Variability in the largescale motion field

The mean annual characteristics of ice motion across the Arctic have already been briefly covered. Here, we show some examples of seasonal and interannual variability. That patterns of sea ice drift exhibit pronounced seasonality is broadly understood from the seasonality in the mean sea level pressure field. During winter there is a low-pressure trough in the Atlantic sector of the Arctic with higher pressures over the central Arctic Ocean. This is seen as a strong Beaufort Gyre, Transpolar Drift Stream, and a pronounced ice flux through Fram Strait (Figure 7.6). The winter pattern is quite similar to that for the annual average (Figure 7.3). In summer, the pressure trough over the Atlantic sector is essentially gone and a weak ridge develops over the Barents and Kara seas. Weak low pressure is found centered near the pole, while anticyclonic conditions prevail in the Beaufort Sea. In turn, the Beaufort Gyre retreats south into the Beaufort Sea. Ice motion is more cyclonic over the Eurasian side of the Arctic, and the Fram Strait outflow is weaker (Figure 7.7). It is nevertheless evident from the mean annual, winter and summer plots that the relationships between mean drifts and mean pressure fields are only general.

As illustrated in the examples provided in Figure 7.8 and Figure 7.9, mean drift patterns differ considerably from month to month. These two figures, showing mean drifts for January 1989 and 1991 with corresponding sea level pressure overlays, employ the displacement of ice features observed in SSM/I passive microwave imagery (Agnew et al., 1997; Emery et al., 1997). The advantage of these satellite records is that gridded fields of ice motion can be obtained at 25-km spatial resolution and daily temporal resolution. While daily fields can be calculated from the IABP data, the low density of stations for any one day means that finer-scale features are missing. The SSM/I data also provide fuller spatial coverage, providing ice drift information in areas such as Baffin Bay. A disadvantage of passive-microwave ice motion products is that the accuracy of the retrievals degrades during summer due to melt effects. Passive microwave algorithms differentiate sea ice and open water from differences in emissivity in centimeter wavelengths. Liquid water on the sea ice reduces this emissivity contrast.

Neither January shows evidence of the mean Beaufort Gyre circulation. In January 1989 there is generally cyclonic drift over most of the central Arctic Ocean around the broad pressure trough. By contrast, for January 1991, the dominant feature is a motion of ice from the Eurasian coast, across the Arctic and toward Alaska. In Baffin Bay, there is no obvious relationship between the ice drift and the inferred wind field, pointing to a strong role of ocean currents.

Figure 7.6 Mean pattern of sea ice drift in the Arctic for winter, 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 7.6 Mean pattern of sea ice drift in the Arctic for winter, 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).

0 0

Post a comment