Sample maps of the polynya / thin ice distribution around Antarctica for July 31 2000 (left) and August 2 2007 (right). Polynya areas are denoted blue (predominantly open water) and dark cyan (thin ice, < 10-20 cm). The data set focusses on coastal polynyas; areas far off the coast are flagged.
RESTRICTED only accessable in CEN/MPI net or via CliSAP login What does that mean?
Data access via file system: /data/icdc/ice_and_snow/ssmi_polynya
Data of the Special Sensor Microwave / Imager (SSM/I) were used to identify and map polynyas; these are areas of open water and/or thin (< 20 cm) sea ice in regions of typically thick sea ice.
For this purpose a classification method was applied to gridded 37 GHz and 85 GHz SSM/I brightness temperature measurements of each SSM/I overpass to distinguish between three surface types: thick ice, thin ice, and open water.
Polynya maps were obtained for each SSM/I overpass (inter-sensor difference corrected data of DMSP-platforms f10, f13 and f15), quality controlled and merged to maps showing the daily winter-time polynya distribution.
The offered data set (version 01) contains a mean daily composite of the polynya distribution as well as an estimate of the minimum polynya distribution - as a conservative estimate of a likely lower bound of the polynya area.
The basic algorithm is described in Markus and Burns (1995) and Hunewinkel et al. (1998). The modifications that were required to obtain a circum-Antarctic polynya distribution and further details can be found in Kern et al. (2007) and the project report.
From May 2016 onwards NRT polynya maps are created from single swath DMSP-f18 SSM/IS observations and offered via ICDC. The maps below are dummies of the backward running animations of the last 20 polynya maps of the respective region. These are, from left to right: "Central Weddell Sea (CWS)", "Eastern Indian Ocean (EIS)", "Western Pacific Ocean (WPS)", "Eastern Pacific Ocean", "Western Ross Sea (WRS)", "Western Bellingshausen/Amundsen Sea (WBS)", and "Eastern Bellingshausen/Amundsen Sea (EBS)". White, light and dark blue denote thick (> 0.2 m) and thin ice (< 0.2 m) and open water, respectively. Via clicking on the maps you can start the animation; these are updated daily. The land mask is sub-optimal and needs to be updated. Wrong classification results are discarded and cause temporal gaps. Jumps in the spatial distribution of the areas showing valid data are caused by using swath data. Please read the documentation below for further details and limitations. For other days and other Antarctic regions contact Stefan Kern.
We note that the maps derived during the summer and fall months do not show the polynya extent or a proper ice-type distribution. On the one hand, during this time of the year the definition of what a polynya is is not fulfilled and one should not speak of a polynya when the open water areas are connected with the ice-free Southern Ocean. On the other hand, the PSSM only works properly (in terms of discriminating between open water, thin and thick ice) during the freezing season. During summer and fall the class "thin ice" can rather be regarded as an open ice cover and class "thick ice" can be regarded as a mostly closed ice cover.
|Mean polynya distribution||none||0: open water|
|30: thin ice (< 10-20 cm)|
|90 & 100: thick ice|
|220: missing & flagged data|
|Minimum polynya distribution||none||see above|
|Number of satellite overpasses||1/day|
Period and temporal resolution:
- 1992-2008, May to September, respectively
Coverage and spatial resolution:
- Southern hemisphere (without northern part of Antarctic Peninsula and the region between 130°W and 155°W)
- Spatial resolution: 5 km x 5 km; polar-stereographic grid, tangential plane at 70°S
- Geographic Longitude: -180°E to 180°E
- Geographic Latitude: about 40°S to 90°S
- Dimension: 1520 columns x 1660 rows
- Altitude: 0.0 m
Upon request we offer data of the polynya distribution for every subregion used, e.g. the Ross Sea, also as a daily composite or for every SSM/I overpass (depending on the region up to 6/day) for the same time period on the same grid with the same grid resolution also in netCDF format.
The data set does not contain uncertainty estimates*.
According to Markus and Burns (1995) changes in polynya area as small as 50 km² can be identified with a mean accuracy of the obtained polynya area of 80 km². We suggest a more conservative estimate of both numbers of 200 km². Error contributions come from (see also: Publications)
- inaccurate / too old coastline**
- dynamically changing shelf ice borders**
- influence of fast ice, shelf ice, and ice bergs on the classification
- weather influence
- Smearing of strong contrasts due to coarse spatial resolution
*) The daily polynya distribution data available upon request for the single sub-regions contain besides the mean daily composite polynya distribution also an estimate of the maximum and the minimum polynya distribution.
**) In order to mitigate this influence, a new landmask with 5 km grid resolution was created from clear-sky NOAA-AVHRR imagery. This landmask is included in the data set. It is kind of a dynamic coastline as it has been updated every two years for 1992-2004.
When using this data please cite like follows:
Kern, S. (2012) Antarctic daily winter-time polynya distribution from SSM/I brightness temperature data, http://icdc.cen.uni-hamburg.de/polynya_ant.html , (years used), Integrated Climate Data Center (ICDC), CEN, University of Hamburg, Hamburg, Germany, Digital Media.
PLUS the paper Kern et al., 2007 from the References.