Proceedings Template - WORD - Bedford Institute of Oceanography

The ups and downs in developing an under-ice moored profiler called the ICYCLER Simon Prinsenberg, Roger Pettipas, George A. Fowler and Greg Siddall Bedford Institute of Oceanography, Department of Fisheries & Oceans, Dartmouth, Nova Scotia, Canada

ABSTRACT

MOORING DESCRIPTION

The ICYCLER is a moored oceanographic profiler designed to measure surface layer water properties under mobile ice cover. The instrumentation is deployed in the Canadian Arctic Archipelago to measure the surface properties passing from the Arctic Ocean to the Atlantic Ocean. The profiler is designed to provide daily 50-meter salinity-temperature-chlorophyll profiles for a full year. A description of the ICYCLER design was presented at the ISOPE2003 conference (Fowler et al., 2004). An ICYCLER prototype was successfully used in the Canadian Arctic Archipelago during a year-long deployment. A second re-designed ICYCLER was deployed in the summer of 2004 but was not recovered until 2 years later. Simultaneous ice and oceanographic events caused the mooring to move 11miles eastwards into deeper waters where its buoyancy tank collapsed and the entire mooring sank to the bottom. Data presented showed that ice may have snagged the sensor float when it remained near the surface for a day because of excessive cable resulting from strong ocean currents.

The ICYCLER consists of a two-float assembly. The main (lower) float serves as a mid-water platform below the ice hazard zone from which profiling is initiated (Fig. 1). Below this depth, traditional moorings can be used. The main float reduces energy consumption by enabling a much smaller float to lift the sensors the rest of the way towards the surface and collect profiles of the surface layer.

KEY WORDS: ICYCLER; Profiler; Mooring; Canadian Arctic; Mobile Ice. INTRODUCTION It is generally accepted now that due to climate change, the polar ice caps are melting and that the additional freshwater being introduced could affect ocean processes and possibly change the global ocean circulation. To quantify this, oceanographers need to measure the seasonal variability of freshwater flux from the Arctic Ocean through the Canadian Arctic Archipelago. To do this accurately using current sensor and mooring technologies, the water near the surface must be sampled in situ, since this is where the freshwater is concentrated. However the surface of the Arctic Ocean is frequently covered with moving pack ice having ridges with depths of 25-30 meters which represent hazards to in situ surface mooring instrumentation. To overcome this problem, the Bedford Institute of Oceanography (BIO) has been developing a new moored profiler called “ICYCLER” and the progress of this development is reported here.

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Fig. 1, Typical ICYCLER mooring configuration showing two-float assembly. The ICYCLER’s sensor float (Fig. 2) uses a streamlined OpenSeas “SUB” enclosure (Hamilton, Fowler, Belliveau, 1997) to house a Seabird 19+ CTD with pump and a Wetlabs fluorometer to collect data, while a Datasonics echo sounder monitors the distance to the underside

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of the ice during each profile. Sensor data are relayed to electronics located on the mid-water platform via electro-mechanical cable. The electronics provide data storage, and control the height to which sensors profile.

It shows that fresher surface water (salinity of 26-30 PSU) is mixed down to 50 meters in late summer, but becomes saltier during the winter months due to brine rejection from the growing pack ice. The biweekly variation in profiling height and range is caused by tidal action when monitoring sea level height at the same time each day. In late winter, a ridged area of landfast ice with depths to 10m is stationed above the mooring. During this period the profiling range is smaller as the profiler avoids this ice hazard. The surface layer freshening for the following season started near the end of the time series when the mooring was recovered. THE SECOND-GENERATION ICYCLER The ICYCLER prototype (ICYCLER #1) also returned engineering data that pointed out several shortcomings in the original design. Using this information, a new ICYCLER #2, (Fig. 4), was built and deployed in August 2004 as part of the Canadian Archipelago Through-Flow Study (CATS) under the Arctic/Subarctic Ocean Flux program (ASOF, 2004).

Fig. 2, Sensor float with fairing and buoyancy spheres removed. Round float has 40cm diameter and is 140cm long. To further reduce energy requirements, the ICYCLER mechanically stores the energy gained from the ascent portion of a profile by winching the lower float down. It then uses this stored energy to power the descent phase. The only energy consumed during the profiling cycle is that which is lost to mechanical inefficiencies and imbalanced float drag. Further details of the ICYCLER mechanical design can be found in Fowler et al., 2004).

