The Ice-Tethered Profiler : Argo of the Arctic

Author Posting. © Oceanography Society, 2011. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 24 no. 3 (2011): 126–135, doi:10.5670/oceanog.2011.64.

. The new development efforts use a single instrument package traveling up and down along a tether to return repeated high-vertical-resolution profiles, akin to those obtained by free-drifting Argo floats.Here, we present a status report for one of these systems-the Woods Hole Oceanographic Institution (WHOI) Ice-Tethered Profiler-along with highlights of some scientific analyses based on data obtained by these instruments.
We note that complementary data are being collected in the Arctic by Polar Ocean Profiling Systems (POPS) buoys, an instrument system developed jointly by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Metocean Services International (Kikuchi et al., 2007).
Indeed, Argo has been very successful this last decade (Roemmich et al., 2009), with a maintained array of more than 3,000 operational floats that has to date returned some 700,000 usable temperature and salinity profiles that have fueled an exponential growth in the number of published science papers that use those data (total number of papers is now around 700), though the challenge of sustaining the array is ever present (Roemmich and the Argo Steering Team, 2009).Due to the requirement that floats surface for positioning and data telemetry, Argo has not ventured very far north into the Arctic because of the extensive sea ice cover there.However, data are now being obtained regularly from selected seasonal and marginal ice zones using innovative approaches, including instrumented marine mammals (Boehme et al., 2009) Remarkably, eight ITP systems that had been deemed lost after a year or more (and in one case, approximately 2.5 years) of silence, reinitiated data telemetry.We believe these surface units were buried by ice during rafting events, then re-emerged in subsequent ice deformation events or melted out.
The ITP surface unit is designed to store data when its satellite telemetry system is unable to relay information to shore.
In these cases, many more months of additional CTD profile data were recovered from these reborn systems (acquired while the surface unit was buried in ice-obviously, the rafting did not immediately sever the tether in these cases).However, these GPS receivers, when buried by ice, were unable to fix the locations of these instruments, requiring use of data from collocated instruments or independent ice-drift estimates to derive positions for those CTD profiles.Similarly, the ITP underwater unit will store data if the inductive modem subsystem fails.Full records of profile data were extracted from those recovered ITP systems that had experienced modem problems.The recovered data from reborn and recovered ITPs are included in the statistics reported above.

The iCe-BaSed OBSerVaTOrY CONCepT
As frequently as possible, ITPs are deployed together with other autonomous buoy systems to sample a wide range of variables characterizing the Arctic ocean-ice-atmosphere-ecosystem.We term such collection of buoys an Ice-Based Observatory (IBO; see Proshutinsky et al., 2004) et al., 1998;Krishfield et al., 2002).
In similar spirit, the unprecedented

Figure 1 .
Figure 1.(a) Schematic drawing of the ice-Tethered profiler system with components labeled.photographs of (b) the underwater profiling module being attached to the wire-rope tether during deployment and (c) the final buoy installation (present surface buoy design), the latter with CCgS Louis S. St-Laurent in the background.
describe the Ice-Tethered Profiler (ITP) technology in detail.Briefly, the system consists of three main components: a surface instrument package that typically sits atop an ice floe, a weighted, wire-rope tether of arbitrary length (up to 800 m) suspended from the surface package, and an instrumented underwater unit that travels up and down the wire tether (Figure 1).The current design of the ITP surface expression is a conical-shaped buoy that houses a controller, inductive modem electronics, a GPS receiver, and an Iridium satellite phone with associated antennae and batteries within a watertight aluminum housing capped by an ultra-highmolecular-weight (UHMW) polyethylene dome.The electronics case sits within a foam body designed to provide buoyancy for the plastic-jacketed wire rope tether and end weight should the ice fracture or melt, and to provide modest protection in the event of ice ridging.The ITP tether is constructed from conventional ¼ in (.6 cm) diameter plastic-jacketed wire rope commonly used in ocean mooring applications.The upper 5 m of the wire tether is cast within a 3.5 in (8.75 cm) thick protective urethane jacket that also houses an electrical ground lead for the inductive modem.A custom termination is used to mechanically join the tether to the surface unit and preserve the electrical isolation of the wire tether from seawater.A 250 lb (112.5 kg) ballast weight (made up of 50 lb [22.5 kg] plates to facilitate transportation) is fixed to the bottom wire termination to add tension to the wire and minimize its catenary when the supporting ice floe moves.The profiler unit (much like an Argo float in shape and size) mounts on the tether and cycles vertically along it.Unlike a float or the profiling module of the POPS instrument, which adjust buoyancy to profile, ITP uses a small traction drive wheel mounted midway iNTrOduCTiON At the turn of the last century, the international Argo float program was beginning to make good on its promise to return upper-ocean water properties and circulation information at ~ 300 km horizontal resolution and ~ 10-day intervals throughout the temperate seas Figure 2. (a) location of full-or partial-depth conductivity-temperature-depth (CTd) profiles acquired by ice-Tethered profilers (iTps) in the arctic.The close spacing of the profile locations makes these points appear as continuous lines on the scale of this figure.(b) Bin-totals of the number of full-depth or partial CTd profiles acquired by iTps since 2004 (left), the corresponding figure for all available casts from the 1970s (right), and the subset of 1970s data where the ocean depth exceeded 760 m (middle).in all cases, the bins for the analyses were 55.5 km 2 .Bar coloring as well as bar height indicate the number of profiles in a bin; a reference bar is given for 200 casts in a bin.The 1970s station information is derived from Timokhovand Tanis (1997and Tanis ( , 1998)).

Figure 3 .
Figure 3. (a) histogram of the number of conductivity-temperature-depth (CTd) profiles exceeding 700 m in vertical extent obtained from individual ice-Tethered profiler (iTp) units.(b) lifetimes of individual iTp systems.The blue bars mark the time span during which data were received from the underwater vehicle; the gray bars show the lifetimes of the surface buoys.iTp units #1 and #2 are shown swapped in this panel for visual esthetics.gaps in the figure represent iTp systems deployed in the Southern Ocean or Crater lake, Or.

Figure 4 .
Figure4.drift track of an ice-Based Observatory (iBO) between april 19, 2010, and November 10, 2011, indicating, in color, the salinity at 10 m from the ice-Tethered profiler in the instrument cluster.The 1,000, 2,500, and 3,000 m isobaths have been plotted using the iBCaO grid.images are from the NOaa/pMel web camera installed on the same ice floe (see http://www.arctic.noaa.gov/gallery_np.html).Black pointers indicate the iBO location at the start and end of the melt period.FromTimmermans et al. (2011)

Figure 5 .Figure 7
Figure 5. Objectively mapped observed freshwater inventory from the surface to the depth of the 34 isohaline for the deep arctic Ocean during July-august-September (JaS): (a) 1992-1999 and (b) 2006-2008.The anomaly of 2006-2008 relative to 1992-1999 is shown in (c).The locations of measured salinity profiles used for the mapping are shown as black dots in (a) and (b).FromRabe et al. (2011)