Feature Articles—October 2009 Issue
Autonomous Daily CTD Profiles Between 3,700 Meters and the Ocean SurfaceA Moored CTD Profiling Installation Covers the Complete Water Column Between the Deepsea Ocean Floor and the Ocean Surface
By Dr. Gereon Th. Budéus
Senior Scientist,
Physical Oceanography
and Instrument Development
Alfred Wegener Institute
for Polar and Marine Research
Bremerhaven, Germany
In the summer of 2008, an autonomously profiling moored installation was successfully established to provide conductivity, temperature, depth (CTD) measurements between the deepsea ocean bottom and the surface for an entire year. It was moored in the subarctic waters of the Greenland Sea at about 75° N and 4° W, where the ocean is roughly 3,700 meters deep. The installation aimed to provide daily profiles over the entire depth range.
Oceans exhibit a complex multiscale and variable vertical structure that classical in-situ measurements cannot resolve. Detailed profiles can be obtained from a ship only at very specific times, fixed instruments can record continuously only at discrete points in the vertical column, and moored deepsea profilers cannot operate in the uppermost ocean layer or reach the ocean surface. Drifters and floats are restrained closely to the initial water package into which they have been deployed and can perform only comparatively few profiles, and satellites are of little help as they can only observe the ocean surface, since the ocean is opaque to electromagnetic waves.
It is important, however, to include the surface layer of the ocean in field measurements because the important freshwater component is concentrated there, as well as the heat content during most times of the year. At the mentioned mooring site, the properties of the uppermost layers greatly determine how exactly winter convection works and to which depths it penetrates. Continuous profiles between the deepsea bottom and the sea surface are therefore high on the wish list of field researchers, particularly in the polar regions.
Evidently, principal difficulties arise from the energy demands of such an installation. This is common to all deepsea profiling installations. A second, not less important difficulty is reaching the ocean surface. In order to overcome the effect of surface waves on the vertically moving vehicle, much greater forces are required in the upper 100 meters of the ocean than in its deeper parts. This can similarly consume huge amounts of energy.
Due to these issues, the profiling installation was divided into two parts: one moving between the ocean bottom and a nominal depth of about 100 meters and a second covering the upper part between about 160 meters and the surface. The first, the deepsea yoyo, operated with very small applied forces and covered the largest part of the water column. The latter, the shallow-water yoyo, utilized larger forces, but only had to cover a small fraction of the water column.
Downcasts to 3,700 Meters
The Externally Powered/ Compress-ibility Compensated (EP/CC)-Yoyo, which was designed by the Alfred Wegener Institute and has been used operationally for a number of years, served as the deepsea yoyo. It is driven mechanically by the application of small weights dispensed by a control unit on the top of the mooring that are thrown off (and collected) when the vehicle has reached the ocean bottom.
The general philosophy is to use a slim vehicle in order to reach high speeds with a small amount of force. The vehicle’s diameter is 110 millimeters with a length of 2.4 meters. A small titanium basket on the vehicle holds the weights that drive the movement. Downward movement is driven by about four newtons, resulting in a downcast velocity of roughly one meter per second. Upward movement is forced by about 1.2 newtons.
The vehicle moving up and down consists of two elements: a buoyancy module and a self-contained CTD. The CTD is a special version of a Sea-Bird Electronics (Bellevue, Washington) Seacat SBE 19-plus that has been further modified to reduce its weight. The buoyancy module serves a twofold purpose: It provides buoyancy and at the same time adjusts the overall compressibility of the vehicle so it resembles the compressibility of seawater. Without such an adjustment, half of the force applied for the downcast would vanish in the deep ocean due to the increase in the vehicle’s buoyancy.
Both requirements—correct buoyancy and correct compressibility—are achieved by the right mix of the heavy and almost incompressible CTD instrument, the main buoyancy elements (custom-made ceramic spheres by San Diego, California-based DeepSea Power & Light), and very compressible and light fluids that fill the space between the spheres in the tubular buoyancy module.
While the equipment has been operational for quite some time, a completely redesigned new generation of the vehicle was deployed in 2008. This new buoyancy module avoids the use of volatile and combustible liquids by replacing them with greasy fluids that are less compressible but still suited for the application. Compromises in terms of compressibility could be accomplished because the new CTD uses a pumped conductivity sensor and records pressure values for every scan. A further improvement was the increase in measurement frequency from one to four hertz. All of these improvements are due to the new low-energy-consumption design of the CTD.
Challenges of Reaching the Surface
The shallow-water yoyo consisted of an underwater winch, the AES-3 (automatic elevating system) manufactured by Saitama, Japan-based Nichiyu Giken Kogyo (NGK), and a CTD, a custom design by Optimare (Wilhelmshaven, Germany) that utilizes a Sea-Bird Electronics SBE41.