A new self-contained drive mechanism, called “SeaMotor”, (Fig. 4), was developed to simplify power transmission and enhance reliability. SeaMotor’s watertight exterior case functions as one of the profiling winch drums. This saves space and eliminates the need for underwater slip-rings since the drum always rotates with the cable stored on it. Elimination of the slip-rings reduces the energy loss thereby reducing battery requirements. The drive motor and battery are mounted together on a pendulum inside the pressure case. The pendulum is suspended below bearings supported at the centre of both end caps and the drive motor engages a gear on the winch drum. The streamlined fairings for ICYCLER #2 are made from molded fiberglass, and are designed to avoid flat surfaces which cause hydrodynamic instability. A cylindrical tail aligns the float with the direction of water flow.

THE ICYCLER PROTOTYPE – ICYCLER #1 The Bedford Institute of Oceanography developed a working prototype (ICYCLER #1) that spent two years in Lancaster Sound of the Canadian Arctic Archipelago. It successfully completed 350 cycles over each of the year-long deployments, however a programming error limited the height of each profile during the 2002/03 deployment. The year-long salinity data record from the 2003/04 deployment is shown in Figure 3.

Fig. 4, ICYCLER #2 on storage stand with “SeaMotor” in foreground and with drum cover shown below the ICYCLER. Overall length of float is 250cm and it has a diameter of 100cm.

Fig. 3, Year-long salinity record collected by ICYCLER #1 in Lancaster Sound from August 2003 to August 2004.


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The ICYCLER #2 was deployed during August 2004 but could not be located the following year in August 2005. However, after several days searching for its acoustic beacon in the summer of 2006, it was located 11 miles east of its deployment location and recovered by dragging it up

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from the bottom (206m). The following paragraphs will describe the environmental data that indicate what happened to the mooring over its two year deployment. ICYCLER #2 TWO-YEAR DEPLOYMENT A MicroCat on the mooring line below the ICYCLER #2 main float provided two years of salinity, temperature and pressure data. The pressure data showed that there were three events where the depths of the MicroCat changed. Such events usually indicate that the mooring moved to another location where water depths are different. For the ICYCLER #2 case, the depths were progressively greater. The dates of these events were Aug. 22 and Aug. 31, 2004 and a half year later on April 6, 2005.

Fig. 6,.Surface ocean current speed and direction from mooring site near the ICYCLER mooring in southern Lancaster Sound.

Fig. 5, Wind speed and direction for August and September 2004 observed at Resolute Bay. Local wind data were examined to see if extreme wind events on those particular days could generate large wind drifts and/or waves that could affect the ICYCLER functions. Fig. 5 shows the wind speed and direction for the nearby observing station at Resolute Bay, 60km NW of the mooring site. The data covers the August-September, 2004 period when the first two events occurred (Aug. 22 and 31). Wind speeds on those two days were normal or below normal; so wind forcing alone can be ruled out for the mooring motion. Surface ocean currents at a depth of 10m were measured at the same mooring site with an Acoustic Doppler Current Profiler (ADCP). These data (Fig. 6) show that both mooring motions occurred during large easterly current events. However, similar current events occurred on previous days when the mooring did not move. Currents at the depth of the lower float (50m) were of similar magnitude and considered large for the region. The large currents resulted when both the tidal currents and non-tidal currents had amplitudes of 35 to 40 cm/sec and were in the same direction when the tide was ebbing (eastward current). In addition, the lunar and solar tidal forcing were in phase, producing maximal bi-monthly tidal currents.


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Fig. 7, Daily profiling range of ICYCLER #2 for August and September, 2004. The ICYCLER #2 software was set up to collect one profile per day from the depth of just above the lower float at 46m, to 3m below the ocean surface or bottom of the ice, as recognized by the upward looking echo sounder within the profiling sensor float. Fig. 7 shows the profiling range achieved by the ICYCLER #2 for August and September, 2004. The normal range was achieved 3 times after deployment before the surface float remained near the surface for two days (Jdays 221 and 222). After this, it went back to the normal profiling range. The surface float is left on the surface when the force to pull down the sensor float exceeds a critical level. When the surface float was again left on the surface on Aug. 22 and on Aug. 31, the mooring moved. The software command leaving the float at the surface was removed for the present 2006/07 deployment. Now the main float will continue to try to bring the sensor float down to its docking position, as the pull-down required force quickly reduces away from the surface. The data in Fig. 6 show that strong current events at the time of the profile resulted in leaving the surface float near the surface. However the mooring moved during only two of several such events.

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Acoustic Doppler Current Profilers (ADCP) in their bottom tracking mode can provide ice drifts when ice is present above their moorings. During both mooring motion events, ice was detected by the ADCP and ice drifts were determined (Fig.10). Evidence that mooring motion occurred is inferred in the figure by the change in depth of the MicroCat that is located just beneath the main ICYCLER float. Ice motion was very large (60cm/sec) and eastwards moving along with the strong ocean surface currents that occurred at this time (Fig. 6). Fig. 10 shows that on Aug. 22 the MicroCat depth increased from 64m to 76m and on Aug. 31 the depth increased further to 112m. Available data thus seem to indicate that most likely the surface float became snagged on the ice and the mooring was dragged eastwards into deeper waters. After August 31, the mooring appeared to remain at this location (inferred from the MicroCat depth) until the buoyancy tank collapse on April 6, 2005 (Fig. 11) and the mooring including the MicroCat sank to the bottom (205m).

