Seasonal change in floe size distribution at the northern entrance of the Arctic Nares Strait derived from Sentinel-1 images: retrieval method and case study in 2019

Floe size distribution (FSD) is a parameter that describes the geometric features of discrete sea ice floes. It plays a crucial role in the momentum, energy and mass exchanges between atmosphere and ocean over the frozen waters, through regulating the ice field consolidation and lead distribution. Previous studies on characterizing the Arctic FSD mainly focused on the marginal ice zone (MIZ) in the peripheral waters. However, continuous ice thinning and enhanced response of ice motion to wind forcing lead to more intense sea ice fragmentation in the Arctic pack ice zone (PIZ). This, in turn, further strengthens sea ice mobility and contributes to the Arctic sea ice outflow. Therefore, this study focuses on the FSD at the northern entrance of the Nares Strait, a region characterized by severe ice conditions and high ice dynamics, which is essential for the outflow of Arctic multi-year ice. We firstly developed a parameter-free method based on a deep convolutional neural network and graph partitioning algorithm to retrieve floes geometry in this specific region from Sentinel-1 synthetic aperture radar images. Using manual produced ground truth data as the benchmark, the proposed method outperformed four conventional algorithms, including K-means, dynamic local thresholding, watershed, and kernel graph cut, in visual assessment and quantitative evaluation. It achieved an overall accuracy and F1-score of 97.0% and 97.6%, respectively. Subsequently, the method was applied to obtain the FSD at the northern entrance of the Nares Strait in 2019. Result showed that the FSD exhibited a power-law distribution for floes with mean caliper diameter ranging from approximately 3 to 53 km for most time. Exceptions occurred in early autumn at the onset of ice freezing, where invalid power law exponent α emerged due to the finite image size and/or abnormal ice advection. We further found that the variations of α were primarily regulated by sea ice thickness, ice advection, and events of floe breakup and welding. Compared with that identified in the Arctic MIZ, seasonal change in FSD in this region appeared relatively moderate, owing to high sea ice concentration and different regulating mechanisms.This work provides a practical algorithm for floe geometry retrieval and a rare case study on the seasonal change in FSD in the Arctic PIZ.