Iridium two-way communication is essential for ice floats as well as the ability to internally store profiles over winter when the float cannot surface due to ice cover. When the sea ice first melts in spring the float will surface and telemeter all of the stored under-ice profiles from the previous winter period, however the profile position data are lost. A challenge for the community is how to best estimate float positions when the float is under ice. A number of efforts are underway to improve estimation of under-ice profile location.
Ice floats in the Weddell Sea that are equipped with a RAFOS receiver can be geolocated through an array of acoustic moorings. Through the analysis of received acoustic signals the position data can be calculated in post processing. A proposal to deploy an acoustic array of sound sources and RAFOS-equipped floats is under consideration for the Ross Sea.
Active ice sensing is being tested on the Pro-Ice floats deployed in the Baffin Bay. All floats carry an upward-looking altimeter to detect ice-conditions and one float was additionally equipped with an optical ice detector and was tested in May 2014 under the floe at Qikiqtarjuaq with the sailing ship Vagabond as basecamp. Optical ice detection uses the light depolarizing properties of sea ice and tries to discriminate between different layers of ice. Based on the analysis of these data, further adaptions to the ice-sensing algorithm will be performed.
Another challenge is to minimize potential damage to the floats surfacing in partially ice covered conditions. To avoid shocks from contact with the ice, some float have an additional cage at the top which protects the CTD, any additional sensors and strengthened antennas.
Southern Ocean Ice Argo – floats south of 60°S
Ice floats from the early 2000’s had a higher failure rate and lower life expectancy with approximately 40% achieving 100 cycles and just 15% reaching 200 cycles. Iridium APEX ice floats (mostly deployed since 2010) have very good survivability and lifetimes, with 90% achieving 120-200 cycles (comparable to regular floats).
The two figure panels below show the number of profiles observed south of 60°S from Argo floats (between 2000-2016) on the left and those returned from ship-board hydrography (1900-2013) on the right. In just 16 years, more than 45,000 profiles have been collected by Argo floats including 20,000 winter profiles, this can be compared to 2,700 winter profiles available from ships.
Twenty four percent of profiles south of 60°S are under ice (black dots). The open water profiles are indicated by green dots. The growth of the Southern Ocean array has currently stalled at around 40% of the required number of operational floats.
The Polar Argo community always welcomes contributions of floats for the Southern Ocean array. We are able to assist countries that are new to Argo to partner with countries that have expertise and deployment opportunities in the Southern Ocean.
Acoustically-tracked RAFOS floats in the Weddell Sea
An array of acoustically-tracked floats equipped with RAFOS receivers is operational in the Weddell Sea. The array of acoustic sound sources (with an effective range of around 500km) is maintained by the Alfred-Wegener Institute in Germany. The correlation height depends primarily on distance between the float and the sound source but also strongly on season. In summer, there are large correlation heights for distances up to 700km, which decreases to 300km in winter, mainly due to differences in stratification in the upper water column and interference from sea ice (Klatt et al., 2007). However, poorer signals can still be used if they match up with higher correlation heights, resulting in an effective year-round range of approximately 500km (Klatt et al., 2007).