Observations of the Terra Nova Bay (Antarctica) polynya by MODIS ice surface temperature imagery from 2005 to 2010

Ice surface temperature (IST) imagery from the thermal infrared bands of the satellite-borne Moderate Resolution Imaging Spectroradiometer (MODIS) has been used to analyze the activity of the Terra Nova Bay polynya from 2005 to 2010. In clear sky conditions, the shape of the open water area of the polynya (i.e. the portion of the ocean free of sea-ice) is revealed by the skin sea-ice temperatures close to the water freezing point provided by the MODIS-derived IST product. The swaths of the MODIS sensors onboard Terra and Aqua satellites over the Terra Nova Bay consist of about 5500 scenes per year, useful to estimate the cloud cover too. The open water area was extracted from clear sky scenes and from scenes affected by fog but providing continuity of the shape of the polynya. Using about 40% of the available swaths, the polynya was detected for more than 80% of the sea-ice period with a revisit time below 24 h and for more than 50% with a revisit time below 3 h. The open water area was extracted using thresholds of IST based on the IST values detected within the open water area and within the band of sea-ice surrounding the polynya, obtained scene by scene. An ice free area larger than 100 km2 was observed near the Nansen Ice Shelf for more than 70% of time (larger than 500 km2 for 50% of time). The mean annual open water area, around 900 km2, was close to the lower limit of the reported range except during 2006, around 600 km2, due to the limited activity in the second half of the cold season. The largest extents, between 6000 and 8500 km2, occurred in different periods of the Antarctic winter and exceeded maxima estimated in previous studies by thermal infrared data. Polynya openings developed from a few hours to several days, in the latter case sustained by subsequent katabatic events visible in the IST sequence. Peaks of growing rate above 300 km2/h were observed during intense katabatic events with gusts of 50 m/s. The fluctuations of the open water area resulted well correlated with wind records available from local Automatic Weather Stations within the main katabatic routes, and followed katabatic events at intervals between 0 and 6 h. The analysis of the IST data set derived from the thermal infrared bands of MODIS provides new polynya observations of high horizontal resolution (1 km) and frequent revisit time during the winter season, complementing previously available observations with passive microwave imagery.