Polar sea ice plays an important role in
the climate system, which is, however, not well elucidated due to difficulties
in getting regular information about the state of sea ice cover, in particular
in the Antarctic. The availability of measurement data in the Antarctic has
increased with the advancement of remote sensing methods. Whereas the extent and
concentration of sea ice are regularly derived with reasonable accuracy from satellite
information since 1979, data on sea ice motion has not been
compiled and analysed homogeneously over longer time scales.
The vector of sea ice motion is of special importance among the quantities describing the state of an oceanic ice cover, since it couples the vertical momentum fluxes in the lower atmosphere and in the upper ocean, it causes opening and closing of the ice cover, which affects the heat exchange, and it transports the ice from the areas of prevailing freezing to that of melting and so impacts the thermohaline structure of the ocean as well as convection by changing the density of water. Ice covered parts of the ocean with their high albedo change the surface heat balance of this areas by the high amount of reflected radiation.
The first direct atmospheric and sea ice-related information from the Weddell Sea in Austral winter was provided by the involuntary drifts of the vessels Deutschland in 1911 and Endurance in 1915/1916, which got stuck in the pack ice (Meinardus, 1938).
During the First Global Experiment of the Global Atmospheric Research Program (GARP) in 1979, meteorological buoys were deployed by parachute drop into the western Weddell Sea for the first time, giving meteorological data every 1 or 2 days (Ackley and Holt, 1984). Buoys were used more frequently in the years after 1986. They are capable of monitoring atmospheric pressure, air and ice temperatures, as well as position, and they provide some new and detailed information on geostrophic winds and ice motion from the Weddell Sea. Recent wintertime ship operations of the Winter Weddell Gyre Projects 1986 (WWSP86), the Winter Weddell Gyre Studies 1989 and 1992 (WWGS89, WWGS92), the Ice Station Weddell in 1992 (ISW), and the Antarctic Zone Flux Experiment (ANZFLUX) in 1994 were used to launch buoys, which lasted far beyond the ship cruises and which covered large regions of the Antarctic sea ice belt. The buoys are frequently arranged within groups of three to seven to allow calculation of reliable estimates of geostrophic winds and ice motion and under favorable conditions their spatial derivatives.
Descriptions of the individual buoy programs and on related process studies have been published in Allison (1989); Crane (1990); Hoeber (1991); Kottmeier et al (1992); Kottmeier (1992); Vihma and Launiainen(1993); Ackley (1981); Wadhams (1989); Martinson and Wamser(1990). Since 1994, the International Programme for Antarctic Buoys (IPAB) coordinates the acquisition of data with drifting buoys in the Antarctic. The IPAB data base forms an important part of present day information about Antarctic sea ice motion.
Regarding satellite information on ice drift, various schemes and algorithms have been developed and tested to determine ice motion out of pairs of sequential radar or passive microwave radiometer images (Kwok et al., 1990; Kwok et al., 1998; Liu and Cavalieri, 1998).They base on detection of significant features in the satellite images, which move with the mean drift and appear at different places in the following image. The displacement vector of the single features is calculated with cross correlations methods after moving a search window in a defined region of the target image. Errors can be caused mainly by weather effects (cloud cover) and surface change of the tracked feature (ridging, melting).A comprehensive comparison of different ice motion products and algorithms has been done by Maslanik et al, (1998), giving an overview of the accuracy and main findings.
For the Atlas of Antarctic Sea Ice Drift, ice drift data from both measurement principles are used: