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Platelet ice is a unique type of sea ice; its occurrence has numerous implications for physical and ecological systems. Mostly, platelet ice has been reported from the Antarctic where ice crystals grow in supercooled ice shelf water and accumulate below sea ice to form sub-ice platelet layers. In the Arctic however, platelet ice formation has only been sparsely documented so far. The associated formation processes and morphology differ significantly from the Antarctic, but currently remain poorly understood. Here, we present the first comprehensive, repeat in-situ observations of a decimeter thick sub-ice platelet layer under drifting pack ice of the Central Arctic in winter. Observations carried out with a remotely operated underwater vehicle (ROV) during the midwinter leg of the MOSAiC drift expedition provided clear evidence of the growth of platelet layers from supercooled water present in the ocean mixed layer. This process was observed under all ice types present during the surveys. Oceanographic data from autonomous observing platforms leads us to the conclusion that platelet ice formation is a widespread yet overlooked feature of Arctic winter sea ice growth.

The Beaufort Gyre (BG) is the largest liquid freshwater reservoir of the Arctic Ocean. The liquid freshwater content (FWC) significantly increased in the BG in the 2000s during an anticyclonic wind regime and remained at a high level despite a transition to a more cyclonic state in the early 2010s. It is not well understood to what extent the rapid sea ice decline during this period has modified the trend and variability of the BG liquid FWC in the past decade. Our numerical simulations show that about 50% of the liquid freshwater accumulated in the BG in the 2000s can be explained by the sea ice decline caused by the Arctic atmospheric warming. Among this part of the FWC increase, 60% can be attributed to surface freshening associated with the reduction of the net sea ice thermodynamic growth rate, and 40% to changes in ocean circulation, which makes freshwater more accessible to the BG for storage. Thus, the rapid increase of the BG FWC in the 2000s was due to the concurrence of the anticyclonic wind regime and the high freshwater availability. We also find that if the Arctic sea ice had not declined, the liquid FWC in the BG would have shown a stronger decreasing tendency at the beginning of the 2010s owing to the cyclonic wind regime. From our results we argue that changes in sea ice conditions should be adequately taken into account when it comes to understanding and predicting variations of BG liquid FWC in a changing climate.

Freshwater in the Arctic Ocean plays an important role in the regional ocean circulation, sea ice, and global climate. From salinity observed by a variety of platforms, we are able, for the first time, to estimate a statistically reliable liquid freshwater trend from monthly gridded fields over all upper Arctic Ocean basins. From 1992 to 2012 this trend was 600±300 km3 yr−1. A numerical model agrees very well with the observed freshwater changes. A decrease in salinity made up about two thirds of the freshwater trend and a thickening of the upper layer up to one third. The Arctic Ocean Oscillation index, a measure for the regional wind stress curl, correlated well with our freshwater time series. No clear relation to Arctic Oscillation or Arctic Dipole indices could be found. Following other observational studies, an increased Bering Strait freshwater import to the Arctic Ocean, a decreased Davis Strait export, and enhanced net sea ice melt could have played an important role in the freshwater trend we observed.

AUTOSUB‐2, an autonomous underwater vehicle (AUV) developed by the Southampton Oceanography Centre, was used for high resolution hydrographic surveys in the Sicily Strait. A combination of “seasoar” type profiling and terrain following missions were undertaken and velocity and hydrographic measurements taken from AUTOSUB‐2 were compared with concurrent shipboard hydrographic and velocity profiles. Even though shipboard stations were separated by just 5 to 8 km along the mission path, data from the AUV showed small scale variability that was missed by the shipboard sampling. In this paper we present the example of an intense jet, with maximum speed greater than 0.50 m s−1, less than 4 km wide.