18 profiles of seawater temperature, salinity and dissolved oxygen were obtained in a lead next to the MOSAiC Leg5 ice camp between 23 August and 17 September 2020. A Rinko handheld CTD (JFE Advantech Co., Ltd., Japan) was carefully lowered into the lead on a regular basis to determine the spatial distribution and temporal evolution of the stratification in the upper 5 m. The several meter wide lead had formed from an initially small crack through the main ice floe on 24 August 2020. Ice dynamics caused the lead to successively open and close throughout the study period. Lead widths were recorded via a laser distance meter during each profile. A fresh meltwater layer was detected at several locations beneath the floe, and was also observed in the lead. As part of a dedicated study, the present dataset documents the evolution of this layer.

The data were produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020 (grant number AWI_PS122_00). We thank the crew of the RV Polarstern, and in particular the chief scientist Markus Rex. We express our gratitude to all participants of the expedition, particularly to the logistic support for making this endeavor possible, the polar bear guards for keeping us safe and to Alison Webb, Manuel Dall'osto, Katrin Schmidt, Elise Droste, and Emelia Chamberlain for the field support. This study was supported by the Japan Society for the Promotion of Science (grant numbers: JP18H03745; JP18KK0292; JP17KK0083; JP17H04715; JP20H04345; JP16H01596) and by a grant from the Joint Research Program of the Japan Arctic Research Network Center.