An improved sea ice detection algorithm using MODIS: application as a new European sea ice extent indicator

Joan Antoni Parera-Portell,Charles Gignac,Raquel Ubach

doi:10.5194/tc-15-2803-2021

Abstract

Abstract. The continued loss of sea ice in the Northern Hemisphere due to global warming poses a threat to biota and human activities, evidencing the necessity of efficient sea ice monitoring tools. Aiming at the creation of an improved sea ice extent indicator covering the European regional seas, the new IceMap500 algorithm has been developed to classify sea ice and water at a resolution of 500 m at nadir. IceMap500 features a classification strategy built upon previous MODIS sea ice extent algorithms and a new method to reclassify areas affected by resolution-breaking features inherited from the MODIS cloud mask. This approach results in an enlargement of mapped area, a reduction of potential error sources and a better delineation of the sea ice edge, while still systematically achieving accuracies above 90 %, as obtained by manual validation. Swath maps have been aggregated at a monthly scale to obtain sea ice extent with a method that is sensitive to spatio-temporal variations in the sea ice cover and that can be used as an additional error filter. The resulting dataset, covering the months of maximum and minimum sea ice extent (i.e. March and September) over 2 decades (from 2000 to 2019), demonstrates the algorithm's applicability as a monitoring tool and as an indicator, illustrating the sea ice decline at a regional scale. The European sea regions located in the Arctic, NE Atlantic and Barents seas display clear negative trends in both March (−27.98 ± 6.01 × 103 km2yr−1) and September (−16.47 ± 5.66 × 103 km2yr−1). Such trends indicate that the sea ice cover is shrinking at a rate of ∼ 9 % and ∼ 13 % per decade, respectively, even though the sea ice extent loss is comparatively ∼ 70 % greater in March.

Highlights

The Arctic sea ice cover has been changing rapidly over the last decades, with its overall extent declining steadily since the first satellite observations in the late 1970s (e.g. Cavalieri and Parkinson, 2012; Massonnet et al, 2012; Meier et al, 2014; Parkinson, 2014; Serreze and Stroeve, 2015; Comiso et al, 2017), reaching its historical minimum on September 2012
Sea ice extent maps have been used to determine the sea ice extent trends between 2000 and 2019 in the NE Atlantic–Barents region and the Baltic Sea separately. Both March and September trends have been obtained for the NE Atlantic–Barents, that is, the trends of the maximum and minimum sea ice cover, respectively
Quality assessment revealed the most common error is sea ice commission caused by unmasked clouds, manifesting the key role that cloud masking plays on the overall accuracy of the algorithm

Summary

Introduction

The Arctic sea ice cover has been changing rapidly over the last decades, with its overall extent declining steadily since the first satellite observations in the late 1970s (e.g. Cavalieri and Parkinson, 2012; Massonnet et al, 2012; Meier et al, 2014; Parkinson, 2014; Serreze and Stroeve, 2015; Comiso et al, 2017), reaching its historical minimum on September 2012. The same decreasing trends are evidenced by other parameters such as sea ice thickness (Kwok, 2018; Liu et al, 2020), which has decreased as much as 65 % in the period extending from 1975 to 2012 (Lindsay and Schweiger, 2015). This massive loss of ice is unprecedented in the last few thousand years (Polyak et al, 2010) and is attributed both to climatic variability and to external forcing caused by the anthropogenic release of greenhouse gases (e.g. Myhre et al, 2013; Stroeve and Notz, 2018). Passive microwave sensors typically provide data at resolutions above 15 km, hindering their use for local and re-

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Conclusion