170
M.R. DRINKWATER
is highly influenced by tidal currents and high-frequency motions. Hourly GPS positions afford the possibility to investigate correlations on shorter timescales than Argos
buoys, since the positional errors which accompany hourly interpolated Argos positions are unacceptably large. This limits spectral analysis in Argos buoy data to typically 3-h or greater sampling intervals, since in practice typically only around 18-20
Argos fixes are obtained daily for each buoy in the Southern Oceans.
This combined hourly GPS and wind record indicates that ice on the continental shelf
also responds largely to wind fluctuations on timescales of 120 h (i.e., S days) or more.
Both the ice and wind auto-spectra indicate a predominant peak at around SOO-lOOO h
(i.e. 20-40 days), which represents a dominant response of drift speed to wind fluctuations on times cales associated with synoptically driven pressure systems. Therefore, the
monthly mean pressure situation over the Weddell Sea is responsible for the primary
forcing of northwards drift in the western limb of the Weddell Gyre. The spectrum for
ISW drift speed in Fig. 109 shows a relatively high degree of coherency for fluctuations
of a period exceeding 100 h, but the phase spectrum indicates a mean of zero with a high
degree of variance in the angle between the wind and drift directions for periods up to
around 12 h. This indicates that the ice does not drift in a consistent fashion on short
timescales. Short term fluctuations in coherency and phase (below 12 h) appear to be
caused largely by aliasing of motions induced by tidal currents and breakdown of the
relationship with wind. Though the geostrophic wind is the predominant forcing mechanism on long times cales, short-term fluctuations in velocity caused by internal ice
forces and tides must therefore be considered with respect to satellite observations of
ice drift in these regions.
In order to resolve the specific tidal influence, one must focus on nonnormalized ice
velocity components. Power spectral density results indicate strong diurnal (M2) and
semi-diurnal (K1 and 01) tidal current forced ice velocity fluctuations, with strong peaks
at periods close to 12 and 24 h. Current meters suspended beneath ISW also demonstrate
similar fluctuations (Muench and Gordon 1995). This result compares favorably with the
findings of Vie hoff and Li and recent studies ofVilima et al. (1996) and Kottmeier and
Sellmann (1996). It may be expected, therefore, that tidal fluctuations on times cales of 1
day or less have an important impact on the variance in velocity over the period of satellite repeat orbits. Exact repeat orbits will suffer no serious impact from tidally induced
ice-drift velocity variance, whereas image pairs acquired at fractions or non-multiples
of 24 h, suffer from aliasing of tidally induced velocity components. The impact of tides
is discussed in more detail in the context of SAR results later.
8.5.3
Large-Scale Sea-Ice Tracking Using Visible Wavelength Data
Several remote sensing methods have been used to track sea-ice drift in Antarctica, the
most common of which utilizes visible or near-infrared wavelength data. During the
WWGS '92 experiment a shipborne high resolution picture transmission (HRPT)
receiving station was used to collect data from the Advanced very High Resolution
Radiometer (AVHRR). During this period data from the NOAA-9, NOAA-lO, NOAA-ll
and NOAA-12 satellites were received (Lemke 1994). Because of missing daylight during the austral winter, overpass information is limited to the infrared channels (3, 4,
and S). Similarly, cloud-free data are relatively sparse. Despite these restrictions, some
M.R. DRINKWATER
is highly influenced by tidal currents and high-frequency motions. Hourly GPS positions afford the possibility to investigate correlations on shorter timescales than Argos
buoys, since the positional errors which accompany hourly interpolated Argos positions are unacceptably large. This limits spectral analysis in Argos buoy data to typically 3-h or greater sampling intervals, since in practice typically only around 18-20
Argos fixes are obtained daily for each buoy in the Southern Oceans.
This combined hourly GPS and wind record indicates that ice on the continental shelf
also responds largely to wind fluctuations on timescales of 120 h (i.e., S days) or more.
Both the ice and wind auto-spectra indicate a predominant peak at around SOO-lOOO h
(i.e. 20-40 days), which represents a dominant response of drift speed to wind fluctuations on times cales associated with synoptically driven pressure systems. Therefore, the
monthly mean pressure situation over the Weddell Sea is responsible for the primary
forcing of northwards drift in the western limb of the Weddell Gyre. The spectrum for
ISW drift speed in Fig. 109 shows a relatively high degree of coherency for fluctuations
of a period exceeding 100 h, but the phase spectrum indicates a mean of zero with a high
degree of variance in the angle between the wind and drift directions for periods up to
around 12 h. This indicates that the ice does not drift in a consistent fashion on short
timescales. Short term fluctuations in coherency and phase (below 12 h) appear to be
caused largely by aliasing of motions induced by tidal currents and breakdown of the
relationship with wind. Though the geostrophic wind is the predominant forcing mechanism on long times cales, short-term fluctuations in velocity caused by internal ice
forces and tides must therefore be considered with respect to satellite observations of
ice drift in these regions.
In order to resolve the specific tidal influence, one must focus on nonnormalized ice
velocity components. Power spectral density results indicate strong diurnal (M2) and
semi-diurnal (K1 and 01) tidal current forced ice velocity fluctuations, with strong peaks
at periods close to 12 and 24 h. Current meters suspended beneath ISW also demonstrate
similar fluctuations (Muench and Gordon 1995). This result compares favorably with the
findings of Vie hoff and Li and recent studies ofVilima et al. (1996) and Kottmeier and
Sellmann (1996). It may be expected, therefore, that tidal fluctuations on times cales of 1
day or less have an important impact on the variance in velocity over the period of satellite repeat orbits. Exact repeat orbits will suffer no serious impact from tidally induced
ice-drift velocity variance, whereas image pairs acquired at fractions or non-multiples
of 24 h, suffer from aliasing of tidally induced velocity components. The impact of tides
is discussed in more detail in the context of SAR results later.
8.5.3
Large-Scale Sea-Ice Tracking Using Visible Wavelength Data
Several remote sensing methods have been used to track sea-ice drift in Antarctica, the
most common of which utilizes visible or near-infrared wavelength data. During the
WWGS '92 experiment a shipborne high resolution picture transmission (HRPT)
receiving station was used to collect data from the Advanced very High Resolution
Radiometer (AVHRR). During this period data from the NOAA-9, NOAA-lO, NOAA-ll
and NOAA-12 satellites were received (Lemke 1994). Because of missing daylight during the austral winter, overpass information is limited to the infrared channels (3, 4,
and S). Similarly, cloud-free data are relatively sparse. Despite these restrictions, some