Fig. 8, Salinity profile data for August 2004 collected by the ICYCLER #2. Fig. 8 shows the short salinity time series of profiles collected during August 2004 by the ICYCLER #2. The surface freshwater layer (salinity 31- 28 PSU) above 30m is present as was seen in the 2003-04 data (Fig. 3) at the same location. Since traditional in situ moorings can only be used below 30m to avoid ice damage, all information on this surface freshwater content would be missed. Data profile gaps show where the sensor float remained at the surface. After the first mooring motion on August 22, the ICYCLER was capable of reaching the surface from its deeper location resulting in a large profile range (Aug. 25 and 26).

Fig. 11, Collapsed buoyancy tank of ICYCLER #2. Round floats are 20cm in diameter and overall length of the tank is 170cm.

CONCLUSIONS

Fig. 10, Ice drift velocities (eastwards) during the time the mooring moved on Aug. 22 (top panel) and on Aug. 31 (bottom panel). On the same time scale, the depth variation of the MicroCat below the main float reflects the mooring motion. Although the pack ice breaks up during the summer from east to west in the Northwest Passage, some ice is present most of the time around the mooring site. Ice charts from the Canadian Ice Service show landfast ice along the southern shore of Lancaster Sound and mobile pack ice west of the mooring in Lancaster Sound (Can Ice Service, 2004).


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The ICYCLER’s unique efficient design features enable surface layer water properties to be measured year-round under mobile ice cover using a logistically manageable package. The technology has been proven and extremely rare oceanographic time series data are starting to be returned. The returned engineering data are being used to keep improving the instrumentation technology and data collection efficiency. Development has been frustrated by the fact that no major hardware modification can be made in the field when one instrument is retrieved and one deployed. Only software changes can be made in the field if one feels that those are necessary from inspecting the data from the recovered instrument. Major hardware modifications are thus not verified as quickly in comparison to instrumentation developments in southern latitudes. The experience and data analysis of the ICYCLER #2 deployment for 2005-06 presented here indicated that currents and ice drifts are larger than expected and occur when non-tidal and tidal currents are in the same direction and when tidal currents are at their bi-monthly maxima. This caused extreme winching pull forces and forced the profiler to leave the sensor float near the surface for several days. On two occasions, August 22 and August 31, the ice present at the mooring site

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appeared to have snagged the sensor float and moved the mooring eastwards into deeper waters. The second mooring motion put the main float at over 100m water depth (twice the deployment depth), and after a half year its buoyancy tanks collapsed under the increased hydrostatic pressure, taking all the instrumentation to the bottom . Some operational changes were made to the ICYCLER #3 now deployed in the Canadian Archipelago. A heavier anchor was used, the “stop limit” to the ice cover was increased from 3m to 5m, and the software statement of the limit force to pull the profiler down was eliminated. With these and other hardware changes, the third ICYCLER now in the Arctic is expected to demonstrate increased efficiency of the system and enhanced data recovery reliability. ACKNOWLEDGEMENTS The authors would like to thank ICYCLER’s development team at the Bedford Institute of Oceanography for the fabrication of prototypes and Brian Beanlands for software development, Murray Scotney for mooring and logistic support, and Jim Hamilton for modeling assistance and managing the Archipelago field work. Internal and external reviewers are thanked for their helpful comments on the various drafts of the manuscript. Personnel of Canadian Coast Guard icebreakers are thanked for their continued support during field operations. This work was supported by the Canadian Program of Energy Research Development (PERD), NOAA Grant no. NA17RJ12323 (P.Rhines) and the Department of Fisheries and Ocean’s High Priority Program.

REFERENCES ASOF (2004). “Arctic/Subarctic Ocean Fluxes, Newsletter No2”, March 2004. (http:asof.npolar.no). Can. Ice Service (2004). http://ice-glace.ec.ca. Fowler, GA, Siddall, GR and Prinsenberg, SJ (2004) “A energy conserving oceanographic profiler for use under mobile ice cover; ICYCLER,” Int. Journal of Offshore and Polar Engineering. Int. Society of Ocean Polar Engineers, ISOPE, Vol.14, No3: 176-181. Hamilton, JM, Fowler, GA, and Belliveau, DJ (1997). “Mooring Vibration as a Source of Current Meter Error and its Correction,” J.Atmos. and Oceanic Tech., Vol.14, No3, pp. 644-655.


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