Improved algorithm for determining the freeze onset of Arctic sea ice using AMSR-E/2 data

Over the past several decades, Arctic sea ice has experienced a significantly longer summer melt season, with an earlier melt onset and delayed freeze onset. The early melt onset/melt onset date (EMO/MO) and early freeze onset/freeze onset date (EFO/FO) products developed using the passive microwave (PMW) algorithm derived from Special Sensor Microwave/Imager (SSM/I) passive microwave data are widely used in the study of the variability in Arctic sea ice freezing melting. Here, we apply the PMW algorithm to the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E/2) dataset at a higher spatial resolution and improve the algorithm. We first limit the start and end dates according to the sea ice concentration (SIC). Then, we exclude false freezing signals by identifying high variability in SIC; finally, we consider changes in the distribution of sea ice types (first-year ice and multiyear ice) over time to obtain an improved algorithm. The early freeze onset (EFO) and FO derived from the AMSR data are then compared with results derived from the surface air tem-perature and sea skin temperature (SAT & SKT) and SSM/I products. The spatial distribution of the new EFO results shows greater consistency with SAT & SKT than with the results of SSM/I, with a mean error, absolute error, and root mean square error for the SAT & SKT comparison of 3.7 days, 10.4 days, and 16.0 days, respectively, for the AMSR2 period. The improved FO and EFO correct the small values caused by misidentified summer melt signals and the large values caused by sea ice dynamic processes after the melt season. There is a constant change in ice type over 20%-30% of the ice cover, and the improved FO and EFO correct for anomalies due to misidentified ice types. The improved results correlate better with the interannual variability in EFO derived from SAT & SKT in subregions, especially in ice marginal regions. The Arctic sea ice EFO north of 70 degrees N was delayed at a rate of 6.4 day/dec from 2003 to 2021, with remarkable trends found in the Laptev (14.6 day/ dec) and Kara (14.1 day/dec) Seas.