The AES consists of a sophisticated combination of an underwater winch, a control unit, multiple parallel power supplies and a frame and buoyancy structure. It is intended for use in the open ocean at a depth of less than 500 meters, and it enables measurements to be taken right up to the ocean surface.
The depth restriction is due to the type of flotation used—the winch can be used as a bottom-standing version in water depths of up to 2,000 meters.
When the NGK system pays out rope, a buoyant instrument ascends until it reaches the water’s surface. The winch is then halted and reverses its rotational direction, thus moving the instrument back to its parking position close to the winch. A tension sensor in the elevator system detects the instrument’s arrival at the surface.
The shallow-water yoyo in the Greenland Sea was the first use of the NGK system outside of Japan.
The NGK system’s buoyancy is about 500 newtons, and buoyancy needed to be added to carry the mooring rope and apply a reasonable tension to it. To achieve this, a modular set of cylindric syntactic foam elements (produced by Biddeford, Maine-based Flotation Technologies), which have the same diameter as the NGK frame, were used. Three slices of syntactic foam were placed just beneath the winch and one mounted on its top. Each of these four foam elements provides a buoyancy of about 900 newtons, so the total buoyancy was about 4,000 newtons. The 8.6-millimeter Aramid rope was made by Lippmann (Hamburg, Germany).
The shallow-water yoyo’s underwater winch resided at 160 meters’ depth, paying out rope at roughly 0.1 meters per second.
The combination of the CTD and flotation had a nominal buoyancy of 80 newtons to overcome the friction imposed on it by wave action, and six 24-volt battery pressure cases with a total of 300 ampere-hours supplied the energy necessary for daily profiles over the one-year deployment.
Once the vehicle reached the ocean surface, the winch immediately reversed its rotational direction to minimize the amount of time spent in the adverse conditions at the surface.
The AES and the CTD were synchronized only by their clocks; there was no communication between them. The sample rate of the self-contained CTD was one hertz.
Deployment and Operation
The two parts of the system for the deep ocean and shallow upper layer were independent moorings, each comprising a releaser set and ground weight of its own. Their mooring locations were as close together as logistically possible—they were moored with a distance of 1.8 kilometers between them.
Both yoyos were scheduled to perform one cast per day, and they both operated successfully and provided an unprecedented data set that covers the complete winter convection phase in the Greenland Sea—one of the few regions worldwide where the densest waters of the global ocean conveyor belt are formed.
Results
The shallow-water yoyo performed more than 300 casts over a period of 312 days, reaching the surface routinely. After this time, the instrument’s memory and the energy supply for the winch were used up. The winch dove to depths greater than 160 meters only a few times, predominantly in the latest part of the record. Such dives were caused by ocean currents that led to an inclination of the mooring rope. The perfect operation of the NGK winch throughout the observation period is particularly remarkable, as it had already operated on a test deployment for a full year, and its batteries were only exchanged once at sea.
The performance of the deepsea yoyo was equally good. Occasionally there were short pauses in the profiles, and after operating for some time, every fourth profile was missed. The pauses were caused by outstanding current events that prevented the vehicle from profiling due to increased friction between the instrument and the mooring rope. As soon as the current event passed, the profiling continued due to the self-healing nature of the design. The principal operation of the system was not affected by this.
The two data sets overlap between 100 and 160 meters. After correcting and despiking them, they were merged by slicing the related profiles at 130 meters. It turned out that despite the small distance between the two mooring locations, the profiles do not combine seamlessly. This indicates that a small-scale local variability exists in the ocean, even far away from frontal regions, and in summer as well as winter. Water parcels do pass both mooring sites consecutively on occasion, but not necessarily. No attempt has been made to artificially smooth the junction between the yoyo profiles.
Conclusions
The measurements, which began on June 28, 2008, show that cooling at the surface starts in October, but substantial deepening of the surface mixed layer (best seen in salinity data) does not take place before January. Then the subsurface salinity maximum, which stems from an extension of the Gulf Stream, is entrained in the deepening, low-saline mixed layer, and the salt of this maximum helps to achieve densities high enough to let the water parcels sink to greater depths.
It is immediately apparent that ventilation depths exceed 1,200 meters, but a more detailed analysis is needed for an exact determination. Temperatures remain far above the freezing point of roughly -1.8° C throughout the winter, thus providing adverse conditions for ice formation in this region. In contrast to the conventional wisdom in the 1990s, scientists now regard the latter as normal.
The mooring installation described here overcomes for the first time the combined problem of performing many dives to great depths (the instrument has traveled a total of about 2.6 million meters) and reaching the ocean surface proper. The result is a top-quality and detailed data set from one of the few regions of the global ocean suited for deep convection.
Dr. Gereon Th. Budéus has been performing oceanographic research in the Arctic region for more than 10 years. He is involved in related instrument development at the Alfred Wegener Institute for Polar and Marine Research, and his work group designed and produced the Externally Powered/Compressibility Compensated-Yoyo. He holds a Ph.D. from the University of Hamburg.
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