Geoscience Reference
In-Depth Information
wind stress and to increase the turning angle between the geostrophic wind and the
wind stress vector (Albright,
1980
). On annual and longer time scales, variations in
ice motion are approximately 50 percent wind driven and 50 percent current driven.
Because winds are highly variable, their forcing tends to cancel out on longer time
scales, so that relatively steady current effects play a stronger role.
7.3.2
Variability in the Large-Scale Motion Field
The mean annual characteristics of ice motion across the Arctic have already been
covered. Here, we show some examples of seasonal and interannual variability.
That mean patterns of sea ice drift exhibit pronounced seasonality is broadly under-
stood 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, a Transpolar
Drift Stream, and a pronounced ice flux through the Fram Strait (
Figure 7.11
). The
winter pattern is quite similar to that for the annual average (
Figure 7.4
). In sum-
mer, 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 cen-
tered near the pole, and 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 7.12
). 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 7.13
and
Figure 7.14
, mean
drift patterns differ considerably from month to month. These two figures, show-
ing mean drifts for January 1989 and 1991 with corresponding sea level pressure
overlays, employ the displacement of ice features observed in SSM/I passive micro-
wave imagery (Agnew, Le, and Hirose,
1997
; Emery, Fowler, and Maslanik,
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. These are the
same data used to track ice age. Whereas daily fields can be calculated from the
IABP data, the low density of stations for any one day means that finer-scale fea-
tures 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
because of melt effects.
Neither of the two Januarys show evidence of the mean Beaufort Gyre circu-
lation. In January 1989, there is generally cyclonic drift over most of the central
Arctic Ocean around a 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.
