Quantifying spatiotemporal variability of ice algal blooms and the impact on surface albedo in southwest Greenland

• Main Authors: Wang, Shujie, Tedesco, Marco, Alexander, Patrick, Xu, Min, Fettweis, Xavier
• Format: Text
• Language: English
• Published: 2019
• Subjects: Greenland; The Sentinel; glacier; ice algae; Ice Sheet; Jakobshavn
• Online Access: https://doi.org/10.5194/tc-2019-226; https://tc.copernicus.org/preprints/tc-2019-226/

Albedo reduction due to light-absorbing impurities can substantially enhance ice sheet surface melt by increasing surface absorption of solar energy. Ice algae have been suggested to play a critical role in darkening the ablation zone in southwest Greenland. It was very recently found that the Sentinel-3 Ocean and Colour Instrument (OLCI) band ratio R 709 nm /R 673 nm can characterize the spatial patterns of ice algal blooms. However, Sentinel-3 was launched in 2016 and current data are only available over three melting seasons. Here, we demonstrate the capability of the MEdium Resolution Imaging Spectrometer (MERIS) for mapping ice algae from space and extend the quantification of ice algal blooms over southwest Greenland back to the period 2004–2011. Several band ratio indices (MERIS chlorophyll indices and impurity index) were computed and compared with each other and against field measurements. The results indicate that the MERIS two-band ratio index (2BDA) R 709 nm /R 665 nm is very effective in capturing the spatial distribution and temporal dynamics of ice algal growth on bare ice in July and August. We analyzed the interannual (2004–2011) and summer (July–August) trends of algal abundance and found significant increasing trends of ice algae close to the Jakobshavn Isbrae Glacier and along the middle dark zone between the altitudes of 1200 m and 1400 m. Using broadband albedo data from the Moderate Resolution Imaging Spectroradiometer (MODIS) we quantified the impact of ice algal growth on bare ice albedo, finding a 0.02∼0.04 reduction rate in albedo for each algal population doubling. Our analysis indicates the strong potential for the satellite algal index to be used to reduce bare ice albedo biases in regional climate model simulations.