Observation of the 2018 North Greenland polynya with a new merged optical and passive microwave sea ice concentration dataset

Abstract. Observations of sea ice concentrationare available from satellites year-round and almost weather-independently using passive microwave radiometersat resolutions down to 5 km. Thermal infrared radiometersprovide data with a resolution of 1 km, but only under cloud-free conditions. We use the best of the two satellite measurements and merge thermal infrared and passive microwave sea ice concentrations. This yields a merged sea ice concentrationproduct which combines the gap-free spatial coverage of the passive microwave sea ice concentrationsand the 1 km resolution of the thermal infrared sea ice concentrations. The benefit of the merged product is demonstrated by observations of a polynya which opened north of Greenland in February 2018. We find that the merged sea ice concentrationproduct resolves leads as sea ice concentrationbetween 60 % and 80 %. They are not resolved by the coarser passive microwave sea ice concentrationproduct. Next, the environmental conditions during the polynya event are analysed. The polynya was caused by unusual southerly winds during which the sea icedrifted northward instead of southward as usual. The daily displacement was 50 % stronger than normal. The polynya was associated with a warm-air intrusion caused by a high- pressure systemover the Eurasian Arctic. Surface air temperatures were slightly beneath 0 °C and thus more than 20 °C above the average. Two estimates of thermodynamic growth yield accumulated growth of 60 and 65 cm at the end of March. This compares well with airborne sea ice thicknessmeasurements. 33 km 3 of sea icewere produced thermodynamically.