150
M.L. Van Woert
Nova Bay; it was not until infrared images from the first Earth-observing satellites
became available in the mid-1960s that it became possible to examine the temporal
and spatial évolution of Antarctic Coastal polynyas [22, 23]. These studies indeed
confirmed Priestly’s earlier finding that open water exists throughout the winter
months. Kurtz and Bromwich [6, 24] elaborated on these early satellite studies by
examining the extensive archive of visible and infrared images at the National Snow
and Ice Data Center in Boulder, Colorado. They concluded on the basis of their
analysis that the mean polynya area is 1300 km2 (65 km north/south by 20 km
east/west), but that it varied in size from near 0 to 5000 km2 (65 km north/south by
75 km east/west). This maximum polynya area corresponds roughly to a polynya
with an offshore extent equal to the length of the Drygalski Ice Tongue (~83 km in
1979 [11]). Kurtz and Bromwich [6] also noted that the open water nucléus was surrounded by a roughly 25000-km2 area of thin or loosely Consolidated ice.
Unfortunately, the Antarctic Océan and ice surfaces are often obscured by
clouds in visible and infrared satellite imagery, thus producing a potentially
biased account of the actual polynya fluctuations. In this study, daily sea ice
concentration estimâtes from Spécial Sensor Microwave Imager (SSM/I) are
used to examine temporal fluctuations in ice concentration in the vicinity of
Terra Nova Bay. The SSM/I is a seven-channel microwave radiometer that sensés the Earth's emitted radiation at 19, 37, and 85 GHz vertical and horizontal
polarization and at 22 GHz vertical polarization [25]. The first of these sensors
was launched June 19, 1987, onboard the Block 5D-2 DMSP satellite F-8 and
remained operational through December 1991. The satellite flew in a sun-synchronous, near-polar orbit at a height of -860 km, providing 4 to 5 passes daily
at high polar latitudes.
Antarctic, daily-averaged, 25-km resolution, gridded sea ice concentration data
products were obtained from the National Snow and Ice Data Center [26]. These ice products are derived by applying the NASATEAM algorithm [27,28] to the
19- and 37-GHz vertical polarization data and 19-GHz horizontal polarization
data from the SSM/I. Higher resolution methods of estimating open water fraction in Coastal régions hâve been proposed using data from the 85-GHz channels
[29], Unfortunately, 85-GHz data are not available for the entire period of this
study and therefore are given no further considération.
To quantify temporal changes in the local ice concentration, a région representing the apparent maximum extent of polynya expansion (Fig. 1; defined here
as a contiguous région of low ice concentration extending northeast from Terra
Nova Bay [e.g. 30]) was extracted from the individual daily ice concentration
images. Areal ice concentration, C, was computed by summing the total ice concentration within the région and dividing by the total study area (14000 km2).
Using an areal average damps smaller polynya events, but, given the inhérent
inaccuracies of the présent sea ice algorithms (-15% [31-34]), this level of
smoothing seems warranted. Open water fraction in percent was computed as
(1-C) x 100. The time sequence of daily-averaged open water fraction for the period July 1987 through December 1990 is shown in Fig. 2.
Figure 2 indicates that during winter 1988, roughly defined here as the period
May to October, the ice open water fraction was typically around 20% except for
brief periods in August and early November when the open water fraction under-
M.L. Van Woert
Nova Bay; it was not until infrared images from the first Earth-observing satellites
became available in the mid-1960s that it became possible to examine the temporal
and spatial évolution of Antarctic Coastal polynyas [22, 23]. These studies indeed
confirmed Priestly’s earlier finding that open water exists throughout the winter
months. Kurtz and Bromwich [6, 24] elaborated on these early satellite studies by
examining the extensive archive of visible and infrared images at the National Snow
and Ice Data Center in Boulder, Colorado. They concluded on the basis of their
analysis that the mean polynya area is 1300 km2 (65 km north/south by 20 km
east/west), but that it varied in size from near 0 to 5000 km2 (65 km north/south by
75 km east/west). This maximum polynya area corresponds roughly to a polynya
with an offshore extent equal to the length of the Drygalski Ice Tongue (~83 km in
1979 [11]). Kurtz and Bromwich [6] also noted that the open water nucléus was surrounded by a roughly 25000-km2 area of thin or loosely Consolidated ice.
Unfortunately, the Antarctic Océan and ice surfaces are often obscured by
clouds in visible and infrared satellite imagery, thus producing a potentially
biased account of the actual polynya fluctuations. In this study, daily sea ice
concentration estimâtes from Spécial Sensor Microwave Imager (SSM/I) are
used to examine temporal fluctuations in ice concentration in the vicinity of
Terra Nova Bay. The SSM/I is a seven-channel microwave radiometer that sensés the Earth's emitted radiation at 19, 37, and 85 GHz vertical and horizontal
polarization and at 22 GHz vertical polarization [25]. The first of these sensors
was launched June 19, 1987, onboard the Block 5D-2 DMSP satellite F-8 and
remained operational through December 1991. The satellite flew in a sun-synchronous, near-polar orbit at a height of -860 km, providing 4 to 5 passes daily
at high polar latitudes.
Antarctic, daily-averaged, 25-km resolution, gridded sea ice concentration data
products were obtained from the National Snow and Ice Data Center [26]. These ice products are derived by applying the NASATEAM algorithm [27,28] to the
19- and 37-GHz vertical polarization data and 19-GHz horizontal polarization
data from the SSM/I. Higher resolution methods of estimating open water fraction in Coastal régions hâve been proposed using data from the 85-GHz channels
[29], Unfortunately, 85-GHz data are not available for the entire period of this
study and therefore are given no further considération.
To quantify temporal changes in the local ice concentration, a région representing the apparent maximum extent of polynya expansion (Fig. 1; defined here
as a contiguous région of low ice concentration extending northeast from Terra
Nova Bay [e.g. 30]) was extracted from the individual daily ice concentration
images. Areal ice concentration, C, was computed by summing the total ice concentration within the région and dividing by the total study area (14000 km2).
Using an areal average damps smaller polynya events, but, given the inhérent
inaccuracies of the présent sea ice algorithms (-15% [31-34]), this level of
smoothing seems warranted. Open water fraction in percent was computed as
(1-C) x 100. The time sequence of daily-averaged open water fraction for the period July 1987 through December 1990 is shown in Fig. 2.
Figure 2 indicates that during winter 1988, roughly defined here as the period
May to October, the ice open water fraction was typically around 20% except for
brief periods in August and early November when the open water fraction under-