166
M.R. DRINKWATER
buoy data in 1992. Corresponding length scales are computed from European Centre
for Medium Range Weather Forecasting (ECMWF) analysis fields of atmospheric pressure and geostrophic wind. The length scales of sea ice drift and atmosphere appear to
vary seasonally and the ice-drift field is especially dependent upon location and primary drift direction with respect to the coast.
8.5.1.1
Coastal Regime
In the northwards drifting perennial ice regime of the western Weddell Sea, ice motion
is forced by both wind and currents. Periodic deviations from this relatively constant
drift pattern are caused by passing cyclones or "polar lows:' Because of the large fractions of thick ice, ice conditions most closely resemble those of the Arctic basin, and
thus the ice drift correlation length scales may be expected to vary seasonally and interannually with ice concentration. In 1992, Argos buoys were spaced at intervals in the
east-west direction with respect to the Antarctic peninsula, thereby allowing characterization of the east-west correlation functional form. Vihma et al. (1996) find longitudinal (Uice) and lateral (Vice) integral correlation length scales of 700 and 550 km,
respectively. Comparative results of Thorndike (1985) for the Arctic Beaufort Sea are
800 and 600 km for Uice and Vice respectively. In contrast the east-west integral correlation length scale of the scalar atmospheric pressure was 890 km during this period,
while the geostrophic wind integral correlation length scales were UG = 1050 and VG =
330 km, respectively. These results indicate that the region of predominantly northward ice motion off the continental shelf exhibits a large decorrelation in velocity in
the east-west direction, and that there is confirmed existence of a large gradient in
northward velocity components with distance from the Antarctic coast. Indeed, the
synoptic pressure pattern also reflects a large gradient in velocity in this same direction.
8.5.1.2
Central Basin
Kottmeier et al. (1992) investigated seasonal and spatial variations in the correlation
length and integral length scales of ice motion to find lateral integral correlation length
scales ranging between 270 and 540 km and longitudinal integral correlation length
scales of between 490 and 680 km. Generally, the correlation length scales become
smaller with greater mobility of the ice, or vicinity to the ice margin, and the smallest
values correspond to summer ice drift near the ice edge in ice concentrations less than
70%. Thus, less compact summer ice tends to have smaller length scales than, for
instance, winter ice or ice drifting close to the coastline, as above. In the winter, length
scales found by Kottmeier et al. (1992) in the central Weddell are somewhat smaller than
those observed in the western Weddell. Though the basin-scale circulation in the Weddell and Beaufort Seas is of a similar scale (approximately 1500 km), the difference
between these central Weddell length scales and those obtained in the Arctic Basin by
Thorndike (1985) suggest that the ice drift differences are caused largely by smaller
scales of forcing. Ice motion data derived from satellite radar images must be used to
test this hypothesis.
M.R. DRINKWATER
buoy data in 1992. Corresponding length scales are computed from European Centre
for Medium Range Weather Forecasting (ECMWF) analysis fields of atmospheric pressure and geostrophic wind. The length scales of sea ice drift and atmosphere appear to
vary seasonally and the ice-drift field is especially dependent upon location and primary drift direction with respect to the coast.
8.5.1.1
Coastal Regime
In the northwards drifting perennial ice regime of the western Weddell Sea, ice motion
is forced by both wind and currents. Periodic deviations from this relatively constant
drift pattern are caused by passing cyclones or "polar lows:' Because of the large fractions of thick ice, ice conditions most closely resemble those of the Arctic basin, and
thus the ice drift correlation length scales may be expected to vary seasonally and interannually with ice concentration. In 1992, Argos buoys were spaced at intervals in the
east-west direction with respect to the Antarctic peninsula, thereby allowing characterization of the east-west correlation functional form. Vihma et al. (1996) find longitudinal (Uice) and lateral (Vice) integral correlation length scales of 700 and 550 km,
respectively. Comparative results of Thorndike (1985) for the Arctic Beaufort Sea are
800 and 600 km for Uice and Vice respectively. In contrast the east-west integral correlation length scale of the scalar atmospheric pressure was 890 km during this period,
while the geostrophic wind integral correlation length scales were UG = 1050 and VG =
330 km, respectively. These results indicate that the region of predominantly northward ice motion off the continental shelf exhibits a large decorrelation in velocity in
the east-west direction, and that there is confirmed existence of a large gradient in
northward velocity components with distance from the Antarctic coast. Indeed, the
synoptic pressure pattern also reflects a large gradient in velocity in this same direction.
8.5.1.2
Central Basin
Kottmeier et al. (1992) investigated seasonal and spatial variations in the correlation
length and integral length scales of ice motion to find lateral integral correlation length
scales ranging between 270 and 540 km and longitudinal integral correlation length
scales of between 490 and 680 km. Generally, the correlation length scales become
smaller with greater mobility of the ice, or vicinity to the ice margin, and the smallest
values correspond to summer ice drift near the ice edge in ice concentrations less than
70%. Thus, less compact summer ice tends to have smaller length scales than, for
instance, winter ice or ice drifting close to the coastline, as above. In the winter, length
scales found by Kottmeier et al. (1992) in the central Weddell are somewhat smaller than
those observed in the western Weddell. Though the basin-scale circulation in the Weddell and Beaufort Seas is of a similar scale (approximately 1500 km), the difference
between these central Weddell length scales and those obtained in the Arctic Basin by
Thorndike (1985) suggest that the ice drift differences are caused largely by smaller
scales of forcing. Ice motion data derived from satellite radar images must be used to
test this hypothesis.