Ross Sea polynyas: Response of ice concentration retrievals to large areas of thin ice

Abstract

For a 3‐month period between May and July of 2005, we examine the response of the Advanced Microwave Scanning Radiometer (AMSR‐E) Enhanced NASA Team 2 (NT2) and AMSR‐E Bootstrap (ABA) ice concentration algorithms to large areas of thin ice of the Ross Sea polynyas. Coincident Envisat Synthetic Aperture Radar (SAR) coverage of the region during this period offers a detailed look at the development of the polynyas within several hundred kilometers of the ice front. The high‐resolution imagery and derived ice motion fields show bands of polynya ice, covering up to ∼105 km2 of the Ross Sea, that are associated with wind‐forced advection. In this study, ice thickness from AMSR‐E 36 GHz polarization information serves as the basis for examination of the response. The quality of the thickness of newly formed sea ice (<10 cm) from AMSR‐E is first assessed with thickness estimates derived from ice surface temperatures from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. The effect of large areas of thin ice in lowering the ice concentration estimates from both NT2/ABA approaches is clearly demonstrated. Results show relatively robust relationships between retrieved ice concentrations and thin ice thickness estimates that differ between the two algorithms. These relationships define the approximate spatial coincidence of ice concentration and thickness isopleths. Using the 83% (ABA) and 91% (NT2) isopleths as polynya boundaries, we show that the computed coverage compares well with that using the estimated 10‐cm thickness contour. The thin ice response characterized here suggests that in regions with polynyas, the retrieval results could be used to provide useful geophysical information, namely thickness and coverage.