The Terra Nova Bay (Antarctica) polynya observed by MODIS ice surface temperature imagery from May to June 2009

Abstract

We have examined the development of the Terra Nova Bay polynya (TNBP) using a two-month sequence of Moderate Resolution Imaging Spectroradiometer (MODIS) ice surface temperature (IST) images, complemented by a set of synthetic aperture radar (SAR) images. Results of a comparison between the clear-sky ISTs and the contemporaneous SAR images revealed that the polynya is characterized in the ISTs by a warm core and in the SARs by a bright backscattering area, due to the presence of seawater and grease ice. The activity of the polynya during the period was highly dynamic, with opening and closing phases that had similar temporal length scales, sometimes of less than 24 h. The recent activity of the polynya is evidenced in the SARs by alternate bright and dark backscatter bands formed during openings and calm periods and transported offshore by the ice drift. The thermal fronts that separate the warm core of the polynya from the surrounding ice sheet appeared sharp during the opening phase and smooth during the closing phase. The spatial resolution of 1 km of MODIS ISTs, compared with space-borne passive microwave systems with maximum resolution of approximately 10 km, provides a smaller area for the TNBP than do passive microwave data. The polynya area was detectable up to 80% of time with a revisit time of a few hours, giving an average area of ¡«1300 km2 and peaks up to 3000 km2. The polynya is forced by katabatic winds that have a warm signature, which is also visible in the IST sequence. Katabatic airflows descending from the Reeves Glacier valley contribute to the opening of the polynya in the central part of the bay, while airflows descending from the surrounding glacier valleys contribute to the opening in the north and south of the bay

A New Approach for Monitoring the Terra Nova Bay Polynya through MODIS Ice Surface Temperature Imagery and Its Validation during 2010 and 2011 Winter Seasons

“…Generally, the retrieved mean winter area of about 900-1000 km 2 agrees with previous estimates based on thermal infrared data acquired by AVHRR ranging between 1000 and 1300 km 2 for different years [15,32,39]. Still, it is not far also from the average area of 1300 km 2 in May and June 2009, with a maximum extent of about 3000 km 2 , found by [11] using MODIS IST products. As for 2010, our results completely agree with corresponding scene by scene observations reported by figure 6 in [11] for the same winter season but using MODIS-derived IST products through a different processing scheme.…”

“…Including the two sensors operational on Aqua and Terra satellites, MODIS ice surface temperatures (IST) can be retrieved with a revisit time of few hours over polar regions, a spatial resolution of about 1 km, and a limited root mean square error widely sufficient to discriminate between the warm polynya core and the surrounding cold ice pack. Despite this shorter revisit time and the finer spatial resolution, up to 2010 TIR imagery has not been commonly used for monitoring TNBP; atmospheric disturbance is a basic limit and processing cloud corrections over many scenes requires a considerable amount of time and computing resources [11,27].…”

“…Cloudy scenes have been completely discarded from our analyses, while both clear-sky and fog-contaminated scenes have been included, thus getting a revisit time of few hours in absence of thick clouds (see details in Section 3.4). In fact, even though the actual accuracy of the IST retrieval remains substantially unknown [51], it has been previously assessed that it is possible to map the sea ice also through fog [11]. In total, 1630 valid MODIS scenes, 868 in 2010 and 762 scenes in 2011, have been used for the wintertime TNBP extent estimation (Table 1).…”

“…MODIS scenes allow us to identify the traces of the wind field too, since the katabatic drainage has a warm signature detected by IST. This effect, due to the adiabatic compression and to the strong vertical mixing within the wind layer [34], make the katabatic events visible in the IST maps by their sharply defined wakes as the air stream leaves the glacier valleys [11]. However, these traces do not allow the calculation of the true wind speed but only suggest the main katabatic pathways and the presence or absence of wind forcing at a given time.…”

“…Sensible heat polynyas are thermally driven by upwelling warm waters which prevent new ice formation. Conversely, latent polynyas form adjacent to a lee shore where strong offshore winds, or currents, advect and maintain the adjacent pack ice away from the Antarctic coast [11], yielding rapid formation of new ice; therefore coastal polynyas are often regarded as ice-production factories [12] and represent important hot spots for ice production, deep-water formation and ventilation of the ocean [13,14]. It was estimated that about 10% of the Antarctic sea ice is produced within the major coastal polynyas [15].…”