Introduction


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:

  • All accessible in-situ measurement data from drifting buoys having been deployed in the Antarctic since 1985 provide a high temporal and spatial resolution, but are only available for specific regions and periods at much different spatial resolution.
  • Satellite ice motion estimates resulting from various schemes and algorithms applied on pairs of passive microwave radiometer images, are more continuous, and cover large portions of the Southern Ocean.
Data are placed into a Geographic Information System (PCI) to allow flexible access to data from different regions and times, to display data at arbitrary resolution and combination of variables, and to assure safe archiving of data, in particular of data from drifting buoys of widespread origin and from various  sources. The vector of sea ice motion data u,v as well as spatial derivatives du/dx, du/dy, dv/dx, dv/dy at daily or two-daily resolution and 100 km spatial resolution out of SMMR and SSM/I data provide the basic data. Monthly averages, variances and covariances as well as DKPs (differential kinematic properties, see section “Data Preparation”) are calculated from these, such that the highest resolution of data is daily, with monthly statistics being presented in the figures in the Atlas is from statistics based on data . Various averaging schemes are applied for quick interannual and seasonal visual comparison. The spatial distributions of variables are shown for the whole Antarctic, statistics of derivatives being combined to covariance ellipses.

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