at rest, and this was observed. We point out at this point that
the response observed with drifting buoys agrees with the baseline values.
The details of this information are given by Pritchard et al. (1977). We
see in Figure 8 that the ice response predicted for 30 January tends to
match the observed response. However, the width of the region that is nearly
at rest along the North Slope around to Banks Island is quite different in
the three cases. The predicted values from 24 and 36 hours show this region
at rest to be much larger than what was observed. Although the 48-hour prediction
provides a velocity field nearly identical to the observed field and
baseline field, we must consider this fortuitous, and we do not expect this
result to be typical. In Figure 9 weshowthe ice velocity field during 2 February,
when the direction has changed. All velocity fields generated using 24-,
36-, and 48-hour predicted air stresses are essentially the same.
In this winter simulation when the ice strength is important, errors in
the air stress do not appear to be transmitted directly into errors in the ice
response. Under these conditions it appears that the predicted ice response
is relatively insensitive to local errors in air stress. It is suggested that
the ice responds to a spatial average of the air stress field,and so local
details of the air stress are less important. A comparison with Figure 2 shows
that the average air stress on 30 January was only about half the observed
value, so that not only are local errors evident, but the large-scale average
error over this entire region is considerable.
We must be careful not to lean too heavily on the results for 30 January
because of an error that occurred during the baseline calculation. As has been
discussed by Pritchard et al. (1977), erroneous data were used to generate the
baseline pressure map; they increased the geostrophic flows in the region of
the manned array to values approximately 25% greater than observed. We must,
therefore, reduce the observed motions accordingly in this simulation, which
would tend to reduce the size of the area brought into motion by the air stress
and reduce the magnitude of the velocities within the area as well.
In summary, then, we must say that, when ice stress has a significant
effect on ice response, a difference in air stress between what is predicted
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