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Abstract:
To realize an autonomous winter time-series observation of the water column in a coastal polynya, we developed a new profiler platform for a fixed location. We deployed a tethered mid-depth float, whose buoyant capability was significantly enhanced by increasing bladder volume. The float system obtained profiles of temperature and salinity over an entire winter in the Cape Darnley Polynya, East Antarctica.The profiling float used in this study sinks and rises by changing its buoyancy. It was tethered to another intermediate float moored at a constant location and depth. Its length was 2600 mm, with a diameter of 200 mm, and weight (in air) of 85 kg. The volume of the bladder was 3500 cc, and the parking depth was set to 150 dbar, tethered with a 900 m long Dyneema rope. To avoid the significant departure from the parking depth, the profiling float checks its depth every five minutes and correct the position if necessary. The profiling float was manufactured by IHI Corporation. The float was equipped with a temperature and salinity sensor (Seabird Scientific SBE37) and pressure sensor (Yokokaya Ltd. FP201A-E21-L20A*B). The sampling interval was set to one second during the profiling mode and 20 minutes during the parking mode. The profiling float is designed to sink to a certain depth (180 dbar in the present case) before it ascends towards the surface. Ascent time varied significantly among profiles but took anywhere from 30 to 260 min to fully ascend the 160 dbar depth range from 180 to 20 dbar. The float was enabled with satellite data transmission so that it can send acquired data in near-real time and receive a new command when it surfaced. To avoid potential collisions with sea ice, it stopped ascending when it detected a near-freezing temperature near the surface. The criterion was set to -1.5 dgreeC at 20 dbar depth, and the sea-ice avoiding protocol functioned perfectly.The system was placed at S 67.649167, E 68.830167 , at a depth of 225 m in the middle of the Cape Darnley Polynha. The profiling float was deployed on 26 February 2017 and retrieved on 26 February 2018. However, the profiling float stopped working at 18 November due to the battery shortage.