Seasonal variability of the mixed layer depth determined using an improved maximum angle method in the Arctic basins

To investigate the spatiotemporal variations in the mixed layer depth (MLD) in the Arctic basins, a new criterion to determine the MLD, called the improved maximum angle method (IMAM), was developed. A total of 45 123 potential density profiles collected using Ice-Tethered Profilers (ITPs) in the Arctic basins during 2005–2021 were used to demonstrate the method’s effectiveness. By comparing the results obtained by the fixed threshold method (FTM), percentage threshold method (PTM), and maximum gradient method (MGM) for profiles in the Canada Basin, Makarov Basin, and Eurasian Basin, we determined that the quality index (1.0 for perfect identification of the MLD) of the IMAM regarding the assessment of the MLD determination method reached 0.94, which is much greater than those of other criteria. Moreover, two types of the density profiles were identified based on the mixed layer development stage. The MLDs of the typical profiles determined using the IMAM were found to have better consistency with the original definition. By utilizing the new mixed layer criterion, the seasonal variations and regional differences in the MLD in the Arctic basins were analyzed. Spatially, the summer and winter MLDs in the Canada Basin were the shallowest (13.55 m in summer, 26.76 m in winter) than those in the Makarov (29.51 m in summer, 49.08 m in winter) and Eurasian (20.36 m in summer, 46.81 m in winter) basins due to the stable stratification in the upper ocean and the subsequent small effects of dynamic and thermodynamic processes (wind-driven stirring and brine rejection) in the Canada Basin. Seasonally, in the three Arctic basins, the average MLD was shallowest (22.77 m) in summer; it deepened through autumn and reached a winter maximum (41.12 m).